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									                                                                                                                                                                   OS13B - 0536
                                                                                                                             Modeling the Distribution and 15N of Nitrogen Gas and Nitrogen Species in the Black Sea
                                                                                                                                                                                                                                                                                                                                             S. K. Konovalov1, C. A. Fuchsman2 and J.W. Murray2
                                                                                                                                                                                    1. Marine Hydrophysical Institute, Kapitanskaya St. 2a, Sevastopol 99000 Ukraine; and 2. School of Oceanography, University of Washington, Box 355351 Seattle WA 98195-5351
Abstract                                                                                                                                                                                                                                                                                                                                    Fig. 1                                                                                                                                                                                                        A m m o n iu m ,  M
                                      -,
Distributions of NO3 NO2 NH4 N2 and PON and their-,              +,
                                                                values in the                                                              15N                                                                                                                                                                                                                                                                                                            7. Data and Model Simulations                                                                                                                           10. Model #2 – Vertical Zonation of Processes:                                                                                      11. Model #3 – Vertical Zonation of Processes:
Black Sea were measured in order to unravel the cycling of nitrogen, especially
                                                                                                                                                                                     1. Cruises and Samples                                                                                                                                                                                                                                                                                                                                         0      10       20          30         40         50
                                                                                                                                                                                     Data for this study was collected during recent cruises in the Black Sea:                                                                                                                                                                                             The model can describe all the data.                                                                                                                    Reactions with S, HS-                                                                                                              Scavenging by MnO2
the role of anammox (NO2- + NH4+ = N2). We have constructed a steady state
                                                                                                                                                                                     •R/V Bilim September 2000                                                                                                                                                                                                                                                                                                                       N itrite (N O 2 - ) & S u lfu r (S 0 ),  M
coupled physical-biogeochemical model of the water column. The physical
                                                                                                                                                                                     •R/V Knorr May 2001
model is a vertical transport model that includes simulation of entrainment                                                                                                                                                                                                                                                                                                                                                                                                                                                                        0 .0     0 .1         0 .2            0 .3         0 .4         We allowed the reaction between NO3- and NH4+ to occur in the region of the NO3-                                                    Heavy NH4+ is removed, possibly by preferential adsorption of NH4+ by MnO2.
                                                                                                                                                                                     •R/V Knorr April 2003 (Track and Station Locations shown in Fig. 1)
and intrusions from the Bosporus Plume. The biogeochemical model includes                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       O2                                 gradient (Fig. 11).                                                                                                                 Light NH4+ is left to react with NO2- to make light N2. The adsorbed (and
mass balance equations for 14N and 15N. Comparison of observations and                                                                                                                                                                                                                                                                                                                                                                                     We have found that PON (~50%)                                                                                                                           The origin of the NH4+ is from regenerated PON and its concentration remains low                                                    heavy NH4+) is released back to the water when the MnO2 is reduced in the
                                                                                                                                                                                     The cruise web sites with data are located at:                                                                                                                                                                                                                                                                                                       1 5 .0
model results show:                                                                                                                                                                                                                                                                                                                                                                                                                                        and DON (~40%) are the main                                                                                                                             because it is rapidly utilized.                                                                                                     anoxic layer (Fig. 13). 15% of the upward flux of NH4+ is removed to the
                                                                                                                                                                                     http://oceanweb.ocean.washington.edu/cruises/Knorr2001                                                                                                                                                                                                                                                                                                                                               N2
1. The chemical distributions can be fit by adjusting the respective reaction                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       N O 3-




