Deconvolving the biogeochemical controls on coral Sr ∕ Ca and Ba ∕ Ca proxies: new perspectives from paired stable Ca, Sr and Ba isotope compositions

This study introduces a novel approach to disentangle the biogeochemical controls on <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">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="b9671c4809874f370bab27d3c7829fa1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00007.svg" width="33pt" height="14pt" src="bg-23-3195-2026-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">Ba</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="36pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="ed113f79720d60f60681160ab4411116"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00008.svg" width="36pt" height="14pt" src="bg-23-3195-2026-ie00008.png"/></svg:svg></span></span> signatures in coral skeletons using paired stable Ca, Sr and Ba isotopes to assess their specific uptake dynamics during coral biomineralization. The observed seasonal variations in stable Ca (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi mathvariant="italic">δ</mi><mrow><mn mathvariant="normal">44</mn><mo>/</mo><mn mathvariant="normal">42</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8c9ab176f741ebbff056fc7e2cac8265"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00009.svg" width="32pt" height="14pt" src="bg-23-3195-2026-ie00009.png"/></svg:svg></span></span>Ca) and Sr isotopes (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi mathvariant="italic">δ</mi><mrow><mn mathvariant="normal">88</mn><mo>/</mo><mn mathvariant="normal">86</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="7ed313445203f6458bcdad28ae1839db"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00010.svg" width="32pt" height="14pt" src="bg-23-3195-2026-ie00010.png"/></svg:svg></span></span>Sr) underscore the capability of corals to actively mediate the transport of Ca<span class="inline-formula">²⁺</span> and Sr<span class="inline-formula">²⁺</span> ions to the calcifying fluid prior to aragonite precipitation. We suggest that while the individual concentrations of Ca and Sr in the calcifying fluid vary seasonally, the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" 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="7a25d1af185cb50eba192ad4c421755f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00011.svg" width="33pt" height="14pt" src="bg-23-3195-2026-ie00011.png"/></svg:svg></span></span> ratio of the fluid is likely comparable to that of seawater due to similar ion uptake dynamics. In contrast, the observed coral stable Ba isotope compositions (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi mathvariant="italic">δ</mi><mrow><mn mathvariant="normal">138</mn><mo>/</mo><mn mathvariant="normal">134</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="40pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="ca3f520676cc8c529481b6d1d32f4cb4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00012.svg" width="40pt" height="14pt" src="bg-23-3195-2026-ie00012.png"/></svg:svg></span></span>Ba) remain essentially constant, suggesting a passive transport mechanism of Ba<span class="inline-formula">²⁺</span> ions, possibly through direct seawater leakage. The contrasting ion transport behaviours of Ba and Ca elucidate the underlying cause of the temperature-dependent variations in coral <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Ba</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="36pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0a2a4dea70a0c9544728a0498a216c70"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00013.svg" width="36pt" height="14pt" src="bg-23-3195-2026-ie00013.png"/></svg:svg></span></span> records. By evaluating the uptake dynamics of Ca, Sr and Ba via their respective isotope systems, this study provides useful implications for the accurate application of coral <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" 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="12e8b7fafaea969e93efa36876130d92"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00014.svg" width="33pt" height="14pt" src="bg-23-3195-2026-ie00014.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Ba</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="36pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="54a5f6dcac65f890aaad756bb858e7be"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-23-3195-2026-ie00015.svg" width="36pt" height="14pt" src="bg-23-3195-2026-ie00015.png"/></svg:svg></span></span> as proxies for paleoclimate reconstructions.