A key to the past? Element ratios as environmental proxies in two Arctic bivalves

Understanding rapid climate change in the Arctic and its ecosystem implications requires more information on the environment at temporal resolutions and time-periods not available from instrumental records. Such information can be acquired through geochemical proxy records, but sub-annual records are rare. We analyzed shell material of bivalve mollusks (Serripes groenlandicus and Ciliatocardium ciliatum) placed on oceanographic moorings for one year in two Arctic fjords to assess the potential use of shell elemental ratios as environmental proxies. Li/Ca, Mg/Ca, Li/Mg, Li/Sr, Mn/Ca, Sr/Ca, Mo/Ca, and Ba/Ca were determined using Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry. Combining data from moorings with previously derived sub-annual shell growth models allowed us to relate the elemental ratio patterns to oceanographic parameters (temperature, salinity, and fluorescence). Shell Ba/Ca profiles were characterized by abrupt peaks occurring 11 to 81 days after the phytoplankton bloom, as indicated by an index of seawater fluorescence. Li/Ca and Mg/Ca values exhibited significant logarithmic relationships with shell growth rate, indicated by marginal R2 values of 0.43 and 0.30, respectively. These ratios were also linearly related to temperature, with marginal R2 values of 0.15 and 0.17, respectively. Mn/Ca and Sr/Ca ratios exhibited variability among individuals and their temporal pattern was likely controlled by several unidentified factors. Mo/Ca patterns within the shells did not demonstrate correlations with any of the oceanographic parameters. Our results reflect complex relationships between elemental ratios and bivalve metabolism, methodological limitations, as well as contemporaneous environmental processes, suggesting that none of the studied elemental ratios can be used as unequivocal proxies of seawater temperature, salinity, paleoproductivity, or shell growth rate. Despite this, Ba/Ca and Li/Ca can be used as sub-annual temporal anchors in further studies because the variability of these elements was synchronized in each fjord.