Sulfate reduction idealized stoichiometry pathways and substrates case studies Cape Lookout Bight – extreme SR Southwest African margin – subtle SR Microbial mats – aerobic SR? But sulfate reducers are obligate anaerobes! Sulfur stable isotopes systematics geologic S cycle implications for C and O cycles

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 (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.

But sulfate reducers can only oxidize a limited suite of simple organic substrates. They typically function as part of a community that includes fermenters, acetogens, and methanogens, as well as sulfate reducers. (Fenchel and Finlay, Ecology and Evolution in Anoxic Worlds)

Syntrophy (more next Tuesday)

The pathways are complex and variable, the processes are tightly linked, production and consumption of intermediates are rapid and in balance, … Jorgensen, 1983

Cape Lookout Bight, NC Martens et al….

Klump and Martens, 1987 Sulfate near zero by 10 – 25 cm! 60 mM DIC!!

mM ammonia

Direct measurements of sulfate reduction rates (Crill and Martens): Closed-tube incubations (decrease in sulfate over time) 35 S-sulfate tracer method Inject cores with 35 S-sulfate After incubation, acidify and collect acid-volatile sulfide (AVS) (HS -, H 2 S, FeS). 35 S activity in the AVS fraction reflects sulfate reduction 35 S activity in residual sulfate to check total tracer recovery caveats: (35) SO 4 -2 addition; isotope equilibration; re-oxidation of labeled sulfides; pyrite, S° formation (non-volatile sulfides)

mM per day SR! Crill and Martens, 1987

Strong seasonal cycles in sulfate penetration, sulfate reduction rate, driven by temperature. Crill and Martens, 1987

Winter – low SR rates in top few cm (oxic) Summer – highest rates near sediment surface. Second maximum at depth: artifacts? Shifts in microbial community?

Seasonal Fe cycle too

25% of organic C rain is remineralized, 75% is buried. Of the remineralized fraction, 70% due to sulfate reduction 30% due to methanogenesis O 2 fluxes (calculated) dominated by oxidation of H 2 S and sulfide minerals Chanton et al., 1987

Ferdelman et al., 1999 Sulfate reduction in the Southeast Atlantic

SR rates determined by 35 S tracer injection. Rates 100x lower than Cape Lookout Bight; little sulfate depletion

In situ microelectrode oxygen flux estimates (Glud)

Assuming that the oxygen flux includes both oxic respiration and reoxidation of SR products, SR accounts for 20-95% of O 2 flux at shallow sites, 5-15% at deep sites.

Canfield and Des Marais (1991) Aerobic sulfate reduction in microbial mats 1 uM/min ~ 1.5 mM/day but the volume is small relative to CLB Sulfate reduction O2O2 O 2 production

noon midnight noon High SR rates during the day, in the O 2 production zone? Aren’t sulfate reducers obligate anaerobes? Novel pathways or communities…

Sulfur stable isotopes: 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

Strong (5 to 45 o/oo) depletion in 34 S of sulfides, relative to sulfate, during sulfate reduction. Canfield and Teske (1996)

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.

Sulfate – large reservoir, small fluxes Sulfate – two similar sinks, one (pyrite) strongly depleted in 34 S; seawater sulfate is enriched in 34 S. The sulfate residence time is long (20 My), but the sulfate isotopic residence time is shorter than the concentration residence time, due to the large SR / H 2 S reoxidation cycle

Carbon (DIC) – small reservoir, large fluxes Carbon – only the smaller sink (organic C) is strongly depleted in 13 C; seawater DIC is only slightly enriched in 13 C

Large, rapid changes in the  34 S of seawater sulfate. Paytan et al., 1998 Barite-based  34 S record

Even stronger signal in the Cretaceous. Paytan et al., 2004

Large, rapid changes in the  34 S of seawater sulfate – two hypotheses for inferred changes in sulfide burial: (1)O 2 (atm) fairly constant in Cenozoic, so 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). (2)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). Paytan et al., 1998 Barite-based  34 S record

In fact, there is no obvious correlation – positive or negative – between  34 S (sulfate) and  13 C (DIC). Paytan et al., 1998

Why are sedimentary sulfides much more strongly depleted in 34 S than the sulfide produced in culture experiments? Canfield and Teske (1996)

Canfield and Thamdrup 1994 Bacterial disproportionation of elemental sulfur produces sulfate that is enriched in 34 S and sulfide that is depleted in 34 S.

Bacterial disproportionation of elemental sulfur: 4Sº + 4H 2 O => 3H 2 S + SO H + (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 H + (4)

If sulfide oxidation to elemental sulfur does not fractionate sulfur isotopes, repeated disproportionation and reoxidation will result in more strongly depleted sulfides.

Shift to lower  34 S sulfide after 1Ga reflects oxygenation of atmosphere and ocean.