Chasing Precambrian Paleo-redox

Yanan Shen

Harvard University

PHANEROZOIC

NEOPROTEROZOIC

MESOPROTEROZOIC

PALEOPROTEROZOICPR

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ARCHEAN

HADEAN

OUTLINEI: 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, 1950. Phys. Rev.)

However, the abundances of stable isotopes may vary from theiraverage values as a result of biological and inorganic reactions.

Biological sulfur cycle

organic sulfur

assimilatory sulfate reduction dissimilatory sulfate reduction

Microbial Sulfate Reduction

Dissimilatory sulfate reduction

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

δ notation

Standard: I: troilite(FeS) from the Cañon Diablo meteorite, CDTII: IAEA-S-1 (Ag2S), V-CDT

S-isotopic fractionation during sulfate reduction(Shen and Buick, 2003. Earth-Sci. Rev., in pres

s)

Fractionation (‰)

Num

ber o

f occ

urre

nces

Non-limiting sulfate

pure cultures

natural populations

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)

Typical δ34S 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 oceanchemistry

The continuous deposition of BIFs during Archean requires that an ocean was rich in dissolved Fe and anoxic, implying low O2 in Archean atmosphere.

◎

◎ Low sulfate in Archean ocean, supporting low sulfate reduction rates only.

Chert bedFe-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 suggestlow sulfate concentration

Summary I

During most of Archean time, the ocean was anoxic and rich in dissolved Fe, by implication, low O2 in Archean atmosphere.

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The persistent deposition of BIFs suggest that the Archean ocean contained little sulfate.

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Reading Proterozoic Geological Record

No BIFs!Little biological change

(1.8-1.0 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 O2

oxic

The ocean was oxic and Fe was sweptby O2 anddeposited as Fe2O3,requiring high PO2,probably higher than 50% PAL,according tomodelling results.

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Mid-Proterozoic ocean chemistry:Sulfidic/Euxinic model

Low atmospheric O2The ocean was sulfidic (H2S rich) and Fe was titrated by H2S produced by sulfate reduction and deposited as FeS2 (PO2<25-50% PAL).

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Working Models

Sulfidic oceans

Oxic oceans High atmospheric

Low atmospheric

Paleo-redox indicators

others

many

biomar

ker

trace metalspecia

tion

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 FeHR, with high DOP and FeHR/FeT values.

Stratigraphy of the Roper Group

McMinn

Velkerri

Bessie Creek

Corcoran

Munyi

Hodgson

Jalboi

Arnold

Crawford

Mainoru

Rop

er

Gro

up

Sampling the Roper Group

Marginal marine

Inner shelfDistal shelf

Deep basinal

Fe speciation data of the Roper Group

Summary II

Fe speciation data from the Roper Groupprovide compelling evidence for sulfidicconditions in middle Proterozoic oceans.

Stage III Oxic ocean?

Stage I

Stage IIOcean: sulfidic Atmosphere: low O2

Ocean: Anoxic, Fe-rich

Atmosphere: little O2

HADEAN

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