Sulfur (S)
| Atomic number: |
16 |
| Atomic weight: |
32.066 |
| Average concentration in ocean: |
28 mmol/kg |
| Residence time: |
8,700,000 years |
Speciation
Sulfur is present in oxygenated seawater as sulfate anion SO42-.
In the absence of oxygen, bacteria may reduce the sulfur in SO42-
from the +VI oxidation state to the -II oxidation state in order to metabolize
organic carbon. This forms hydrogen sulfide H2S, which can
accumulate in anoxic basins, such as the Black Sea, or sediment pore
fluids. On mid-ocean ridges, seawater may come in contact with molten
lava. As the seawater is heated, the SO42- is lost
when CaSO4 is precipitated. Water-rock reactions at higher
temperatures will then add large amounts of H2S.
Distribution in ocean
Sulfur is the fourth most abundant element dissolved in
seawater. As sulfate, it has a conservative distribution
that varies only due to differences in evaporation or
precipitation in the water that sank to form intermediate or deep waters in the
sea (data). Concentrations are higher in
the Atlantic because that ocean is saltier.
Locally high concentrations of hydrogen sulfide can be found in areas such as
deep-sea hydrothermal vents.
Residence time
The sulfate residence time was calculated from the global average river input
after correcting for mans influence on the composition of modern rivers (Berner
and Berner, 1987).
Analysis
Sulfate is determined gravimetrically as BaSO4 for very high precision
measurements, or by ion chromatography, for routine measurements in regions of
large concentration changes. Sulfide is determined colorimetrically as
methylene blue.
References & Notes