1. The Biogeochemical Sulfur Cycle

2. Contents Introduction The global sulfur cycle Sulfur isotopes
Example: the use of sulfur isotopes to predict the early history of atmospheric oxygen

3. Introduction Sulfur 14th most abundant element
Reduced  FeS2 (-2 or –1)
Oxidized  SO42- (+6)
Intermediate valences can occur (transitory)

4. The global sulfur cycle

5. 34S (‰) = (34S/32S)sample -1/ (34S/32S)CDT x 1000
Sulfur Isotopes
Four stable isotopes: 32S, 33S, 34S and 36S
Abundance 95 %, 0.76 %, 4,22 %, 0,014%
Standard: Canyon Diablo Troilite (CDT, a meteorite)
34S (‰) = (34S/32S)sample -1/ (34S/32S)CDT x 1000

6. Fractionation mechanisms
Exchange reactions between sulfates and sulfides.
Kinetic isotopic effects in the bacterial reduction of sulfate.
Precipitation of sulfates in seawater.

7. Fractionation
Organic matter oxidation by sulfate reducing bacteria (f.e. Desulfovibrio desulfuricans)
CH2O + SO4  H2S + 2 HCO3-
Formation of pyrite FeS2

8. Fractionation Pool 34S (‰) Igneous rocks SO4 (sea) +20 SO4 (rain)
SO4 (sea)
+20
SO4 (rain)
+2 to +8 HS
-20 to -40
FeS2
-10 to -40
Algea
+19
Plants +5
Seagrass
-11 to +15

9. Example: The use of sulfur isotopes to predict the early history of atmospheric oxygen
Two scenarios:
Atm O2 reach present day levels by the earliest Archean (3.8 Ga ago).
Atm O2 began to accumulate around 2.2 /2.3 Ga in the early Proterozoic.

10. The sulfur isotope record
Sedimentary sulfides between 3.4 & 2.8 Ga
small isotopic differences
34Ssed sulfides=5‰ against 34Ssolubl sulphate= 2-3 ‰
Formation such sediments under high rates of sulphate reduction in a warm sulphate rich environment.
Model needs extension

11. Figure 1

12. Figure 2

13. Conclusion
Rapid rates of sulphate reduction with abundant SO4 and at higher temperatures up to 85°C, should produce sedimentary sulfides depleted in 34S by about 13 to 28‰ compared with seawater sulphate.
At 2.2 Ga : 34S depleted sulfides of biological origin become a continuous feature of the geological record.