ESS 109C Isotope Geochemistry Notes

May 30, 2007

 

Sulfur isotope geochemistry

 

  1. Class notes & homeworks are available online –
                            http://www2.ess.ucla.edu/~schauble/Isotope_geochemistry/

  2. Sulfur isotopes
    1. 32S: 95.02%, 33S: 0.75%, 34S: 4.21%, 36S: 0.02%.
    2. Standardized to FeS (troilite) from the Ca–on Diablo (iron) meteorite. The isotopic composition of Ca–on Diablo troilite is thought to be similar to the bulk composition of the Earth.
    3. Most measurements are reported in terms of 34S/32S, i.e., d34S.
    4. Sulfur isotope ratios are measured by a number of different methods, most commonly in gas phase mass spectrometers as SO2 or SF6. Some minerals can be analyzed readily in situ with ion microprobe (as at UCLA).
  3. Mechanisms of sulfur isotope fractionation
    1. Sulfur at the EarthÕs surface is present in two dominant oxidation states, S6+ (as sulfate, SO42–) and S2– (sulfide). Intermediate oxidation states also occur, but are generally less stable at low T.
    2. Redox fractionation is potentially very large (~80ä at room temperature), if reduced and oxidized sulfur equilibrate.
    3. Fractionations between species with the same oxidation state are usually much smaller, but may be significant.
    4. Exchange/equilibration/partial reaction can occur at high temperatures abiologically, or at low temperature (usually requiring biological activity).
    5. Microbial metabolism.

                                               i.     At low pO2 (i.e., soil, bog, Black Sea, digestive tracts) some organisms can use sulfate, rather than oxygen, to oxidize organic matter for energy: Sulfate reduction.

                                             ii.     Other organisms oxidize sulfide to sulfate: Sulfide (or Sulfur) oxidation.

                                            iii.     Sulfur disproportionation:
S8 + 8H2O ˆ 6H2S + 2SO42– + 4H+.

    1. Structural/protein chemistry in all organisms: Fixation/transformation/release of organic sulfur
      e.g., cysteine: C3H7NO2S
  1. Biogenic sulfur isotope fractionation (sulfate reduction: most potent)
    1. Kinetic (rate) controlled, involving a mixture of diffusion and bond-breaking steps (analogous to 13C fractionation during photosynthesis).
    2. 1st step: diffusion of SO42– into cell/site of reduction

                                               i.     32SO42– typically diffuses faster

    1. 2nd step: breaking of S-O bond

                                               i.     32S-O bond has more zero-point energy: easier to break.

    1. Both processes lead to preferential reduction of 32SO42– ˆ H232S
    2. Net fractionation is variable, depending on metabolic pathway, organism, growth conditions, and sulfate abundance, etc.

                                               i.     ~20-60ä, can be larger or smaller.

    1. S-isotope exchange between sulfate and sulfide is very slow at ~25¼C, so products and reactants are usually unable to equilibrate

                                               i.     Rayleigh fractionation likely.

    1. Sulfate reduction considered most important fractionation at EarthÕs surface – main determinant of S-isotope composition of chemical sediments and surface reservoirs.
  1. Sulfur isotope records.
    1. Modern redox gradients (up = high pO2, down = low pO2)

                                               i.     Photosynthesis, diffusion add O2 at the surface

                                             ii.     Decomposition, respiration remove O2 at depth

                                            iii.     Sulfate reduction Òturns onÓ once other oxidants disappear (O2, NO3–, Mn3+, Fe3+).

                                            iv.     Preferential reduction of 32SO42– leads to production of H232S

                                             v.     Fe2+ (from Fe3+ reduction) binds with sulfide to make FenS (eventually FeS2)

                                            vi.     Rayleigh distillation leads to progressively 34SO42–-rich residue, last reduction yields 34S -FeS2.

    1. Geologic (sedimentary) records of SO42– and H2S.

                                               i.     Geologic records show strong temporal variation