Coral-inferred high seasonal sea surface temperature in the South China Sea at 7.4 thousand years ago

Under the background of global warming, investigating the seasonal characteristics of climate change during the middle Holocene Thermal Maximum holds critical significance for understanding the future trend. Monthly-resolved Porites coral oxygen isotope (δ¹⁸O) records and annual growth rates of three modern samples from 1962‒2006 CE and one fossil specimen dated from 7.4 ka (7486‒7407 yr BP) from the northern South China Sea (SCS) were used to quantitatively reconstruct variabilities of seasonal and annual sea surface temperatures (SSTs) in the middle Holocene. Compared with modern values, results show that the middle Holocene SSTs were 0.83 ± 0.44 (1σ) °C higher. Seasonally, all four seasons exhibited warmer conditions relative to modern SSTs, with maximum warming occurring in summer and autumn, while minimum warming occurred in winter. This finding implies that proxy reconstructions based solely on summer or autumn SST would overestimate the annual average by 10.3% and 33.8%, respectively, whereas using winter SST would underestimate it by 48.5%. Under sustained global warming, the climatic characteristics observed at 7.4 ka may potentially reemerge in future climate regimes.在全球变暖的背景下，研究全新世中期暖期（Middle Holocene Thermal Maximum）的气候变化季节特征，对理解未来气候趋势具有至关重要的科学意义。本研究采集了南海北部（SCS）3件1962—2006年现代滨珊瑚样品的月分辨率氧同位素（δ¹⁸O）记录与年生长率，以及1件年代为7.4 ka（距今7486—7407年）的化石滨珊瑚样品，以此定量重建了全新世中期的季节与年际海表温度（SST）变化。与现代海表温度基准值相比，全新世中期的海表温度整体偏高0.83±0.44（1σ）℃。从季节尺度分析，全新世中期四个季节的海表温度均较现代偏暖，其中夏季与秋季的升温幅度最大，冬季升温幅度最小。本研究结果显示，若仅基于夏季或秋季海表温度开展代用指标重建，会分别高估年平均海表温度10.3%与33.8%；而若采用冬季海表温度进行重建，则会低估年平均海表温度48.5%。在全球持续变暖的背景下，7.4 ka时期的气候特征可能会在未来气候系统中重现。