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              S igm a-t
                                                                                                                                                                                     http://oceanweb.ocean.washington.edu/cruises/Knorr2003                                                                                                                                                                                                                sources of N to the box included                                                                                                                        Light 14N-NO3- would be preferentially used to make light N2.                                                                       anoxic zone and the isotopic fractionation (e) required is -20‰.
rate constants                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     The heavy 15N-NO3- that remains diffuses downward to be reduced by S to NH4+
                                                                                                                                                                                                                                                                                                                                                                                                                                                           in this model (s = 14.5 to 17.2)                                                                        NO2                             S u b o x ic
2. This steady state model can not simultaneously fit the concentrations and                                                                                                                                                                                                                                                                                                                                                                                                                                                              1 6 .0                                                    zo n e         in the lower layer (thus making heavy NH4+).
stable isotope data without including some process that removes heavy                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               S0
nitrogen                                                                                                                                                                                                                                                                                                                                                                                                                                                   The main N sinks are the upward
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            N H 4+
                                                                                                                                                                                                                                                                                                                                                                                                                                                           fluxes of N2 (~70%) and
                                                                                                                                                                                     4. Nitrogen Isotope Distributions                                                                                                                                                                                                                                                                                                                    1 7 .0                                                                                                                                                                                                       Fig. 13                                                A m m o nium ,  M
                                                                                                                                                                                     (Fig. 5) 15N-NO3- increases to a maximum of ≈ 18‰ in the suboxic zone then
                                                                                                                                                                                                                                                                                                                                                                                                                                                           NO3 (~30%)                                                                                           H 2S                                               Fig. 11
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             A m m o n ium ,  M
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    0           100                 200              300                                                                                                                                                                                                0      10            20          30         40             50
                                                                                                                                                                                     decreases with depth. Above the suboxic zone values are about 8‰ which is                                                                                                                                                                                                                                                                   O x y g en (O 2 ) & S u lfid e (H 2 S ),  M                                                                                      0           10           20          30          40            50
2. Nitrogen Species                                                                                                                                                                  higher than average NO3- in seawater (15N-NO3- ≈ 4.5 to 6‰)
Fig. 2 shows an example of distributions of NO3-, NO2- and NH4+                                                                                                                      (Fig. 6) 15N-NH4+ is very low in the deepwater (≈ 2‰) and increase to a                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            N itrite (N O 2 - ) & S u lfu r (S 0 ), M
•NO3- increases to a maximum at st = 15.6 then decreases to 0 at st = 15.95                                                                                                          maximum of 7‰ at the base of the suboxic zone, then decrease slightly.
                                                                                                                                                                                                                                                                                                                                                                                                                                                               Fig. 9                                                                              0 .0      4 .0            8 .0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      N itrate (N O 3 - ) & N 2 ex . g as,  M
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          1 2 .0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    N itrite (N O 2 ) & S u lfur (S ),  M
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        -                                0

•NO2- has a small maximum centered at st = 15.8                                                                                                                                      (Fig. 7) 15N-N2 in the suboxic zone maximum is extremely light           (15N-                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               0.0            0.1             0.2             0.3             0.4
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  0 .0         0 .1              0 .2          0 .3              0 .4
•NH4+ starts to increase at st = 15.95                                                                                                                                               N2≈0.0‰) compared to SW in equilibrium with the atmosphere        ( 15N-N ≈
                                                                                                                                                                                                                                                                 2                                                                          5. Physical Biogeochemical Model                                                                                                                                                                                                                                                                                                                                                                                                                 15.0
                                                                                                                                                                                                                                                    15N-NO - ≥                                                                                                                                                                                                                                                                                                                                                                                           1 5 .0                                                                                                                                             NO2               O2                   N2
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  15N
                                                                                                                                                                                     0.7‰) and to the other forms of N that are sources for N2    (                                                                                        We utilized a 1.5-D physical transport model (Fig. 8) that includes simulation of the                                                                                                                                                                                                                                        NO2
The N2/Ar data from 2000, 2001 and 2003 are plotted as the ratio of                                                                                                                  8‰; 15N-NH4+ ≥ 8‰; 15N-PON = 2.7 to 9.0‰).
                                                                                                                                                                                                                                                             3
                                                                                                                                                                                                                                                                                                                                            Bosporus Plume and assumes isopycnal distribution of all properties (Samodurov
                                                                                                                                                                                                                                                                                                                                                                                                                                                          8. Simulation of                                      distributions                                                                                                                                                               O2                 N2
sample/saturation versus density in Fig. 3.                                                                                                                                                                                                                                                                                                 and Ivanov, 1998; Ivanov and Samodurov, 2001). The resulting 1-D vertical                                     To simulate 14N/15N fractionation, all reactions were written in terms of both 14N                                                                                                                                                                                                                                                                                 N O 3-
•The water column is usually supersaturated with N2 with a maximum                                                                                                                                                                                                                                                                          balance equations can be written as:                                                                          and 15N. The rate of 15N transformations is proportional to the rate of 14N                                                                                                                                                       N O 3-




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 S igm a-t
centered at st ≈ 16.0.                                                                                                                                                                                                                                                                                                                                                                                                                                    transformations, to the ratio of [15N]/[14N], and fractionation factors a=e/1000 + 1,




