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WEI, CHING-LING, AND JAMES W. MURRAY. Temporal variatio Powered By Docstoc
					Limnol. Oceanogr., 37(2), 1992, 296-314
0 1992, by the American Society of Limnology and Oceanography, Inc.

Temporal variations of 234Thactivity in the water column of
Dabob Bay: Particle scavenging
Ching-Ling Wei’ and James W. Murray
School of #Oceanography,WB-10, University of Washington, Seattle 98 195

           Monthly measurements of total, dissolved, and particulate 234Th,along with fluxes of total mass
        and particulate 234Th, were made throughout 1987 in Dabob Bay, Washington. Vertical distri-
        butions of total 234Thshowed a large deficiency relative to 238Uthroughout the water column during
        all sampling periods.
           Total 234Thactivities in surface waters were generally bigher in winter and lower in summer, in
        response to seasonal biological activity. In spite of this variation, the steady state assumption seems
        valid for modeling 234Thof the bay because the magnitude of variation is insignificant in the mass
        balance calculation. The range of residence times of total, dissolved, and particulate 234Thestimated
        horn an irreversible scavenging model are 4-70, <: l-10, and 3-60 d, respectively. The residence
        time of 234This controlled by the suspended particles whose distribution is determined by biological
        production in the surface layer and resuspension of bottom sediments.
           Vertical fluxes of 234Thmeasured by sediment traps also showed wide seasonal variation and
        generally increased monotonically from 200-l ,000 dpm me2 d-l at 20 m to 2,000-4,000 at 100
        m. The flux of 234Thpredicted with the steady state scavenging model tends to be about twice the
        directly measured 234Th flux. This difference may be due either to low trap efhciency for 234Th-
        containing particles or to boundary scavenging at the sides of the basin. The residence times of
        particulate 234Th and total suspended matter agree well, and both are influenced by biological

    Because of their high particulate matter elide tracers. Among those radionuclides,
concentrations, coastal waters have en- the 238U-234Thpair is e!;pecially suitable to
hanced scavenging properties relative to the study dynamic systems j n which scavenging
open ocean. Consequently, ocean margins time scales range from #days months.               to
play an important role in controlling the                       234Th (half-life - 24.1 d) has been used
geochemical cycling of many elements, es- successfully to study sca.venging in the open
pecially those that are particle reactive. Be- ocean (e.g. Bhat et al. 1!>69;Bacon and An-
cause particles serve as a conveyor belt for derson 1982; Bruland and Coale 1986) and
many elements, it is important to under- in coastal waters (e.g. K.aufman et al. 198 1;
stand the processesthat control their cycling McKee et al. 1984, 1986; Huh and Beasley
and removal in coastal waters. Knowledge                      1987). Coastal systems are more compli-
of scavenging is based largely on distribu-                   cated because scavenging particles originate
tions of naturally occurring radioactive nu- from both biological processes and sedi-
                                                              ment resuspension. Both of these processes
                                                              are usually seasonal and not necessarily in
---                                                           phase. The particles also have different
    l Present address: Institute of Oceanography, Na-         points of origin. The importance of these
tional Taiwan University, P.O. Box 23-l 3, Taipei, Tai-       different particle sources varies depending
wan, Republic of China                                        on location. For example, Santschi et al.
Acknowledgments                                               (1979) estimated that the residence time of
    This research was supported by NSF grant OCE 85- dissolved 234Th with respect to scavenging
 11580. C.-L. Wei acknowledges support from the Na-           in Narragansett Bay ir; only 2-22 d. Nar-
 tional Science Council of the Republic of China (NSC-
 80-0209-M002a- 13).                                          ragansett Bay is shallow and dominated by
    We th;ank B. Paul for help in all aspects of this project sediment resuspension, Extensive studies of
 and A. Devol, R. Citterman, M. Peterson, W. Clark,           234Th scavenging in Funka Bay, a semi-
 and H. Jannasch for help in sample collection. Jennifer closed coastal bay, have also been reported
 Young also made valuable contributions. We thank P. (Minagawa and Tsunogai 1980; Tsunogai et
 Crawford and R. McQuin for ship operation. Reviews
 of the manuscript by L. Balistrieri, G. Shen, and M. al. 1980, 1986; Tanaka et al. 1983). Funka
 Bacon are gratefully acknowledged.                           Bay differs from Narragansett Bay in that
                               Dabob Bay 234Thscavenging                                  297

the seasonal biological effects are more im-     tern like Dabob Bay where the assumption
portant than sediment resuspension.              of steady state needs to be evaluated. There
   The residence times of 234Th in Funka         are very few previous data sets that include
Bay vary seasonally and range between 30         analyses of both the water column and sed-
and 210 d (Tanaka et al. 1983). The first        iment traps. Thus, the third goal of our study
objective of this study was to use 234Th to      was to compare measured fluxes with those
study scavenging in a coastal location (Da-      predicted by scavenging models.
bob Bay) known to be influenced by both             In order to address the goals listed above
biological particle production and sediment      we conducted an intensive, monthly sam-
resuspension. Previous work in Dabob Bay         pling program for 234Th in the bay (Fig. 1)
has indicated that biological production oc-     from January to December 1987. This
curs primarily     in spring and summer          coastal marine environment presented the
(Welschmeyer and Lorenzen 198 5; Downs           opportunity to study the sensitivity of 234Th
and Lorenzen 1985), while sediment resus-        to time-dependent variables like primary
pension is most important in the fall and        productivity and sediment resuspension.
winter period of strong wind and high rain-         Dabob Bay is a coastal fjord that has
fall (Bennett 1980; Hedges et al. 1988a).        served as an ideal natural laboratory for bi-
   In seasonally variable systems, the steady    ological oceanographers and chemists to
state assumption used in modeling 234Th          study coastal ocean processes (e.g. Welsch-
scavenging may not be valid. It is important     meyer 1982; Welschmeyer et al. 1984;
to evaluate this assumption at each location     Hedges et al. 1988a,b). A sill located at the
before estimating vertical fluxes of 234Th       southern end of the bay makes it a semi-
from 234Th/238Udisequilibria in the water        closed system with a negligible horizontal
column. Buesseler et al. (in press) showed       component of advective transport. Gener-
that a nonsteady state model including the       ally, productivity maxima are observed in
time rate of change of Th was necessary to       spring and fall (Welschmeyer and Lorenzen
model the 234Thdata from the U.S. JGOFS           1985; Downs and Lorenzen 1985). Our
North Atlantic Bloom Experiment. In a sea-       chosen station was in the northern end of
 sonal study of 234Thin Funka Bay, Tanaka        the bay at an average water depth of 115 m
et al. (1983) analyzed total 234Th and thus      (Fig. 1). Detailed description of the study
studied total removal rates from the water       area can be found elsewhere (Bennett 1980).
column. There have been few studies of the
seasonal dynamics of Th scavenging that          Methods
include dissolved and particulate samples.           Water-column samples -Seawater was
The advantage of this kind of sampling is        collected from six depths (1, 10, 20,40, 70,
that Th scavenging can be broken into its         110 m) for measurements of salinity, nu-
component steps of uptake by particles and       trients, total suspended matter concentra-
removal of the particles from the water col-     tion, and total, dissolved, and particulate
umn. The second objective of our study was       234Th. Eighty liters of seawater were col-
to sample for dissolved and particulate 234Th    lected with 30-liter Niskin samplers. Two
in Dabob Bay in order to evaluate the sea-       20-liter subsamples were used for total, dis-
sonality of their residence times.               solved, and particulate 234Th analysis. The
   Scavenging models have been developed         rest of the sample was used for 210Pb/210Po
to calculate dissolved, particulate, and total   and 226Ra measurements to be discussed
residence times of 234Th(e.g. Coale and Bru-     elsewhere. Upon recovery, samples were
land 1985, 1987). Many of the assumptions        drained into cubitainers, salinity and nutri-
commonly used in such scavenging models          ent samples were taken, and the cubitainers
can be tested with sediment trap measure-        were transported back to the laboratory.
ments of the particulate flux of 234Th. In       Nutrients (NO,-, Si02, and P043-) were
addition, the predicted 234Th flux can be        measured by autoanalyzer. The average pre-
compared with that measured to calculate         cisions of N03-, Si02 and P043- concentra-
efficiency of the traps. Such evaluation is      tions obtained from duplicate samples were
especially important in a time-varying sys-      3, 7, and 2%.
298                                       Wei and Murray



       Fig. 1. Dabob Bay and the Puget Sound area. Sampling station (0) is shown in the insert.

