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32S 96%34S 4%
Sulfur isotope systematics
Controls on the d34S of marine sulfide minerals
geologic S isotope cycle - implications for C and O cycles
Sulfur stable isotopes
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Can imagine a Redfield-type
sulfate reduction stoichiometry:
(CH2O)106(NH3)16(H3PO4) + 53SO4-2 =>
106(HCO3-) + 16NH3 + H3PO4 + 53(H2S)
Or even just:
2(CH2O) + SO4-2 => 2(HCO3
-)+ H2S
Production of ammonia, H2S, and alkalinity at the depth of SR.
If NH3 and H2S diffuse up and are reoxidized;
consume O2, release H+ close to sediment-water interface
If H2S reacts with Fe++, reduced sulfur and Fe are buried.
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Strong (5 to 45 o/oo) depletion
in 34S of sulfides, relative tosulfate, during sulfate reduction.
Canfield and Teske (1996)
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Most (90%?) sulfide produced
by SR in coastal sediments is
reoxidized.
Elemental sulfur is an important
sulfide oxidation product.
So can undergo microbial
disproportionation.
Canfield and Teske (1996)
Why are sedimentary sulfides
much more strongly depleted in34S than the sulfide produced in
culture experiments?
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Bacterial disproportionation of elemental sulfur:
4S + 4H2O => 3H2S + SO4-2 + 2H+ (1)
is often followed by sulfide scavenging by iron oxides
and sulfide reoxidation:
H2S + 4H+ + 2Fe(OH)3 => 2Fe2+ + S + 6H2O (2)
and
2H2S + 2Fe2+ + => 2FeS + 4H+ (3)
Yielding an overall reaction of :
3S + 2Fe(OH)3 => 2FeS + 2H2O + SO4-2 + 2H+ (4)
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Canfield and Thamdrup 1994
Sediment, ammended with So,
yielded both sulfate and sulfide.This bacterial disproportionation of
elemental sulfur produced sulfate
that was enriched in 34S and sulfide
that was depleted in 34S.
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If sulfide oxidation to elemental sulfur does
not fractionate sulfur isotopes, repeateddisproportionation and reoxidation will
result in more strongly depleted sulfides.
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Large, rapid changes in the
d34S of seawater sulfate.
Lower d34S in sulfate implies
reduced burial of sufide.
Paytan et al., 1998
Barite-based (BaSO4) d34S
record reflects the isotopic
composition of seawater
sulfate
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Even stronger
signal in the
Cretaceous.
Paytan et al., 2004
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Sulfatelarge reservoir, small fluxes
Sulfate
twosimilar sinks, one
(pyrite) strongly
depleted in 34S due
to fractionation
during sulfatereduction; seawater
sulfate is enriched
in 34S w.r.t
weathering input.
Includes
vulcanismand HT
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The sulfate residence time
is long (20 My)
(reservoir/flux), but the
sulfate isotopic residencetime is shorter than the
concentration residence
time, due to the large SR /
H2S reoxidation cycle
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Large, rapid changes in the d34S of seawater
sulfatetwo hypotheses related to
changes in sulfide burial:
Sulfide burial (in margin sediments) should
be linked to organic C burial. Times of
low sulfate d34S (low sulfide burial)
would be times of low DIC d13C (low
organic C burial).
Both sulfide burial and organic C burial
linked to O2 (atm). Since O2 fairly
constant in Cenozoic, sulfide burial and
organic C burial for some reason offset
each other. Times of low sulfate d34S
(low sulfide burial) would be times ofhigh DIC d13C (high organic C burial).
Paytan et al., 1998
Barite-based d34S record
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Carbon (DIC)small reservoir, large
fluxes, short residence time (O 100ky)
Carbon
only thesmaller sink
(organic C) is
strongly depleted
in 13C; seawater
DIC is onlyslightly enriched
in 13C
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In fact, there is no obvious
correlationpositive or
negativebetween d34S
(sulfate) and d13C (DIC).
Paytan et al., 1998
Non-steady-state behavior
of S isotope budget.
Important terrestrialcomponent to C org burial
and d13C (DIC) budget?
Together: Hard to
reconstruct atmospheric O2
fromd34
S (sulfate) andd13C (DIC).