Towards a biologically available strontium isotope baseline for Ireland

Abstract

Strontium isotopes are used in archaeology, ecology, forensics, and other disciplines to study the origin of artefacts, humans, animals and food items. Strontium in animal and human tissues such as bone and teeth originates from food and drink consumed during life, leaving an isotopic signal corresponding to their geographical origin (i.e. where the plants grew, the animals grazed and the drinking water passed through). To contextualise the measurements obtained directly on animal and human remains, it is necessary to have a sound baseline of the isotopic variation of biologically available strontium in the landscape. In general, plants represent the main source of strontium for humans and animals as they usually contain much higher strontium concentrations than animal products (meat and milk) or drinking water. The observed difference between the strontium isotope composition of geological bedrock, soils and plants from the same locality warrants direct measurement of plants to create a reliable baseline. Here we present the first baseline of the biologically available strontium isotope composition for the island of Ireland based on 228 measurements on plants from 140 distinct locations. The isoscape shows significant variation in strontium isotope composition between different areas of Ireland with values as low as 0.7067 for the basalt outcrops in County Antrim and values of up to 0.7164 in the Mourne

Drinking Locally: A Water 87Sr/86Sr Isoscape for Geolocation of Archeological Samples in the Peruvian Andes

“…Second, different species can also show distinct 87 Sr/ 86 Sr from the same location, due in part to their root systems accessing discrete pockets of strontium in the soil (Dijkstra, 2003;Poszwa et al, 2004). Third, differential contributions from sea spray and the acidity or other molecular differences of the microhabitat can cause plant 87 Sr/ 86 Sr to differ even when the same organ and species are sampled from the same location (Snoeck et al, 2020). A fourth complicating factor is the use of non-local fertilizers in either modern or ancient times, which can contribute a non-local geological signature to plant and soil samples (Bentley et al, 2004;Bentley, 2006;Evans et al, 2012;Thomsen and Andreasen, 2019).…”

“…However, a recent study of Danish surface waters (Frei et al, 2020) shows that much of agricultural lime is retained by soils, so agricultural contamination may not influence surface water 87 Sr/ 86 Sr as much as previously thought. Second, proximity to the ocean also impacts water 87 Sr/ 86 Sr. Because modern ocean water is uniform across the planet ( 87 Sr/ 86 Sr = 0.7092) (Veizer, 1989), sea spray and brackish water can drive up otherwise low 87 Sr/ 86 Sr in coastal plains (Whipkey et al, 2000;Bentley, 2006;Snoeck et al, 2020), particularly for water samples closest to the ocean. This can lead 87 Sr/ 86 Sr in coastal regions to deviate from expectations based on bedrock.…”

“…The potential of using water 87 Sr/ 86 Sr for archeological skeleton provenience has varied according to study location and approach. Many baseline studies including water do not directly compare water and archeological human 87 Sr/ 86 Sr (Hodell et al, 2004;Bentley and Knipper, 2005;Frei, 2011, 2013;Lengfelder et al, 2019;Ladegaard-Pedersen et al, 2020;Pacheco-Forés et al, 2020), although some incorporate water into a mixed or predictive model and evaluate archeological data points against that model (e.g., Montgomery et al, 2007;Snoeck et al, 2020). Among studies in mountainous regions comparing skeletal 87 Sr/ 86 Sr directly to local water measurements, a relationship is reported between water elevation, sample variability, and fit of archeological samples to local water.…”

“…Methods for defining the expected local 87 Sr/ 86 Sr range have shifted over the last few decades from statistically parsing human skeletal 87 Sr/ 86 Sr around the sample mean (e.g., Wright, 2005;Slovak et al, 2009;Price et al, 2015) to testing archeological and modern fauna with small home ranges (Price et al, 2002(Price et al, , 2007Evans and Tatham, 2004;Hedman et al, 2009Hedman et al, , 2018Knudson and Tung, 2011). More recently, many scholars have argued that the only reliable way to determine the local 87 Sr/ 86 Sr range is by analyzing the bioavailable strontium in local environmental reference or baseline materials like soils, plants, and local fauna, and then aggregating those reference 87 Sr/ 86 Sr to geological units, soil units, or statistical groupings of baseline and/or skeletal materials (Valentine et al, 2008;Evans et al, 2010;Maurer et al, 2012;Knudson et al, 2014;Kootker et al, 2016;Grimstead et al, 2017;Willmes et al, 2018;Barberena et al, 2019;Pacheco-Forés et al, 2020;Snoeck et al, 2020). Unfortunately, because underlying bedrock geology is highly variable even at a short distance, and because rock weathers unevenly throughout catchments, these methods do not enable the sourcing of a sample to any location beyond the immediate vicinity of the sampling location.…”

“…This is important since studies have generally found that soil and bedrock 87 Sr/ 86 Sr alone are poor predictors for human tissue 87 Sr/ 86 Sr (Price et al, 2002, pp. 120-122;Knudson et al, 2014;Grimstead et al, 2017;Snoeck et al, 2020). Establishing accurate isoscapes of bioavailable 87 Sr/ 86 Sr variation in the natural environment and understanding what proxy materials best reflect human 87 Sr/ 86 Sr are prerequisites for accurately identifying non-locals in archeological assemblages and constraining their likely geographic origins to the most probable locations.…”