Symbiotic Zooxanthellae Drive the δ¹³C Changes of Tridacna gigas Shell in the Southern South China Sea

The carbon isotope (δ 13 C) in marine biogenic carbonate is a valuable tool for comprehending the marine carbon cycle. However, the absence of monthly resolved δ 13 C records before the instrumental era (since 1850) hinders us to fully understand the global carbon cycle. In this study, we present a δ 13 C record from a modern Tridacna gigas shell (δ 13 C shell ) collected from the southern South China Sea (SCS) with an average resolution of ∼10 days. The δ 13 C record illustrates seasonal and interannual cycles which are superimposed by long‐term decreasing trend. We investigate the environmental controls influencing the δ 13 C shell on different time scales. We suggest that more positive δ 13 C shell values observed during the juvenile stage (first 8–10 years), which correspond to a high growth rate, can be attributed to 13 C enrichment of the internal dissolved inorganic carbon pool. This resulted from the exponential increase of symbiotic zooxanthellae, and preferential 12 C sequestration by the accelerated buildup of productive tissues. On the interannual time scale, changes in δ 13 C shell were likely affected by both internal (vital effect) and external processes (sea water δ 13 C), as well as being disturbed by the super El Niño‐La Niña event. Seasonal changes of δ 13 C shell were mainly dominated by the intensity of photosynthesis and respiration of symbiotic zooxanthellae, which depend on the seasonal solar irradiance variations in association with precipitation and the South Asian summer monsoon. Our study sheds light on the controlling factors of δ 13 C shell variations on seasonal timescale and the long‐term trend, providing insight into the carbon cycle over geological history.