Isotopes_in_Ecosystem_Ecology_Workshop

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					Isotopes in Ecosystem Ecology
          Workshop
              8 and 12Jan09
 Terry Loecke (loecket@caryinstitute.org)
    Jon Cole (colej@caryinstitute.org)
      Isotope Workshop Outline
• Introductions
  – experiences with isotopes
• motivation
  – confirm biochemical pathways
  – understand terms in assigned reading
• introductory isotope lecture
  – natural abundance variation
  – why?
• applications
  – fluxes and ecological relationships, e.g., food webs
                   Photosynthesis
• where do plants get their mass?
  – soil? early 1600’s - Jan Baptista van Helmont
       • mass balance
  –   water? late 1600’s – Woodward
  –   air? 1771 – Prestley
  –   Lavoisier – modern chemical theory
  –   Ingenhousz – CO2 was split and O2 was waste
       • hypothesis stood for 150 years
  – How could we use isotopes to test this hypothesis?


                                http://science.jrank.org/pages/5189/Photosynthesis-History-research.html
     Net photosynthesis reaction
6CO2(g) + 6H2O(l) + photons → C6H12O6(aq) + 6O2(g)
            Isotope “have-tos”
•   Define isotope
•   Prevalence among elements
•   Abundance on the planet
•   Isotope Ratio Mass spectrometry
•   δ notation
•   Differences between isotopes
•   Natural abundance
•   Fractionation
•   End member mixing models
Isotopes are just atoms of the same element that
differ in mass number.


           Carbon-12                 Carbon-13
                         electrons


                         neutrons


                         protons


            6 protons                 6 protons
            6 neutrons                7 neutrons



                                                   R. Doucett
Some isotopes are stable, while others are unstable,
or radioactive.

 Carbon-12                 Carbon-13       Carbon-14




  (6P + 6N)                 (6P + 7N)        (6P + 8N)



         Stable isotopes                Radioactive isotope

                                                      R. Doucett
The lightest stable isotope is usually the most
abundant.
    Element         Isotopes             Abundance
    Hydrogen        1H, 2H               1H = 99.985%
                                         2H = 0.015%


    Carbon          12C, 13C             12C = 98.89%
                                         13C = 1.11%


    Nitrogen        14N, 15N             14N = 99.633%
                                         15N = 0.366%


    Oxygen          16O, 17O, 18O        16O = 99.759%
                                         17O = 0.037%

                                         18O = 0.204%


    Sulfur          32S, 33S, 34S, 36S   32S = 95.00%
                                         33S = 0.76%

                                         34S = 4.22%

                                         36S = 0.014%



                                                         R. Doucett
Isotope Ratio Mass Spectrometry




                         m/z 4
                  12C+16O+16O   = 44
              m/z 3 (HD) 16O = 45
                  13C+16O+
                  12C+18O+16O   = 46
Mass spectrometers can measure very small
differences in isotope ratios.


•   13C/12C = 0.011225
•   13C/12C = 0.011071

•   13C/12C = 0.010918




                                            R. Doucett
      FYI: Analytical Methods




Staddon, P.L. 2004. Carbon isotopes in functional soil ecology. Trends Ecol. Evol. 19:148-154.
             Stable Isotope Units
• Isotope ratio - R = [HX] / [LX]
   – e.g., 13C/12C
• Fractional abundance - F = R/(R+1)
   – e.g., F = 13C/(13C+12C) = molar fraction abundance
• Atom % = F*100
• δ = “delta”
   – δ‰ = ((Rsample – Rstd)/(Rstd))*1000
   – each isotope has an international standard
   – e.g.,                13 12       13 12
                           C/ Csample - C/ Cstandard
            d13Csample =                               x 1000
                                13C/12C
                                       standard
                           FYI: Standards
Element    d value    Ratio Measured (R)        International Standards             R, International Standards



