CAN ELEMENT CHEMICAL IMPURITIES IN ARAGONITIC SHELLS OF MARINE BIVALVES SERVE AS PROXIES FOR ENVIRONMENTAL VARIABILITY?

In many biogenic and geogenic materials, ion impurities can provide serviceable environmental proxies. However, the element/Ca ratios of bivalve shells are notoriously challenging to interpret. Due to vital effects, nonclassical nucleation and growth mechanisms, kinetic processes and/or non-lattice bound and microstructure-specific element content, the concentration of trace and minor elements in marine shells typically remains below values observed in inorganic CaCO₃ precipitated from a solution resembling seawater chemistry, but above those expected for thermodynamic equilibrium. If environmental conditions were still encoded in the shells, they should result in reproducible element/Ca chronologies between contemporaneous specimens from the same site. Here, we tested this hypothesis using twelve modern specimens of Arctica islandica collected from four different localities in the North Atlantic. Annual B, Na, Mg, K, Mn, Sr and Ba/Ca ratios measured in the shells were reproducible between most specimens from the same site, supporting the hypothesis that the incorporation of these elements was at least partly controlled by environmental forcings. However, the agreement between shell element/Ca ratios and environmental quantities was weaker than the inter-specimen element/Ca ratios suggesting that the available information on temperature, salinity, food, dissolved oxygen (DO) concentration and water chemistry did not properly reflect the in-situ conditions to which the bivalves were exposed or other extrinsic factors were at work. As in inorganic aragonite – but in contrast to thermodynamic expectations –, annual Sr, Mg and B/Ca ratios were negatively correlated to T (up to 40 % explained variability). The link between Ba/Ca and bulk phytoplankton often remained below the significance threshold, but was otherwise positive. Salinity and DO explained up to 18 % of Na/Ca and 12 % of Mn/Ca variations, respectively. Quantitative environmental reconstructions based on ion impurities in bivalve shells will remain challenging or impossible unless the chemistry of the extrapallial fluid from which the shell actually formed is known, including temporal changes thereof. This information is crucial to compute representative partition coefficients required to calibrate transfer functions.