Sulfur is an abundant element in nature. It is found
in a variety of different forms in nature because
it can possess a +6 to -2 oxidation state. This
gives sulfur the ability to bond to a great number
of other elements, and to participate as both an
electron acceptor and an electron donor in redox
reactions. Sulfur compounds can be found extensively
throughout the hydrosphere.
Sulfur has 16 isotopes. However, except for 35S
= 87.2 days), which has a limited application
for hydrologic processes that occur within 1 year
(Miche 2000), the radioactive isotopes of sulfur
all have half-lives that are too short to be of
any use in hydrologic studies.
In contrast, due to the reactive nature of sulfur
and the large mass differences between isotopes,
natural variation in stable isotope ratios prove
to be valuable tools. 32S
are the most abundant stable isotopes; it their
ratio that is most often looked at in isotopic
studies. The ratio is reported as a d34S
is defined as follows:
The original standard for this sulfur isotope ratio
was based on triolite (FeS) in the Cañon
Diablo iron meteorite; the 32S/34S
ratio of this standard is 22.22 (Attendorn and Bowen
1997). Sulfur isotope values are still reported
relative to the Cañon Diablo triolite. However,
the scarcity of this sample and its lack of homogeneity
(on the order of 1) led the International
Atomic Energy Agency (IAEA) to develop a new standard.
The IAEA standard for d
-0.30 is used in sample analysis.
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Isotope Ratio Mass Spectometry (IRMS): $70 to
$80 for sulfates and sulfides; around $125 for
Analytical for an estimate)
of Isotope Geochemistry, UA Dept. of Geosciences)
Because of the reactive nature of sulfur, many
processes impact its global cycling and isotopic
composition. Important processes include: 1) weathering
of rocks rich in sulfur; 2) deposition of evaporites;
3) sulfate from sea spray; 4) anthropogenic fluxes
from burning of fossil fuels; and 5) cosmogenic
entering the atmosphere/
The sulfur cycle consists of the weathering of
rocks rich in sulfur that are eventually deposited
in the ocean from runoff. The atmosphere obtains
sulfate from sea spray, which is eventually rained
out over the land. Anthropogenic fluxes and cosmogenic
also enter the atmosphere. Sulfur then travels
back to the ocean with the runoff that is dissolving
more lithospheric sulfur along the way.
1) atmospheric sulfur (-5 < d34S
> 25) (Krouse and Mayer 2000)
More than two-thirds of the atmospheric sulfate
in northern industrialized areas is of anthropogenic
origin. Anthropogenic input into the atmosphere
is in the form of sulfur gases (i.e., SO2,
DMS) from burning of fossil fuels. These sulfur
gases usually oxidize to form sulfate once in
the atmosphere. The range of d34S
values for anthropogenic sulfate in northern hemisphere
precipitation usually range from -3 to +9
(Krouse and Mayer 2000). In northern industrial
areas, fossil fuel burning is greater in the winter
months and thus seasonal fluctuations in d34S
values can been seen.
Volcanoes also contribute to atmospheric sulfur;
values from this input are around +5. The
final input to the atmosphere is from marine sulfate.
This is incorporated into the atmosphere from
sea spray and the d34S
values from this input range from +15 to +21
(Krouse and Mayer 2000).
2) lithospheric sulfur (-10 <
> 35) (Krouse and Mayer 2000)
Lithospheric sulfur mainly comes from the weathering
of metamorphic and sedimentary rocks. Most igneous
rocks contain little sulfur and therefore are
not a major contributor to dissolved sulfur compounds.
Sedimentary rocks that are marine evaporites are
the main source of sulfur in groundwater. Two
major evaporites are gypsum and anhydrite. The
range for these rocks is dependent on which geologic
era they are from. These evaporites, if deposited
quickly, reflect the 34S
value of the ocean from the time that they were
deposited. However, they can contain significantly
values compared to the ocean water, due to bacterial
fractionation. When these sedimentary rocks are
metamorphosed, the S will be retained. As a result,
metamorphic rocks have a similar d34S
range of -10 to +25 (Krouse and Mayer 2000).
