Growth rate rather than temperature affects the B / Ca ratio in thecalcareous red alga Lithothamnion corallioides

Abstract. The B / Ca ratio in calcareous marine species is informative of past seawater CO32− concentrations, but scarce data exist on B / Ca in coralline algae (CA). Recent studies suggest influences of temperature and growth rates on B / Ca, the effect of which could be critical for the reconstructions of surface ocean pH and atmospheric pCO2. In this paper, we present the first LA-ICP-MS analyses of Mg, Sr, Li and B in the CA Lithothamnion corallioides collected from different geographic settings and depths across the Mediterranean Sea and in the Atlantic Ocean. We produced the first data on temperature proxies (Mg, Li and Sr / Ca) and B / Ca in a CA species grown in different Basins (the Mediterranean Sea and the Atlantic Ocean), from shallow to deep waters (12 m, 40 m, 45 m and 66 m depth). We tested the B / Ca correlation with temperature proxies and growth rates, in order to evaluate their possible effect on B incorporation. Our results showed a growth rate influence on B / Ca, especially in the deepest sample (Pontian Isl., Italy; 66 m) and in the shallowest sample (Morlaix, Atlantic coast of France; 12 m), where the growth rates were respectively 0.11 mm/yr and 0.13 mm/yr and the B / Ca was respectively 462.8 ± 49.2 μmol/mol and 726.9 ± 102.8 μmol/mol. A positive correlation between B / Ca and the temperature proxies was found only in Morlaix, where the seasonal temperature variation (ΔT) was the highest (8.90 °C). These pieces of evidence suggest that growth rates, triggered by the different ΔT and light availability across depth, affect the B incorporation in L. corallioides.