Sulfate reduction

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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
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Can imagine a Redfield-type sulfate reduction
stoichiometry   (CH2O)106(NH3)16(H3PO4)
53SO4-2 gt 106(HCO3-) 16NH3 H3PO4
53(H2S)   Or even just   2(CH2O) SO4-2 gt
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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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)
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Syntrophy (more next Tuesday)
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Jorgensen, 1983
  The pathways are complex and variable, the
processes are tightly linked, production and
consumption of intermediates are rapid and in
balance,  
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Cape Lookout Bight, NC Martens et al.
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Sulfate near zero by 10 25 cm! 60 mM DIC!!
Klump and Martens, 1987
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mM ammonia
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Direct measurements of sulfate reduction rates
(Crill and Martens)   Closed-tube incubations
(decrease in sulfate over time)   35S-sulfate
tracer method Inject cores with
35S-sulfate After incubation, acidify and
collect acid-volatile sulfide (AVS) (HS-, H2S,
FeS). 35S activity in the AVS fraction reflects
sulfate reduction 35S activity in residual
sulfate to check total tracer recovery
  caveats (35)SO4-2 addition isotope
equilibration re-oxidation of labeled
sulfides pyrite, S formation (non-volatile
sulfides)
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mM per day SR!
Crill and Martens, 1987
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Strong seasonal cycles in sulfate penetration,
sulfate reduction rate, driven by temperature.
Crill and Martens, 1987
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Winter low SR rates in top few cm
(oxic) Summer highest rates near sediment
surface. Second maximum at depth
artifacts? Shifts in microbial community?
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Seasonal Fe cycle too
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25 of organic C rain is remineralized, 75 is
buried. Of the remineralized fraction, 70 due to
sulfate reduction 30 due to methanogenesis O2
fluxes (calculated) dominated by oxidation of H2S
and sulfide minerals
Chanton et al., 1987
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Ferdelman et al., 1999 Sulfate reduction in the
Southeast Atlantic
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SR rates determined by 35S tracer injection.
Rates 100x lower than Cape Lookout Bight little
sulfate depletion
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In situ microelectrode oxygen flux estimates
(Glud)
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Assuming that the oxygen flux includes both oxic
respiration and reoxidation of SR products, SR
accounts for 20-95 of O2 flux at shallow sites,
5-15 at deep sites.
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Canfield and Des Marais (1991)
Aerobic sulfate reduction in microbial mats
O2 production
Sulfate reduction
O2
1 uM/min 1.5 mM/day but the volume is small
relative to CLB
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High SR rates during the day, in the O2
production zone?
Arent sulfate reducers obligate anaerobes? Novel
pathways or communities
noon
noon
midnight
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Sulfur stable isotopes   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
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Strong (5 to 45 o/oo) depletion in 34S of
sulfides, relative to sulfate, during sulfate
reduction.
Canfield and Teske (1996)
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Sulfate two similar sinks, one (pyrite)
strongly depleted in 34S due to fractionation
during sulfate reduction seawater sulfate is
enriched in 34S w.r.t weathering input.
Sulfate large reservoir, small fluxes
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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 / H2S reoxidation cycle
Sulfate two similar sinks, one (pyrite)
strongly depleted in 34S seawater sulfate is
enriched in 34S.
Sulfate large reservoir, small fluxes
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Carbon only the smaller sink (organic C) is
strongly depleted in 13C seawater DIC is only
slightly enriched in 13C
Carbon (DIC) small reservoir, large fluxes
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Barite-based d34S record
Large, rapid changes in the d34S of seawater
sulfate.
Paytan et al., 1998
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Even stronger signal in the Cretaceous.
Paytan et al., 2004
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Barite-based d34S record

• Large, rapid changes in the d34S of seawater
  sulfate two hypotheses for inferred changes in
  sulfide burial
• O2 (atm) fairly constant in Cenozoic, so sulfide
  burial and organic C burial for some reason
  offset each other. Times of low sulfate d34S (low
  sulfide burial) would be times of high DIC d13C
  (high organic C 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).

Paytan et al., 1998
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In fact, there is no obvious correlation
positive or negative between d34S (sulfate) and
d13C (DIC).
Paytan et al., 1998
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Why are sedimentary sulfides much more strongly
depleted in 34S than the sulfide produced in
culture experiments?
Canfield and Teske (1996)
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Bacterial disproportionation of elemental sulfur
produces sulfate that is enriched in 34S and
sulfide that is depleted in 34S.
Canfield and Thamdrup 1994
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Bacterial disproportionation of elemental
sulfur   4Sº 4H2O gt 3H2S SO4-2 2H
(1)   is often followed by
sulfide scavenging by iron oxides and sulfide
reoxidation   H2S 4H 2Fe(OH)3 gt 2Fe2
Sº 6H2O (2)   and   2H2S 2Fe2 gt
2FeS 4H
(3)   Yielding an overall reaction of   3Sº
2Fe(OH)3 gt 2FeS 2H2O SO4-2 2H (4)
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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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Shift to lower d34S sulfide after 1Ga reflects
oxygenation of atmosphere and ocean.
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