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

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

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

Biological sulfur cycle organic sulfur assimilatory sulfate reductiondissimilatory sulfate reduction Microbial Sulfate Reduction

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

δ notation Standard: I: troilite(FeS) from the Cañon Diablo meteorite, CDT II: IAEA-S-1 (Ag 2 S), V-CDT

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

Pyrite formation and S-isotope preservation little fractionation Therefore, δ 34 S of pyrites in sedimetary rocks provide indication: I: the activity of SRB (Life) II: conditions of sulfide formation (Environment)

Typical δ 34 S values of some geological material (relative to CDT) ocean water sedimentary rocks metamorphic rocks granitic rocks basaltic rocks

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

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 O 2 in Archean atmosphere. ◎ ◎ Low sulfate in Archean ocean, supporting low sulfate reduction rates only. Chert bed Fe-rich layer

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

Summary I During most of Archean time, the ocean was anoxic and rich in dissolved Fe, by implication, low O 2 in Archean atmosphere. ◎ The persistent deposition of BIFs suggest that the Archean ocean contained little sulfate. ◎

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

Mid-Proterozoic ocean chemistry: oxic model High atmospheric O 2 oxic The ocean was oxic and Fe was swept by O 2 and deposited as Fe 2 O 3, requiring high PO 2, probably higher than 50% PAL, according to modelling results. ＊

Mid-Proterozoic ocean chemistry: Sulfidic/Euxinic model Low atmospheric O2 The ocean was sulfidic (H 2 S rich) and Fe was titrated by H 2 S produced by sulfate reduction and deposited as FeS 2 (PO 2 <25-50% PAL). ＊

Working Models Sulfidic oceans Oxic oceans High atmospheric Low atmospheric

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

Fe speciation in sedimentary rocks Total Dithionite Pyrite

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

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

Pyrite formation in oxic and sulfidic environments Diagenetic pyrites Syngenetic pyrites & Diagentic pyrites Therefore, sulfidic sediments are enriched in Fe HR, with high DOP and Fe HR /Fe T values.

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

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

Fe speciation data of the Roper Group

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

Stage III Oxic ocean? Stage I Stage II Ocean: sulfidic Atmosphere: low O 2 Ocean: Anoxic, Fe-rich Atmosphere: little O 2 HADEAN ARCHEAN PALEOPROTEROZOIC PHANEROZOIC PROTEROZOIC MESOPROTEROZOIC NEOPROTEROZOIC