Stable isotopes and trace element profiles of modern giant clams Tridacna squamosa from the Karimata Strait : a calibration study.

Fossil shells of marine bivalves such as giant Tridacna provide unique information on past environments with seasonal to daily resolutions. Changes in mean seasonal cycles and inter-annual variability can be reconstructed by sequentially analyzing the composition of the annual layers of calcium carbonate. We present a new calibration study of 5 modern Tridacna squamosa which have been collected in Belitung Island, Indonesia, on the path of the Karimata Strait during different years 2016-2025. At this locality, the seasonal cycle is characterized by a double seasonal peak in sea surface temperature due to the inversion of surface currents associated with the SE and NW monsoons. We analyzed both the stable isotope (del-18O, del-13C) and trace element profiles (Mg/Ca, Ba/Ca). The aim of this study was to demonstrate the reproducibility of the trace elements between samples collected on different dates in the same site. The lifespan of the specimen collected were 2 to 5 years long and geochemical profiles showed significant overlap between modern dead-collected specimen. The results show that Ba/Ca records are highly reproducible between samples exhibiting a single seasonal peak during the period May-June. Mg/Ca show a good reproducibility between samples exhibiting a double seasonal peak. We compared our records to local hydrology: sea surface temperature, rainfall and productivity. Mg/Ca shows a clear relationship with SST. Ba/Ca has previously been shown to reflect both productivity and/or riverine sources of Barium. At Belitung site there are 2 seasonal increases in productivity, and the shell Ba/Ca only increases during the may-june period highlighting a more complex relationship than previously observed