Introduction to Sr Isotope Geochemistry

Isobar strontium geochemistry

Compilation of 87Sr/86Sr ranges for various earth system materials and environments. Data after Capo et al, 1998 and Bentley, 2006.

The element strontium has four naturally occurring isotopes – all are stable: 84Sr, 86Sr, 87Sr, and 88Sr. Only 87Sr is radiogenic – produced by the β-decay of 87Rb with a half-life of 48.8 billion years. In geologic and environmental investigations, the ratio of 87Sr/86Sr is typically used because of their similar abundances – 7% and 10%, respectively. Strontium separation from interfering matrix elements is done by extraction chromatography and high-precision isotope dilution mass spectrometry. Multi-collector inductively coupled plasma mass spectrometery (i.e. MC-ICP-MS) provides precise measurements of the isotopes of Sr (87Sr/86Sr). Sr isotopic high precision measurements can be conducted on small sample sizes and sample preparation is easy compared to other isotopes.

Strontium is a ubiquitous trace element present in rocks, soil, waters, plants, and animals. The 87Sr/86Sr ratio is a function of the geochemical origin, Rb concentration, and age of the source rock. Because of the high mass of Sr, the ratio is essentially unchanged as the rock weathers and moves through the water cycle and food chain. Thus, the 87Sr/86Sr ratio is extensively used in geochemical fingerprinting, source tracking, contamination prediction, and migration/mobility studies. A systematic change in 87Sr/86Sr ratio of seawater through time has provided a basis for strontium stratigraphy and strontium chronology of marine biogenic carbonates and phosphates.

Sr Isotopic Ratio Applications

Archaeology

Strontium ratio human teeth

Strontium isotopic ratio (87Sr/86Sr) of human deciduous teeth showing human mobility in southern Italy during the Gravettian and Epigravettian periods (32157-19097 yr cal BP), across the Last Glacial Maximum. Adopted from Lugli et al., 2019.

Each region in the world presents a specific 87Sr/86Sr ratio based on the surrounding geology and water sources. Humans, plants and other animals inherit the Sr isotopic signature of their habitat as Sr is incorporated in their cells along with other elements (i.e. Ca). As humans and animals move from one habitat to another, they may be exposed to different sources of water and food (plant and animal). Measuring the 87Sr/86Sr ratio in human and animal bone and hair is a powerful tool for tracking mobility and migration.

Groundwater

Strontium ratio of groundwater

Saltwater intrusion in the Biscayne aquifer as indicated by strontium isotopic ratio (87Sr/86Sr) of water samples collected in Miami-Dade County during 2009. Data from Prinos et al., 2014.

As physical processes do not cause fractionation in strontium isotopes, the 87Sr/86Sr ratio of groundwater reflects the isotopic signature of the source water plus the rock formations along its flow path. Using the Sr isotopic ratio along with the Sr concentration in the water is a powerful method to trace groundwater mixing and contamination, as well as saltwater intrusion.

Geochronology-Stratigraphy

Strontium ratio of seawater

Strontium isotopic ratio (87Sr/86Sr) of seawater during geological time based on analyses of bulk sediment, unaltered brachiopods, belemnites, conodonts and foraminifera samples from various locations around the world. Red area denotes the 95-percent confidence interval. Adopted from McArthur et al., 2001.

Precipitation of marine biogenic carbonates and phosphates involves uptake of strontium from seawater with the 87Sr/86Sr ratio identical to that of the oceanic value at the time of incorporation. By measuring the 87Sr/86Sr ratio in the sample and placing it on the standard seawater Sr-curve the age of the sample can be determined. This method works best for time periods with long-term unidirectional shifts in the 87Sr/86Sr ratio, such as the Tertiary, but technically can work over the past 600 million years. This method gives a maximum time resolution of 1 million year.

Forensics

Geochemical fingerprinting food

87Sr/86Sr ranges of geological substrates (open boxes) and wines (filled boxes) from the vineyards on the same substrate in different regions of Italy. Redrawn from Marchionni et al., 2013.

The concept of “We are what we eat” is very true when it comes to the isotopic signature of living organisms.  The cells of humans, plants, and other animals reflect the isotopic signature of the food and water from their habitats. This quality has been used successfully in forensic science for the geochemical fingerprinting of narcotics, to study the ethnic background of an individual living in a society, to track the migration pattern of communities, and to trace food adulteration, especially in wine industry.

Oil Industry

Strontium fingerprint oil studies

Strontium isotope ratios of residual salts in the oil leg from the lower Jurassic Tilje formation, Norway, show that shale layers compartmentalize oil reservoirs. Some layers are more effective barriers to mixing of petroleum than the others. Modified after Peters et al., 2005.

Sr geochemical fingerprinting has a variety of applications in the oil industry. Since different geological formations present specific Sr isotopic signatures, the 87Sr/86Sr ratio can be used to study oil migration and mixing between geological units and their compartmentalization efficiency. The Sr isotopic ratio is also broadly used for production water tracing and isotope stratigraphy.

References

Bentley, R. A., 2006: Strontium Isotopes from the Earth to the Archaeological Skeleton: A Review. Journal of Archaeological Method and Theory, vol. 13, No. 3, pp. 135-187. DOI: 10.1007/s10816-006-9009-x

Capo, R.C.; Stewart, B. W.; Chadwick, O. A., 1998: Strontium isotopes as tracers of ecosystem processes: theory and methods. Geoderma, vol. 82, pp. 197-225.

Lugli, F.; Cipriani, A.; Capecchi, G.; Ricci, S., Boschin, F.; Boscato, P.; Iacumin, P.; Badino, F.; Mannino, M. A.; Talamo, S.; Richards, M. P.; Benazzi, S.; Ronchitelli, A., 2019: Strontium and stable isotope evidence of human mobility strategies across the Last Glacial Maximum in southern Italy. Nature Ecology & Evolution, vol. 3, pp. 905–911. DOI:10.1038/s41559-019-0900-8.

Marchionni, S.; Braschi, E.; Tommasini, S.; Bollati, A.; Cifelli, F.; Mulinacci, N.; Mattei, M.; Conticelli, S., 2013:High-Precision 87Sr/86Sr Analyses in Wines and Their Use as a Geological Fingerprint for Tracing Geographic Provenance. Journal of Agriculture and Food Chemistry, vol. 61, no. 28, pp. 6822-6831.

McArthur, J. M.; Howarth, R. J. and Bailey, T. R., 2001: Strontium Isotope Stratigraphy: LOWESS Version 3: Best Fit to the Marine Sr‐Isotope Curve for 0–509 Ma and Accompanying Look‐up Table for Deriving Numerical Age. The Journal of Geology , vol. 109, no. 2, pp. 155-170.

Peters, K. E.; Walters, C. C. and Moldowan, J. M., 2005: The Biomarker Guide, Second Edition, Volum I Biomarkers and Isotopes in the Environment and Human History. Cambridge University Press, 471 p. ISBN 0 521 78158 2.

Prinos, S.T.; Wacker, M. A.; Cunningham, K. J.; Fitterman, D. V., 2014: Origins and Delineation of Saltwater Intrusion in the Biscayne Aquifer and Changes in the Distribution of Saltwater in Miami-Dade County, Florida. U.S. Geological Survey, Scientific Investigations Report 2014–5025, 101 p.