Indirect sea spray effects on radiocarbon data of coastal plants – how physiological reactions in plants can be visualized by a combined investigation of stable, radiogenic, and radiocarbon isotopes in a greenhouse experiment

The so-called sea spray effect is known to considerably influence the stable isotope fingerprint of coastal samples (plants, soil, bones, teeth). However, the impact of sea spray on radiocarbon analyses in environmental samples from coastal sites has not been investigated, yet. Sea spray aerosols, containing, e.g., HCO₃^- or CO₃^2- of marine origin, enter the terrestrial environment, shifting stable isotope values of terrestrial samples towards a seemingly marine isotope signature. Moreover, the sea spray is always accompanied by physiological effects in the sprayed plants, e.g., due to the salinity of the incorporated water, also visible in, e.g., stable carbon isotope data. A terrestrial herbivore, never consuming any marine food, can show a marine isotope signal due to the sea spray effect. While the marine reservoir effect, resulting from the consumption of marine food sources, can be investigated by calculating isotopic mixing models based on the δ¹³C_collagen and δ¹⁵N_collagen values of archaeological animal and human bones, the sea spray effect remains undetected in the δ¹³C_collagen and δ¹⁵N_collagen values of individuals consuming terrestrial protein (i.e., plant sources) influenced by marine aerosols. Therefore, it is important to investigate the influence of the sea spray on the radiocarbon signature.

The impact of either the direct or the accompanied, indirect sea spray effect can be visualized by an artificial sea spray experiment which was performed in a greenhouse. European beach grass (Ammophila arenaria, L.) was sprayed with mineral salt solution of different ion concentration but only traces of NaCl, with salty water from the Schlei inlet, collected next to the archaeological site of Haithabu (Germany), and with seawater from the Baltic Sea, collected at the western coast of Fehmarn island (Germany), respectively. Plants of all treatment groups were irrigated with Munich tap water (mainly originating from Mangfall valley). Radiocarbon analyses (F¹⁴C), stable as well as radiogenic isotope analyses (δ¹³C_DIC, δ¹³C_bulk, δ¹³C_cellulose, δ¹⁸O_cellulose, δ¹⁸O_sulfate, δ³⁴S_sulfate, δ³⁴S_{total S}, ⁸⁷Sr/⁸⁶Sr), and (trace) elemental analyses were conducted on the plant, soil, and water samples (spray water, irrigation water) using mass spectrometry (AMS, IRMS, TIMS, ICP-MS, IC).

Radiocarbon analyses of the plants showed an impact due to the artificial sea spray. The indirect sea spray effect, resulting from either salinity (NaCl) or HCO₃^- stress, has an impact on plants’ F¹⁴C. The study demonstrates that a substantial proportion of ¹⁴C, which is taken up by the sprayed plants, originates from either irrigation or spray water. Stomatal conductance is markedly reduced due to both salinity and bicarbonate stress. Accordingly, less atmospheric ¹⁴CO₂ can enter the plants via their stomata, while H¹⁴CO₃^-/¹⁴CO₃^2-/¹⁴CO_{2 (aq.)} can still be incorporated via the roots.

A multi-dimensional approach with a combined analysis of stable (δ¹³C, δ¹⁸O, δ³⁴S), radiogenic (⁸⁷Sr/⁸⁶Sr), and radiocarbon isotopes in environmental samples allows to depict a detailed image of biochemical and physiological processes associated with the sea spray effect and will help to reveal new insights into the sea spray impact on the isotopic fingerprint of plants, animals, and humans, including potential caveats for radiocarbon analyses in coastal regions.