Chasing Precambrian Paleo-redox Yanan Shen Harvard Univers ity PHANEROZOIC NEOPROTEROZOIC MESOPROTEROZOIC PALEOPROTEROZOIC PROTEROZOIC ARCHEAN HADEAN
Feb 08, 2016
Chasing Precambrian Paleo-redox
Yanan Shen
Harvard University
PHANEROZOIC
NEOPROTEROZOIC
MESOPROTEROZOIC
PALEOPROTEROZOICPR
OTE
RO
ZOIC
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.
◎
The persistent deposition of BIFs suggest that the Archean ocean contained little sulfate.
◎
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.
*
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).
*
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
ARCHEAN
PALEOPROTEROZOIC
PHANEROZOIC
PR
OTE
RO
ZOIC
MESOPROTEROZOIC
NEOPROTEROZOIC