Strontium

Strontium has 4 stable isotopes. ⁸⁷Sr is radiogenic, being produced through the decay of ⁸⁷Rb. Natural variation in the abundance of ⁸⁷Sr/⁸⁶Sr ratio can be used to trace strontium sources to surface and groundwater.

Cost of Analysis

Thermal Ionization Mass Spectrometer (TIMS): $250-$375 per sample. For information and detailed prices, see, for example:

Geochron Laboratories

Origin

The alkali earth metal strontium (Sr) has four naturally occurring stable isotopes: ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr. Of these, ⁸⁷Sr is the only radiogenic isotope, produced through the beta decay of ⁸⁷Rb. Consequently, the strontium present in nature is the sum of that present when the earth was created plus the daughter product from the decay of ⁸⁷Rb. Variation in the relative abundance of ⁸⁷Sr is usually expressed using ⁸⁷Sr/⁸⁶Sr

Measurement Techniques

The Sr ratios of natural waters are controlled in large part by rock-water interactions. Chemical reactions such as ion exchange and mineral dissolution determine the Sr isotopic value of the fluid, and mineral precipitation affects the Sr concentration. Sr is normally reported as the absolute ⁸⁷Sr/⁸⁶Sr ratio. However, like many other isotopes, variation in the Sr isotope system can be expressed using delta notation:

where the ⁸⁷Sr/⁸⁶Sr ratio of seawater is the typical standard used.

Analysis of ⁸⁷Sr/⁸⁶Sr ratios is done by thermal ionization mass spectrometry, an important method for analysis of elements that are not easily converted to a gas.

Hydrological Applications

The ⁸⁷Sr/⁸⁶Sr ratio is a useful indicator in groundwater studies of source water/rock interactions. However, the kinetics of chemical reactions and mineral precipitation must be taken into account. A large number of mineral phases can affect the ⁸⁷Sr/⁸⁶Sr values in the water phase. Different minerals release Sr at different rates and may have different ⁸⁷Sr/⁸⁶Sr ratios. Flow rate and flow path are other important factors to consider when attempting to interpret the ⁸⁷Sr/⁸⁶Sr value of any groundwater sample (McNutt 2000).

Sr closely mimics the behavior of Ca²⁺, for example substituting for Ca²⁺ in the lattices of minerals and replacing Ca in the cell walls of plants and animals. Because of this, Sr isotope ratios are useful to trace Ca sources and cycling in oceans, watersheds, and ecosystems.

Sr isotopic analysis has some appreciable advantages:

Sr does not fractionate appreciably during chemical reactions (including phase changes, chemical and biological processes, etc.); hence, measured Sr ratios are good indicators of the original source of Sr. Furthermore, Sr isotope analysis uses the ⁸⁸Sr/⁸⁶Sr ratio, which is constant, as an internal standard to correct for fractionation during measurement. Thus, even if fractionation naturally did occur it would be corrected during measurement.
The atmospheric concentration of Sr is very low, so there is little risk of any atmospheric contamination to a given Sr ratio and, therefore, no atmospheric correction is required.

The ⁸⁷Sr/⁸⁶Sr ratio analysis can be used:

to obtain information on the provenance of a water
to determine mixing relationships within bodies of water
by measuring the ⁸⁷Sr/⁸⁶Sr ratio of lake sediments, to determine the changes in the source history of a drainage or watershed
to determine controls on source contributions to water bodies
to trace nutrient pathways and availability of nutrient pools in ecosystems
to fingerprint and quantify the sources of salinity in river systems (See SAHRA's project on the Solute Balance of the Rio Grande).
to indicate preferential flowpaths within groundwater systems (See Johnson et al. abstract)

Strontium (Sr) is an alkali earth metal and chemically reactive. Strontium commonly occurs in nature and is the 15^th most abundant element on earth. Strontium is found mainly as the mineral SrSO₄ and SrCO₃. Strontium is soluble in aqueous solution as the Sr²⁺ ion and is geochemically similar to Ca. The main source of Sr in waters is from chemical weathering.

Measuremment Techniques

Sr²⁺ is commonly measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) or Ion Chromatography (IC)

⁸⁷Sr/⁸⁶Sr is measured using Thermal Ionization Mass Spectrometer (TIMS)

Applications

Strontium ion
Strontium ion (Sr²⁺) can be used a tracer to indicate chemical weathering and groundwater residence time. For example, in carbonate systems, Sr, unlike Ca is usually not solubility limited and the increase of Sr²⁺ concentration along the flow line is due to the incongruent reactions of carbonate minerals and/or the dissolution of gypsum or anhydrite. Using the ratio of Sr/Ca shows diagnostic trend along groundwater flow line.

