Millennial calcification trends in Porites corals: Resilient skeletal density and vulnerable linear extension in response to environmental change

Global coral reefs are currently facing widespread declines in calcification rates, a trend that underscores the need to understand variations between calcification in contemporary and ancient corals. This study investigates the changes in coral calcification over the last millennium by analyzing 21 subfossil and 11 modern coral samples from a marginal reef in the northern South China Sea. Our results reveal that although coral skeletal density has remained relatively stable, significant differences are evident in linear extension rates between the Medieval Climate Anomaly (MCA, 900–1300 CE) and the Little Ice Age (LIA, 1550–1850 CE), as well as between the pre- and post-industrial periods. This finding suggests that inshore Porites corals have greater resilience in their skeletal density compared to linear extension rates when responding to environmental changes. Our analysis indicates that sea surface temperature (SST) is the primary environmental driver influencing coral calcification, with a positive correlation observed between SST and both modern coral linear extension and calcification rates. In contrast, factors such as seawater pH, salinity, and total solar irradiance appear to have minimal impact on coral skeletal growth. Therefore, the lower linear extension and calcification rates observed in the LIA corals, as compared to the MCA corals, are likely due to the cooler temperatures during the LIA. We also hypothesize that the extended period of cold SST from the LIA to the early 20th century may have led to the reduced linear extension and calcification rates in modern corals, with recent warming insufficient to facilitate a return to the optimal skeletal growth observed during the MCA.