Seasonal Mg/Ca-inferred temperatures of brackish water ostracods

The common brackish water ostracods Cyprideis torosa and Loxoconcha elliptica frequently occur together in high abundances in marginal marine environments. Seasonality of calcification differs between species and can have important implications for palaeotemperature reconstructions. There are existing palaeotemperature calibrations for both genera. However, the Loxoconcha spp. Mg/Ca temperature calibration has not thus far been applied to L. elliptica. The equation for Loxoconcha spp. does not rely on a known Mg/Ca_water value, unlike the calibration for C. torosa, suggesting it may be possible to reconstruct temperatures without an estimation of Mg/Ca_water, which is potentially particularly beneficial in environments that have highly dynamic Mg/Ca_water. However, the calibration has only been applied in environments with marine-like Mg/Ca_water. Demonstrating the applicability of the equation in marginal marine environments (with Mg/Ca_water 3–5 mol/mol) and tracking the seasonal calcification of L. elliptica alongside C. torosa, therefore, has the potential to improve uncertainty in seasonal palaeotemperature reconstructions. Here, we compare previous monitoring of C. torosa with L. elliptica from the same collections. We demonstrate that the Mg/Ca temperature calibration for Loxoconcha spp. is appropriate to use with L. elliptica. Mg/Ca-inferred temperatures broadly track spring temperatures and suggest spring calcification. Cyprideis torosa Mg/Ca-inferred temperatures record the range of expected temperatures between spring and autumn. When analysing multiple single valves of L. elliptica and C. torosa simultaneously, the maximum Mg/Ca_C.torosa can, therefore, be used to reconstruct maximum summer temperatures, the minimum Mg/Ca_C.torosa to reconstruct autumn temperatures, and the Mg/Ca_L.elliptica to reconstruct the range in spring temperatures.