The Biogeochemical Sulfur Cycle

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

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

The global sulfur cycle

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

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

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

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

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.

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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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.