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            S ig m a-t
                                                                                                                                                                                       Fig. 5                                           15N-NO -                 versus density                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  N O 2 - + N H 4 +   
•Supersaturation in the shallow layers results from upward flux of N2 from the                                                                                                                                                                3                                                                                                                                    C                                                                     where e is the isotopic enrichment factor.                                                                                                                                                                                          N O 3 - (N O 2 - ) + N H 4 +   
                                                                                                                                                                                                                                                                                                                                                                   F lu x =  k           wC                                                                                                                                                                                                                                                                                                                                                                                                16.0
suboxic zone.                                                                                                                                                                                                                                                                                              15 N -N O 3 -                                                           z
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         1 6 .0                                                                                                                                                              N H 4 + M n (II) + O 2  (N H 4 ) x M n (IV )
                                                                                                                                                                                                                   14                    SWavg
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          15N-PON                                                                                                                                                                           N O 3 (N O 2 ) + S (H 2 S )    
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    -          -         0                   
•There is significant interannual variability.                                                                                                                                                                                                                                                                                                                                                                                                            Thus, the rate of oxidation of                                 is                                                                                                                                                                                                                                                                                            S0
                                                                                                                                                                                                                                                                                                       N ear B o sp oru s K n o rr 2 0 03                 C    C      w                                                                                                                                                                                                                                                                                                     S0                                                                                                                                       (N H 4 ) x M n (IV ) + H 2 S  N H 4 + M n (II)
•The concentrations can be reset to atmospheric saturation after intense                                                                                                                                                                                                                               C en tra l G y re K n o rr 2 0 03                k    w    = R    Cb  C                                                                                  N  PON    N  PON   N  PON
                                                                                                                                                                                                                                                                                                                                                                                                                                                                            15            14          15
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 1    15                   O2 
storms.                                                                                                                                                                                                          14 .5                                                                                 C en tra l G y re K n o rr 2 0 01            z     z    z      z                                                                                                    
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      =
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    14
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   =     N  PON  
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    O
                                                                                                                                                                                                                                                                                                                                                                                         C                                                                                    t            t      N  PON                    a   a                  2 1 / 2   O 2 
                                                                                                                                                                                                                                                                                                       N ear B o sp oru s K n o rr 2 0 01   k = vertical diffusion coefficient;  k    = diffusive flux,
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  +                                                                           H 2S                                       N H 4+
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      NH4
                                                                                                                                                                                               S igm a T h eta




                                                                                                                                                                                                                                                                                                                                                                                    z                                                                    Published data on fractionation (a) or isotopic enrichment (e) factors are limited.                                                                                                                             H 2S
                                                                                                                                                                                                                   15                                                                                                                       w = vertical velocity and wC = the advective flux occurring due to displacement of                                                                                                                                                                                                                                                                                                                                                               17.0
 Fig. 2   Example of NO3-, NO2- and NH4+ for R/V Knorr 2003                                                                                                                                                                                                                                                                                 the Black Sea deep waters with the waters from the Bosporus plume,                                                                                                                                                                                                                                           1 7 .0
                                                                                                                                                                                                                                                                                                                                                                                                                                                          It is often difficult to identify, if the published values are specific for individual                                                                                                                                                                                                                                                        0              100                200                      300
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   0                100                   200                 300
                                                  Center of western Gyre
                                                                                                                                                                                                                                                                                                                                            R = rate of biogeochemical production-consumption,
                                                                                                                                                                                                                                                                                                                                                                                                                                                          reactions or processes that may involve a number of reactions.                                                                                                                        O x y gen (O 2 ) & S u lfid e (H 2 S ),  M                                                                                         O x y gen (O 2 ) & S u lfid e (H 2 S ),  M
                                                                                                                                                                                                                 15 .5                                                                                                                      Cb = concentration in the “Bosporus plume”
                                             0             1                                     2          3                    4                                                                                                                                                                                                          C = concentration in the ambient water.                                                                       Still, the published values for the isotopic enrichment factors (e) generally vary
                                                                                                                                                                                                                                                                                                               suboxic zone                                                                                                                               from 0 to 40‰.
                                                                  N O 3- M                                                                                                                                        16                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               0.0            4.0              8.0             12.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  0 .0          4 .0               8 .0            1 2 .0
                                             0             0.1                                  0.2        0.3               0.4
                                                                                                                                                                                                                                                                                                                                            The model has been verified using 137Cs data from 1986 to 2003.
                                                                                                                                                                                                                                                                                                                                                                                                                                                          The enrichment factors (e) required to fit the data are given below:                                                                                                                        N itrate (N O 3 - ) & N 2 ex . gas,  M                                                                                            N itrate (N O 3 ) & N 2 ex . gas,  M
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         -
                                     13 .6
                                                                  N O 2- 
                                       14
                                                                                                                                                                                                                 16 .5                                                                                                                      6. Biogeochemical Transformations                                                                             •nitrate (NO3-) reduction, for all reductants (e.g. CH2O, S) e = 25 - 30‰
                                                                                               B la ck S ea C en tral G y re
                                                                                                                                                                                                                                                                                                                                                                                                                                                          •nitrite (NO2-) reduction and oxidation e = 10 - 15‰
                                                                                                                                                                                                                                  4              8                      12               16             20                                                                                    -,
                                                                                                                                                                                                                                                                                                                                            Transformations of O2, PON, DON, NO3 NO2 N2, NH4 H2S, and dissolved   -,        +,       S0
                                     14 .4                                                             N H 4+                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Fig. 12                                                                                                                             Fig. 14
                                                                                                                                                                                                                                                                                                                                   and particulate Mn(II) and Mn(IV) and Fe(II) and Fe(III) are included in the model.                           •reactions of particulate organic nitrogen to NH4+ e = 2 -5‰
                   S igm a T h eta




