Transcript The Biogeochemical Sulfur Cycle

The Biogeochemical Sulfur
Cycle
Contents
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Introduction
The global sulfur cycle
Sulfur isotopes
Example: the use of sulfur isotopes to
predict the early history of atmospheric
oxygen
Introduction
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Sulfur 14th most abundant element
Reduced  FeS2 (-2 or –1)
Oxidized  SO42- (+6)
Intermediate valences can occur (transitory)
The global sulfur cycle
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
1. Exchange reactions between sulfates and
sulfides.
2. Kinetic isotopic effects in the bacterial
reduction of sulfate.
3. 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
Igneous rocks
SO4 (sea)
SO4 (rain)
HS
FeS2
Algea
Plants
Seagrass
34S (‰)
0
+20
+2 to +8
-20 to -40
-10 to -40
+19
+5
-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
Figure 1
Figure 2
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.