   The total 234Thsamples were weighed and          passing the sample through three anion ex-
acidified with -50 ml of 12 N HCl and               change columns preconditioned with 8 N
spiked with 230Thand 236Uyield tracer along         HN03. The sample was evaporated down
with 60 mg of Fe carrier (as FeCl,). An iso-        to one drop and was thlen ready for extrac-
topic equilibration time of at least 5 h was        tion. 234Th and the yield tracer 230Th were
allowed. About 50 ml of concentrated                extracted into a TTA ([thenoyltrifluoroace-
NH40H was then added to raise the pH to             tone)-benzene solution and stippled on a
precipitate Fe(OH)3, which was separated            stainless steel disk. 238Uin the sample along
from the supernatant by centrifugation and          with the yield tracer 21W was plated on a
dissolved in concentrated HCl.                      stainless steel disk by electrodeposition.
   Sample digestion and purification pro-               Samples were weighed and then separated
cedures were modified slightly from An-             into dissolved and particulate fractions by
derson and Fleer (1982). The sample was             filtration through preweighed 0.45~pm Nu-
digested with perchloric acid to break down         clepore filters. Dissolved samples were pro-
organic material and hydrofluoric acid to           cessedin the same way as the total samples,
dissolve silicate minerals. After digestion,        except that the digesting procedure was ex-
the sample was reprecipitated with concen-          cluded. Filters were rinsed with -30 ml of
trated NH40H in a SO-ml centrifuge tube.            deionized distilled water, dried in a desic-
The precipitate was then rinsed three times         cator at room temperature, and then weighed
by repeating centrifugation, decanting, and         to estimate the concentration of total sus-
resuspension with deionized distilled water.        pended material. They were soaked in con-
The rinsed precipitate was dissolved in con-        centrated NH,OH for a few hours to break
centrated HCl to make a 9 N HCl solution.           them down and then digested and purified
   Separation of U and Th was accom-                following the same procedures as for total
plished by passing the solution through an          234Th samples. In order to minimize in-
anion ‘exchangecolumn ( 1X-8 Dowex resin)           growth of 234Th from 238U, filtration and
preconditioned with 9 N HCl. Th passed              Fe(OH), precipitate separation were typi-
through the column while U and Fe were              cally completed within 1 d after the sample
retained by the resin. U was then eluted with       was taken.
0.1 N HCl into a separate Teflon beaker.                234Th activity was counted with a low-
Th anld U samples were purified further by          level, anti-coincidence beta counter with a
evaporating the sample on a hot plate and           background of 0.2-0.3 cpm. Counting effi-
                                     Dabob Bay 234Thscavenging                                       299

 Table 1. 234Thactivity (dpm kg-‘) of multiple samples collected on selected cruises.
     Cruise         (m)        Xl          x2             x3        x4       ?I         s      SD(W
  Mar/Apr           110      0.812       0.957           0.910      -       3      0.893     0.074 (8)
  Ott                 0      0.395       0.286           0.320      -       3      0.334     0.056 (17)
  Nov                 0      0.322       0.289           0.304    0.289     4      0.301     0.016 (5)

ciency was determined by counting a 234Th                  drained through 47-mm, 0.45~pm pore-size
standard plate prepared from 238U. The                     Nuclepore filters mounted on the bottom of
counting efficiencies of two detectors were                the traps and then rinsed with -30 ml of
monitored throughout the experiment and                    deionized distilled water to remove salts.
showed no significant variation. Selected                  Zooplankton “swimmers” were found in all
234Th samples were also counted through                    the traps and were removed by hand pick-
time to confirm that the decay curve was                   ing. The 234Th associated with the zoo-
followed. Alpha activity of 230Th was ob-                  plankton that were hand picked in April
tained with silicon surface-barrier detectors              samples was determined and found to be
(EG&G Ortec 576). U assay of the total                     negligible compared to total 234Thactivities
sample was also obtained with the same                     of the trap samples. The filters were dried
detectors. The counting efficiency was cal-                in a desiccator at room temperature and
ibrated against an 241Am standard disk.                    weighed to estimate total mass flux.
Total counts for the samples were generally                    Analytical procedures used to measure
 > 1,000 to achieve satisfactory counting sta-             234Thactivity on these filters were the same
tistics (SE = 3%). The chemical yield of Th                as those for particulate 234Th.Blank values
was determined by 230Thactivity on the disk                for mass flux and 234Th flux were deter-
and was typically in the range of 30-60%.                   mined with two dummy traps left on board
   Blank determinations for Nuclepore fil-                  ship during deployment of the other traps.
ters and reagents were carried out for each                 Most of the blank values of the mass and
batch of samples and were generally negli-                  234Th fluxes were < 7 and 1% of the total
gible. Beta contamination from the Nucle-                   mass and 234Th fluxes. These blank values
pore filters was found, however, in partic-                 were subtracted from measured values. Pre-
ulate samples of January, February, and                     cision is expressed as + 1 SD and was ob-
March-April.     The measured particulate                   tained by propagating counting error.
234Thdata for these months were discarded
and particulate 234Thvalues were estimated                 Results
by subtracting dissolved 234Th from total                     The precision obtained from multiple
234Th.                                                     measurements (Table 1) averaged 10%. The
   Sediment-trap samples-Modified MLML                     sum of the dissolved (DTh) and the partic-
sediment traps (Knauer et al. 1979;                        ulate (PTh) 234Th activities are not exactly
Martin et al. 1987) were deployed at five                  equal to total 234Thactivities (TTh) (Tables
depths on an anchored mooring line near                    2 and 3). Most of the discrepancies between
the water-column sampling station. Each has                (DTh + PTh) and TTh are < lo%, however
a trapping area of -50 cm2. The deepest                    (i.e. within the the average precision of the
traps were 5 m from the anchor weight. Each                analysis). The least-squares fit of (DTh +
set was composed of two or four traps, and                 PTh) and TTh gives a slope of 0.9 1-to.07
sets were separated by 20 m. Typical de-                   (n = 48).
ployment lasted -3 d. The trap solutions                                                             and
                                                              Salinity ranges from 19.83 to 3 l.O2o/oo
were prepared by adding - 1,000 g of NaCl                  increases with depth (Fig. 2). The 3OcroO iso-
and -60 ml of 37% formaldehyde to filtered                 haline fluctuates from 25-40 m in winter to
surface seawater from the bay. This trap                   90 m in summer, reflecting river runoff and
solution was used to minimize resuspension                 the degree of seawater exchange with Hood
losses and microbial degradation of trapped                Canal. In periods of high wind, like Decem-
particles.                                                 ber 1987, the water column was homoge-
    Upon recovery, trap solutions were                     neous and overall salinity was higher.