                                                                                                       N O 3-
                                     14 .8
                                                                                                       N O 2-                                                                                                                                                                                                                               Dissolved organic N is split into labile (DON(L)) and refractory (DON(R)                                      •ammonium (NH4+) consumption or oxidation (e.g. Anammox) e = 0 to a few ‰
                                     15 .2                                                                                                                                                                                                                                                                                                  fractions. Particulate organic nitrogen is split into the fraction sinking from the                                                                                                                                                                                                                                                                                                                                       15                                                                 15
                                     15 .6                                                                                                                                                                                                                                                                                                  euphotic zone (PON) and the fraction that is bacterially produced (PON(B)).                                                                                                                                                                                                          15                                                                       15
                                                                                                                                          Suboxic Zone                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              M o d el #3




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          S igm a-t
                                       16                                                                                                                                                                                                                                                                                                    Equations that parameterize biogeochemical transformations are written to follow                                                                                                                                                                                                                                            M od el # 2
                                                                                                                                                                                      Fig. 6




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     S igm a-t
                                                                                                                                                                                                                                                                                                                                            either “chemical” or Michaelis-Menten kinetics.                                                               9. Model #1 Simulation                                                                                                                                                                                                    D ata
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                D ata
                                     16 .4                                                                                                                                               15N-PON                                                                                        15N-NH +                                                                                                                                                                                                                                                                                                                                                                                                                                                                      16                                                                 16
                                                                                                                                                                                                                                        versus density                                        4               versus depth                                                                                                                                The model was tuned to simulate 15N profiles in the oxic and anoxic layers (Fig.                                                                                      16                                                 M o d el              16
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                M o d el
                                     16 .8
                                             0             10                                   20         30                40
                                                                                                                                                                                                                                                                                                                                                                                                                                                          10).
                                                                                                                                                                                                                                                                                                                                            Thus, oxidation of sulfide by oxygen is parameterized as a chemical process.                                  The initial conditions derived from the real observations are shown as dashed lines.
                                                                 N H 4+ 
                                                                                                                                                    Leg 8 Station 30                                                                                                                                                                                                                                                                                      Simulations are shown for 90 days, 1 year and 13 years.                                                                                                                                                                                                                                                     17                                                                 17
                                                                                                                                                                                                                                                                                                                                                                          R = k   H 2 S   O2 
                                                                                                                                                                                                                                                                                                                                                                                                                                                          Variations in a (e) for denitrification and anammox did not change the final result.                                                                                   17                                                                       17

                                                                                                                                                                                                                                                                                                                                                                                                                                                          Complete consumption of NO3- (NO2-) and NH4+ does not result in any                                                                                                                                                                                                                                              0.2   0.4          0.6       0.8        1         1.2              -4          0              4    8   12
                                                                                                                                                                                                                                                                                                                                            Microbiologically mediated processes, like oxidation of PON by oxygen are                                     fractionation.                                                                                                                                                              0.2      0.4           0.6         0.8        1        1.2               0             4          8    12   16                                      15 N -N 2 , ‰                                                   15 N -P O N , ‰
 Fig. 3
                                                                                                                                                                                                                                                                                                                                            parameterized by Michaelis-Menten kinetics.                                                                                                                                                                                                                                                                    15 N -N 2 , ‰                                                     15 N -P O N , ‰
                     N2/Ar solubility ratio vs density
                                                                                                                                                                                                                                                                                                                                                                    PON 
                                                                                                                                                                                                                                                                                                                                                                                                                                                          Steady State profiles are easily obtained for 15N-NO3- and 15N-NH4+
                                                                                                                                                                                                                                                                                                                                                                                                                 O2 
             Date from Bilim 2000, Knorr 2001 and Knorr 2003                                                                                                                                                                                                                                                                                                                  =   PON                                                                but 15N-N2 decreases initially then increases rapidly. At steady state (after several
                                                                                                                                                                                                                                                                                                                                                                     t                         O 2 1 / 2   O 2                                                                                                                                                                                                                                                                                                                                                  15                                                                 15
                                                                                                                                                                                                                                                                                                                                                                                                                                                          decades?) the 15N-N2 must equal 8‰, much higher than the observed values of                                                                                           15                                                                       15
                                                                                                                                                                                                                                                                                                                                                                                                                                                          0.0‰.




