The Biogeochemical Sulfur Cycle

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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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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
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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 sulfides5 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 85C, 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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