070412 Lecture 15 S Isotopes Only DM 21983

8/3/2019 070412 Lecture 15 S Isotopes Only DM 21983

1/14

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


2/14

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.


3/14

Strong (5 to 45 o/oo) depletion

in 34S of sulfides, relative tosulfate, during sulfate reduction.

Canfield and Teske (1996)


4/14

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?


5/14

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)


6/14

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.


7/14

If sulfide oxidation to elemental sulfur does

not fractionate sulfur isotopes, repeateddisproportionation and reoxidation will

result in more strongly depleted sulfides.


8/14

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


9/14

Even stronger

signal in the

Cretaceous.

Paytan et al., 2004


10/14

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


11/14

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


12/14

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


13/14

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


14/14

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).

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070412 Lecture 15 S Isotopes Only DM 21983