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          S igm a-t
                                                                                                                                                                                                                                                                                                                                             The specific (k) and maximum () rates were taken from publications, when




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     S igm a-t
                                                   Atm Eq
                                                                                                                 Bilim
                                                                                                                 B ilum 2000 M 10L 41
                                                                                                                                                                                                                                                                                                                                            available, and adjusted to fit the data.                                                                                                                                                                                                                                                                                                                                                                                  16                                                                 16
                                     12                                                                          K norr 2001 S tation 2                                                                                                                                                                                                     All values of k and  were assumed constant.                                                                                                                                                                                                                                         16                                                                       16
                                                                                                                 K norr 2001 S tation 14
                                               F reshly                                                          K norr 2003 C entral G yre                                                                                                                                                                                                                                                                                                                    Fig. 10
                                     13
                S ig m a-T h eta




                                             intensively                                                         K norr 2003 S tation 20
                                              ventilated                                                         100% saturation
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              V ersio n 2.4 .14 .2.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      17                                                                 17
                                                                                                                 T he core of the C IL
                                     14                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          17                                                                       17
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     1 5 N -N 2 -ga s                           1 5 N -N H 4 +                                        N -N O 3 -
                                                                                                                                                     CIL                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   0         2              4          6             8                0          20          40      60   80
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           In 0 d ay s.                                      In 0 d ay s.                                       In 0 d ay s.
                                     15                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               0           2           4          6          8        10                0             20         40   60   80
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      N -N H 4 , ‰
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         15
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           N -N O 3 , ‰
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              15
                                                                                                                                                                                                                                                                                                                                               Fig. 8   Physical Model                                                                                                                     In 9 0 day s.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           In 1 y ear.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             In 9 0 day s.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             In 1 y ear.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                In 9 0 day s.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                In 1 y ear.                                               15 N -N H 4 , ‰                                                    15 N -N O 3 , ‰
                                     16
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           In 1 3 years.                                     In 1 3 years.                                      In 1 3 years.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       Conclusion:
                                                                                                                                                                                                                                         15N-N2
                                     17                                                                                                                                                                                                                                                                                                                                                                                                                                 14.5                                  14.5                                              14.5
                                                                                                                                                                                        Fig. 7
                                                                                                                                                                                                                                                                      versus density                                                                                                                            V ertical velocity (w ), m /day                                                                                                                                                                                                                                                                                                        •The simulation results in distributions close to those
                                      0.98            1.00                                       1.02               1.04                                                                                                                                                                                                                                                                                             -0.02       -0.01       0.00                       15.0                                  15.0                                              15.0                                               Conclusions:
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       observed (Fig. 14).
                                          S a m p le/S a tu ra tion N 2 /A r                                                                                                                                                                                                 Atmospheric                                                                                                                     14.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                            S igm a-t
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   •This model correctly simulates 15N-N2 and 15N-NO3- but
                                                                                                                                                                                                                                                                             equilibrium                                                                                                                                                                                15.5                                  15.5                                              15.5                                                                                                                                                                                   •Light N2 is made.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   not 15N-NH4+ and 15N-PON which are too heavy (Fig.
                                                                                                                                                                                                                                        12           B ilim 2 0 0 0 M 1 0 L 4 1
                                                                                                                                                                                                                                                                                                                                                                                                             15.0                                                       16.0                                  16.0                                              16.0                                                                                                                                                                                   • 15N-NH4+ reaches a maximum at the right depth, then
                                                                                                                                                                                                                                                     K n orr 2 0 0 1 S tation 2                               Enriched surface                                                                                                                                                                                                                                                                                     12).
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       decreases to minimum values that help to produce light N2
                                                                                                                                                                                                                                                                                                                                                                                                   S ig m a -t