Strontium isotopes
The ⁸⁷Sr/⁸⁶Sr ratio is a useful indicator in groundwater studies of source water/rock interactions. However, the kinetics of chemical reactions and mineral precipitation must be taken into account. A large number of mineral phases can affect the ⁸⁷Sr/⁸⁶Sr values in the water phase. Different minerals release Sr at different rates and may have different ⁸⁷Sr/⁸⁶Sr ratios. Flow rate and flow path are other important factors to consider when attempting to interpret the ⁸⁷Sr/⁸⁶Sr value of any groundwater sample (McNutt, 2000).

Sr isotopic analysis has some appreciable advantages:

Sr does not fractionate appreciably during chemical reactions (including phase changes, chemical and biological processes, etc.); hence, measured Sr ratios are good indicators of the original source of Sr. Furthermore, Sr isotope analysis uses the 88Sr/86Sr ratio, which is constant, as an internal standard to correct for fractionation during measurement. Thus, even if fractionation naturally did occur it would be corrected during measurement.
The atmospheric concentration of Sr is very low, so there is little risk of any atmospheric contamination to a given Sr ratio and, therefore, no atmospheric correction is required.

The use of 87Sr/86Sr ratio analysis can be summarised:

to obtain information on the provenance of a water
to determine mixing relationships within bodies of water
by measuring the ⁸⁷Sr/⁸⁶Sr ratio of lake sediments, to determine the changes in the source history of a drainage or watershed
to determine controls on source contributions to water bodies
to trace nutrient pathways and availability of nutrient pools in ecosystems
to fingerprint and quantify the sources of salinity in river systems.
to indicate preferential flow paths within groundwater systems

References and Further Reading

Benson, L., and Z. Peterman, Carbonate deposition, Pyramid Lake subbasin, Nevada: 3. The use of ⁸⁷Sr values in carbonate deposits (tufas) to determine the hydrologic state of paleolake systems, Palaeogeography, Palaeoclimatology, Palaeoecology, 119, 201-213, 1995.
Bouchard, D.P., D.S. Kaufman, A. Hochberg, and J. Quade, Quaternary history of the Thatcher Basin, Idaho, reconstructed from the ⁸⁷Sr/⁸⁶Sr and amino acid composition of lacustrine fossils: implications for the diversion of the Bear River into the Bonneville Basin, Palaeogeography, Palaeoclimatology, Palaeoecology, 141, 95-114, 1998.
Clark, I., and P. Fritz, Environmental Isotopes in Hydrogeology, Lewis Publishers, Boca Raton, 1997.
English, N.B., J. Quade, P.G. DeCelles and C.N. Garzione, Geologic control of Sr and major element chemistry in Himalayan Rivers, Nepal, Geochimica et. Cosm. Acta, 64, 2549-2566, 2000.
Faure, G., Principles of Isotope Geology, 2nd ed., John Wiley & Sons, New York, 1986.
Gosz, J.R., and D.I. Moore, Strontium isotope studies of atmospheric inputs to forested watersheds in New Mexico, Biogeochemistry, 8, 115-134, 1989.
Graustein, W.C., and R.L. Armstrong, The use of strontium-87/strontium-86 ratios to measure atmospheric transport into forested watersheds, Science, 219, 289-293, 1983.
Jones, L.M. and G. Faure, Strontium isotope geochemistry of Great Salt Lake, Utah, Geo. Society of Amer.Bulletin, 83, 1875-1880, 1972.
Kendall, C., and J.J. McDonnell, editors, Isotope Tracers in Catchment Hydrology, Elsevier, NY, 1998.
Mazor, E., Applied Chemical and Isotopic Groundwater Hydrology, Halsted Press, 1991.
McNutt, R.H., Strontium isotopes, in Envirionmental Tracers in Subsurface Geology, ed. by P.G. Cook and A.L. Herczeg, pp. 234-260. Kluwer, Boston, 2000.
Miller, E.K., J.D. Blum, and A.J. Friedland, Determination of soil exchangeable-cation loss and weathering rates using Sr isotopes, (Letters to…) Nature, 362, 438-441, 1993.
Pedone, V.A., Negative covariance between lake volume and strontium isotope ratio in the Great Salt Lake, Utah, GSA Annual Meeting Abstracts, Session 174, p. A-389, 2000.
Quade, J., Strontium ratios and Lake Bonneville chronostratigraphy, Late Quaternary Paleoecology in the Bonneville Basin (Bulletin 130, Utah Geological Survey), pp. 21-23, 2000.
Quade, J., Strontium ratios and the origin of early Homestead Cave biota, Late Quaternary Paleoecology in the Bonneville Basin (Bulletin 130, Utah Geological Survey), pp. 44-46, 2000.
Walker, F.W., J.P. Parrington, and F. Feiner, Nuclides and Isotopes, 14th edition, General Electric Company, San Jose, CA, 1989.

Internet Resources

Johnson, T.M., et al, abstract of: Groundwater "fast paths" in the Snake River Plain aquifer: radiogenic isotope ratios as natural groundwater tracers, Geology, 28(10): 871-874.