CALCIUM, MAGNESIUM AND STRONTIUM CONCENTRATIONS IN THE CALCITE SCLERITES OF MEDITERRANEAN GORGONIANS (COELENTERATA, OCTOCORALLIA)

The mineralogical analysis of the sclerites of four Mediterranean gorg onians revealed magnesium calcite as the only calcium carbonate polymo rph. The Mg/Ca ratio (0064-0.098) and the Sr/Ca ratio (0.0014-0.0025) in the sclerites of the investigated species were the lowest ever repo rted in octocoral calcite. Calcium concentrations generally did not: v ary with water depth or colony region, whereas there was a high intras pecific 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 sc lerites cannot be explained by the chemical composition of the seawate r, as the Mg/Ca and Sr/Ca ratios of seawater were constant over the de pths and seasons investigated (4.0 +/- 0.04 and 0.0084 +/- 0.00015). T he comparison with inorganic precipitated calcite revealed a biologica l enrichment of Mg and a discrimination against Sr in octocoral calcit e. Our data do not support the hypothesis that the Mg concentration is influenced by the growth rate. According to our data on Mg concentrat ions in gorgonian sclerites and ambient water temperature, the relatio nship between Mg content in octocorals and ambient water temperature d erived by Chave (1954) should probably be revised. A positive correlat ion 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 ra te is discussed as a potential factor influencing Sr concentrations.