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

Giant clams (Tridacna) are the largest marine bivalves, and their carbonate shells can be used for highr-esolution paleoclimate reconstructions. In this contribution, delta O-18(shell) 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 delta O-18(shell). The results obtained reveal that delta(18)O(shell )seasonal variation is mainly controlled by SST and appears to be insensitive to local SSS change. Thus, the delta O-18 of Tridacna shells can be roughly used as a proxy of local SST: a 1 parts per thousand delta O-18(shell ) change is roughly equal to 4.41 degrees C of SST. The r -monthly delta O-18 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 delta O-18 of the fossil Tridacna shell (delta O-18 = -1.34 %0) and the modern Tridacna specimen (delta O-18= -1.15 %0) probably implies a warm climate, roughly 0.84 degrees 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 NinO-Southern Oscillation (ENSO) variation (based on the reconstructed SST series) indicates reduced ENSO frequency but increased ENSO-related variability and extreme El NinO winter events 3700 years ago.