300                                                     Wei and Murray

   Table Z!. Salinity, nutrients, total suspended material, 238U,and total, dissolved, and particulate 234Thfrom
Dabob B;ly for January-December 1987. Standard deviations are based on propagated counting error (f 1 SD).
                        PO,‘-       SiO,        NO,-                238”           TTh                 DTh            PTh
Depth    Salinity   _                                     TSM ---
 Cm)      (73                   @mol liter ‘)           (mg kg-‘)                              (dpm-
    1 28.832            2.51      73.28         27.57    0.48                  0.589t-0.017       0.130f0.005
   10 29.279            2.33      70.03         25.03    0.45                  0.626+-0.018       0.1231k0.004
   20 29.952            2.64      68.52         28.46    0.31                  0.856kO.029        0.399kO.012
   40 30.444            2.79      67.87         28.68    0.17                  1.37720.049        0.137~0.005
   70 30.580            2.48      69.67         28.28    0.44                  0.895aO.023        0.110f0.004
  110 30.804            2.42      78.57         26.45    0.96                  0.910f0.026        0.113~0.004
    1 24.170            0.61      67.30          2.75    4.19                  0.392t-0.009       0.08 1k 0.003
   10 29.142            2.31      69.95         25.71    0.96                  0.552-t-0.030      0.283 k 0.007
   20 29.859            2.51      73.89         28.44    1.03                  0.770+-0.016       0.156k 0.004
   40 30.132            2.56      75.03         28.90    0.72                  l.O02t-0.038       0.150* 0.005
   70 30.435            2.72      70.63         29.60    1.25                  1.0121c_O.O26      0.131~0.004
  110 30.744            3.20      83.25         30.37    0.84                  0.934a0.023        0.086 2 0.003
    1 26.553                                             1.89                  0.375 -co.009      0.097 5 0.003
   10 29.181                                             0.44                  0.5701t0.017       0.192? 0.006
   20 29.496                                             0.35                  0.774-cO.020       0.170~.0.006
   40 29.625                                             0.39                  1.097-c0.030       0.137+.0.004
   70 30.841                                             1.02                  0.997f0.032        0.101 kO.004
  110 30.188                                             1.07                  0.893-eO.027       0.141 kO.003
     1   27.487         0.75       7.81          1.11    1.20                  0.317-c0.011       0.08 1kO.003    0.225kO.005
   10    28.938         1.39      20.87          7.36    0.66                  0.401 kO.012       0.148 kO.006    0.361 +O.O14
   20    29.394         2.17      43.20         18.22    0.97                  0.573kO.019        0.175+:0.007    0.404+0.0 10
   40    29.802         2.37      63.73         28.07    0.42                  0.723zkO.013       0.214kO.007     0.487kO.O 15
   70    29.936         2.54      67.65         28.57    1.08                  0.695 kO.023       0.156k0.005     0.589kO.017
   90    29.948         2.75      71.60         30.24    3.70                  0.750a0.029        0.055 k0.002    0.494+0.016
     1   28.350         0.39       4.85          0.72    1.15 1.820            0.254-tO.007       0.09 1kO.003    0.226+0.006
   10    28.744         1.05      16.62          5.48    2.06 1.910            0.474-to.013       0.104~I0.003    0.293kO.011
   20    29.533         2.39      45.94         22.70    1.05 1.970            0.48 1kO.020       0.140~r0.005    0.248 zkO.008
   40    29.834         2.56      55.03         26.43    0.51 2.030            0.8631kO.032       0.129 kO.003    0.579kO.018
   70    29.97 1        2.70      63.53         28.88    0.82 2.010            0.839a0.029        0.078kO.002     0.701 kO.021
  110    30.145         3.07      74.8 1        31.42    3.15 1.940            0.824a0.031        0.040~:0.00 1   0.759f0.024
                        0.62      34.64          0.07    0.68 1.750            0.363a0.013        0.154-1~0.007   0.134~0.005
   10                   1.21      38.26          5.80    1.37 1.990            0.365kO.010        0.074~~0.003    0.272+0.012
   20                   2.48      53.00         25.76    0.57 1.900            0.61OkO.019        0.131~~0.005    0.369+0.010
   40                   2.70      60.30         29.00    0.37 1.760            0.896a0.036        0.1541t0.008    0.509-tO.021
   70                   2.78      62.35         29.29    0.61 2.040            0.929 kO.038       0.0541kO.002    0.807kO.033
  110                   3.20      78.75         33.33    0.40 1.740            0.782-tO.025       0.062kO.003     0.79 1kO.027
    1 29.069            0.82      33.86          0.08    1.28 1.970            0.278+0.011        0.08 1kO.003    0.214+0.009
   10 29.768            2.19      43.41         15.07    0.76 2.003            0.5251kO.019       0.131:t0.006    0.223 kO.008
   20 29.924            2.69      45.19         24.75    0.31 1.898            0.525kO.019        0.128k0.006     0.417+0.015
   40 30.095            2.85      54.94         28.08    0.45 2.015            0.933-1-0.037      0.116:t0.004    0.743kO.025
   70 30.342            2.62      53.9 I        28.53    0.61 2.037            0.738zkO.028       0.113:+0.005    0.6001kO.019
  110 30.369            2.79      58.86         27.88    0.78 1.889            0.6241kO.020       0.086:+0.004    0.615+0.019
    1 29.224            0.53      20.17          0.57         2.00 1           0.191f0.008        0.006.kO.000    0.145+0.005
   10   29.840          1.oo       2.58          7.77    1.39 2.04 1           0.321+0.012        0.090~+0.003    0.244+0.011
   20 30.132            2.73      57.61         29.44    0.84 1.665            0.843 kO.026       0.185 kO.006    0.830+0.026
                                             Dabob Bay 234Thscavenging                                               301

 Table 2. Continued.
                     PO,‘-       SiO,        NO,-                 23qJ       TTh               DTh             PTh
Depth     Salinity                                     TSM
 (4         (W               &mol liter-‘)           (mg kg-‘)                          @pm kg-‘)
   40     30.461     2.66      56.70         28.80    0.28       2.028   0.646kO.019      0.020*0.001      0.497zk0.015
   70     30.479     2.58      58.59         28.08    0.10       2.262   0.517kO.016      0.107~0.004      0.55O-cO.018
  110     30.65 1    2.19      49.74         21.79    0.44       2.193   0.438+0.014      0.043+0.001      0.497kO.026
    1     29.27      1.07       9.91          2.05    0.93       1.792   0.395f0.014      0.121-tO.004     0.223+_0.008
   10     30.27      2.68      65.92         21.89    0.32       1.898   0.473kO.014      0.24OkO.009      0.124+0.004
   20     30.59      2.60      67.83         22.57    0.20       1.923   0.694kO.017      0.133~0.004      0.685f0.024
   40     30.71      2.72      51.82         24.68    0.20       1.924   0.6161kO.013     0.197f0.007      0.419+0.015
   70     30.79      2.69      55.17         24.82    0.35       1.946   0.599+0.015      0.070f0.002      0.436f0.014
  110     30.81      2.58      45.00         20.38    0.59       2.038   0.532-eO.018     0.083+_0.003     0.479kO.016
    1 19.380         2.32      46.64         20.63     0.25 1.787        0.413+0.011      0.180~0.007      0.146+0.006
   10 23.602         2.20      19.37          9.95     0.31 1.831        0.503f0.021      0.281 kO.011     0.165f0.005
   20 25.164         2.37      40.00         18.18     0.02 1.811        0.328kO.008      0.209 kO.008     0.155Ik0.005
   40 29.860         2.55      35.96         17.62     0.42 1.745        0.469kO.015      0.023 t-O.00 1   0.399+0.015
   70 29.925         2.76      51.15         21.69     0.78 1.978        0.968kO.033      0.029+_0.001     0.77320.026
  110 30.7 10        2.43      52.55         20.61     1.14 1.778        0.545kO.020      0.0061kO.000     0.482f0.013
    1 28.277         2.56       63.67        25.52     1.97 1.828        0.55920.02 1     0.037f0.001      0.317kO.012
   10 28.258         2.60       61.99        25.82     1.88 1.829        0.4131kO.015     0.028~0.001      0.356kO.012
   20 30.220         2.77       60.94        27.19     0.59 1.901        0.428f0.018      0.087 zkO.004    0.468-eO.016
   40 3 1.022        2.77       59.71        27.34     0.70 1.963        0.735kO.023      0.039+0.002      0.473+0.018
   70 31.005         2.71       59.65        27.39     0.75 2.149        0.7OlkO.022      0.068kO.003      0.502kO.018
  110 31.022         2.69       65.55        26.94     0.93 2.204        0.696kO.025         dO.001        0.68 1kO.026