3) marine sulfur (modern ~21)
The ocean is huge sink for sulfur. It is estimated
to contain approximately 1.3 x 1015
tons of sulfate (Attendorn and Bowen 1997). The
range of the d34S
values for the oceans has changed over the geologic
4) groundwater sulfur (-10 <
> 35) (Krouse and Mayer 2000)
values of groundwater are highly variable and
depend on the nature of the sulfur inputs to the
water. Sulfur takes on many forms in groundwater,
but is mainly found in the form of sulfates and
sulfides. The main source of sulfate is from the
dissolution of gypsum and anhydrite. Some dissolved
organic sulfur (DOS), elemental sulfur and mineral
sulfur might also be present in groundwater.
5) cosmogenic origin
is formed by cosmic ray spallation of 40Ar
in the atmosphere and falls to earth in precipitation.
Because of its relatively short half-life (87
days), its presence indicates the water is from
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Sulfur isotope fractionation occurs by two processes
in nature. First, there is equilibrium fractionation
during inorgranic reactions between sulfur bearing
ions, molecules and solids. In this case, 34S
is concentrated in the compounds with the highest
oxidation state or the greatest bond strength (Bowen
Secondly, and more important, is fractionation
due to the reduction of sulfate ions. This can
occur by inorganic process or by biogenic processes.
This fractionation occurs because the 32S
form weaker bonds than the 34S
and therefore the reduced product tends to be
depleted in 34S.
The amount of inorganic reduction fractionation
is dependent on external factors affecting the
system. These factors will vary the rate at which
S-O bonds in sulfate are broken (Bowen 1988).
The reduced sulfur species produced by this process
can be depleted in 34S
by up to 73 from the starting sulfate (Clark
& Fritz 1997). This reduction process is very
uncommon in the environment however. The process
proceeds very slowly, requiring high temperatures
and low pH.
Biogenic reduction is the dominant form of sulfur
fractionation in nature. Biogenic fractionation
is mainly a product of sulfur reducing bacteria
such as Desulfovibrio desulfuricans.
Slide of spiral shaped bacteria,
These bacteria live in the ocean and lake sediment
deposits. The bacteria are able to achieve greater
rates of sulfate reduction because the process
is enzyme catalyzed. Depletions of up to 50
have been reported for reduced products (Krouse
and Mayer 2000). The extent of biogenic fractionation
is inversely proportional to rate at which the
bacteria reduce the sulfate. This in turn is a
function of the surrounding temperature and the
concentration of sulfate (Attendorn and Bowen
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Sulfur may exist in many forms in a water sample.
To avoid the complexities of performing isotope
analysis on all of these species, one usually
is only concerned with two most dominant forms:
sulfate and sulfide. It is best to extract these
species in the field to avoid any further chemical
or biogenic interconversions between species.
If this is not possible, a bacteriocide may be
added to the sample to minimize sulfate reduction.
Caution should be taken to avoid exposing the
sample to air. One way this can be done is by
immersing a sample bottle that has previously
been purged with nitrogen gas for sample collection
(Krouse and Mayer 2000).
The first step in sample preparation is to filter
the sample through a 0.45 µm filter. The
sulfides in the water sample are analyzed first.
They are reacted with cadmium acetate (CdAc) to
from CdS. The CdS is then reacted with CuO or
at temperatures around 1000oC
to form SO2
gas. This gas is often the preferred form for
sulfur analysis for its ease of preparation. The
sulfates follow the same fate, but they must first
be reduced to sulfides.
gas is relatively easy to prepare for isotopic
analysis, it does have some drawbacks. First is
that this gas tends to absorb to surfaces, creating
memory effects in the mass spectrometer. Heating
the inlet of the mass spectrometer can reduce
this effect. Second, the mass spectrometer cannot
determine if the SO2
mass 66 peak is from 32S16O18O+
or from 34S16O2+.
