Abstract. Shells of Cyprideis, a widespread euryhaline ostracod, have commonly been used in geochemical investigations involving determinations of trace elements (especially magnesium and strontium) and isotopes (of oxygen, carbon and strontium). In this paper, we evaluate geochemical signatures in Cyprideis based on new and previously published data. Mg/Ca and Sr/Ca determinations of fossil shells that calcified in marine-type water have potential to reconstruct palaeotemperature and past water composition using empirical relationships derived from living ostracods recovered from in vitro cultures or natural settings. For shells that calcified in non-marine waters of contrasting composition, partitioning of trace metals from water into ostracod shells may differ, meaning that relationships developed for marine waters do not apply. However, variations in Mg/Ca and Sr/Ca in Cyprideis in continental settings may still provide valuable palaeohydrological information. Determinations of oxygen isotopes in Cyprideis shells are consistent with positive offsets from equilibrium, in common with other ostracod taxa: carbon-isotope values reflect the fact that Cyprideis is a detritivore. Oxygen-isotope analyses of Cyprideis shells from continental settings provide important palaeohydrological information. Strontium-isotope analyses of Cyprideis shells provide valuable records of mixing of marine and continental water in marginal-marine settings. Geochemical analyses of different morphotypes of Cyprideis lend support to suggestions that ecophenotypic variations are controlled by factors other than, or additional to, salinity.

Supplementary material: descriptions of sites, materials and methods for previously unpublished studies are available at https://doi.org/10.6084/m9.figshare.c.3248710.v1