Evidence from giant-clam δ¹⁸O of intense El Ninõ-Southern Oscillation-related variability but reduced frequency 3700 years ago

Giant clams (Tridacna) are the largest marine bivalves, and their carbonate shells can be used for highresolution paleoclimate reconstructions. In this contribution, δ¹⁸Oshell 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 (r-monthly) resolution of Tridacna gigas δ¹⁸O_shell. The results obtained reveal that δ¹⁸Oshell seasonal variation is mainly controlled by SST and appears 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% δ¹⁸Oshell change is roughly equal to 4.41 °C of SST. The r-monthly δ¹⁸O of a 40-year-old Tridacna squamosa (3673±28 BP) from the North Reef of the Xisha Islands was analyzed and compared with the modern specimen. The difference between the average δ¹⁸O of the fossil Tridacna shell (δ¹⁸O D -1:34 %) and the modern Tridacna specimen (δ¹⁸O D -1:15 %) probably implies a warm climate, roughly 0.84 °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 r-monthly and r-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.