Magnesium in the lattice of calcite-shelled brachiopods

Abstract

Palaeoclimate information is often extracted from Rhynchonelliform brachiopod shell calcite, in particular the inner secondary layer, via the δ18O composition, which is a proxy for seawater temperature. Compared to δ18O, the potential for Mg/Ca ratio, as a brachiopod seawater temperature proxy, has been neglected. The use of Mg/Ca ratio as a temperature proxy assumes that, with increasing temperature, more Mg substitutes for Ca in the calcite lattice. Brachiopod shells, like all biominerals, are composites of organic and inorganic components. This raises the possibility that magnesium is hosted by the organic components. Alternatively, magnesium may be present as a separate mineral phase, rather than a true component of the calcite lattice, or incorporated into calcite in non-ideal or variable coordination. Here we use synchrotron X-ray absorption Near Edge Spectroscopy (XANES) at the Mg K-edge to determine the local environment of magnesium in two species of brachiopod with low Mg-calcite shells, Terebratulina retusa and Notosaria nigricans and one species with a high Mg-calcite shell, Novocrania anomala. XANES at the Mg K-edge of a suite of Mg-bearing standards fingerprints the local environment of magnesium in the brachiopod shell powders as well as in situ analyses. In all cases, it is evident that magnesium is not hosted by organic components but is within the inorganic component of the shell. These data support the possibility of using brachiopod Mg/Ca ratios as a temperature proxy.