SULFUR CYCLE

1
SULFUR CYCLE

• Sulfur reservoirs
• Driving forces for sulfur transformation
• Sulfur in the atmosphere
• Sulfur in rivers
• Sulfur in the ocean
• Global sulfur cycle
• Sulfur budget for the ocean
• Pyrite formation in sediments
• Diagenetic modelling

2
Global Sulfur Cycle

• Valence states 6 (SO42-) to -2 (sulfides)
• Original pool - pyrite FeS2
• Reservoirs,1018 g S
• Deep oceanic rocks 2375 820
• Sediments 78001700
• Freshwater 0.0030.002
• Ice 0.0060.002
• Atmosphere 3.6
• Sea 128055
• Organic 5.62x 10-3

3
Driving Forces Microbial Transformation

• Anaerobic conditions
• sulfate reduction 2H SO42-
  2(CH2O)-gt2CO2H2S2H2O (Desulfovibrio sp. or
  Desulfotomaculum sp.))
• bacteria produce a variety of gases hydrogen
  sulfide (H2S), dimethyl-sulfide (CH3)2S, carboxyl
  sulfide COS
• H2S reacts with Fe2 to precipitate FeS, which
  can be converted to pyrite FeS2
• FeS H2S gt FeS2 2H 2e-
• H2S diffuses though zone of F3
• 2Fe(OH)3 3H2S 2H gt FeS2 6H2O Fe2
• sulfur-based photosynthesis (thought to be one of
  first forms of photosynthesis on the Earth)
• Plant uptake
• assimilatory SO42- reduction and incorporation
  of carbon-bounded sulfur into the amino acids
  cysteine and methionine.
• Aerobic conditions
• reduced sulfur compounds oxidized by microbes,
  oxidation usually coupled to reduction of CO2 in
  relations of S-based chemosynthesis.

4
Sulfur in the Atmosphere

• Gaseous component
• no sulfur gas is a long-lived or major
  constituent of the atmosphere, oxidation of SO42-
  ? short residence time, all expressions in g S

• Aerosols
• particles lt 1um are held a loft by Brownian
  motion
• long transport
• sources volcanic eruptions, ocean, water
  evaporates from bubbles, the salt crystallizes
  to form sea-salt aerosols

5
Sources of Sulfur in the Atmosphere
Eriksson (1960) - SO42- deposition on land from
ocean 4x1012 g S , Jung (1960) - SO42- in
rainfall in land 73x1012 g S, ? other sources as
sea.

• Volcanic eruptions
• average over many years 12-30x1012 g S
• e.g. Tambora (Indonesia) in 1815, 1816 - year
  without summer in England, USA, Canada, 50x1012
  g S

• Soil dust

• Biogenic gases
• H2S, dimethyl-sulfide (CH3)2S, carbonyl sulfide
  COS

• Anthropogenic emissions
• without human effects, net transport through the
  atmosphere carries S from sea to land

6
Sulfur in Rivers

• Natural river load
• from weathering of pyrite4FeS215O28H2O?
  2Fe2O38H2SO4
• and gypsum, rainfall

• Human activities affect the transport of S in
  rivers
• 28 of the current content of S in rivers is
  derived from air pollution, mining, erosion, and
  the other human activities
• the current transport is supposed to be about
  double that of pre-industrial conditions

7
Marine sulfur cycle

• Ocean is large source of aerosols (sea salts)
  that contains SO42-.
• Most of the flux is re-deposited in the ocean in
  precipitation and dry-fall
• Dimethyl-sulfid (CH3)2S or DMS is the major
  biogenic gases emitted from sea
• annual flux is about 15
• mean residence time about 1-2 days - most of S
  from DMS is also re-deposited in the ocean
• The net transport of S from sea to land is about
  20x1012 g S/yr. Ocean receives a net input of S.

8
Global Sulfur Cycle
all values in 10 12 g S/yr
Rivers
72
Natural weathering and erosion
From Schlesinger W.H. 1997
9
Sulfur budget for the ocean
all values in 1012 g S/yr
Other reduced gases
SO2
DMS
Precipitation dry fall
11
40
lt6
247
Sea salt
Rivers
144
131
12 x 10 20 g
Hydrothermal vents
Pyrite
96
39
From Schlesinger W.H. 1997
10
Marine sulfur cycle

• Content 12x1020 g S/ yr., residence time gt 3 000
  000 years.
• Major marine sinks metallic sulfides
  precipitated at hydro-thermal vents, biogenic
  pyrites, the formation of organic sulfur.

11
Dimethylsulfid (CH3)2S or DMS
DMS is the major one of biogenic gases emitted
from sea

• mean residence time is about 1-2 days - most of S
  from DMS is also re-deposited in the ocean

• is produces during decom-position of
  dimethyl-sulfonpropionate (DMSP) from dying
  phytoplankton

• only small fraction lost into the atmosphere

12
DMS and climate

• oxidation of DMS to sulfate aerosols increases
  the abundance of cloud condensation nuclei ? to
  greater cloudiness

• layer of sulfate aerosols (known as Junge layer)
  is about 20-25 km altitude, source SO2 and
  carbonyl sulfate COS

• clouds over sea reflect incoming sunlight ?
  global cooling

• production of DMS - net primary production.

• if higher NPP is associated with warmer sea
  surface, then DMS-flux would have negative
  feedback on global warming

13
Pyrite Formation in Sediments
Sulfate reduction SO42- 2(CH2O)-gt2HCO3- H2S
Two steps reaction

• Reaction H2S with Fe2 or reactive Fe-mineral
• 4Fe2O39H2S -gt8FeSSO42-8H2O2H

• Reaction of iron sulfide with elemental sulfur
• FeS S0 -gt FeS2

incomplete oxidation of H2S or FeS by O2, NO3-,
MnO2 or FeOOH
Recently

• In strictly anoxic sediments FeSH2S -gtFeS2 H2

• In salt sediments Fe2 S0 -gtFeS2

From Schultze/Zabel 2000
14
Pyrite Formation in Sediments(after Berner 1972)
ORGANIC MATTER
SO42-
BACTERIA
Fe MINERALS
H2S
BACTERIA
BACTERIA
FeS
S0
FeS2
PYRITE
15
SULFUR IN LAKES
Plants
Bacteria
Fe 2
FeS