                                           Vienna Standard Mean Ocean Water
Hydrogen     dD             2H/1H                                                          0.00015575
                                                       (VSMOW)



                            2H/1H          Standard Light Antarctic Precipitation
                                                                                           0.000089089
                                                          (SLAP)



 Carbon     d   13C         13C/12C         Vienna Pee Dee Belemnite (VPDB)                 0.0112372



Nitrogen    d   15N        15N/14N                          Air                             0.003676



                                           Vienna Standard Mean Ocean Water
Oxygen      d   18O        18O/16O                                                          0.0020052
                                                       (VSMOW)


                           18O/16O          Vienna Pee Dee Belemnite (VPDB)                 0.0020672



                           18O/16O         Standard Light Antarctic Precipitation
                                                                                            0.0018939
                                                          (SLAP)



 Sulfur     d   34S         34S/32S            Canyon Diablo Triolite (CDT)                 0.045005
               Relationship d13C and
                     atom%  13C
                           40000
                                                                         R² = 0.992
                           35000
                           30000
Enrichment studies         25000

- atom% only               20000
                      δ‰
                           15000
                           10000
                            5000
                               0
                                   0   5          10     15       20         25             30   35
                           -5000
                                                             Atom %
                            100                                                R² = 1
                             80
                             60
                             40
                             20
Natural abundance -
                      δ‰




                              0

atom% or δ                   -20 1         1.05        1.1            1.15            1.2        1.25

                             -40
                             -60
                             -80
                            -100
                                                             Atom %
Relationship d13C and atom%13C



                            d notation is primarily used
                            for natural abundance
                            isotope ratios, not
                            experimental enrichments




“depleted”       d13C               “enriched”
     or
             _          +               or
 “lighter”                           “heavier”
 What effect do extra neutrons have?
• Since neutrons have no charge, the possible
  chemical reactions of an element are constant
• Since the element has a greater mass,
  – heavier isotope ~ lower velocity
     • kinetic energy = 1/2MV2
  – atomic attraction is greater, leading to stronger
    chemical bonds.
  – zero point energy is lower for heavier isotopes
Spatial variability of O isotopes in hydrologic fluxes




                                          taken from David Myrold
C isotopic variation
C3 vs. C4
    Both Kinetic and
    Equilibrium fractionation




    Kinetic fractionation




Staddon, P.L. 2004. Carbon isotopes in functional
soil ecology. Trends Ecol. Evol. 19:148-154.
      Variation of C isotope ratios in nature




                                 Soil Organic
                                 Matter -12


Pool values = δ13C‰
Arrows = isotope fractionation                  Fry 2006
Carbon Isotope Variation
Sulfur Isotope Variation
             Fractionation Mechanisms
Only when reactant pools are not completely exhausted

• Equilibrium fractionation
    –   closed systems
    –   Reversible reactions
    –   more energy is needed to break a bond with greater mass
    –   Rule of Thumb – oxidized states tend to be enriched
         •   δ18O(S) > δ18O(L) > δ18O(V)
         •   CO2 < HCO3- < CaCO3
         •   δ13CO2 > δ13CH4
         •   Higher the temperatures minimize equilibrium fractionation
         •   e.g., CO16O + H218O <-> CO18O + H216O
• Kinetic fractionation
    –   KE = 1/2MV2
    –   Heavy isotopes diffuse slower
    –   open systems
    –   Irreversible reactions
    –   difficult to predict in complex biological reactions

• *Sub-disciplines differ in notation for fractionation factors
                Discrimination
• You are not exactly (isotopically) what you eat.
• Isotopic fractionation can also be expressed as
  the discrimination (Δ) against an isotope during a
  reaction or physical process:
   – This is also known as the fractionation factor
• Δ = (δsource - δproduct)/(1 + δproduct)
• Discrimination is the result of fractionation.
 Fractionation
 Example
    Both Kinetic and
    Equilibrium fractionation




