Invited review articleAdvances in global bioavailable strontium isoscapes
Introduction
Strontium (Sr) isotope ratios (87Sr/86Sr) display a unique and predictable patterns of variability on the Earth's surface that follow the geological age and lithology of bedrocks (Bataille and Bowen, 2012). As rocks interact with the hydrosphere, atmosphere and biosphere, bedrock Sr is transferred to other reservoirs on the Earth's surface, such as soils and plants. Geologists have long recognized and capitalized on this natural 87Sr/86Sr variability to trace the provenance of geological materials (Reviewed in Banner, 2004; Capo et al., 1998; Peucker-Ehrenbrink and Fiske, 2019). In the last few decades, researchers have also recognized the potential for 87Sr/86Sr data to solve new questions in ecology, paleoecology, and archeology (Reviewed in Åberg, 1995; Bentley, 2006; Crowley et al., 2017a; Hobson et al., 2010; Makarewicz and Sealy, 2015). This uptick of interest in 87Sr/86Sr geochemistry has coincided with analytical advances and the development of multi-collector inductively coupled plasma mass spectrometers (MC-ICPMS). This instrumentation and its greater global availability has made 87Sr/86Sr analysis more mainstream by accelerating throughput and enhancing cost-effectiveness while also facilitating the development of new applications in the life sciences such as laser ablation of incrementally growing tissues (e.g., fin rays and otoliths; Brennan et al., 2015b; Willmes et al., 2016). With these advances, 87Sr/86Sr geochemistry has become a critical tool for tracing the mobility and/or geographic origin of biological material in ecology (Reviewed in Hobson et al., 2010), paleoecology (Reviewed in Crowley et al., 2017a), archeology (Reviewed in Bentley, 2006), forensic sciences (Reviewed in Makarewicz and Sealy, 2015), and food sciences (Reviewed in Coelho et al., 2017). All of these applications rely on comparing the 87Sr/86Sr of a given substrate with the isotopic signatures of its potential sources. To facilitate the interpretation of 87Sr/86Sr data in these applications, it is critical to constrain the spatial variability of 87Sr/86Sr in the geosphere, hydrosphere and biosphere.
Isoscapes are spatially explicit predictions of isotopic variations. These predictions can be produced either through geostatistical interpolation of observed isotopic data, or through mechanistic model based on first principles of isotope geochemistry (Bowen and West, 2008). Over the last few decades, isoscapes of hydrogen, carbon, oxygen, and nitrogen have been developed, building upon the growing number of isotopic observations (Bowen and Wilkinson, 2002; Still and Powell, 2010; West et al., 2010a). These isoscapes have become a routine tool to understand movement patterns of animals and humans and environmental and biological processes (West et al., 2010b). Isoscape science has recently contributed to research on many high-profile science questions, from partitioning the global hydrological cycle (Good et al., 2015) to assessing the population dynamics of critical species (Brennan et al., 2019). As such, the field of isotope provenancing is rapidly expanding and entering the realm of data science, for example through large initiatives to integrate relevant data in centralized repositories (Pauli et al., 2017), and community efforts to make modeling products widely accessible (Bowen et al., 2014). Historically, interest in, and development of, Sr isoscapes has lagged hydrogen, oxygen or carbon isotopic systems. The primary reasons are that 87Sr/86Sr analysis is challenging, relatively expensive, and relies on instrumentation that is not as widely available as that needed for conducting light stable isotope analyses. However, 87Sr/86Sr analyses have progressively emerged as a powerful complementary tool in provenance studies due to their unique spatial patterns of isotopic variability, with pioneering work having been conducted in archeology (Ezzo et al., 1997; Price et al., 1994; Sillen et al., 1998), paleoecology (Hoppe et al., 1999), ecology (Chamberlain et al., 1997; Kennedy et al., 2000, Kennedy et al., 2002; Koch et al., 1995a, Koch et al., 1995b; Thorrold and Shuttleworth, 2000), and ecosystem dynamics (Blum et al., 2000; Gosz et al., 1983). In the last decade, the development and application of 87Sr/86Sr isoscapes has grown exponentially, driven by high-profile applications in archeology (e.g., Copeland et al., 2011), paleoecology (e.g., Price et al., 2017), ecology (e.g., Brennan et al., 2019; Glassburn et al., 2018), and forensic science (e.g., Bartelink and Chesson, 2019; Kramer et al., 2020).
This review synthesizes the current state of the rapidly evolving and interdisciplinary research associated with 87Sr/86Sr isoscapes. We begin by reviewing spatial 87Sr/86Sr trends on the Earth surface with a focus on large-scale patterns derived from the interactions of the geosphere, hydrosphere, atmosphere and biosphere. We then compare different approaches for making 87Sr/86Sr isoscapes in terrestrial and freshwater environments. In an effort to better integrate interdisciplinary 87Sr/86Sr data, we present the first global compilation of 87Sr/86Sr data from different environmental substrates. We use this compilation to produce a global model for predicting bioavailable 87Sr/86Sr and demonstrate the potential of using this approach to generate 87Sr/86Sr isoscapes at the regional scale in two regions: Europe and Madagascar. We conclude by discussing key knowledge gaps and new research avenues opened by this global data science approach.
