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

Abstract. Giant clams (Tridacna) are the largest marine bivalves, and their carbonate shells can be used for high-resolution paleoclimate reconstructions. In this contribution, δ¹⁸O_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 δ¹⁸O_shell. The results obtained reveal that δ¹⁸O_shell 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？‰ δ¹⁸O_shell 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 $=-\mathrm{1.34}$？‰) and the modern Tridacna specimen (δ¹⁸O