Nutrient concentrations showed large vari-                          weight basis are 9-22 and 0.5-5.4%, re-
ations, depending on physical conditions                            spectively. Only one observation was
and biological activity. Generally, in winter                       available to show vertical variation in the
and early spring the nutrient content of the                        composition of suspended matter. A par-
 bay is higher and more homogeneous than                            ticulate sample taken from 50 m in winter
 in other seasons.In early fall (e.g. September                     contained significantly lower (50% lower)
 and October) broad maxima of all three ma-                         organic C and higher (60% higher) Al than
jor nutrients are found at middepths.                               the sample taken from 5 m.
    The vertical variation of total suspended                          238U activities in the unfiltered samples
 matter (TSM) is characterized by a mini-                           were determined only from June to Decem-
 mum at middepth, which is sandwiched be-                           ber. Despite poor recovery efficiencies
 tween maxima resulting from production by                          (< 5%) the average value of the normalized
 biological activity in the surface layer and                       238Uto 35%0salinity is 2.3320.22 dpm kg-‘,
 resuspension of bottom sediments in deep                           consistent with the open-ocean value of 2.43
 water (Fig. 3). Bennett (1980) gave a slightly                     dpm kg-’ at 35sloo   salinity (Ku et al. 1977).
 different picture of the vertical distribution                     Thus, we assume a conservative distribu-
 of TSM (i.e. generally constant values in the                      tion of 238Uin the bay with the same 238U-
 upper 50 m increasing toward bottom). He                            salinity correlation as the open ocean, 238U
 pointed out, however, that his TSM mea-                            (dpm liter-l) = 0.07081 X salinity (?&).
 surements may have been biased by the                                 The 234Thdistribution in the bay (Fig. 4)
 small-volume (50- 100 ml) samples used.                            is different from that of open and coastal
 Bennett determined the compositions of                             oceans in several ways. First, total 234Th
 suspended matter collected on four cruises                         below the euphotic zones of open and coast-
 in the bay. He found that the concentration                        al oceans is generally in secular equilibrium
 of organic C and particulate Al on a dry-                          with 238U, but in Dabob Bay total 234Th
302                                                Wei and Murray

    Table 3. Total mass and 234Thfluxes determined              Table 3. Continued.
from sediment-trap samples collected in Dabob Bay                                                            C
from February to December 1987. Standard deviations               Depth     Mass flux     234Thflux       *-Th
are based on propagated counting error (+ 1 SD). Spe-              b-4     (gm-*d-l )   (dpm rnd2d-l)   @pm mg-9
cific 234Thactivities are calculated by the ratio of 234Th         68        3.36       1,1601t32        0.35
flux to mass flux.                                                 88        5.39       2,751+83         0.51
--                                                                108        8.17       3,642+ 125       0.45
        Depth      Mass flux       234Thflux        234Th
         (ml      (g nl-2 d-l)   (dpm IT-~ d-l)   @pm mg-‘)     December
    February                                                       25       22.85       2,146+73         0.09
       20            2.98          693k 18          0.23           45       23.80       3,442+82         0.14
       50            2.76        1,181&36           0.43           65       25.55       4,343+83         0.17
       90           10.02        3,105+110          0.31           85       36.40       7,473f 175       0.21
                                                                  105       42.85       9,832-+ 170      0.23
       19            0.96          365+ 14          0.38
       39            1.13          352+6            0.31
       59            0.94        1,229&27           1.31
       79            5.03        1,835+49           0.36      activities are below the level of secular equi-
       99           15.48        3,513+115          0.23
                                                              librium with 238Uthroughout the water col-
   May                                                        umn. 238Uactivity in Dabob Bay waters is
      25             1.41          168+5            0.12      - 1.9-2.0 dpm kg-l and is relatively ho-
      45             1.12          29028            0.26
      6!j            1.36          4492 17          0.33      mogeneous throughout the water column.
      85             3.89          907+36           0.23      Hence, the 234Th: 238Uratio of total samples
     1O!j           16.14        1,945-e37          0.12      ranges from -0.2 in the surface layer to
   June                                                        -0.5 in the deep layer, and the vertical dis-
      23             4.82          635-t-24         0.13      tribution of 234Th*238Uis similar to the 234Th
      43             4.15          977+-34          0.24      activity profiles. Second, 234Th in the open
      63             4.91        1,248+37           0.25      and coastal ocean is primarily in dissolved
      t3:3           5.59        1,790+45           0.32      form, but in Dabob Bay most of the 234Th
     103            11.61        2,722+- 100        0.23
                                                              (>90%) is incorporated into filterable par-
   July                                                       ticles.
       23            2.86          722223           0.25          The partition coefficient, &, of 234Th
      4.3            2.76        1,23St-47          0.45
       6.3           2.21        l,266t-43          0.57      ranges from 1O6to 1O8ml g- ’ . Generally, Kd
       83            5.10        1,657t65           0.33      in the euphotic zone is about an order of
     103            12.36        3,722f 133         0.30      magnitude less than that below the euphotic
   August                                                     zone, and a broad maximum in Kd is found
      20            1.98           16817            0.08      in the intermediate layer of Dabob Bay. This
      40            2.08           97Of25           0.47      pattern is due to lower concentration and
      60            2.52         1,676f60           0.67      longer residence time of TSM at interme-
      81)           4.81         2,197-t84          0.46
     1010          11.39         3,060&85           0.27      diate depth relative to the surface and bot-
                                                              tom layers. Third, the vertical variation in
   September                                                  234Th is more complicated than that of ei-
      24             3.44          98Ok30           0.28      ther the open or coastal ocean. Total 234Th
      44             2.09          999+33           0.48
      64             2.52        1,1s3+47           0.47      profiles in the bay have their lowest values
      84             7.53        1,706-t54          0.23      at the surface and a subsurface maximum,
     104            15.45        1,74 1f47          0.11      usually at 20-40 m. Dissolved 234Th (Fig.
   October                                                    4b) activities are low, CO.2 dpm kg-‘, and
      25             1.26          632f15           0.50      show less vertical variation than those of
      4.5            1.51        1,160+39           0.77      total 234Th.A slight m.aximum of dissolved
      65             2.42        1,239a49           0.5 I     234Th is also found at intermediate depth
      85             2.86        2,042+55           0.72
     105             5.97        2,553-t43          0.43      for most of the samp’ling months. Particu-
                                                              late 234Thactivities (Fig. 4c) are much high-
   November                                                   er than the dissolved fraction and show a
      ;!8            2.39          358k 12          0.15
      48             1.95          683+22           0.35      vertical distribution similar to total 234Th.
                                                                  The range of total mass fluxes measured
                                                                     Dabob Bay 234Thscavenging                                                                    303

                                                                               Salinity, %bo                                                        TSM, mg kg”
                     FMAMJJASOND                                                                                   JFMAMJJA8OND


