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Published byBridget Webb Modified over 8 years ago
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32 S 96% 34 S 4% Sulfur isotope systematics Controls on the 34 S 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: (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 53SO 4 -2 => 106(HCO 3 - ) + 16NH 3 + H 3 PO 4 + 53(H 2 S) Or even just: 2(CH 2 O) + SO 4 -2 => 2(HCO 3 - )+ H 2 S Production of ammonia, H 2 S, and alkalinity at the depth of SR. If NH 3 and H 2 S diffuse up and are reoxidized; consume O 2, release H + close to sediment-water interface If H 2 S reacts with Fe ++, reduced sulfur and Fe are buried.
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Strong (5 to 45 o/oo) depletion in 34 S of sulfides, relative to sulfate, 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. S o can undergo microbial disproportionation. Canfield and Teske (1996) Why are sedimentary sulfides much more strongly depleted in 34 S than the sulfide produced in culture experiments?
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Bacterial disproportionation of elemental sulfur: 4Sº + 4H 2 O => 3H 2 S + SO 4 -2 + 2H + (1) is often followed by sulfide scavenging by iron oxides and sulfide reoxidation: H 2 S + 4H + + 2Fe(OH) 3 => 2Fe 2+ + Sº + 6H 2 O (2) and 2H 2 S + 2Fe 2+ + => 2FeS + 4H + (3) Yielding an overall reaction of : 3Sº + 2Fe(OH) 3 => 2FeS + 2H 2 O + SO 4 -2 + 2H + (4)
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Canfield and Thamdrup 1994 Sediment, ammended with S o, yielded both sulfate and sulfide. This bacterial disproportionation of elemental sulfur produced sulfate that was enriched in 34 S and sulfide that was depleted in 34 S.
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If sulfide oxidation to elemental sulfur does not fractionate sulfur isotopes, repeated disproportionation and reoxidation will result in more strongly depleted sulfides.
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Large, rapid changes in the 34 S of seawater sulfate. Lower 34 S in sulfate implies reduced burial of sufide. Paytan et al., 1998 Barite-based (BaSO 4 ) 34 S 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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Sulfate – large reservoir, small fluxes Sulfate – two similar sinks, one (pyrite) strongly depleted in 34 S due to fractionation during sulfate reduction; seawater sulfate is enriched in 34 S w.r.t weathering input. Includes vulcanism and HT
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The sulfate residence time is long (20 My) (reservoir/flux), but the sulfate isotopic residence time is shorter than the concentration residence time, due to the large SR / H 2 S reoxidation cycle
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Large, rapid changes in the 34 S of seawater sulfate – two hypotheses related to changes in sulfide burial: Sulfide burial (in margin sediments) should be linked to organic C burial. Times of low sulfate 34 S (low sulfide burial) would be times of low DIC 13 C (low organic C burial). Both sulfide burial and organic C burial linked to O 2 (atm). Since O 2 fairly constant in Cenozoic, sulfide burial and organic C burial for some reason offset each other. Times of low sulfate 34 S (low sulfide burial) would be times of high DIC 13 C (high organic C burial). Paytan et al., 1998 Barite-based 34 S record
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Carbon (DIC) – small reservoir, large fluxes, short residence time (O 100ky) Carbon – only the smaller sink (organic C) is strongly depleted in 13 C; seawater DIC is only slightly enriched in 13 C
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In fact, there is no obvious correlation – positive or negative – between 34 S (sulfate) and 13 C (DIC). Paytan et al., 1998 Non-steady-state behavior of S isotope budget. Important terrestrial component to C org burial and 13 C (DIC) budget? Together: Hard to reconstruct atmospheric O 2 from 34 S (sulfate) and 13 C (DIC).
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