Changes in continental ostracode shell chemistry; uncertainty of cause

Ostracode shell chemistry is a powerful tool for recording changes of certain dissolved ions in their environment. Such data are commonly interpreted in terms of change in temperature and salinity, which are then used to offer insights into paleohydrology and from that paleoclimate. In particular, the changes in Sr/Ca_shell are believed to reflect changes in salinity, and the changes in Mg/Ca_shell to reflect both salinity and water temperature. However, the established application of this proxy too often ignores the complexities involved in the chemical changes accompanying solute evolution, hydrologic change, physical chemistry, and the autoecology and biology of ostracodes. Chemical changes occur: (1) in the dissolved Mg/Ca and Sr/Ca ratios during solute evolution, (2) as a consequence of multiple sources of water, but especially ground and surface water interchange as well as evaporation, (3) as a consequence of different dissolved cation-anion pair formation at various concentrations, and (4) because of bio/ecologic effects, including seasonality of molting and taxa-specific ionic regulation capabilities. We suggest that changes in shell chemistry in a stratigraphic sequence tell us when changes occurred in the solutes, but cannot identify what caused those change(s) in the chemical environment. Inclusion of information from other paleoenvironmental and paleoclimatic proxies that co-occur with ostracode shells may help to limit the choices amongst possible causes.