       .--                                                                                                   Fig. 3.   As Fig. 2, but of total suspended matter.
                                                                       NO$mol                  litei’
                 J       F   M       A   M   J       J       A        S        0           N            D   tal mass flux at 55-60 m in 1977 and 1978
                                                                                                            and found temporal variability of up to a
                                                                                                            factor of three (2-5 g m-2 d-l). This range
Lo-.                 .           .       .       .       .       .         .       .   .        .
                                                                                                            of variation was reproduced by our 60-m
a            -                                                                                      H       trap if the anomalous December data were
fb-                                                                                                         excluded. The anomalous mass fluxes ob-
                                                                                                            served in December were as high as 43 g
                                                                                                            m-2d-1 at 100 m. These fluxes were possibly
                                                                                                            a result of active resuspension of bottom
                                                                                                            sediment by strong winds. The consistency
                                                                                                            between our data and others indicates that
                 J       F   M       A   M   J       J       A        S        0           N            D
                                                                                                            our trap deployment period of only 3 d was
                                                                                                            sufficient to reproduce the seasonal patterns
                                                                                                            found by Bennett and Hedges et al. ( 19886)
                                                                                                            (15-30-d deployments). The agreement be-
                                                                                                            tween mass fluxes measured by duplicate
                                                                                                            traps on the same trap holder is excellent
                                                                                                            (flux A = -0.5 + 1.05 x flux B, r = 0.99,
                                                                                                            n = 78).
                                                                                                                Mass fluxes obtained from our sediment
                                                                                                            traps were relatively constant or decreased
                                                                                                            slightly in the upper 50 m. Below 50 m a
                                                                      SlO,, pm01 Iltei’
                 J       F   M       A   M   J       J       A         S       0           N            D   dramatic increase was observed. The mass
                                                                                                            fluxes for our deepest trap (N 100 m) were
                                                                                                            up to a factor of 5 higher than in the shal-
                                                                                                            lowest trap (-20 m). The increasing mass
                                                                                                            flux with depth was probably the result of
                                                                                                            added resuspended sediment to our traps
                                                                                                            deeper than 50 m. This resuspension phe-
                                                                                                            nomenon is supported by the findings of
                                                                                                            Cowie and Hedges (1984), who showed that
                                                                                                            the carbohydrate composition of particles
                                                                                                            in their deepest traps (90 m) was the same
   Fig. 2. Vertical distributions    of salinity, nitrate,                                                  as in underlying sediments. Furthermore,
 phosphate, and silicate vs. time. The regions with high
 and low concentrations are represented by H and L.                                                         all mass fluxes deeper than 60 m were higher
                                                                                                            than the average sedimentation rate of
                                                                                                             3.2OkO.6 1 g mm2d-l (Carpenter et al. 1985;
 from February to November was 0.5-16.0                                                                     Furlong 1986). Resuspension must be an
 g m-2 d-l (Fig. 5a), consistent with previous                                                              important factor controlling the distribu-
 studies in the bay (Hedges et al. 1988b; Ben-                                                              tion of particles in the deep water of Dabob
 nett 1980). Bennett (1980) measured the to-                                                                Bay.
                                          Dabob Bay 234Thscavenging                                    305

pared with a measured trap flux. In this way,                         J Th   =   xTh(u     -   DTh),    (7)
234Th can be used to test the efficiency of
sediment-trap collection.                                                    DTh
                                                                       T,, = -                          (8)
    In the irreversible   scavenging model                                        J Th ’
adopted from Coale and Bruland (1985),
diffusive and advective transport rates are                           PTh    =   xTh(u     -   TTh),    (9)
ignored because the turnover of 234Th in
seawater is rapid as a result of its brief mean            and
lifetime (35 d) and fast scavenging onto par-
ticles. If advection and diffusion terms are                           rp = -                          (10)
ignored, then the mass-balance equations                                         PTh ’
for total, dissolved, and particulate 234Th
are written as                                                The scavenging model can be applied to
                                                           a given data set by different, but equally
                                                           valid approaches. The choice depends on
  dTTh                                                     the purpose of the model. Values of X,, .&,,
  -    = X,,U            - &TTh      - X,TTh,        (1)
        dt                                                 PTh, and T can be calculated on a point-by-
                                                           point basis using U and Th activities in units
  -            = XT&     - &,DTh      - JTh,         (2)   of dpm kg-l. These parameters can also be
    dt                                                     calculated for the water column for one or
                                                           a number of compartments or boxes, with
                                                           the activities in each box expressed as in-
   dPTh                                                    ventories in dpm m-2.
  dt=               JTh - X,,PTh   - PTh             (3)      Coale and Bruland (1985, 1987) applied
                                                           the steady state version of this model to
where TTh is the total 234Th (dpm kg-l),                   vertical profiles of water-column and sedi-
DTh the dissolved 234Th (dpm kg-‘), PTh                    ment-trap data. Values of JTh and PTh were
the particulate 234Th (dpm kg-l), U the 238U               calculated point-by-point     through the water
(dpm kg-‘), &, the decay constant of 234Th                 column. The resulting values of residence
(d-l), X, the first-order scavenging rate con-             time tended to be erratic. In order to cal-
stant for 234Th (d-l), and JTh and PTh the                 culate a Th flux (dpm m-2 d-l) that could
rate of removal of dissolved and particulate               be compared with the measured trap flux
(and total) 234Th, respectively, by processes              they integrated the values of PTh (in dpm
other than radioactive decay (dpm kg-l d-l).               kg-’ d-l) from the sea surface to the trap
   The removal rate constants for total 234Th              depth.
and the nonradiogenic        production-con-                  Buesseler et al. (in press) obtained a time
sumption of dissolved and particulate 234Th                series during the UWJGOFS          North Atlan-
are easily calculated by assuming steady                   tic Bloom Experiment. They divided the
state:                                                     water column into a series of boxes that
                                                           corresponded to the floating sediment-trap
                                                           in each box. Changes in the inventories be-
             -c-=-c     dDTh       dPTh      o             tween sampling times were used to calculate
               dt         dt         dt          ’   (4)
                                                           depths and calculated the inventories (dpm
                                                           m-2) of dissolved, particulate, and total 234Th
The total, dissolved, and particulate 234Th                time-rate-of-change    terms in Eq. 1, 2, and
residence times (TT, TP, and TP) are calcu-                3. The resulting values of JTh and P-,-hhave
lated as                                                   units of dpm m-2 d-‘. The calculated flux
                                                           at the bottom of each box was equal to the
                    x = A U-J - TTh)                       PTh produced in that box plus the PTh input
                     C   Th
                              TTB    ’               (5)
                                                           from the overlying box. Only the surface
                                                           box has no PTh input from above. This ap-
                          1                                proach is useful to determine the flux of
                    TT = -,                          (6)
                         xc                                234Th produced in different parts of the wa-
306                                       Wei and Murray

ter column. If the goal is simply to predict          Table 4. Residence times, (in days) of total and dis-
the flux of 234Th at some depth, the multi-        solved 234Thin the surface water calculated from the
                                                   irreversible scavenging model with the assumptions of
box approach is not necessary and an in-           steady state and nonsteady state.
tegration from. the surface to the trap depth
will give the correct value. The only data
necessary to predict a 234Th flux at a given
depth are the activities of total 238U and
234Th (‘seeEq. I and 9). Because disequili-
                                                      14 Jan        14.6                  2.4
bria represent removal, the model flux (PTh)          18 Feb        10.7        8.4
                                                                                          1.8       1.5
is the net of all processes, including aggre-          1 Apr         8.9       10                   2.1
                                                                                7.2       1.9       1.6
gation and disaggregation. This approach              20 May         6.8
                                                                                5.6       1.6       1.8
assumes that all the 234Th is collected in            24 Jun         5.6                  1.8
                                                      29 Jul         9.1        8.7                 3.2
sampling total Th (i.e. that no significant                                     6.6       3.4       1.7
                                                      26 Aug         5.6                  1.5
particulate pool is excluded). The residence          23 Sep         3.7        3.7                 0.1
                                                                                9.9       0.1       2.5
times for the single- or multibox approach            26 Ott         9.8                  2.5
                                                      10 Nov        10.5       10.6                 3.9
are calculated by dividing the inventories                                     17.3       3.9
                                                       7 Dee        15.3                  0.7       0.7
of 234Th by JTh and I’Th as appropriate.
    Tanaka et al. ( 1983) doubted the validity
of the steady state assumption in the coastal
environment     and retained dTThldt in Eq.         state is an acceptable assumption for all
 1 to solve for TTh analytically:                   depths in the bay.
                                                       In Funka Bay, the mean residence time
                                                   of total 234Th was l-7 months rather than
Th2                                                 5-30 d as observed in Dabob Bay. The lon-
            Xc    +     b-h
                                                   ger residence times (or slower removal rates)
                                                   of234Th in Funka Bay make dTThldt more
                                                   significant in the mass-balance equation. In
                                                   some months, dTTh/dt was even greater
                                                   than the particulate sinking term of 234Th.
                      -=I+(&   + h,)Ar]     (11)   Hence, the steady state assumption is in-
                                                   appropriate for conditions in Funka Bay.
where Thi is the total 234Th activity from         Dabob Bay is different from Funka Bay in
successive observations, and AT the inter-         two aspects: the removal rate of 234Th is
val between sampling time of Th, and Th2.          faster and temporal variations of 234Th are
                                                   smaller. Radioactive decay and other re-
Discussion                                         moval processes are fast enough to balance
  Steady state vs. nonsteady state- We test-       the production rate of 234Th. Accordingly,
ed the steady state assumption in Dabob            the steady state assumption is applicable for
Bay by comparing steady state and non-             our Dabob Bay data. The difference be-
steady state approaches. We used total and         tween the two bays may be due to physical
dissolved 234Th data of the surface water for      mixing. Active exchange of water masses in
this test because those samples showed the         Funka Bay has been reported (Ohtani and
largest variations in 234Th activity with time.    Akiba 1970). On the other hand, the pres-
Total and dissolved 234Th should bracket           ence of a sill at the southern end of Dabob
particulate 234Th in terms of activity and         Bay inhibits water exchange with the main
removal rate. It is clear (Table 4) that there     body of Puget Sound, which is also some-
is good agreement between the two ap-              what isolated from the North Pacific Ocean
proaches. The same calculation was applied         by a sill.
to the: subsurface total and dissolved 234Th         Residence time of 234Th in Dabob Bay-
data but is not shown. There was no differ-        One of the goals of this study was to utilize
ence between the residence times calculated        our dissolved and particulate analysis of
from steady state and nonsteady state ap-          234Th to evaluate the seasonal variation in
proac.hes. These results suggest that steady       dissolved Th uptake by particles and re-
                                      Dabob Bay 234Thscavenging                                                                                                     307

