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The carbon isotopic fractionation process in biogenic carbonate is very complex, and the mechanism for seasonal and long-term variations in biogenic carbonate δ13C 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 δ13C 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 δ13C during the mid- to late Holocene and the controlling mechanism. The Tridacnidae δ13C 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 δ13C composition. The decoupling between the Tridacnidae δ13C and solar activity over the past 200 years is most likely due to increased amount of anthropogenic CO 2 and the oceanic δ13C Suess effect. This study improves our understanding about the seasonal and long-term variations of δ13C in Tridacnidae. • The first mid- to late Holocene Tridacnidae δ13C record in South China Sea was reconstructed. • Mid- to late Holocene carbon isotopic records of Tridacnidae is linked to solar activity. • Potential controlling mechanism of long-term Tridacnidae δ13C composition was proposed. [ABSTRACT FROM AUTHOR]