Periodic Table--Sulfur

Sulfur has four stable isotopes: ³²S (95.02%), ³³S (0.75%), ³⁴S (4.21%), and ³⁶S (0.02%). Stable isotopic compositions are reported as ratios of ³⁴S/³²S in ‰ relative to the standard VCDT [Vienna Canyon Diablo Troilite (Coplen and Krouse, in press)]. The general terrestrial range is +50 to -50‰, with rare values much heavier or lighter. The d ³⁴S of the ocean is currently about +20‰, but has ranged from about +10 (beginning of the Mesozoic) to +30 (beginning of the Paleozoic). Both Krouse and Grinenko (1991) and Mitchell et al. (1998) provide very comprehensive evaluations of stable sulfur isotopes as tracers of natural and anthropogenic sulfur. Variations in the d³⁴S values are caused by two kinds of processes: reduction of sulfate to sulfide by anerobic bacteria which results in an increase in the ³⁴S of the residual sulfate, and various kinds of exchange reactions which result in ³⁴S being concentrated in the compound with the highest oxidation state of S.

The principle use of sulfur isotopes has been to understand the formation of sulfide ore deposits, which may originate in either sedimentary or igneous environments. The sulfur associated with sedimentary processes generally reflects the composition of biogenic sulfide produced by bacteria reduction of marine sulfate, and has negative d³⁴S values. On the other hand, the S associated with igneous rocks derived from the mantle is isotopically similar to that of meteorites and has d³⁴S values close to 0‰. Unfortunately, these simpler differences are rarely useful for determining the origin of ore deposits because of their complex histories.

When sulfide minerals are precipitated, isotopic equilibration among solids and liquid may cause small differences in the d³⁴S values of co-genetic minerals. The differences between minerals can be used to estimate the temperature of equilibration. The d¹³C and d³⁴S of co-existing carbonates and sulfides can be used to determine the pH and oxygen fugacity of the ore-bearing fluid during ore formation (Rye and Ohmoto, 1974).

In most forest ecosystems, sulfate is derived mostly from the atmosphere; weathering of ore minerals and evaporites also contributes some sulfur. Because sulfur isotopic ratios are strongly fractionated by biogeochemical processes, there has been concern over whether d³⁴S could be used to separate sources of sulfur in catchments. Some catchments appear to be affected by isotopic fractionation processes, whereas others seem to show only minor effects of watershed processes on d³⁴S in lakes or streams. Stam et al. (1992) suggest that the extent of fractionation might be a function of water residence time in the catchment, with steep catchments showing less fractionation. They note that increases in d³⁴S of stream sulfate during the winter may be a result of micropore flow during the snow-covered period, rather than the more typical macropore flow characteristic of storms.

Intensive investigations of the sulfur dynamics of forest ecosystems (see Mitchell et al., 1998, for a complete review) in the last decade can be attributed to the dominant role of sulfur as a component of acidic deposition. Sulfur with a distinctive isotopic composition has been used to identify pollution sources (Krouse et al., 1984), and enriched sulfur has been added as a tracer (Mayer et al., 1993). Differences in the natural abundances can also be used in systems where there is sufficient variation in the ³⁴S of ecosystem components. Rocky Mountain lakes thought to be dominated by atmospheric sources of sulfate have been found to have different d³⁴S values from lakes believed to be dominated by watershed sources of sulfate (Turk et al., 1993).

Use of a dual isotope approach to tracing sources of sulfur (i.e. measurement of d¹⁸O and d³⁴S of sulfate) will often provide better separation of potential sources of sulfur and, under favorable conditions, provide information on the processes responsible for sulfur cycling in the ecosystem. The rate of oxygen isotopic exchange between sulfate and water is very slow at normal pH levels. Even in acidic rain of pH 4, the "half-life" of exchange is on the order of 1000 years (Lloyd, 1968). Depending on the reaction responsible for sulfate formation, between 12.5 to 100 % of the oxygen in sulfate is derived from the oxygen in the environmental water; the remaining oxygen comes from O₂ (Taylor et al., 1984). At isotopic equilibrium at 0°C, aqueous sulfate is about 30‰ enriched in ¹⁸O relative to water (Mizutani and Rafter, 1969). Reviews of applications of d¹⁸O of sulfate include Holt and Kumar (1991) and Pearson and Rightmire (1980).

Sulfur-35 is a radioisotope formed from cosmic ray spallation of argon- 40 in the atmosphere (Peters, 1959); it has a half-life of 87 days. In the first application of ³⁵S in an aquatic system, Cooper et al. (1991) found that sulfur deposited as precipitation in the Arctic is strongly adsorbed within the watershed and that most sulfur released to streamflow is derived from longer-term storage in soils, vegetation, or geologic materials. Michel and Naftz (1995) report that the combined use of ³⁵S and tritium shows that meltwater from Wind River Range (Wyoming, USA) glaciers contains water from the current year. Furthermore, although lakes in Colorado fed by snowmelt and precipitation contain recent atmospherically-derived sulfate, this atmospherically deposited sulfur takes several months to emerge in springs fed by shallow ground water (Michel and Turk, 1996).

Further information can be found in the section: Sulfur Cycle in Clark and Fritz (1997), Environmental Isotopes in Hydrology (CRC Press):

Source of text: This review was assembled by Carol Kendall and Dan Snyder, primarily from Kendall et al. (1995).