Variation and influencing factors of δ¹³C and δ¹⁸O in the inner and outer layers of modern Tridacna squamosa from the Xisha Islands, South China Sea

Tridacna shells, known for their distinct growth bands, serve as excellent proxies for recording tropical marine climate changes. The δ¹⁸O and δ¹³C in Tridacna shells have been applied in the Quaternary paleoclimate research. However, most previous researches focused on the inner layer, while the outer layer received less attention. We conducted a comprehensive analysis at a monthly resolution δ¹⁸O, δ¹³C tests on both the inner and outer layers, as well as daily growth band scans on the inner layer of modern Tridacna squamosa from the Xisha Islands in the South China Sea. Results reveal that the outer layer exhibited higher δ¹⁸O values than the inner layer. In addition, both shells displayed similar variation patterns, being primarily influenced by sea surface temperature (SST), and indicating that the δ¹⁸O of the outer layer could reliably indicate climate and environmental changes. In contrast, significant differences in δ¹³C value were observed between the inner and outer layers. The inner layer displayed a noticeable annual cycle in δ¹³C value. By comparing the inner layer δ¹³C, climatic and environmental parameters, and daily growth rate (DGR), we determined that the seasonal variations in inner layer δ¹³C were linked to the primary productivity and the life activities of Tridacna. Meanwhile, the δ¹³C of the outer layer, which is relatively negative compared to the inner layer, exhibited a continuous downward trend without seasonal changes. This discrepancy may be attributed to the sampling path deviating from the maximum growth axis.