A review of the utility of Mg/Ca and Sr/Ca ratios of ostracode valves as a tool for paleoenvironmental reconstructions

Abstract

Ostracoda, a crustacean group, have valves composed of low-magnesium calcite. The magnesium/calcium（Mg/Ca）and strontium/calcium（Sr/Ca）ratios of fossil ostracodes have been used as indicators of thepaleoenvironment. The Mg/Ca ratios and partition coefficients ofmarine ostracodes generally correlate with the water temperature ofthe ostracode habitat, and the Mg/Ca and Sr/Ca ratios of freshwaterostracodes have been used as a proxy for water chemistry. Wereviewed previous studies on the Mg/Ca and Sr/Ca ratios of ostracodevalves and obtained the following findings.（1）Diagenesis is likely toaffect the Mg/Ca and Sr/Ca ratios of subfossil and fossil valves ； however,it is difficult to assess the degree of diagenetic alteration usingmethods developed in previous studies, such as that based on the visualpreservation index（VPI）. Our statistic analysis, using publisheddata, revealed no significant correlation between VPI and the Mg/Ca orSr/Ca ratios of artificial dissolved specimens.（2）The partition coefficientsof Mg/Ca and Sr/Ca ratios vary with the taxa, habitat, andphyletic group. Cypridoidea have statistically higher Mg/Ca and lowerSr/Ca partition coefficients than do Bairdioidea and Cytheroidea. Comparedwith marine taxa, freshwater taxa have more variable partitioncoefficients. The Mg/Ca partition coefficient of marine ostracodes isclearly dependent on the phyletic group at the family level.（3）TheMg/Ca and Sr/Ca partition coefficients are independent of sex but maydepend on ontogenic stage.（4）The variable nature of partition coefficientsin marine species indicates that Mg/Ca thermometers have anerror of ± 2－4 ℃.（5）Spatial variations in Mg concentration within an individual valve have an effect on the Mg/Ca ratio of the whole valve.The accuracy of Mg/Ca and Sr/Ca ratios as proxies for temperatureand water chemistry would be improved if the effects of diagenesisand spatial heterogeneity are evaluated quantitatively and if the partitioncoefficients of various taxa are available.