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
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
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
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.,
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).
||Coplen, T. B. and Krouse, H. R. (in press)"A New Scale for Reporting
Relative Sulfur Isotope-Abundance Data--VCDT." Nature.
||Holt, B. D., and Kumar, R. (1991). "Oxygen isotope fractionation
for understanding the sulphur cycle. In: H. R. Krouse and V. A. Grineko
(Eds.), Stable Isotopes: Natural and Anthropogenic Sulphur in the Environment,
SCOPE (Scientific Committee on Problems of the Environment) 43,
||Kendall, C., Sklash, M. G., Bullen, T. D. (1995). "Isotope Tracers
of Water and Solute Sources in Catchments", in Solute Modelling
in Catchment Systems, John Wiley and Sons, New York, pp. 261-303.
||Krouse, H. R., and Grineko, V. A. (Eds.) (1991). Stable Isotopes:
Natural and Anthropogenic Sulphur in the Environment John Wiley, New
York, SCOPE 43, 440 pp.
||Krouse, H.R., Legge, A., and Brown, H.M., (1984). "Sulphur gas
emissions in the boreal forest: the West Whitecourt Case Study V: Stable
sulfur isotopes." Water, Air Soil Poll. 22: 321-347.
||Lloyd, R.M., (1968). "Oxygen isotope behavior in the sulfate-water
system". J. Geophys. Res., 73: 6099-6110.
||Mayer, B., Krouse, H.R., Fritz, P., Prietzel, J., and Rehfuess, K.E.,
(1993). "Evaluation of biogeochemical sulfur transformations in forest
soils by chemical and isotope data". In: Tracers in Hydrology,
IAHS Publ., No. 215, pp. 65-72.
||Michel, R. L., and Naftz, D. L. (1995). "Use of sulphur-35 and
tritium to study runoff from an alpine glacier, Wind River Range, Wyoming."
In: K. Tonnessen, M. Williams, and M. Tranter (Eds.), Biogeochemistry
of Seasonally Snow-covered Catchments, IAHS Publ., July 3-14, 1995,
Boulder, CO, 8 pp.
||Michel, R.L., and Turk, J.T., (1996) "Use of sulphur-35 to study
sulphur migration in the Flat Tops Wilderness Area", IAEA Symposium
on Isotopes in Water Resources Management, Vienna, 20- 24 March, 1995,
||Mitchell, M.J., Krouse, H.R. Mayer, B, Stam, A.C. and Zhang, Y. (1998)
"Use of Stable Isotopes in Evaluating Sulfur Biogeochemistry of Forest
Ecosystems", In: C. Kendall and J.J. McDonnell (Eds.), Isotope
Tracers in Catchment Hydrology, Elsevier, Amsterdam, pp. 489-518.
||Mizutani, Y., and Rafter, T.A. (1969). "Oxygen isotopic composition
of sulphates: Part 4. Bacterial fractionation of oxygen isotopes in the
reduction of sulphate and in the oxidation of sulphur". N.Z. J.
Sci., 12: 60-67.
||Pearson, F. J., and Rightmire, C. T. (1980). "Sulphur and oxygen
isotopes in aqueous sulfur compounds." In: P. Fritz and J. Ch., Fontes
(Eds.), Handbook of Environmental Isotope Geochemistry), Elsevier,
Amsterdam, pp. 179-226.
||Peters, B. (1959). "Cosmic-ray produced radioactive isotopes as
tracers for studying large-scale atmospheric circulation", J. Atmos.
Terr. Phys., 13: 351-370.
||Rye, R. O., and Ohmoto, H. (1974), "Sulfur and carbon isotopes
and ore genesis: A review", Econ. Geol., 69: pp. 826-842.
||Stam, A.C., Mitchell, M.J., Krouse, H.R., and Kahl, J.S., (1992). "Stable
sulfur isotopes of sulfate in precipitation and stream solutions in a northern
hardwood watershed". Water Resour. Res., 28: 231-236.
||Taylor, B.E., Wheeler, M.C., and Nordstrom, D.K., (1984). "Oxygen
and sulfur compositions of sulphate in acid mine drainage: evidence for
oxidation mechanisms". Nature, 308, 538-541.
||Turk, J.T., Campbell, D.H. and Spahr, N.E., 1993. "Use of chemistry
and stable sulfur isotopes to determine sources of trends in sulfate of
Colorado lakes", Water, Air Soil Poll. 67, 415-431.