Magnesium and strontium distributions within valves of a recent marine ostracode, Neonesidea oligodentata: Implications for paleoenvironmental reconstructions

Abstract

Ostracodes are bivalved microcrustaceans that live in nearly all types of aquatic environments. The chemical compositions of the calcified part of their carapaces are frequently used as an empirical proxy in paleoenvironmental reconstructions. We investigated the compositions and the distribution of Mg and Sr within valves of a recent ostracode, Neonesideaoligodentata, using an electron microprobe analyzer. Each of the investigated valves displayed large variations in Mg and Sr concentrations compared to the variations in Ca (and probably O). There were no systematic chemical differences between right and left valves of each carapace, and between male and female carapaces. Although there were some deviations, the valves consisted of, at least, three chemical bands: (1) a narrow (usually <10 μm) high‐Mg, Sr outer band (Outer Band), (2) a heterogeneous low‐Mg, Sr middle band (Middle Band), and (3) a high‐Mg, low‐Sr inner band (Inner Band). The high‐Mg, low‐Sr Inner Band was well developed at the inner part of the margin, including the marginal infold. The high‐Mg, Sr Outer Band is thought to be formed during the early stage of carapace formation when the calcification rate is high, whereas other parts formed at later stages with a relatively slow calcification rates. Since bairdioidean ostracodes, exemplified by Neonesidea oligodentata, have a long geological history and are abundant in many regions, further studies on the mechanism of the Mg/Ca distribution in these ostracodes will provide a suitable tool to estimate paleotemperatures in various marine environments.