USGS -- Isotope Tracers -- Resources (2023)

Periodic Table--Sulfur

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

The principle use of sulfur isotopes has been to understand the formationof sulfide ore deposits, which may originate in either sedimentary or igneousenvironments. The sulfur associated with sedimentary processes generallyreflects the composition of biogenic sulfide produced by bacteria reductionof marine sulfate, and has negative d³⁴Svalues. On the other hand, the S associated with igneous rocks derivedfrom the mantle is isotopically similar to that of meteorites and has d³⁴Svalues close to 0‰. Unfortunately, these simpler differences are rarelyuseful for determining the origin of ore deposits because of their complexhistories.

When sulfide minerals are precipitated, isotopic equilibration amongsolids and liquid may cause small differences in the d³⁴Svalues of co-genetic minerals. The differences between minerals can beused to estimate the temperature of equilibration. The d¹³Cand d³⁴S of co-existing carbonatesand sulfides can be used to determine the pH and oxygen fugacity of theore-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 biogeochemicalprocesses, there has been concern over whether d³⁴Scould be used to separate sources of sulfur in catchments. Some catchmentsappear to be affected by isotopic fractionation processes, whereas othersseem to show only minor effects of watershed processes on d³⁴Sin lakes or streams. Stam et al. (1992) suggest that the extent of fractionationmight be a function of water residence time in the catchment, with steepcatchments showing less fractionation. They note that increases in d³⁴Sof stream sulfate during the winter may be a result of micropore flow duringthe snow-covered period, rather than the more typical macropore flow characteristicof storms.

Intensive investigations of the sulfur dynamics of forest ecosystems(see Mitchell et al., 1998, for a complete review) in the last decade canbe attributed to the dominant role of sulfur as a component of acidic deposition.Sulfur with a distinctive isotopic composition has been used to identifypollution sources (Krouse et al., 1984), and enriched sulfur has been addedas a tracer (Mayer et al., 1993). Differences in the natural abundancescan also be used in systems where there is sufficient variation in the³⁴S of ecosystem components. Rocky Mountain lakes thought tobe dominated by atmospheric sources of sulfate have been found to havedifferent d³⁴S values from lakesbelieved 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. measurementof d¹⁸O and d³⁴Sof sulfate) will often provide better separation of potential sources ofsulfur and, under favorable conditions, provide information on the processesresponsible for sulfur cycling in the ecosystem. The rate of oxygen isotopicexchange between sulfate and water is very slow at normal pH levels. Evenin acidic rain of pH 4, the "half-life" of exchange is on theorder of 1000 years (Lloyd, 1968). Depending on the reaction responsiblefor sulfate formation, between 12.5 to 100 % of the oxygen in sulfate isderived from the oxygen in the environmental water; the remaining oxygencomes from O₂ (Taylor et al., 1984). At isotopic equilibriumat 0°C, aqueous sulfate is about 30‰ enriched in ¹⁸O relativeto water (Mizutani and Rafter, 1969). Reviews of applications of d¹⁸Oof 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. Inthe first application of ³⁵S in an aquatic system, Cooper etal. (1991) found that sulfur deposited as precipitation in the Arctic isstrongly adsorbed within the watershed and that most sulfur released tostreamflow is derived from longer-term storage in soils, vegetation, orgeologic materials. Michel and Naftz (1995) report that the combined useof 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 recentatmospherically-derived sulfate, this atmospherically deposited sulfurtakes several months to emerge in springs fed by shallow ground water (Micheland Turk, 1996).

Further information can be found in the section: SulfurCycle in Clark and Fritz (1997), EnvironmentalIsotopes in Hydrology (CRC Press):

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