moval of particulate Th from the water col-
umn. The range of residence times for total
(TT), dissolved (TD), and partidate         (7~) 234Th
are 4-70, < l-10, and 3-60 d (Fig. 7). There                     2
tends to be a maximum in 73 between 30
and 70 m. Minagawa and Tsunogai (1980)
and Tsunogai et al. (1986) observed a sim-
ilar subsurface residence-time maximum in
Funka Bay. They attributed this distribu-
tion to removal from the surface layer,                            o-
followed by disaggregation and decreased
 settling velocity of particles, without regen-                 loo-                 >20               20                                 (20
 eration of nutrients, at middepth. We be-                                                                   .    .          .       .     .      .        .
 lieve this combination          of processes is also                                                                                                         *(a

 the cause of most of the vertical distribution                                                                                                           TD(d)
                                                                                Jan        Feb   Mar        May   Jun        Jul    Aug   Sep   Ott       Nov       Dee
 of TT in Dabob Bay. Remineralization                of
 particulate material with nutrient regener-
 ation may be important in September and
 October when a broad maximum develops                           2
 in nutrient distributions           at intermediate
 depths. Another possible cause of the max-                z 4
 imUm      in TT iS reSUSpenSiOn      Of bottom sed-       2
 iment into waters below the maximum, per-                 5 6o
 haps from the sides ofthe bay. Resuspended                c1 -                  ’          ’     l          ’     ’          ’      l     l       l        ’        ’
  particles contain less 234Th per gram than                 *O-
  particles found in the overlying water (Car-
  penter et al. 1984). When these sediments
                                                                       -         .          .     .          .     .          .      .     .       .        .        l
  are resuspended into the overlying water,                                                                                                                              (bl
  they may act as efficient scavengers for Th
  in seawater.                                                                                                                                            TP (d)
                                                                                Jan Feb Mar                 May  Jun Jul            Aug Sep     Ott       Nov       Dee
      The residence time of dissolved 234Th with                   0       I1    I I I I I
                                                                                     .   .
                                                                                                              I I I I I,
                                                                                                                  .   .              .
                                                                                                                                           .    I...
                                                                                                                                                     I,     I   ,     , ,
  respect to the net scavenging rate is very                                    .           .     .          .     .          .      .     .       .        .        .
  brief (3 d) throughout the year. Extremely                     20

  short TD relative to TT and rp implies that
  the uptake rate is much faster than the re-                    40
  moval rate of particulate 234Th. A maxi-                 2
  mum of TD was observed below the euphotic                5     60
  zone at the depth of the TSM minimum.                    n
                                                                       -        .           .     .          .
  The coexistence of the TSM minimum and                         80
  the TD maximum is not surprising because
   fewer adsorption sites are available in that                 100                                                    >20
   layer. Honeyman et al. (1988) reviewed                              -        .          .      .          .     .          .      .     .       .        .        .
   234Th field data and found that a positive                   120
   correlation    exists between TSM and the
                                                               Fig. 7. Vertical distributions of residence time of
   scavenging rate constant. Vertical and tem-              total, dissolved, and particulate 234Thvs. time as es-
   poral variations of Tp were similar to rT.               timated by the irreversible scavenging model.
   This finding implies that the removal rate
   of particulate matter is the determinant of
   total residence time of 234Th in Dabob Bay.              (Welschmeyer 1982; Downs and Lorenzen
      234Thin the surface waters-Average            pri-     1985). Welschmeyer and Lorenzen ( 198 5)
   mary production in the euphotic zone of the              estimated that >80% of the chlorophyll
   bay is -600 mg C m-2 d-’ with a seasonal                 produced in the euphotic zone is removed
   variation     of one order of magnitude                  by settling in the form of fecal pellets pro-
308                                        Wei and Murray

                                              Tot.         ing the annual cycle of 1987. Dissolved 234Th
      0.5 -’                                              fluctuates between 0 and 0.2 dpm kg-l and
                                                          particulate 234Th activity generally shows a

      0.4 m-
                                                          pattern similar to that of total 234Th (i.e.
      0.3 -’                                        Part. high in winter and early spring and low in
                                                          other seasons).
      0.2 -’
                                                              The temporal variations of the residence
      a1 .&                                               times of surface 234Th are similar to those
                                                    Diss. of the 234Th activity (Fig. 8b). The ranges
                                            I  I  -I
      o.o-    ’  ’ ’   ’   ’  ’  ’   ’                    of the residence times for total, dissolved,
           J FMAMJJASOND
                                                          and particulate 234Th are 3-15, -K1-6, and
                             Month                         3-l 1 d. Residence times for total and par-
                                                          ticulate 234Th show strong seasonal varia-
                                                   Tot.   tions, while those of dissolved 234Th re-
                                                          mained constant.
                                                              Seasonal variations in the activity and
                                                          residence time of 234Th in the surface layer
                                                   Part. .are closely related to biological processes.
                                                          Primary productivity        data provided by A.
                                                          Devol show phytoplankton blooms in May,
                                                          July, and September. 13ecause these 14C in-
                                                  J       cubation measurements were not carried out
         ‘J FMAMJJASOND                                   at exactly the same d,ates as our Th sam-
                             Month                        pling, the data can serve only in qualitative
                                                          comparison with Th data. In Dabob Bay,