    Kinetic fractionation




Staddon, P.L. 2004. Carbon isotopes in functional
soil ecology. Trends Ecol. Evol. 19:148-154.
           Trophic Fractionation
 15                 15
δ Npredator = 3.0 + δ Nprey (‰)




                            Isotopic fractionation – the light 14N isotope is
                            excreted more than the heavy 15N isotope,
                            leaving the animal enriched by 3‰ in δ15N
                            relative to its food source.
  Trophic Level Fractionation
  - the power of multiple isotope groups
    10
                                Fish



d15N 5            Zooplankton          Snail



                   Pelagic             Benthic
                    Algae               Algae
     0
         -30           -20                     -10
                     d13C
    Characteristics of useful natural
         abundance isotopes
• Large mass differences between rare and
  abundant isotope.
• Rare isotope is rare (0.05 to 5%).
• Element forms covalent bonds.
• Multiple oxidation states.
• Element in compounds with high vapor
  pressure.
   Verbal Mixing Model -
Blue and Yellow make Green
   Two End Member Mixing Model
                             Substitute δ for C
                            Qg  Qb  Q y
                            Qgd g  Qbd b  Q yd y
                            Qb  Qg  Q y
                                    dg  d y
Q = quantity                Fb  Qg
C = concentration
F = fraction
                                    db  d y
  Qg  Qb  Q y                                   Or Atom% for C
                                            Qg  Qb  Q y
  Qg C g  QbCb  Q y C y
                                            Qg % g  Qb % b  Q y % y
  Qb  Qg  Q y
                                            Qb  Qg  Q y
               Cg  C y
  Fb  Qg                                               %g  % y
               Cb  C y                     Fb  Qg
                                                        %b  % y
End Member Mixing Model for Natural
           Abundance
             (δt - δB)
         ƒA =(δ - δ )
               A    B
•   ƒA = fraction of C from end member A
•   δt = 13C signature of the mixed C
•   δA = 13C signature of the end member A
•   δB = 13C signature of the end member B
•   end member signatures need to be determined
    independently
Defining End Members:
     One Example
       Keeling Plot
      Isotope Workshop Outline
• Introductions
  – experiences with isotopes
• motivation
  – confirm biochemical pathways
  – understand terms in assigned reading
• introductory isotope lecture
  – natural abundance variation
  – why?
• applications
  – fluxes and ecological relationships, e.g., food webs
          Isotope Applications
• Natural Abundance:
  – temperature measurements in the past
  – origin of water, rocks and organic matter
  – origin of drugs, wine, etc. (e.g., forensic
    applications)
  – process studies
     • rate, fate, and pathway
     • difficulty with multiple processes
     • food-web studies
• Enrichment:
  – rate, fate, and pathway
  – sensitivity and complexity
     Net photosynthesis reaction
6CO2(g) + 6H2O(l) + photons → C6H12O6(aq) + 6O2(g)

label CO2                   label H2O
C18O2 + H216O ->             C16O2 + H218O ->
  CH218O + 16O2    or             CH218O + 16O2 or
   CH216O + 18O2   or             CH218O + 16O2 or
  mixture                         mixture
Sample tin capsules for solids
Elemental Analyzer - IRMS
                  Fretwell’s Law
“Warning! Stable isotope data may cause severe and
contagious stomach upset if taken alone.

To prevent upsetting reviewers’ stomachs and your own,
take stable isotope data with a healthy dose of other
hydrologic, geologic, and geochemical information.

Then, you will find stable isotope data very beneficial.”

            (Marvin O. Fretwell, USGS, pers. comm. 1983) via
                                                Carol Kendall
                      Resources
• http://wwwrcamnl.wr.usgs.gov/isoig/res/
  – Courses
     •   Jim Ehleringer – U of Utah
     •   Peggy and Nathaniel Ostrom – Michigan State U
     •   Carol Kendall – USGS
     •   Marine Biological Laboratory
     •   Meeting - http://isoscapes2008.org/

				
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