Palaeobiodiversity research based on stable isotopes: Correction of the sea spray effect on bone carbonate δ13C and δ18O by Gaussian Mixture Model clustering
Abstract Transport of sea spray aerosol in coastal areas (“sea spray” effect) can have a marked influence on isotopic ratios of terrestrial ecosystems shifting terrestrial isotopic ratios towards unusual high values masking the original terrestrial signature. It is unclear so far if and to what extend sea spray influences other stable isotopes besides sulphur. In this study, we examined if the effect was also detectable in carbon, nitrogen, and oxygen stable isotopes of bone collagen and carbonate, respectively. Multi-isotope data of mammals sampled from the Viking Haithabu and medieval Schleswig sites in Northern Germany were analysed according to a previously developed approximation procedure and Gaussian Mixture Model (GMM) clustering in order to quantify the sea spray effect in the isotopes under study. While we were able to approximate an influence of the sea spray effect of at least 32.8% and 62.8% in δ13Ccarb and δ18Ocarb, respectively, it was not possible to validate or approximate this effect in δ13Ccoll and δ15Ncoll. Indeed, detection of the sea spray effect not only in δ34Scoll, but also in δ13Ccarb and δ18Ocarb is of particular importance for studies on both prehistoric and recent material. GMM clustering on terrestrial herbivorous and marine piscivorous mammals was used to confirm the existing influence and to validate the approximated correction for the sea spray effect in the respective isotopic ratios (δ13Ccarb, δ18Ocarb, δ34Scoll) and the correction for the limnic influence on δ15Ncoll approximated in a previous study. After correction, the clustering results markedly changed corresponding to the actual diet and habitat preference of the examined species. Although our study focused on palaeoecology, we suggest that GMM clustering also constitutes a very useful tool for modern landscape ecology based on stable isotope analyses.