Mineralogy and Sr–Mg geochemistry of charophyte carbonates: a new tool for paleolimnological research

The carbonate biominerals formed in the oogonial cells and on stems of charophytes reveal intra-specific and species-to-species variations within the genus Chara. Calcite, high-magnesian calcite (HMC) and aragonite may be present in both oogonium and stem calcifications, and for the first time evidence is presented of polymineral calcification (HMC+aragonite) within single oogonial cells. The data indicate strong specific differences in the biological mechanisms of mineralization within this group of plants. Among the various species of the genus Chara studied, there is no correlation between the charophyte carbonate mineralogy and the salinity of water where they formed: calcite, HMC and aragonite may all be present in charophyte calcifications formed in low-salinity waters. On the contrary, there is an overall relationship between the mineralogy of these carbonates and the Mg/Ca ratio of the water in which they formed. Sr content in charophyte calcite and high Mg–calcite reflect directly the Sr/Ca ratio of the host water, whereas Mg content in these carbonates probably depends on the temperature and the Mg/Ca ratio of the water. Partitioning coefficients for Mg and Sr for some widespread species are discussed. Our results indicate that charophyte carbonates may be used as paleohydrochemical proxies and imply that special attention must be paid to the mineralogical control of charophyte carbonates when they are to be used in isotopic studies.