Multi-isotope fingerprints of recent environmental samples from the Baltic coast and their implications for bioarchaeological studies

Abstract

Stable isotopes in coastal regions are influenced by the so-called sea spray effect which masks the actual terrestrial isotope fingerprint with a marine isotope signal. The sea spray impact on plants was investigated by analyzing different stable isotope systems (δ¹³C_cellulose, δ¹⁸O_cellulose, δ¹⁸O_sulfate, δ³⁴S_sulfate, δ³⁴S_{total S}, δ³⁴S_{organic S}, ⁸⁷Sr/⁸⁶Sr) in recent environmental samples (plants, soil, water) collected close to the Baltic Sea. All these isotopic systems are influenced by the sea spray, either by the uptake of ions (HCO₃^-, SO₄^2-, Sr²⁺) of marine origin, thus exhibiting a marine isotopic signature, or by biochemical reactions associated with, e.g., salinity stress. A shift towards seawater values is observed for δ¹⁸O_sulfate, δ³⁴S, and ⁸⁷Sr/⁸⁶Sr. Cellulose becomes enriched in ¹³C and ¹⁸O due to sea spray, further enhanced (δ¹³C_cellulose) or mitigated (δ¹⁸O_cellulose) by salinity stress. The effect differs both regionally and seasonally, probably as a result of, e.g., differences in wind strength or prevailing wind direction, as well as between plants collected only few meters apart, in either the open field or at more wind-protected sites, reflecting samples more or less influenced by sea spray. The stable isotope data of recent environmental samples are compared to previously analyzed archaeological bone samples of animals from the Viking Haithabu and Early Medieval Schleswig sites located close to the Baltic Sea. Potential regions of origin can be predicted based on the magnitude of the (recent) local sea spray effect. This enables the identification of probably non-local individuals. The insights into sea spray mechanisms, biochemical reactions in plants, as well as seasonal, regional, and small-scale differences in stable isotope data will help to interpret multi-isotope fingerprints at coastal sites. Our study demonstrates the usefulness of environmental samples for bioarchaeological studies. Moreover, the detected seasonal and small-scale differences require adjusted sampling strategies for, e.g., isotopic baselines in coastal areas.

Keywords: Aerosol; Carbon; Oxygen; Plant physiology; Strontium; Sulfur.