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<p>The <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="815fcaa8810ee07fc362ef95c42bf7cf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00007.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00007.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Sr</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e8cf9285cd2a29961028b2b379e5bb4f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00008.svg" width="33pt" height="14pt" src="bg-18-1987-2021-ie00008.png"/></svg:svg></span></span> ratios of marine shells have been widely used in environmental paleoreconstructions to understand past marine conditions. Temperature calibrations to ostracod <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="6fecc37bb20730e40ab6853772ac401b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00009.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00009.png"/></svg:svg></span></span> 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, <i>Sinocytheridea impressa</i> and <i>Neomonoceratina delicata</i>, 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="f8b6609c3cfdcd2d94fcafe3f14b5b00"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00010.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00010.png"/></svg:svg></span></span> ratios of both species to the annual (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="d1f58fc3a76bb75dfaa8c6e5d7932caa"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00011.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00011.png"/></svg:svg></span></span><span class="inline-formula">_{<i>S. impressa</i>}=</span>3.7 <span class="inline-formula">⋅</span> <span class="inline-formula"><i>T</i>−62.7</span>; <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="ba1c13ec1a2f7c9d61dc1e30484e1e0a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00012.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00012.png"/></svg:svg></span></span><span class="inline-formula">_{<i>N. delicata</i>}=1.6</span> <span class="inline-formula">⋅</span> <span class="inline-formula"><i>T</i>−16.8</span>) and April (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="92be5ea42e05a3e49a765741d834c1a7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00013.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00013.png"/></svg:svg></span></span><span class="inline-formula">_{<i>S. impressa</i>}=</span>2.8 <span class="inline-formula">⋅</span> <span class="inline-formula"><i>T</i>−39.2</span>; <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e1c6bc0c11d24c196e22ff4718eeeecf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00014.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00014.png"/></svg:svg></span></span><span class="inline-formula">_{<i>N. delicata</i>}=</span>1.6 <span class="inline-formula">⋅</span> <span class="inline-formula"><i>T</i>−15.7</span>) temperatures. The correlation of temperature to the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="4e1d35ef4f243de67764b7f703759b1e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00015.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00015.png"/></svg:svg></span></span> ratio of <i>S. impressa </i>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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="5240528fe19271b9d3b0ff18509ad340"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00016.svg" width="37pt" height="14pt" src="bg-18-1987-2021-ie00016.png"/></svg:svg></span></span> of <i>S. impressa</i> and <i>N. delicata</i> 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. <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Sr</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="fcb882a35b990a83e39588a10abaf6f8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1987-2021-ie00017.svg" width="33pt" height="14pt" src="bg-18-1987-2021-ie00017.png"/></svg:svg></span></span> 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 <span class="inline-formula">^∘</span>C), salinity (23.8–33.7 PSU), dissolved oxygen (4.3–7.1 mg L<span class="inline-formula">⁻¹</span>), suspended solids (1.9–35.4 mg L<span class="inline-formula">⁻¹</span>) and pH (7.7–8.2).</p>