In most cases the oxygen isotopes are then measured
in the same samples in order to correct for this
(Krouse and Mayer 2000). This is why d34S/
diagrams are common, because often both analyses
are performed on the sulfate sample. Examples
of where various end-members fall on these diagrams
are shown in the following diagram:
(Reprinted from Clark and Fritz 1997, p. 143)
Some analyses have been performed by converting
sulfur to the sulfur hexaflouride gas. The benefit
of this is that fluoride has only one stable isotope,
making analysis easy. The difficulties however,
are the tedious sample preparation and that the
sulfur species needed for this conversion tend
to be the less abundant forms (elemental and certain
mineral forms) (Attendorn and Bowen 1997).
measurements are usually performed on the SO2
form of sulfur using an isotope ratio mass spectrometer.
These are usually double inlet mass spectrometers
with a double or multiple collector system (Attendorn
and Bowen 1997).
the IRMS page for
more information about the process)
web site for photos and more specific information
on IRMS instrumentation)
IRMS manufacturers are listed on the ISOGEOCHEM
The sample is measured and reported against a
standard, usually based on the Cañon Diablo
triolite or the IAEA equivalent. The sample size
needed for IRMS analysis is on the order of a
few mg of sulfur.
Some sulfur isotope analysis is performed on
continuous flow isotope ratio mass spectrometers
(CF-IRMS). These machines require only a few µg
of sulfur to sample. The problem with these machines
is that in producing SO2
gas, several sources of oxygen may be incorporated
in the sample and standard inlet procedures. Therefore,
many standards must be run to account for this
difficult problem (Krouse and Mayer 2000).
DeltaPlus Advantage for an example of continuous
Sulfur isotopes are used in hydrology to trace
natural and anthropogenic sources of sulfur, in
particular to study the cycling of sulfur in agricultural
watersheds, the sources of salinity in coastal
aquifers or sedimentary strata, groundwater contamination
by landfill leachate plumes, and acid mine drainage.
There is some concern that the use of d34S
to separate sources of sulfur in catchments is
compromised by the fact that sulfur itosopic ratios
are strongly fractionated through biogeochemical
processes discussed above, with effects varying
considerably from catchment to catchment. Stam
et al. (1992) have suggested that the fractionation
is a function of residence time in the catchment,
with less fractionation occurring in steep catchments
which have relatively short residence times. Increases
of stream sulfate during the winter may also be
due to differences in flow dynamics (storm runoff
in summer vs. winter micropore flow).
Study of the concentration of organically bound
sulfur in fossil fuels can help determine the
age of the petroleum source rocks and help determine
its migration into reservoir rock. Study of isotopic
composition of sulfur in Precambrian rocks provides
information on when life first appeared and the
biological evolution of the earth.
- Attendorn, H.G., and R. Bowen, Radioactive
and Stable Isotope Geology, Chapman &
Hall. New York, 1997.
- Bowen, R., Isotopes in the Earth Sciences,
Elsevier Applied Science. New York, NY, 1988.
- Clark, I., and P. Fritz, Environmental
Isotopes in Hydrology, CRC Press, Boca Raton,
- Cook, P., and A.L. Herczeg, editors, Environmental
Tracers in Subsurface Hydrology, Kluwer
Academic Publishers. Norwell, MA, 2000.
- Krouse, H.R. and B. Mayer, Sulfur and oxygen
isotopes in sulfate, in Environmental Tracers
in Subsurface Hydrology, ed. by P. Cook
and A.L. Herczeg, Kluwer Academic Publishers,
Norwell, MA, 2000.
- Michel, R.L., Sulfur-35, in Environmental
Tracers in Subsurface Hydrology, ed. by
P. Cook and A.L. Herczeg, Kluwer Academic Publishers,
Norwell, MA, 2000.
- Stam, A.C., Mitchell, M.J., Krouse, H.R.,
and Kahl, J.S., Stable sulfur isotopes of sulfate
in precipitation and stream solutions in a northern
hardwood watershed, Water Resour. Res., 28,
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isotope signatures in New Zealand, Water
& Atmosphere Online
Berkeley National Laboratory, Isotopes Project,
Finnegan web site
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