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The Biogeochemical Sulfur Cycle
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Contents Introduction The global sulfur cycle Sulfur isotopes
Example: the use of sulfur isotopes to predict the early history of atmospheric oxygen
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Introduction Sulfur 14th most abundant element
Reduced FeS2 (-2 or –1) Oxidized SO42- (+6) Intermediate valences can occur (transitory)
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The global sulfur cycle
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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
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Fractionation mechanisms
Exchange reactions between sulfates and sulfides. Kinetic isotopic effects in the bacterial reduction of sulfate. Precipitation of sulfates in seawater.
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Fractionation Organic matter oxidation by sulfate reducing bacteria (f.e. Desulfovibrio desulfuricans) CH2O + SO4 H2S + 2 HCO3- Formation of pyrite FeS2
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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
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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.
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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
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Figure 1
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Figure 2
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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.
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