Growth rate rather than temperature affects the B∕Ca ratio in the calcareous red alga <i>Lithothamnion corallioides</i>

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. 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 laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of Mg, Sr, Li, and B in the coralline alga Lithothamnion corallioides collected from different geographic settings and depths across the Mediterranean Sea and in the Atlantic Ocean. We produced the first data on putative temperature proxies (Mg/Ca, Li/Ca, Sr/Ca, Mg/Li) and B/Ca in a coralline algal species grown in different basins from across the photic zone (12, 40, 45, 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 suggested a growth rate influence on B/Ca, which was evident in the sample with the lowest growth rate of 0.10 mm yr−1 (Pontian Isl., Italy; 66 m depth) and in Elba (Italy; 45 m depth), where the algal growth rate was the highest (0.14 mm yr−1). At these two sites, the measured B/Ca was the lowest at 462.8 ± 49.2 µmol mol−1 and the highest at 757.7 ± 75.5 µmol mol−1, respectively. A positive correlation between B/Ca and temperature proxies was found only in the shallowest sample from Morlaix (Atlantic coast of France; 12 m depth), where the amplitude of temperature variation (ΔT) was the highest (8.9 ∘C). Still, fluctuations in B/Ca did not mirror yearly seasonal temperature oscillations as for Mg/Ca, Li/Ca, and Sr/Ca. We concluded that growth rates, triggered by the different ΔT and light availability across depth, affect the B incorporation in L. corallioides.