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GLOBAL SULFUR CYCLE
(1) Global redox: C, S, and O
(2) Sulfur basics: chemistry, redox
(3) Sulfur forms on Earth’s surface
(4) Sulfur in the marine environment
(5) Long-term sulfur cycle a la Berner
(6) Anthropogenic effects on sulfur cycle
REFERENCES:
Brimblecombe, P. (2005) The global sulfur cycle, In Treatise of Geochemistry, vol.
8 (ed. W.H. Schlesinger), pp. 645-682.
Berner, R.A. (2004) The Phanerozoic carbon cycle: CO2 and O2, Chapter 6,
Atmospheric O2 over Phanerozoic time, pp. 100-124.
For oxygen:
Petsch, S.A. (2005) The global oxygen cycle, In Treatise of Geochemistry, vol. 8 (ed.
W.H. Schlesinger), pp. 515-555.
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Brief word about global silicon cycle: continental weathering of silicate rocks,
opaline silica burial, reconstitution into silicate rocks. Oceanic Si balance covered
thoroughly in Marine Geology. Interesting issues about things like Si/C ratios in
diatoms and iron, Si/C ratios in terrestrial plants.
(1) Global redox: C, S, and O
Why does sulfur matter?
 Tenth most abundant element in solar system, not very abundant in
continental crust
 Highly mobile, biochemically active
 Sulfuric acid industrially important
 Forms large number of oxides and oxyanions, polymeric species
 Strong link to atmospheric oxygen and to carbon cycle, long-term
o Sulfur important to atmospheric oxygen balance because of
abundance and stochiometry
o Iron less abundant, oxidation-reduction reactions don’t affect oxygen
as much for given amount
 Anthropogenic pollutant
(2) Sulfur basics
Sulfur isotopes: four stable (Table 1) plus nine known radioactive isotopes (most
very short half-lives)
HANDOUT: Table 3, TOG
Lots of different S forms (allotropes) from readiness to form S-S bonds
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REDOX STATES OF SULFUR
S(-II)
S(0)
S(+II)
S(+VI)
H2S, HS- (hydrogen sulfide)
S (elemental sulfur)
S2O32- (thiosulfate ion)
SO42- (sulfate ion)
(3) Sulfur forms on Earth’s surface
ELEMENTAL SULFUR
 Found in many places
 Usually from sulfate reduction
 Gunpowder ingredient
 Silicy an important source, U.S. now major source (20% of world
production)
 Most (>90%) used in producing sulfuric acid, much in turn used in
phosphate fertilizer production
HANDOUT: Figure 7, TOG
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HANDOUT: from brochure, more details about sulfur production and
consumption
SULFIDES
 Pyrite (FeS2) Volcanogenic and sulfate reduction sources
 Other minerals, including calcopyrite (CuFeS2), galena (PbS), cinnabar (HgS)
 Sulfide ores with Cu, Pb, Ni, Zn, Au, Ag
 Four episodes of massive sulfide deposition
o 2.69-2.72 Ga
o 1.77-1.90 Ga
o Devonian-Early Carboniferous
o Cambrian-Early Ordivician
EVAPORITES
 Deposits from seawater evaporation
o Calcium sulfate as gypsum (CaSO42H2O) or anhydrite (CaSO4)
o Precipitates out before halite
o Temporal variability
HANDOUT: Figure 6, TOG
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(4) Sulfur in the marine environment
Sulfate ion
 one of seven major ions in seawater, one of three major anions
 conservative in seawater
 ~28 mM
Early ocean, likely very different sulfate levels (Earth’s redox state)
Hydrogen sulfide
 Throughout world ocean, at low levels
o Oxidized in less than an hour
 Important form in anoxic regions
o Water column
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 E.g., Black Sea, other anoxic basins (restricted circulation,
usually from sills)
o Sediment: sulfate reduction produces HS 2CH2O + SO42-  2HCO3- + HS- + H+
o Pyrite formation
Organosulfides
 Largest flux of volatile form is dimethyl sulfide (DMS)
 Lovelock et al. (1972)
o Role in cloud condensation nuclei
(5) Long-term sulfur cycle a la Berner
HANDOUT: Berner, Figure 6.1
SOURCES OF SULFUR TO OCEAN:
 Sulfate in rivers, continental weathering
o Oxidative weathering of pyrite
o Dissolution of calcium sulfate minerals
 Reduced sulfur
o volcanic, metamorphic, diagenetic reactions, then oxidized
REMOVAL OF SULFUR FROM OCEAN:
 Sedimentary pyrite, calcium sulfate formation
 Also some pyrite, anhydrite in hydrothermal circulation
REACTIONS with effect on atmospheric O2
15O2 + 4FeS2 + 8H2O2Fe2O3 + 8SO42- + 16H+
Oxidative weathering of pyrites on continents
Net reaction for thermal breakdown of pyrite at depth + oxidation by atm.
O2 of reduced sulfur
2Fe2O3 + 8SO42- + 16H+  15O2 + 4FeS2 + 8H2O
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Could get this as sum of:
Burial of photosynthetic carbon
16CO2 + 16H2O  CH2O +16O2
Bacterial sulfate reduction
16CH2O + 8SO42-8H2S + 16HCO3Sedimentary pyrite formation
2Fe2O3 + 8H2S + O2  4FeS2 + 8H2O
Neutralization of bicarbonate
16H+ + 16HCO3-  16CO2 + 16H2O
REMAINING REACTIONS OF LONG TERM SULFUR CYCLE
CaSO4  Ca2+ + SO42Ca2+ + SO42-  CaSO4
Can translate these into effects on atmospheric oxygen—will do so in next lecture
on global oxygen balance
(6) Anthropogenic effects on sulfur cycle
 Sulfur released from fossil fuel burning, esp. high sulfur coal
o Releases SO2 sulfur dioxide
o Leads to acid rain
 Volatile organosulfur compounds as pollutants
o Sulfur-rich waste waters and sewage sludge
o Aluminum production
o Tire wear
 Sulfate in rivers, increased by agricultural and industrial activities
o Can be sulfide releases, with oxidation
o Sulfur in various forms as fertilizer
o Sulfur-containing pesticides
 Sulfate dusts, e.g., Aral Sea region