Abstract

Brachiopods have been widely used as proxies to reconstruct ancient oceanographic conditions based on the assumption that their shell calcite is precipitated in near isotopic equilibrium with ambient seawater. Few studies, however, have tested the presumption of equilibrium precipitation for specimens from extreme polar environments. Furthermore, reported isotopic values for polar specimens are largely indicative of disequilibrium precipitation, leading to the conclusion that brachiopods living in extreme environments may be poor recorders of ambient oceanography. The results of shell chemistry of the Antarctic brachiopod Liothyrella uva (L. uva) are compared to the local oceanographic data at Cape Armitage, McMurdo Sound, to assess the suitability of extreme environment brachiopods as environmental proxies. Results reveal that significant kinetic fractionation occurs during primary-layer shell secretion, resulting in whole-shell isotopic compositions that do not reflect equilibrium with ambient seawater. Secondary-layer shell calcite, however, is less affected by biological fractionation and exhibits isotopic compositions that fall within the range of predicted equilibrium values. Additionally, whole-shell concentrations of elements including Ba, Cd, Cr, Fe, Mn, Na, Sr, and Zn exhibit trends that are interpreted to reflect their relative concentrations in ambient seawater. Concentrations of Mg and B, however, are found to be largely controlled by physiological processes related to brachiopod growth rate. While this study concludes that the shell chemistry of L. uva does reflect the local oceanographic conditions at McMurdo Sound, the reliability of extreme-environment brachiopods should be assessed on a species basis as differences in brachiopod physiology and microstructure can significantly influence the degree of equilibrium reflected in shell calcite.