Sulfur has four stable isotopes. Natural variation in sulfur isotopes can be used for tracing natural and anthropogenic solute sources.

• Introduction
  
• Cost of Analysis
  
• Origin
• Sulfur Fractionation
• Measurement Techniques
  
• Hydrological Applications
  
• Other Applications
  
• References and further reading
• Internet resources

Introduction (return to top)

Sulfur has 16 isotopes. However, except for ³⁵S (t_1/2 = 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 and ³⁴S 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 value. d³⁴S is defined as follows:

Cost of Analysis (return to top)

Isotope Ratio Mass Spectometry (IRMS): $70 to $80 for sulfates and sulfides; around $125 for organic compounds.

(See ISO Analytical for an estimate)

(See also Geochron Laboratories)

(See also Laboratory of Isotope Geochemistry, UA Dept. of Geosciences)

Origin (return to top)

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 ³⁵S 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 ³⁵S 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, H₂S, 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; the d³⁴S 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‰ < d³⁴S > 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 d³⁴S 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 depleted d³⁴S 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 eras.

4) groundwater sulfur (-10‰ < d³⁴S > 35‰) (Krouse and Mayer 2000)

The d³⁴S 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

³⁵S 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 recent precipitation.

Sulfur fractionation (return to top)

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, Desulfovibrio desulfuricans
Source: Buckman Laboratories

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‰ in ³⁴S 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 1997).

Measurement Techniques (return to top)

Sampling

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).

Sample Preparation

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 V₂O₅ at temperatures around 1000^oC 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.

Although SO₂ 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/ d¹⁸O 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).

Instrumentation

d³⁴S 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).

(See the IRMS page for more information about the process)

(See the ThermoFinnigan web site for photos and more specific information on IRMS instrumentation)

(Other IRMS manufacturers are listed on the ISOGEOCHEM site)

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).

(See ThermoFinnigan's DeltaPlus Advantage for an example of continuous flow IRMS)

Hydrological Applications (return to top)

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 in d³⁴S of stream sulfate during the winter may also be due to differences in flow dynamics (storm runoff in summer vs. winter micropore flow).

Other applications (return to top)

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.

References and further reading (return to top)

• 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, FL, 1997.
  
  
• 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, 231-236, 1992.

Internet resources (return to top)

Cainey, J., Sulfur isotope signatures in New Zealand, Water & Atmosphere Online

Lawrence Berkeley National Laboratory, Isotopes Project, Isotopes of Sulfur

Thermo Finnegan web site

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