Calibration of past water temperature in the Sea of Japan based on Mg/Ca of ostracode shells of two shallow marine species in the genus Cytheropteron

Compared to Mg/Ca-temperature calibrations in deep-sea ostracodes and the use of these calibrations to understand past conditions, there have been few studies of shallow marine ostracode taxa. In the present study, Mg/Ca-temperature calibrations were constructed for two shallow marine ostracode species belonging to the genus Cytheropteron in the recent sediments of the Sea of Japan in order to reconstruct the past water temperature record in shallow sea areas. The Mg/Ca of adult Cytheropteron miurense and Cytheropteron sawanense shells from nine bottom sediment samples was positively correlated with the mean annual water temperature (WT) according to the following equation: WT (°C) = Mg/Ca of ostracode shell (mmol/mol) ∗ 0.94–9.53. Comparison of the regression curves of Mg/Ca versus WT for six genera showed similar temperature sensitivity of Mg uptake in shells of Cytheropteron, Sarsicytheridea and the genus Krithe in most cases, but lower than those for genera Bythocypris and Loxoconcha and Xestoleberis hanaii. The regression equation was applied to Late Pliocene and the resulting past water temperatures were compared to modern oceanographic data from water depths of 50 to 150 m in the Sea of Japan, which is the paleodepth estimated to encompass the habitat of ostracode assemblages. Decreasing variation in the obtained past WT with the increase in the number of fossil ostracode valves from one to four suggests that more than four ostracode valves are required to reconstruct the mean annual WT using the equation established with C. miurense and C. sawanense. Thus, the mean annual WT during the Late Pliocene based on analysis of four specimens ranged from 8.9 to 11.6 °C, which is similar to or slightly lower than the WT observed today.