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

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  • Morishita Tomoaki
    Frontier Science Organization, Kanazawa University/Department of Geology & Geophysics, University of Hawaii at Manoa
  • Yamaguchi Tatsuhiko
    Postdoctoral Fellowship for Research Abroad, Japan Society for the Promotion of Science/Scripps Institution of Oceanography, University of California, San Diego
  • Mashiba Hisakazu
    Division of Earth and Environmental Sciences, Graduate School of Natural Science and Technology, Kanazawa University
  • Kamiya Takahiro
    Division of Earth and Environmental Sciences, Graduate School of Natural Science and Technology, Kanazawa University

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Other Title
  • 貝形虫の殻のMg/Ca比，Sr/Ca比による古環境推定の現状と問題点
  • カイケイチュウ ノ カク ノ Mg Caヒ Sr Caヒ ニ ヨル コ カンキョウ スイテイ ノ ゲンジョウ ト モンダイテン

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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 the paleoenvironment. The Mg/Ca ratios and partition coefficients of marine ostracodes generally correlate with the water temperature of the ostracode habitat, and the Mg/Ca and Sr/Ca ratios of freshwater ostracodes have been used as a proxy for water chemistry. We reviewed previous studies on the Mg/Ca and Sr/Ca ratios of ostracode valves and obtained the following findings. (1) Diagenesis is likely to affect the Mg/Ca and Sr/Ca ratios of subfossil and fossil valves ; however, it is difficult to assess the degree of diagenetic alteration using methods developed in previous studies, such as that based on the visual preservation index (VPI) . Our statistic analysis, using published data, revealed no significant correlation between VPI and the Mg/Ca or Sr/Ca ratios of artificial dissolved specimens. (2) The partition coefficients of Mg/Ca and Sr/Ca ratios vary with the taxa, habitat, and phyletic group. Cypridoidea have statistically higher Mg/Ca and lower Sr/Ca partition coefficients than do Bairdioidea and Cytheroidea. Compared with marine taxa, freshwater taxa have more variable partition coefficients. The Mg/Ca partition coefficient of marine ostracodes is clearly dependent on the phyletic group at the family level. (3) The Mg/Ca and Sr/Ca partition coefficients are independent of sex but may depend on ontogenic stage. (4) The variable nature of partition coefficients in marine species indicates that Mg/Ca thermometers have an error of ±2–4°C. (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 temperature and water chemistry would be improved if the effects of diagenesis and spatial heterogeneity are evaluated quantitatively and if the partition coefficients of various taxa are available.

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