The controlling mechanism of mid- to late Holocene carbon isotopic variations of Tridacnidae in the South China Sea

Abstract The carbon isotopic fractionation process in biogenic carbonate is very complex, and the mechanism for seasonal and long-term variations in biogenic carbonate δ 13 C remains a matter of debate. In this study, 5 Tridacnidae samples were collected from the South China Sea and analyzed in detail to study seasonal variations. The high-resolution δ 13 C records exhibited cyclic variations, likely related to seasonality in environmental parameters (e.g., Chl-a concentration, salinity, SST, rainfall). 57 Tridacnidae were collected and analyzed to study the millennium-scale changes of Tridacnidae δ 13 C during the mid- to late Holocene and the controlling mechanism. The Tridacnidae δ 13 C over the period 500–5500 BP is statistically significantly correlated with solar activity (TSI and sunspot number), likely attributed to the presence of symbiotic zooxanthellae within Tridacnidae 's mantle lobes. Solar activity could significantly affect the metabolic activity of zooxanthellae, Tridacnidae 's microenvironment, and thus Tridacnidae 's δ 13 C composition. The decoupling between the Tridacnidae δ 13 C and solar activity over the past 200 years is most likely due to increased amount of anthropogenic CO 2 and the oceanic δ 13 C Suess effect. This study improves our understanding about the seasonal and long-term variations of δ 13 C in Tridacnidae .