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Sulfur isotopes in the rock record

Sulfur isotopes in the rock record. James Farquhar ESSIC and Department of Geology, University of Maryland. Research presented here supported by ACSPRF, NASA, and NSF. 2 parts. The Geological story told by sulfur isotopes

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Sulfur isotopes in the rock record

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  1. Sulfur isotopes in the rock record James FarquharESSIC and Department of Geology, University of Maryland Research presented here supported by ACSPRF, NASA, and NSF

  2. 2 parts • The Geological story told by sulfur isotopes • Questions about the sulfur isotope record and the processes it records

  3. Rationale for S-isotopes in geochemistry (32S, 33S, 34S, & 36S) (δ34S, Δ33S, Δ36S) As Tracers of biological activity

  4. Diamond Rationale for S-isotopes in geochemistry As Tracers of mass transfer

  5. Oxygen Rationale for S-isotopes in geochemistry Biogeochemistry of oceanic sulfate Atmospheric oxygen 3.0 2.0 1.0 0 Age (Gyr) Modified from Holland (2006) As Tracers of planetary evolution Credit:  NASA Johnson Space Center (NASA-JSC)

  6. Linking sulfur isotope variations to chemical and physical process NOTATION: Express compositions as ratios of isotopic ratios using: d34S = [(34S/ 32S)I/(34S/ 32S)ref – ((34S/ 32S)ref /(34S/ 32S)ref ] D33S ismeasured –predicted 33S/ 32S in a sample D36S ismeasured –predicted 36S/ 32S in a sample D33S = [(33S/ 32S)I/(33S/ 32S)ref-((34S/ 32S)I/(34S/ 32S)ref )0.515] D36S = [(36S/ 32S)I/(36S/ 32S)ref-((34S/ 32S)I/(34S/ 32S)ref )1.9]

  7. D – notation: classical isotope effect reference frame 1st POINT: Mass-dependent effects produce only small variations for Δ33S (Δ36S) CIE arise because isotope mass plays a part determining Vibrational partition functions and internal energy D33S = [(33S/ 32S)I/(33S/ 32S)ref-((34S/ 32S)I/(34S/ 32S)ref )0.515] Mass-dependent effects produce significant variations for 34S

  8. NMD Mass-independent isotopic effects 2nd POINT: Mass-independent effects produce larger Variations for Δ33S (Δ36S) With or without variations for 34S Factors in addition to mass play roles in other types of chemical reactions Thiemens and Heidenreich, 1983 Science

  9. Mass-conservation effects • Variations in Δ3xS also occur because of a linear dependence of isotope ratios when material is added to pools (mixing) OR when material is removed from pools (e.g., Rayleigh effects) • (34S/ 32S)tot = 32Xa(34S/32S)a + 32Xb(34S/ 32S)b • Instead of an exponential dependence that is used to define the reference fractionation arrays • (33S/ 32S) a / (33S/ 32S) b ~ [(34S/ 32S)a/ (34S/ 32S) b] 0.515 • These Principles apply in Biosynthetic Networks and in Biogeochemical Networks • Most significant impact on Δ36S 3rd POINT: Small magnitude signals for Δ33S (larger for Δ36S) produced by biological cycling Δ33S (Δ36S) scale with 34S Desulfomaculum acetoxidans Spring et al., 2009

  10. How are these different types of isotope effects expressed in the geologic record? Increase in fractionation with time

  11. Decrease in Δ33S variation With time

  12. Other aspects of data

  13. Question about Origin of MIF Farquhar et al., 2001

  14. Link to Ozone and Oxygen Why geoscientists care?

  15. Connection: Oxygen and ozone concentrations control available UV radiation Link to atmospheric oxygen levels: Sulfur chemistry and atmospheric transparency

  16. Oxygen and ozone concentrations control available UV radiation

  17. Oxygen and ozone concentrations control available UV radiation

  18. Oxygen and ozone concentrations control available UV radiation

  19. Oxygen and ozone concentrations control available UV radiation

  20. Link to atmospheric oxygen levels: Sulfur chemistry and atmospheric transparency

  21. Second link – cycling of sulfur insufficient to homogenize Δ33S Limits oxidative weathering – consistent with geological evidence P – pyrite R – rutile Z – zircon C - chromite Wacey et al. 2010

  22. Third link to atmospheric oxygen levels: Formation of sulfur aerosols creates a second pathway for transfer of atmospheric signals to surface sulfur pools Kasting JF, SCIENCE, 293: 819-820, 2001 Also Pavlov et al. 2002

  23. Developments in the past 10 years Offset in absorption features conducive for shielding effects Effects related to UV spectrum Danielache et al., 2008

  24. Developments in the past 10 years Reduction of sulfate using amino acids Hypothesis: either an MIE or a new type of isotope effect (may be relevant in geological systems) Possible alternative chemical pathways for MIF Watanabe et al., 2009

  25. Developments in the past 10 years Geochemical Interpretations Possible variations in the signal during the ArcheanOno et al., 2006; Ohmoto et al., 2006; Domagol Goldman et al., 2009; Halevey et al., 2010) More detailed focus on the record and development of models for interpretation

  26. Ono et al (2003, 2009) argued that changes in the MIF-S signal reflect changes in Where the chemistry occurs. the amount of sulfur released to the atmosphere and the oxidation state of the atmosphere (controlled by CH4). Developments in the past 10 years Geochemical Interpretations Ono et al. 2003

  27. Domagal-Goldman argued that climate feedbacks and organic haze controlled the available radiation and the expression of MIF(other studies – Ueno et al., 2009 explored other shielding processes) Archean climate control feedback loop (Pavlov et al. 2001) Developments in the past 10 years Geochemical Interpretations Domagol Goldman et al. 2009

  28. A model that describes the production of MIF in terms of shielding by CO2, the proportion of sulfur that is photolyzed with MIF (assumed SO2), and the proportion of sulfur that is lost by non MIF processes (oxidation and H2S photolysis). And the geologicpreservation of MIF by the homogenization of sulfur in a one box (well-mixed) ocean by metabolic activity Developments in the past 10 years Geochemical Interpretations Halevey et al. 2010

  29. Significant issues remain • Sampling the sulfur isotope record • (representative sample or not?)

  30. Bias toward samples with high Δ33S? Or Missing pool of sulfur with negative Δ33S? Sample density too low

  31. Significant issues remain • Sampling the sulfur isotope record • (representative sample or not?) • Characterizing variability in the early sulfur isotope record

  32. What is the true nature of temporal variablity?

  33. Martian meteorites Difference: lack of anomalous Δ36S Shergotites Other Martians Work of Franz and Kim, unpubl.

  34. Significant issues remain • Sampling the sulfur isotope record • (representative sample or not?) • Characterizing variability in the early sulfur isotope record • Characterization of the source of the effect

  35. Role of shielding and primary photochemical IE Effects related to UV spectrum Offset in absorption maxima, minima, and width, carry implications for isotope effects. Danielache et al., 2008

  36. Issues with experiments Point: Relationships between Isotope effects and UV spectrum

  37. Issues with experiments Masterson et al. 2011 Point: Systematic relationships between Isotope effects and pressure

  38. Issues with experiments Point: Systematic relationships between Isotope effects and identity of oxygen Experiments with S18O2 and S16O2 (Heather Franz, unpub)

  39. END

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