                                                                                                                                                                                                                                                     K n orr 2 0 0 1 S tation 1 4
                                                                                                                                                                                                                                                                                                                                                                                                             15.5
                                                                                                                                                                                                                                                     K n orr 2 0 0 3 C entral G y re
                                                                                                                                                                                                                                                                                                              values                                                                                                                                                    16.5                                  16.5                                              16.5                                               •The 15N-NH4+ maximum is too shallow.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       •The 15N-PON is similar to those observed.
                                                                                                                                                                                                                                        13
                                                                                                                                                                                                                     S ig m a-T h eta




                                                                                                                                                                                                                                                     K n orr 2 0 0 3 S tation 2 0

   3. Nitrogen Balance                                                                                0         1            2             3         4
                                                                                                                                                                                                                                                     A n equ lib riu m v alu e
                                                                                                                                                                                                                                                     T h e co re o f th e C IL                                                                                                                               16.0                                                       17.0                                  17.0                                              17.0
                                                                                                                                                                                                                                        14
                                                                                                                       NO 3 
                                                                                                                             -

 Average Vertical Nitrogen                                                                        0                                                        C en tral S ta tio n s                                                                                                                       CIL                                                                                                  16.5                                                              0     1        2           3          0           20                40      60            0          20    40        60       80
                                                                                                                                                                 N O 3- uM
 Balance in/out of suboxic zone                                                                                                                                                                                                         15                                                                                                                                                                                                                                          N va lu e
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           N va lu e
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 N va lu e
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
                                                                                                                                                                 N H 4+ uM
                                                                                                                                                                                                                                                                                                          During denitrification                                                                             17.0                                                                                                                                                                                                    Conclusion
                                                                      A vera ge D ep th (m )




                                                                                                                                                                 N 2 E xcess u M
                                                                                                 40
 in Central Gyre                                                                                                                                                 N 2 E xcess u M                                                                                                             suboxic      N2 becomes light and
                                                                                                                                                                                                                                        16
                                                                                                                                                                                                                                                                                                                                                                                                                   0.0     0.4    0.8      1.2      1.6
                                                                                                                                                                                      Suboxic                                                                                                             NO3 becomes heavy
        Gradient (mol N m-4)                                                                     80                                                                                                                                                                                                                                                                                                               V ertical diffusion (k b ), m 2 /day
                                                                                                                                                                                                                                        17
 NO3-       0.13                                                                                                                                                                      Zone:
                                                                                                                                                                                                                                              0.0                  0.4                 0.8        1.2                                                                                                                                                     Conclusions (Doesn’t explain all data):                                                                                                                    Steady State model runs are not successful at simulating the observed 15N distributions in the
                                                                                                                                                                                      Depleted
                                                                                                                                                                                                                                                                                                                                                                                                                                                          •Can’t simultaneously model 15N of N2, NO3- and NH4+.                                                                                                     suboxic zone of the Black Sea unless we assume a process to remove heavy 15N-NH4+.
 NH4 +      0.17                                                                               12 0                                                                                                                                                                15
                                                                                                                                                                                                                                                                        N -N 2
 Total N in = 0.30                                                                                                                                                                    values
                                                                                                      0     2           4            6         8     10
                                                                                                                                                                                                                                             Strong Temperal Variability                                                                                                                                                                                  •Can’t make light 15N-N2 by total consumption of NO3-
                                                                                                                    N 2 E x cess  
 N2up      0.21                                                                                                                                                                                                                                                                                                                                                                                                                                           and NH4+.
 N2down   0.08
                                                                                                      0     5           10           15        20    25                                                                                                                                                                                                                                                                                                                                                                                                                                                              Comparison of model and data suggest three possibilities:
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     1. The 15N data we are using are not representative.
                                                                                                                       NH 4 
                                                                                                                             +


 Total N out = 0.59
    There is twice as much flux of N out as N2 than                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  2. There is a process to transport heavy NH4+ from the suboxic zone to the anoxic zone.
    flux of N in as NO3- and NH4+. The difference probably                                                                                                                                                                                                                                        Acknowledgement                                                                                                                                                                                                                                                                                                    3. The 15N profiles are not at steady state.
    comes from remineralization of PON                                                                                                                                                                                                                                                            This work is a part of the next projects: NSF MCB 0132101; NSF OCE #0081118; NATO CLG #9791211
                                                                                                                                                                        Fig. 4

								
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