Mg/Ca and Sr/Ca ratios of Limnocythere inopinata ostracod valves from the Badain Jaran Desert, Northwest China and their implications for paleoenvironmental reconstruction

Abstract

The trace element composition, especially the Mg/Ca and Sr/Ca ratios, of ostracod valves are commonly used as proxies for paleoenvironmental reconstruction; however, the effects of hydrochemical parameters on these compositions remain uncertain. In this study, we report Mg/Ca_valve and Sr/Ca_valve ratios of Limnocythere inopinata, a common and eurytopic ostracod species in the Holarctic, and corresponding water chemistry data from 21 lakes in the Badain Jaran Desert of Northwest China. These selected lakes are distributed in close proximity and have similar water temperatures, allowing us to analyse the influences of water chemistry on the valve partition coefficients separately. The results show that Sr/Ca_valve ratio has a good linear correlation with Sr/Ca_water ratio and that K_d[Sr] is little affected by hydrochemical parameters. These results indicate that Sr enters ostracod valves via isomorphism (a certain proportion of Ca²⁺ in low-Mg calcite crystals is replaced by Sr²⁺). Mg/Ca_water and alkalinity are the two main factors affecting the K_d[Mg] of L. inopinata valves. Increases in the Mg/Ca ratio and alkalinity of host water enhance the calcification of valves, which leads to a decrease in the proportion of organic matter in ostracod valves. This process is not conducive to the preservation of Mg in ostracod valves. In addition, comparative analysis of the data from different areas indicated that the K_d[Mg] and K_d[Sr] values of L. inopinata valves have similar relationships with hydrochemical parameters in different areas, which is favourable for reconstructing the paleoenvironment by using the trace element compositions of L. inopinata valves.