An empirical study of the B/Ca proxy in calcareous red algae

Calcareous red algae have a long and rich fossil record (Basso et al., 2007; Bracchi et al., 2014), and a widely recognized importance as paleoclimate archives (Halfar et al., 2008; McCoy and Kamenos, 2015). Their geochemical composition is used to infer past seawater temperature (Fietzke et al., 2015; Ragazzola et al., 2020), salinity (Kamenos et al., 2012), and pH (Anagnostou et al., 2019). The B/Ca ratio in benthic foraminifera has been used as a proxy for past CO32- concentrations (Yu and Elderfield, 2007; Rae et al., 2011), but scarce data exist on B/Ca in coralline algae, and its driving factors. Recent experiments on cultured Neogoniolithon sp. showed an inverse relationship between seawater dissolved inorganic carbon (DIC) and B/Ca (Donald et al., 2017). B and Ca contents were measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in the maerl species Lithothamnion corallioides (P. Crouan & H. Crouan) P. Crouan & H. Crouan 1867, which has a wide geographic distribution in European waters. Samples were collected in the Mediterranean Sea (Pontian Islands, 66 m depth; Elba, 45 m depth and Aegadian Islands, 40 m depth), and in the NE Atlantic Ocean (Morlaix Bay, 12 m depth). DIC data from the sampling sites have been extracted by CMEMS (E.U. Copernicus Marine Service Information) products, and the Kruskal-Wallis test was run in R 3.6.3 software to evaluate B/Ca differences among samples. The B/Ca ratio showed significant differences among samples (p<0.01). The mean DIC was lower in the Atlantic Ocean, where the alga registered high B/Ca values. In Pontian Isl., the deepest site, the B/Ca was significantly lower than the other Mediterranean sites (p<0.01), despite very similar DIC. The measurements on growth rates resulted in a lower rate in Pontian Isl. (0.10 mm/yr), compared to the other samples (~0.13 mm/yr), likely related to the variations in light availability with depth (Halfar et al., 2011). In general, the differences in growth rates among samples explained the shifts between DIC results and the expected B/Ca (Donald et al., 2017). In the light of this evidence, we suggest a growth rate influence on the B incorporation in this alga. As known for other geochemical signals, the B/Ca is therefore subject to algal growth patterns which should be carefully investigated in the attempt to reconstruct the paleoclimate.