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<p>Giant clams (<i>Tridacna</i>) are the largest marine bivalves, and their carbonate shells can be used for high-resolution paleoclimate reconstructions. In this contribution, <span class="inline-formula"><i>δ</i>¹⁸O</span><span class="inline-formula">_shell</span> was used to estimate climatic variation in the Xisha Islands of the South China Sea. We first evaluate sea surface temperature (SST) and sea surface salinity (SSS) influence on the modern resampled monthly (<span class="inline-formula"><i>r</i></span>-monthly) resolution of <i>Tridacna gigas</i> <span class="inline-formula"><i>δ</i>¹⁸O</span><span class="inline-formula">_shell</span>. The results obtained reveal that <span class="inline-formula"><i>δ</i>¹⁸O</span><span class="inline-formula">_shell</span> seasonal variation is mainly controlled by SST and appears to be insensitive to local SSS change. Thus, the <span class="inline-formula"><i>δ</i>¹⁸O</span> of <i>Tridacna</i> shells can be roughly used as a proxy of local SST: a 1&thinsp;‰ <span class="inline-formula"><i>δ</i>¹⁸O</span><span class="inline-formula">_shell</span> change is roughly equal to 4.41&thinsp;<span class="inline-formula">^∘</span>C of SST. The <span class="inline-formula"><i>r</i></span>-monthly <span class="inline-formula"><i>δ</i>¹⁸O</span> of a 40-year-old <i>Tridacna</i> <i>squamosa</i> (<span class="inline-formula">3673±28</span>&thinsp;BP) from the North Reef of the Xisha Islands was analyzed and compared with the modern specimen. The difference between the average <span class="inline-formula"><i>δ</i>¹⁸O</span> of the fossil <i>Tridacna</i> shell (<span class="inline-formula"><i>δ</i>¹⁸O</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>-</mo><mn mathvariant="normal">1.34</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="2413dcffb35407d3792466e52a1cd2e8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-16-597-2020-ie00001.svg" width="41pt" height="10pt" src="cp-16-597-2020-ie00001.png"/></svg:svg></span></span>&thinsp;‰) and the modern <i>Tridacna</i> specimen (<span class="inline-formula"><i>δ</i>¹⁸O</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>-</mo><mn mathvariant="normal">1.15</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="c2dddad30e09b42cac8a198c1a8d7426"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-16-597-2020-ie00002.svg" width="41pt" height="10pt" src="cp-16-597-2020-ie00002.png"/></svg:svg></span></span>&thinsp;‰) probably implies a warm climate, roughly 0.84&thinsp;<span class="inline-formula">^∘</span>C, 3700 years ago. The seasonal variation 3700 years ago was slightly lower than that suggested by modern instrumental data, and the transition between warm and cold seasons was rapid. Higher amplitudes of reconstructed <span class="inline-formula"><i>r</i></span>-monthly and <span class="inline-formula"><i>r</i></span>-annual SST anomalies imply an enhanced climate variability during this warm period. Investigation of the El Ninõ–Southern Oscillation (ENSO) variation (based on the reconstructed SST series) indicates reduced ENSO frequency but increased ENSO-related variability and extreme El Ninõ winter events 3700 years ago.</p>