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

摘要

Two high-resolution δ~(13)C 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 δ~(13)C 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%o and amplitude of 4.21‰, which revealed higher coral δ~(13)C and increased seasonality in the middle Holocene. Our analysis suggests that the higher coral δ~(13)C was the combined result of higher radiation, salinity and nutrient level and lower atmosphere CO_2 concentration, which might increase coral δ~(13)C 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 δ~(13)C revealed weakened interannual changes of atmospheric CO_2, salinity, and nutrient. By contrast, the increasing deviation of the ～4.4 ka coral δ~(13)C suggests strengthened interannual changes of these variables. The ～5.4 ka coral δ~(13)C showed a long-term increasing trend at a rate of 0.33% increase y~(-1) which was attributed to natural decrease in atmospheric CO_2, and increasing salinity and nutrient level. However, the ～4.4 ka coral δ~(13)C showed a long term decreasing trend at a rate of 0.25% decrease y~(-1) which was ascribed to unusual increase in atmospheric CO_2, and decreasing salinity and nutrient level.