T 60-                                  w                  microphytoplankton        biomass is dominated
                                                          by diatoms (Hedges et al. 19883), which
 $j                                                       may explain the two minima in SiO, con-
=                                                         centration in late spring and late fall (Fig.
 E 4o
 3.                                                       8~). It is interesting that SiO, concentration
   ;                                                      seems to vary with total 234Th activity. Slight
8 20-                                                     offsets of the 234Th and SiO, peaks from
             bd,                                          primary productivity        are probably due to
         O-                                               the role of secondary ,producers.
           J FMAMJJASOND                                      The grazing activities of copepods in the
                             Month                        genera Calanus and Pseudocalanus play an
                                                          important role in the dynamics of algal bio-
    Fig. 8. Variation of 234Thactivities and residence
times in total, dissolved, and particulate samples, and mass in the bay (Shaman 1978; Bennett
SiO, concentrations in the surface waters vs. time.        1980; Welschmeyer :and Lorenzen 1985;
                                                          Welschmeyer et al. 1984). A simulation
                                                          model describing the relationship between
                                                           phytoplankton       biomass and filter-feeding
 ducecl by herbivorous zooplankton or by the zooplankton biomass (Frost 1980) suggest-
 direct sinking of algal cells. Only a small               ed a time lag of - 1 month between the
 proportion of fixed C stays in the euphotic               maxima of phytoplankton            biomass and
 zone and is recycled. These biologically pro- zooplankton biomass. From careful exam-
 duced particles may serve as a conveyor belt ination of the pigment budget at the same
 for Th and other particle-reactive           metals.      site, Welschmeyer and Lorenzen ( 198 5)
 Accordingly , 234Th activity in surface wa- concluded that zooplankton grazing is the
 ters should give some seasonal signals that major process that controls phytoplankton
 reflect biological activity. Generally, the ac- abundance. Hence, a reasonable picture is
 tivity of 234Th is higher in winter than in that herbivorous zooplankton biomass (and
 summer and fall (Fig. 8a). A threefold vari-              activity) starts to increase, in response to
 ation in 234Th activities was observed dur-               increasing phytoplankton          biomass, and
                                Dabob Bay 234Thscavenging                                            309

reaches a maximum - 1 month later than
the primary productivity      peak. Although
zooplankton     grazing activity     increases,
downward fluxes of egested fecal pellet also
increase and reach a maximum at - 1 month
after the primary productivity     peak.
    A lag between the primary productivity
peak and the pheopigment flux was not ob-
served by Welschmeyer          and Lorenzen
(198 5). It should be noted that their trap
data were from 60 to 80 m and thus the                     JFMAMJ                     JASOND

pheopigment fluxes may not be represen-                                       Month

tative of that out of the euphotic zone. Close      Fig. 9. Variation of total, dissolved, and particulate
examination reveals that the pheopigment          234Thinventory for the O-l 10-m depth range vs. time.
fluxes did show significant vertical varia-
tion, up to eightfold, between 20 and 40 m        ever, provided a large amount of particulate
during some of the months of their sampling       matter to scavenge 234Th out of the water
year. In this study, we observed maxima in        column. As a result, the inventory of total
the mass and 234Th fluxes at 20 m in June         234Th in December was lower than in Jan-
and September. Hence, the total 234Th ac-         uary.
tivity in surface water in these 2 months             The dissolved 234Th inventory        is rela-
was probably a minimum as a result of en-         tively constant throughout the year. Partic-
hanced 234Th transport out of the euphotic        ulate 234Th inventory follows the annual
zone during those periods.                         variation of total 234Th inventory. More than
    Inventory of 234Th-The highest total           70% of the 234Th in Dabob Bay is associated
 234Th standing stock was found in January         with filterable particles (Fig. 9).
 (1.08 x 1O5 dpm m-2), and minima, on the                Th
                                                      234 activity on particles-The activity
 order of 0.70 x lo5 dpm m-2, were found          of 234Th per unit of weight of particles is
 in May and September (Fig. 9). Standing          called the specific activity and is a function
 stock of 238U above 110 m is -2.35 x 1O5         of the concentration and residence time of
 dpm m-2 and thus the daily radiogenic pro-       particles. In Dabob Bay the specific activity
 duction of 234Th is 6,800 dpm m-2. Part of       of 234Th in filterable particles ranges from
 this production       (30-45%)     disappears    0.15 to 2.5 dpm mg-l and generally shows
 through radioactive decay. If steady state is    a maximum at middepth (Fig. 1Oa). This
 maintained, the remaining 55-70% must be         range brackets the values found by others
removed by processes other than radioac-          (e.g. Minagawa and Tsunogai 1980). Unlike
tive decay. Annual variations in the inven-       the findings of Minagawa and Tsunogai
tory of total 234Th are generally similar to      (1980), however, the correlation of partic-
 variations in the 234Th activity in surface      ulate 234Th and TSM in our data set is poor.
waters (Fig. 8) with two exceptions: the min-     The maximum specific activities of 234Th
imum in the total and particulate 234Th in-       are found on particles at middepth due to
ventory was found in May instead of in            lower TSM concentrations and longer res-
June and the total 234Th inventory of De-         idence times of particles in that layer. The
cember is only -70% of the January inven-         dilution effect caused by resuspension of
t0I-y.                                            sediment with low 234Th activity into the
    Causes of the variations in the inventory     bottom layers may also result in a relative
are more complicated than that of the ac-         maximum of specific 234Th activity at mid-
tivities in the surface layer. Sediment re-       depth. The dilution effect was especially ev-
suspension needs to be considered as well         ident in late winter when the resuspension
as biological activity. For example, primary      was active, resulting in low specific 234Th
productivity   in December is low and, as a       activity in filterable particles throughout the
result, the 234Th activity in surface waters      water column.
is as high as the January value. Active re-           The specific activity of 234Th in sediment-
suspension caused by strong winds, how-           trap material on a salt-free, dry-weight basis
310                                             Wei and Murray

                                        =Th, dpm mg’        The difference between the scavenging abil-
                                                            ity of these two kinds of particles has also
                                                            been reported for other elements. Bennett
                                                            ( 1980) found that trap :particles have higher
                                                            Al content and lower Mn and organic C
                                                            content than do suspended particles. The
                                                            difference in the 234Th activity of these two
                                                            types of particles is a reflection of the dif-
                                                            ference in their size spectrum, composition,
                                                            and history. Tsunogai e:t al. (1974) proposed
                                                            that most of the particles collected by fil-
                                         P4Th, dpm mg”     tration from seawater do not contribute to
       JFMAMJJASOND                                        the vertical settling flux. The particles caught
                                                           by a trap originate in the euphotic zone.
                                                               Bacon and Anderson (19 82) and Nozaki
                                                           et al. ( 1987) differentiated particles into two
                                                           reservoirs, fine particles with negligible set-
                                                           tling velocity and 1arg;e particles with fast
                                                            settling velocity, to model the distribution
                                                            of 230Th (half-life = 7.5 x 1O4 yr) in the
                                                     ib)   open ocean. Sinking particles are generally
                                                           larger in size and have a smaller surface-
  Fig. 10. As Fig. 2, but of 234Thactivity in filterable   area-to-volume ratio, which results in fewer
particles (a) and trap particles (b) vs. time.
                                                           adsorption sites for Th and other elements.
                                                           Bennett (1980) found that 86 111% of trap
was 0.1-0.5 dpm mg--’ (Table 3 and Fig.                    particles on a dry-weight basis are > 10 pm
 lob). This value is similar to that observed              in size and that suspencled particles are much
for suspended matter in the euphotic zone.                 smaller. He argued that the enrichment of
Our data bracket the 234Th specific activities             fine (< 1 pm) MnO, particles in the sus-
obtained at the same station by Carpenter                  pended pool was explained by the inability
et al. I(1984), who used Lorenzen-type sed-                of Pseudocalanus to feed on particles small-
iment traps (Lorenzen et al. 198 1). A max-                er than 4 pm. Murray ( 1975a, b) and Murray
imum in the activity of 234Th in sediment-                 and Brewer (1977) found that Mn oxides or
trap materials at 60 m was found in all                    hydroxides can be efficient scavengers of
months, although the maximum was not as                    dissolved trace metals. Hunter et al. (1988)
pronounced as that for filterable particles.               extrapolated their laboratory results to con-
These: maxima are probably the net result                  clude that significant adsorption of Th by
of adsorption of 234Th by particles settling               Mn oxides can be expected in the coastal
through the water column and dilution with                 environment.
resuspended particles having lower specific                    The existence of fine Mn02 particles in
234Th activities. The highest activity of ex-              the suspended material may explain why
cess 234Th observed in surface sediments in                the activity of 234Th in suspended particles
the miain basin of Puget Sound is 0.05 dpm                 is higher than that of trap particles. Another
mg-l (Carpenter et al. 1984), which is lower               possible explanation for the lower 234Th ac-
than lthe value of either our trap samples or              tivity on trap particles is their “older” age.
filterable particles. Radioactive     decay of             Almost all the particulate matter caught by
234Th while particles travel through water                 sediment traps is fecal1pellet material egest-
column is probably insignificant because the               ed by zooplankton a:nd those fecal pellets
 settling velocity of fecal pellets egested by              must have undergone recycling before sink-
zoopjtankton is high (143 m d-l, Shuman                     ing (Shuman 1978; Bennett 1980). Bennett
 1978; Lorenzen and Welschmeyer 1983).                      (1980) attributed this recycling to coproph-
    234’rh activity on trap particles is lower              agy which is intensified when the population
than that of particles collected by filtration.             of zooplankton is high.
                                           Dabob Bay 234Thscavenging                                                                                  311

