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Chasing Precambrian Paleo-redox

PHANEROZOIC. NEOPROTEROZOIC. MESOPROTEROZOIC. Chasing Precambrian Paleo-redox. PROTEROZOIC. PALEOPROTEROZOIC. Yanan Shen Harvard University. ARCHEAN. HADEAN. OUTLINE. I: Sulfate Reduction and S-isotopic Fractionation II: Archean Ocean Chemistryand Atmosphere

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Chasing Precambrian Paleo-redox

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  1. PHANEROZOIC NEOPROTEROZOIC MESOPROTEROZOIC Chasing Precambrian Paleo-redox PROTEROZOIC PALEOPROTEROZOIC Yanan Shen Harvard University ARCHEAN HADEAN

  2. OUTLINE I: Sulfate Reduction and S-isotopic Fractionation II: Archean Ocean Chemistryand Atmosphere III: Paleo-redox of mid-Proterozoic oceans

  3. Abundances of stable sulfur isotopes (MacNamara and Thode, 1950. Phys. Rev.) However, the abundances of stable isotopes may vary from their average values as a result of biological and inorganic reactions.

  4. Biological sulfur cycle organic sulfur assimilatory sulfate reduction dissimilatory sulfate reduction Microbial Sulfate Reduction

  5. Dissimilatory sulfate reduction 32S-O bond easier to break than 34S-O Sulfides become depleted in 34S, enriched in 32S

  6. δ notation Standard: I: troilite(FeS) from the Cañon Diablo meteorite, CDT II: IAEA-S-1 (Ag2S), V-CDT

  7. S-isotopic fractionation during sulfate reduction (Shen and Buick, 2003. Earth-Sci. Rev., in press) pure cultures natural populations Number of occurrences Non-limiting sulfate Fractionation (‰)

  8. Pyrite formation and S-isotope preservation little fractionation Therefore, δ34S of pyrites in sedimetary rocks provide indication: I: the activity of SRB (Life) II: conditions of sulfide formation (Environment)

  9. Typical δ34S values of some geological material (relative to CDT) ocean water sedimentary rocks metamorphic rocks granitic rocks basaltic rocks

  10. Reading Archean Geological Record Oldest biomarker Paleoproterozoic snowball Oldest microfossils banded iron formations Time (Ga)

  11. Banded Iron Formations and Archean ocean chemistry The continuous deposition of BIFs during Archean requires that an ocean was rich in dissolved Fe and anoxic, implying low O2 in Archean atmosphere. ◎ Chert bed Fe-rich layer ◎ Low sulfate in Archean ocean, supporting low sulfate reduction rates only.

  12. Sulfate reduction rates vs rates of Fe delivery Inference: The great insolubility of iron sulfide minerals requires that, at normal marine Ph values, both iron and sulfide cannot coexist in abundant in solution, so either sulfide or iron may dominate ocean chemistry, but not both. Low SR rates in Archean oceans suggest low sulfate concentration

  13. Summary I During most of Archean time, the ocean was anoxic and rich in dissolved Fe, by implication, low O2 in Archean atmosphere. ◎ The persistent deposition of BIFs suggest that the Archean ocean contained little sulfate. ◎

  14. Reading Proterozoic Geological Record The dullest time in Earth history!! (Buick, Des Marais, Knoll, 1995) No BIFs! Little biological change No glaciations (1.8-1.0 Ga) banded iron formations Time (Ga)

  15. Mid-Proterozoic ocean chemistry: oxic model The ocean was oxic and Fe was swept by O2 and deposited as Fe2O3, requiring high PO2, probably higher than 50% PAL, according to modelling results. ＊ High atmospheric O2 oxic

  16. Mid-Proterozoic ocean chemistry: Sulfidic/Euxinic model Low atmospheric O2 The ocean was sulfidic (H2S rich) and Fe was titrated by H2S produced by sulfate reduction and deposited as FeS2 (PO2<25-50% PAL). ＊

  17. Working Models Sulfidic oceans Low atmospheric Oxic oceans High atmospheric

  18. Paleo-redox indicators speciation trace metal Fe my stuff size of framboid pyrite green sulfur bacteria many S/C ratios biomarker others

  19. Fe speciation in sedimentary rocks Total Dithionite Pyrite

  20. Degree of Pyritization (DOP) (Raiswell et al., 1988) Highly Reactive Fe Ratio (Raiswell and Canfield, 1998)

  21. The FeHR/FeT and DOP for normal marine, euxinic sediments, and black shales from the McArthur Basin (Shen et al., 2002.AM. J. Sci.)

  22. Pyrite formation in oxic and sulfidic environments Syngenetic pyrites & Diagentic pyrites Diagenetic pyrites Therefore, sulfidic sediments are enriched in FeHR, with high DOP and FeHR/FeT values.

  23. Stratigraphy of the Roper Group McMinn Velkerri Bessie Creek Corcoran Munyi Roper Group Hodgson Jalboi Arnold Crawford Mainoru

  24. Sampling the Roper Group Marginal marine Inner shelf Distal shelf Deep basinal

  25. Fe speciation data of the Roper Group

  26. Summary II Fe speciation data from the Roper Group provide compelling evidence for sulfidic conditions in middle Proterozoic oceans.

  27. PHANEROZOIC Stage III Oxic ocean? NEOPROTEROZOIC MESOPROTEROZOIC Ocean: sulfidic Atmosphere: low O2 Stage II PROTEROZOIC PALEOPROTEROZOIC Ocean: Anoxic, Fe-rich Atmosphere: little O2 Stage I ARCHEAN HADEAN

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