Calibration of Mg ∕ Ca and Sr ∕ Ca in coastal marine ostracods as a proxy for temperature

Abstract

The Mg/Ca and Sr/Ca ratios of marine shells have been widely used in environmental paleoreconstructions to understand past marine conditions. Temperature calibrations to ostracod Mg/Ca ratios are known to be species-specific but only available for a few species, despite the large number of known ostracod species. Here, we develop temperature calibrations for two shallow marine ostracods, Sinocytheridea impressa and Neomonoceratina delicata, using modern sediment samples. Our results show that adult specimens of these two species might be useful as a paleothermometer. We observed significant correlations using the Mg/Ca ratios of both species to the annual (Mg/CaS. impressa=3.7 ⋅ T−62.7; Mg/CaN. delicata=1.6 ⋅ T−16.8) and April (Mg/CaS. impressa=2.8 ⋅ T−39.2; Mg/CaN. delicata=1.6 ⋅ T−15.7) temperatures. The correlation of temperature to the Mg/Ca ratio of S. impressa is more significant and therefore should be preferred for paleoreconstructions. Re-analysis from satellite data allows us to validate our temperature calibration to an extended area around the Pearl River estuary. Our results show that Mg/Ca of S. impressa and N. delicata ostracods can be used to reconstruct water temperature at a regional scale, which provides information on the oceanic circulation in coastal areas of the South China Sea. Sr/Ca ratios of both species do not correlate with any of the 24 water parameters recorded by the Environmental Protection Department of Hong Kong, including temperature (21.7–24.1 ∘C), salinity (23.8–33.7 PSU), dissolved oxygen (4.3–7.1 mg L−1), suspended solids (1.9–35.4 mg L−1) and pH (7.7–8.2).