Section snippets
Strontium isotopes geochemistry
Strontium is a divalent alkaline earth trace element with four naturally occurring isotopes: 84Sr (~0.56%), 86Sr (~9.87%), 87Sr (~7.04%) and 88Sr (~82.53%). 84Sr, 86Sr, 87Sr and 88Sr are all stable isotopes (i.e., do not radioactively decay). Unlike the other Sr isotopes, 87Sr is the radiogenic daughter product of rubidium 87 (87Rb; decay constant λ = 1.42 × 10–11 year−1; Steiger and Jäger, 1977). The ratio of 87Sr to the other isotopes is therefore a function of the variable abundance of 87Sr.
Empirical isoscapes
The most commonly used strategy to create biologically available (bioavailable) 87Sr/86Sr baselines is to analyze 87Sr/86Sr in a series of biological samples representing bioavailable Sr pools in a study area (Lengfelder et al., 2019). However, as outlined above, different substrates (e.g., soils, plants, animals, or waters) can integrate 87Sr/86Sr at different spatiotemporal scales depending on the local geo-environmental conditions. Thus, identifying appropriate substrates can be challenging.
Compilation description
To date, no researchers have attempted global bioavailable 87Sr/86Sr modeling. Here we leverage a new global data compilation, mechanistic models, and auxiliary variables integrated in a multivariate random forest regression framework to predict bioavailable 87Sr/86Sr at the global scale. The dataset used in our study is a compilation of 17,240 published and unpublished 87Sr/86Sr analyses from 278 individual studies spanning 8476 individual locations across the globe (Fig. 2). Unpublished data
Regional dataset
We tested the performance of the model in two regions with different geological settings and sampling density to provide guidance on how to use this global bioavailable 87Sr/86Sr isoscape. First, we used data collected through the GEMAS project (Hoogewerff et al., 2019) to test the performance of the global model in a data-rich region. To date, GEMAS is the most systematic and comprehensive continental-scale dataset of bioavailable 87Sr/86Sr. The dataset includes close to 1200 soil samples from
Guidelines for using the global isoscape
Provenance studies have underlying assumptions specific to the sample type and the question being addressed. The predicted bioavailable 87Sr/86Sr isoscape presented here (Fig. 9) is best suited as a broad scale approach for 1) excluding provenance areas and 2) informing where targeted sampling for a specific research question should occur. When samples in question exhibit a limited range in bioavailable 87Sr/86Sr, as is the case for plants, soils, and animals with small feeding ranges (e.g.,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This paper is dedicated to the memory of Dr. Sean Brennan who sadly passed away in January 2020. Dr. Brennan was a stellar young scientist, and had considerably advanced the Sr isoscape field. His work and his passion will be dearly missed. This work was supported in part by NSERC Discovery Grant RGPIN-2019-05709 (to C.P·B). B.E.C was supported by the UC faculty startup. G.J.B was supported by NSF DBI-1759730. We thank Tom Johnson and Gideon Bartov for analyzing specimens at the University of
References (130)
- et al.
Dust composition changes from Taylor Glacier (East Antarctica) during the last glacial-interglacial transition: a multi-proxy approach
Quat. Sci. Rev.
(2017) - et al.
Strontium isotopes as indicators of mineral weathering in catchments
Catena
(1994) Radiogenic isotopes: Systematics and applications to earth surface processes and chemical stratigraphy
Earth Sci. Rev.
(2004)- et al.
Mapping 87Sr/86Sr variations in bedrock and water for large scale provenance studies
Chem. Geol.
(2012) - et al.
A geostatistical framework for predicting variability in strontium concentrations and isotope ratios in Alaskan rivers
Chem. Geol.
(2014) - et al.
Chemical weathering outputs from the flood plain of the Ganga
Geochim. Cosmochim. Acta
(2018) - et al.
87Sr/86Sr ratios of sierra Nevada stream waters: Implications for relative mineral weathering rates
Geochim. Cosmochim. Acta
(1993) - et al.
Isotope landscapes for terrestrial migration research
Terr. Ecol.
(2008) - et al.
Strontium isotopes in otoliths of a non-migratory fish (slimy sculpin): Implications for provenance studies
Geochim. Cosmochim. Acta
(2015) - et al.
Strontium isotopes as tracers of ecosystem processes : theory and methods
Geoderma
(1998)
Changing sources of strontium to soils and ecosystems across the Hawaiian Islands
Chem. Geol.
Recent developments and trends in the application of strontium and its isotopes in biological related fields
TrAC Trends Anal. Chem.
Strontium isotope investigation of ungulate movement patterns on the Pleistocene Paleo-Agulhas Plain of the Greater Cape Floristic Region, South Africa
Quat. Sci. Rev.
