Carbon isotopic records of middle Holocene corals and environmental causes in northern South China Sea

Two high-resolution δ13C records of ∼5.4 and ∼4.4 ka coral colonies Porites lutea from east coast of Hainan Island in the northern South China Sea (SCS) provide insights into the complex links between coral carbon isotopes and environment changes. The δ13C of the ∼5.4 ka coral offered a 40-year growth history with an average of −2.62‰ and amplitude of 2.96‰, the ∼4.4 ka coral's revealed a 55-year growth history with an average of −3.12‰ and amplitude of 4.21‰, which revealed higher coral δ13C and increased seasonality in the middle Holocene. Our analysis suggests that the higher coral δ13C was the combined result of higher radiation, salinity and nutrient level and lower atmosphere CO2 concentration, which might increase coral δ13C through strengthening photosynthesis of symbiont zooxanthella in the context of kinetic and metabolic fractionations. The increased seasonality was mostly produced by strengthened radiation.The decreasing standard deviation of the ∼5.4 ka coral annual δ13C revealed weakened interannual changes of atmospheric CO2, salinity, and nutrient. By contrast, the increasing deviation of the ∼4.4 ka coral δ13C suggests strengthened interannual changes of these variables. The ∼5.4 ka coral δ13C showed a long-term increasing trend at a rate of 0.33% increase y−1, which was attributed to natural decrease in atmospheric CO2, and increasing salinity and nutrient level. However, the ∼4.4 ka coral δ13C showed a long term decreasing trend at a rate of 0.25% decrease y−1, which was ascribed to unusual increase in atmospheric CO2, and decreasing salinity and nutrient level.