Growth patterns and geochemical characteristics of a colonial scleractinian cold-water coral in the South Chian Sea: A 500-year record of ocean environmental changes

Abstract

Scleractinian cold-water corals (CWCs) are unique and prominent geological archives in the deep ocean, offering crucial insights into ocean dynamics and climate changes over time. Using scleractinian CWC skeletons to reconstruct paleoclimate often relies on combining data from multiple solitary cup corals or branching colonial corals. However, biomineralization strategies vary between species, and even within different skeletal structures (e.g. the corallite and the coenosteum) of the same species, leading to complex geochemical characteristics within coral skeletons, complicating our understanding of CWC growth patterns and the reconstruction of past ocean changes. In this study, we investigated the growth patterns of a colonial scleractinian CWC from the South China Sea using the UTh dating technique and measured the geochemical compositions of the corallite and the coenosteum to evaluate their environmental significance. Our results indicate a budding rate approximately 2.8 polyps per decade, with linear extension rates between 0.47 and 0.57 mm/year, and radial thickening rates ranging from 0.014 to 0.022 mm/year. Significantly, the geochemical compositions, such as δ¹³C, δ¹⁸O, δ¹¹B, Sr/Ca, B/Ca, Ba/Ca, and U/Ca, differ between the corallite and coenosteum. Conversely, Mg/Ca and Li/Mg ratios remain consistent across both structures. These disparities likely reflect varying degrees of vital effects during the formation of different skeletal parts. Among the examined geochemical proxies, Ba/Ca and Li/Mg ratios provided clear signals of environmental changes over the past ∼500 years. Specifically, Ba/Ca ratios in both corallite and coenosteum exhibit a marked secular decline since the 1500s, suggesting a reduction in surface primary productivity or decreased ventilation of North Pacific intermediate waters. Additionally, intermediate water temperatures estimated from the Li/Mg ratios of these two structures collectively show a tendency to increase towards the twentieth century.