                                                          ?l~   Flux   (1000 dpm m”d         -3

   oO   12   3     4   5   6   0   123       4:   6   012              9        4   26     O!      2   3         4   56   01    .2       Cj       4   5   6

                               a                      b                                    A&

                                         MulApr                            May                             Jun                               Jd

                 A’w                                                       od                              NW                                Dee

   Fig. 11. Vertical profiles of the 234Thfluxes measured from sediment-trap data (0) and calculated from the
irreversible scavenging model (0).

  Measured vs. calculated 234Th fluxes-                                depth and calculating the inventories of to-
Minagawa and Tsunogai (1980) observed                                  tal 238U and 234Th. Both fluxes increase with
that measured 234Th fluxes were lower than                             depth and, with the exception of December,
fluxes derived from an irreversible     scav-                          the trap flux is always greater than the model
enging model. They attributed the discrep-                             flux. Overall the ratio of the trap flux to
ancy to poor trapping efficiency and exper-                            model flux is -56%, and there are no sys-
imental error. Tsunogai et al. (1986) applied                          tematic trends in this ratio with water depth.
a nonsteady state scavenging model to the                                 There are some variations from month to
Funka Bay data and concluded that the dif-                             month (Table 5). In February, the ratio is
ference between calculated and measured                                close to 1 and in December it is > 1. In May
234Th fluxes was caused by variations in sta-
bility of the water column. They observed
that the calculated and measured fluxes co-                               Table 5. The monthly averages of the ratio of the
                                                                       measured trap flux of 234Thto that predicted with the
incided in summer and diverged in other                                irreversible scavenging model.
seasons. In open-ocean sutiace waters, Coale
and Bruland ( 198 5) found that the mea-                                                   Month                               Trap/model

sured 234Th flux agreed with the calculated                                              Feb                                         0.84
234Th flux to within 20-40%.                                                             Mar/Apr                                     0.6 1
    In the irreversible scavenging model (Eq.                                            May                                         0.24
3 and 9), PTh is defined as the removal rate                                             Jun                                         0.55
                                                                                         Jul                                         0.64
of particulate 234Th by processes other than                                             Aug                                         0.57
radioactive decay. For comparison with trap                                              Sw                                          0.55
data (Fig. 11) we calculated the fluxes of                                               Ott                                         0.52
234Th (PTh in dpm m -2 d-l) by defining a                                                Nov                                         0.52
                                                                                         Dee                                         1.78
box from the surface to each successive trap                           _
312                                                         Wei and Murray


                  “E   30-
                  u”   25 -

                  g    20-
                  ;    15-

                         0      ,,,‘,‘,,,‘,l,‘,,,‘,
                              Feb              Mar    May     Jun   Jul      Aw    Sep   Ott   Nov   Dee
   Fig. 12. Residence times of particulate 234Th and total suspended matter in the euphotic zone vs. time.
Residence time of particulate 234Thgiven by the ratio of particulate 234Thabove the shallowest trap and the PTh
term defined in the irreversible scavenging model-Cl; residence time of particulate z34Thgiven by the ratio of
inventory of PTh above the shallowest trap (19-28 m) and 234Thflux measured al: that depth-O; residence
time of suspended particles given by the ratio of inventory of TSM above the shallowest trap (19-28 m) and
total mass flux measured at that depth-O.

the ratio averages 24%. For all the rest of                               solve these alternatives with the present data
the year the ratio is - 5 5%. May is a period                             set.
of maximal grazing following the spring                                     Particle dynamics in the euphotic zone-
bloom (Downs and Lorenzen 1985) and it                                    The depth of the euphotic zone, judging from
may be that the low ratio during that month                               a Secchi disk, ranged between 10 and 25 m.
reflects poor trapping efficiency for the type                            We have taken the depth of the shallowest
of sinking particles produced at that time.                               trap (19-28 m) as the lower boundary of the
    December is anomalous because the mea-                                euphotic zone. Particulate 234Th residence
sured fluxes are larger than the modeled                                  times in the euphotic zone (Fig. 12) were
fluxes. The predicted flux is based on water-                             calculated by two approaches: dividing the
column measurements made at the time of                                   inventory    of particulate 234Th above the
trap deployment. After those samples were                                 shallowest trap by the 234Th flux (ThF) de-
collected there were strong winds that may                                termined from sediment-trap data at that
have :resulted in a pulse of resuspended sed-                             depth and dividing the inventory by the flux
iment. This possibility is supported by the                               predicted by the irreversible      scavenging
low values of the specific: 234Th activities                              model (Eq. 10). A similar approach was used
observed (Table 3). The value of 55% ob-                                  to calculate residence time of TSM by di-
served for the rest of the year may represent                             viding the inventory of TSM above the trap
the efficiency of our sediment traps for 234Th-                           by the measured mass flux (A@‘).
containing particles. This low ratio may be                                   In general, the residence time of partic-
due either to hydrodynamic effects or to the                              ulate 234Th calculated by trap and model
nature of the sinking particles. An alterna-                               fluxes agree to within a factor of 3. Partic-
tive explanation is that because Dabob Bay                                ulate 234Th residence times calculated from
is a fjord, boundary scavenging, as de-                                    the trap data are anolmalously long in May
 scribed by Anderson et al. (1983) may be                                  and August. These points are at the time of
 an important removal process. The bound-                                  the spring and fall blooms and the partic-
 aries of the bay would be a sink for 234Th                                ulate 234Th residence times are long because
 and, as a result, the vertical flux would be                              the trap fluxes for those two months were
less than expected. It is n.ot possible to re-                             unusually low. The connection        between
                                        Dabob Bay 234Thscavenging                                                313

these low fluxes and the bloom conditions                  DOWNS,J. N., AND C. J. LORENZEN. 1985. Carbon:
is not clear. The residence times of TSM                        pheopigment ratios of zooplankton fecal pellets as
                                                                an index of herbivorous feeding. Limnol. Gcean-
and particulate 234Th agree well, suggesting                    ogr. 30: 1024-1036.
that 234Th is a good tracer for the mass flux              FROST,B. W. 1980. The inadequacy of body size as
at this location. During 1977 and early 1978                    an indicator of niches in the zooplankton. Am.
the residence time of particulate organic C                     Sot. Limnol. Oceanogr. Spec. Symp. 3: 742-753.
                                                                New England.
 (POC) ranged between 8 and 36 d with lon-
                                                           FURLONG, T. 1986. Sediment geochemistry of pho-
ger residence times in spring and early sum-                    tosynthetic pigments in oxic and anoxic marine
 mer (Bennett 1980). Similarities in the time                   and lacustrine sediments: Dabob Bay, Saanich In-
 scale and seasonal variation      pattern be-                  let, and Lake Washington. Ph.D. thesis, Univ.
 tween the residence time of particulate 234Th                  Washington. 2 14 p.
 and POC implies that 234Th may also be a                  HEDGES,      J.I.,W.A.   CLARK,ANDG.C.COWIE. 1988a.
                                                                Organic matter sources to the water column and
 useful tracer for organic C in coastal envi-                    surficial sediments of a marine bay. Limnol.
 ronments like Dabob Bay.                                       Oceanogr. 33: 1116-l 136.
                                                           -           -,AND-.                1988b. Fluxes and reac-
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