Calcium, magnesium and strontium concentrations in the calcite sclerites of Mediterranean gorgonians (Coelenterata: Octocorallia)

The mineralogical analysis of the sclerites of four Mediterranean gorgonians revealed magnesium calcite as the only calcium carbonate polymorph. The Mg/Ca ratio (0•064–0•098) and the Sr/Ca ratio (0•0014–0•0025) in the sclerites of the investigated species were the lowest ever reported in octocoral calcite. Calcium concentrations generally did not vary with water depth or colony region, whereas there was a high intraspecific variation of Mg/Ca and Sr/Ca ratios related to water depth and colony region. This variation of the Mg/Ca and Sr/Ca ratios in the sclerites cannot be explained by the chemical composition of the seawater, as the Mg/Ca and Sr/Ca ratios of seawater were constant over the depths and seasons investigated (4•0 ± 0•04 and 0•0084 ± 0•00015). The comparison with inorganic precipitated calcite revealed a biological enrichment of Mg and a discrimination against Sr in octocoral calcite. Our data do not support the hypothesis that the Mg concentration is influenced by the growth rate. According to our data on Mg concentrations in gorgonian sclerites and ambient water temperature, the relationship between Mg content in octocorals and ambient water temperature derived by Chave 1954 should probably be revised. A positive correlation between Mg and Sr values in octocoral calcite (including data from literature) intimates that Mg is an important factor influencing the incorporation of Sr into the calcium carbonate matrix. No correlation between Sr concentration and temperature was found, however, growth rate is discussed as a potential factor influencing Sr concentrations.