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. Giant clams (Tridacna) are the largest marine bivalves, and their carbonate shellscan be used for high-resolution paleoclimate reconstructions. In thiscontribution, δ18O shell was used to estimate climaticvariation in the Xisha Islands of the South China Sea. We first evaluate seasurface temperature (SST) and sea surface salinity (SSS) influence on themodern resampled monthly ( r -monthly) resolution of Tridacna gigas δ18O shell . The results obtained reveal that δ18O shell seasonal variation is mainly controlled by SST andappears to be insensitive to local SSS change. Thus, the δ18O of Tridacna shells can be roughly used as a proxy of local SST: a 1 ‰ δ18O shell change is roughly equal to4.41  ∘ C of SST. The r -monthly δ18O of a 40-year-oldTridacna squamosa ( 3673±28  BP) from the North Reef of the Xisha Islands was analyzedand compared with the modern specimen. The difference between the average δ18O of the fossil Tridacna shell ( δ18O = - 1.34  ‰) and the modern Tridacna specimen ( δ18O = - 1.15  ‰) probably implies a warm climate, roughly0.84  ∘ C, 3700 years ago. The seasonal variation 3700 years agowas slightly lower than that suggested by modern instrumental data, and thetransition between warm and cold seasons was rapid. Higher amplitudes ofreconstructed r -monthly and r -annual SST anomalies imply an enhanced climatevariability during this warm period. Investigation of the ElNino–Southern Oscillation (ENSO) variation (based on the reconstructedSST series) indicates reduced ENSO frequency but increased ENSO-relatedvariability and extreme El Nino winter events 3700 years ago.

climate of the past

Evidence from giant-clam &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O of intense El Ninõ–Southern Oscillation-related variability but reduced frequency 3700 years ago