Trace element composition of cold-water corals from coastal Antarctic surface sediments: Implications for biomineralization mechanisms and paleoceanographic reconstructions

The Southern Ocean is one of the most sensitive regions to ongoing climate change, and plays an important role in the global ocean circulation and carbon cycle through a range of physical, chemical and biological processes. Understanding long-term environmental changes in the Southern Ocean can help us better resolve the key processes influencing its impact on global climate. Paleoceanographic reconstructions in the Southern Ocean, however, are often challenging due to a lack of feasible archives and well-calibrated environmental proxies. Cold-water corals are a type of benthic organism of relatively high abundance in the Southern Ocean, with their calcium carbonate skeletons that can potentially record environmental changes at relatively high temporal resolution. Here we report measurements of the trace element composition of cold-water corals and a tubeworm collected from surface sediments in the Ross Sea and west of the Amundsen Sea. We find that spatial distribution of calcitic and aragonitic corals in the Southern Ocean is likely controlled by the bottom water carbonate saturation state. Trace element composition of the biogenic carbonates is mainly determined by their respective mineralogy and displays significant vital effects. Different element-to‑calcium ratios show strong correlations in individual organisms that are similar to cold-water corals found in other regions, and can generally be explained with existing biomineralization models. Several elemental ratio proxies show relations with environmental conditions that are consistent with global calibrations, most notably the Li/Mg-temperature relation in both biogenic aragonite and high-Mg calcite, yet large uncertainties remain in terms of their use in paleoceanographic reconstructions. Understanding the biomineralization mechanisms underlying the vital effects in different types of cold-water corals with in-situ analytical techniques and numerical models is a key step in improving the proxies that are suitable for the extreme environmental conditions in the Southern Ocean.