Hydrogeochemistry of seasonal flow regimes in the Chena River, a subarctic watershed draining discontinuous permafrost in interior Alaska (USA)
Chem. Geol.
Analytical perspectives on prehistoric migration: a case study from East-Central Arizona
J. Archaeol. Sci.
Some aspects of the geochemistry of strontium and calcium in the Hudson Bay and the Great Lakes
Geochim. Cosmochim. Acta
The geographic distribution of strontium isotopes in Danish surface waters - a base for provenance studies in archaeology, hydrology and agriculture
Appl. Geochem.
Shallow retardation of the strontium isotope signal of agricultural liming - implications for isoscapes used in provenance studies
Sci. Total Environ.
The Sahara-East Mediterranean dust and climate connection revealed by strontium and uranium isotopes in a Jerusalem speleothem
Earth Planet. Sci. Lett.
The strontium isotopic budget of Himalayan rivers in Nepal and Bangladesh
Geochim. Cosmochim. Acta
Environmental and dietary exposure of young children to inorganic trace elements
Environ. Int.
Antarctic (Dome C) ice-core dust at 18 k.y. B.P.: isotopic constraints on origins
Earth Planet. Sci. Lett.
Mapping and defining sources of variability in bioavailable strontium isotope ratios in the Eastern Mediterranean
Geochim. Cosmochim. Acta
Variation in strontium isotope ratios of archaeological fauna in the Midwestern United States: a preliminary study
J. Archaeol. Sci.
Predicting strontium isotope variation and fish location with bedrock geology: Understanding the effects of geologic heterogeneity
Chem. Geol.
Bioavailable 87Sr/86Sr in European soils: a baseline for provenancing studies
Sci. Total Environ.
Stable isotope stratigraphy and paleoclimatology of the Paleogene Bighorn Basin (Wyoming, USA)
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Spatial variation of biologically available strontium isotopes (87Sr/86Sr) in an archipelagic setting: a case study from the Caribbean
J. Archaeol. Sci.
Modelling strontium isotopes in past biospheres – Assessment of bioavailable 87Sr/86Sr ratios in local archaeological vertebrates based on environmental signatures
Sci. Total Environ.
Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin)
Geochim. Cosmochim. Acta
Dietary reconstruction, mobility, and the analysis of ancient skeletal tissues: Expanding the prospects of stable isotope research in archaeology
J. Archaeol. Sci.
Bioavailable 87Sr/86Sr in different environmental samples - effects of anthropogenic contamination and implications for isoscapes in past migration studies
Sci. Total Environ.
Evaluating the use of strontium isotopes in tree rings to record the isotopic signal of dust deposited on the Wasatch Mountains
Appl. Geochem.
A conditioned Latin hypercube method for sampling in the presence of ancillary information
Comput. Geosci.
Isotopic evidence for eolian recycling of pedogenic carbonate and variations in carbonate dust sources throughout the Southwest United States
Geochim. Cosmochim. Acta
Controls over the strontium isotope composition of river water
Geochim. Cosmochim. Acta
Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico
Geochim. Cosmochim. Acta
A continental perspective of the seawater 87Sr/86Sr record: a review
Chem. Geol.
The use of natural strontium isotopes as tracers in environmental studies
Water Air Soil Pollut.
Spherical harmonic modelling to ultra-high degree of Bouguer and isostatic anomalies
J. Geod.
Recent applications of isotope analysis to forensic anthropology
Forensic Sci. Res.
Mapping multiple source effects on the strontium isotopic signatures of ecosystems from the circum-Caribbean region
Ecosphere
A bioavailable strontium isoscape for Western Europe: a machine learning approach
PLoS One
Strontium isotope composition of skeletal material can determine the birth place and geographic mobility of humans and animals
J. Forensic Sci.
Strontium isotopes from the earth to the archaeological skeleton: a review
J. Archaeol. Method Theory
Unexpected dominance of parent-material strontium in a tropical forest on highly weathered soils
Ecology
Carte Géologique de Madagascar
A strontium, neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in the Trois Seigneurs Massif, Pyrenees, France
Contrib. Mineral. Petrol.
Controls on the 87Sr/86Sr ratio of carbonates in the Garhwal Himalaya, Headwaters of the Ganges
J. Geol.
Changes in Sr/Ca, Ba/Ca and 87Sr/86Sr ratios between trophic levels in two forest ecosystems in the northeastern U.S.A
Biogeochemistry
Cited by (108)
A reanalysis of strontium isotope ratios as indicators of dispersal in South African hominins
2024, Journal of Human EvolutionMobility and connection among the Early Bronze Age Syrian elite
2023, Journal of Archaeological Science: ReportsCombining stable isotope ratios with elemental concentrations to improve the estimation of terrestrial carnivore diets
2023, Global Ecology and ConservationThe use and abuse of Pb in bioarchaeological studies: A review of Pb concentration and isotope analyses of teeth
2023, Journal of Archaeological Science