Abstract
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).
Highlights
Element / calcium ratios (E/Ca) of secreted biogenic calcium carbonate by marine organisms, such as foraminifera and corals, have been used as proxies for past environmental marine parameters (Hendy et al, 2002; Lea, 2003; Lea and Boyle, 1989; Linsley et al, 2000; Martin et al, 2002)
We investigated adult specimens of S. impressa (Brady, 1869; Whatley and Zhao, 1988) and N. delicata (Ishizaki and Kato, 1976) from the uppermost 1 cm sediment layer collected in Hong Kong (HK) by the Environmental Protection Department of Hong Kong (EPD) in January and July 2012 (Hong et al, 2019; Rodriguez, 2021)
The strongest linear correlation between the 24 parameters (Table S1) measured by the EPD and Mg/Ca ratio was for the annual mean temperature, which suggests that water temperature is the main control of Mg/Ca uptake in adult ostracods of S. impressa and N. delicata (Fig. 2)
Summary
Element / calcium ratios (E/Ca) of secreted biogenic calcium carbonate by marine organisms, such as foraminifera and corals, have been used as proxies for past environmental marine parameters (Hendy et al, 2002; Lea, 2003; Lea and Boyle, 1989; Linsley et al, 2000; Martin et al, 2002). Mg/Ca of ostracod shells has provided valuable information about water temperatures (Chivas et al, 1983, 1986a; De Deckker and Forester, 1988; Dwyer et al, 2002). Other studies have investigated species found in shallow marine environments, such as those from the genus Loxoconcha (Cronin et al, 2003, 2005a) and Cyprideis (Holmes and De Deckker, 2016; Roberts et al, 2020). There is uncertainty in temperature calibrations to Mg/Ca of marine ostracods due to the low number of calibrations developed in comparison to the number of known species (Holmes and De Deckker, 2012) and the contrasting cali-
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