A multi-proxy, bioavailable strontium isotope baseline for southern Almería, Spain: Using modern environmental samples to constrain the isotopic range of bioavailable strontium

Abstract

The Spanish region of Almería is well known for its rich geological records and its richness in archaeological remains. Sr isotopes have been applied in archaeology as a powerful tracing tool for individual human and animal mobility, but their application requires extensive regional baselines as reference against which the target materials can be compared. This study presents Sr concentrations and 87Sr/86Sr values of modern environmental proxies (plants, soils and surface waters) from southern Almería to establish such a bioavailable Sr isotope baseline for this region. Additionally, this study evaluates differences in bioavailable 87Sr/86Sr signatures of the plants, soil leachates and surface waters and tests three soil leaching agents, ultrapure water (mq), 1M NH4NO3 and 0.1M HNO3, to better understand variations of Sr isotope signatures captured by the different proxies and to evaluate their suitability for baseline constructions. Our results define a wide range of 87Sr/86Sr values ranging from 0.70836 to 0.71630. Our data reveals a strong influence of the local surface lithology on bioavailable 87Sr/86Sr compositions. While the plants and soil leachates generally returned similar 87Sr/86Sr values, surface waters from the same sites sometimes returned significantly less radiogenic values likely due to transported, carbonate-derived Sr from their catchment areas. The different soil leaching procedures returned leachable fractions with similar 87Sr/86Sr values, but a slight bias was observed for soils with a carbonate component signifying the overall strong control of bioavailable Sr by carbonates. We propose to define bioavailable Sr isotope baselines as the average bioavailable 87Sr/86Sr ratio ± double standard deviation (x̅± 2σ) of 1) plants and soil leachates and 2) surface waters for each surface lithology. The soil and plant-based baselines define the narrowest range in 87Sr/86Sr for areas dominated by Cenozoic volcanic rocks while the widest range is seen in areas dominated by high grade Paleozoic metamorphic rocks. Due to the scarcity of surface water run-off in the arid region of southern Almería, surface water based 87Sr/86Sr baselines could only be defined for sites dominated by Cenozoic sediments and high grade Paleozoic metamorphic rocks.