ARE SOME SPECIES BETTER PROXY INDICATORS THAN OTHERS? SOME CASE STUDIES FROM OSTRACODA

In the past two decades, ostracodes have become important proxy indicators for reconstructing previous environmental and climatic conditions, particularly in nonmarine settings. This has been possible because many ostracodes have specific physical and chemical tolerances and are thus highly responsive to environmental change. Their responses often include changes in biogeographic distribution, shell morphology and shell chemistry. Despite this utility, there is still the question of whether or not some species of ostracodes make better proxy indicators than others and why.

We examined ostracode faunas from two field areas: San Salvador Island, Bahamas and Lake Issyk-Kul, Kyrgyzstan to determine the geochemical variability of various ostracode species within these systems. On San Salvador, we sampled eleven saline lakes. Of the three species analyzed (Cyprideis americana, Hemicyprideis setipunctata and Perissocytheridea bicelliforma) P. bicelliforma had a different uptake ratio for Mg and possibly Sr than the other two species. In addition, there was a negative trend and decrease in variance between [Mg/Ca_(valve)/Mg/Ca_(water)] and mean lake salinity. Within species, the males and females of C. americana have different Mg/Ca ratios but not Sr/Ca ratios, suggesting that there is a gender difference in uptake ratios.

In Lake Issyk-Kul, preliminary stable isotope analyses comparing the d¹⁸O composition of C. neglecta to C. caudata and C. candida from several depths in a piston core (core #IK00-11P) indicate that while there is little difference between the d¹⁸O composition of C. neglecta and C. caudata, coeval specimens of C. candida may be up to 0.3‰ more positive than C. neglecta. Others workers have not noted this difference between members of Candona. While our results may be due to our limited data set, it suggests that there may be a species-specific geochemical signature for different members of the Candona clade. This is also suggested in the trace element data for the different genders of C. americana. Thus, ostracode species have a varied responses to geochemical conditions and therefore, it is important to choose the least labile species as an environmental proxy.