Mid-Late Holocene coral calcification dynamics: Deciphering climatic and environmental effects

Over the past four decades, a marked decrease in coral calcification has occurred across the world's tropical reefs, closely linked to climate change and human-activity impacts. However, how natural and human-induced factors influence coral calcification remains unclear due to limited understanding of the geological past. This study addresses this gap by investigating the calcification parameters of 82 Porites corals from the northern South China Sea, with growth periods covering distinct climatic epochs during the Mid-Late Holocene, including the Holocene Climate Optimum, 4.2 ka BP event, Medieval Climate Anomaly, Little Ice Age, and Current Warm Period. Our findings show a gradual increase in coral skeletal density towards the present, and varied linear extension and calcification rates between warm and cold phases and between pre- and post-industrial periods. This suggests that temperature plays a pivotal role in controlling coral calcification, with contingent influences from volcanic activity and solar radiation. Notably, the linear extension and calcification rates were significantly reduced during the Current Warm Period, suggesting a surpassing impact of contemporary human activities over the natural variability on coral calcification. This raises concerns about the future prospects of coral reefs in the face of ongoing climate change and increasing human-activity impacts.