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Sulfur is an important element in the Earth’s lithosphere, biosphere, hydrosphere and atmosphere. Like carbon and nitrogen, sulfur occurs in reduced and oxidized forms. Biological processes, such as the reduction of sulfate by bacteria, change the oxidation state of sulfur.
Sulfur Isotope Geochemistry
The chemical characteristics of a single stable sulfur isotope are influenced by the amount of protons and neutrons in its nucleus (and therefore its atomic mass). A heavier isotope is more likely to form than a lighter one due to the presence of more neutrons. The most common isotopes of sulfur are 32S and 34S, with 32S being the most abundant.
Sulfur-32 is a light isotope of sulfur, with 16 protons and 16 neutrons in its nucleus. sulfur-34 is a heavier isotope of sulfur, with two extra neutrons in its nucleus.
d34S values in marine sulfate or sedimentary sulfides provide a record of the changing sulfur cycle. The d34S value is reported in parts per thousand, per mil or per mille of a standard (such as the Vienna Canyon Diablo Troilite, VCDT) and is interpreted as a record of the rate of sulfur fractionation.
In euxinic systems, the rate-limiting parameter may be the amount of sulfate available for bacterial reduction. Several studies have demonstrated that modern sulfate-reducing bacteria produce sulfur isotopic fractionations in a range of d34S values from zero to equilibrium, depending on the amount of sulfate available.
Despite this, the use of sulfur isotopic ratios for determining euxinia has received little attention. However, d34S fractionations have been found in several Proterozoic euxinic basins and are considered a useful proxy for understanding sulfate-rich conditions.