The ocean plays a central role in regulating Earth’s climate by redistributing heat and storing carbon, yet our understanding of its long-term response to climate change is hindered by the scarcity of deep-ocean records. This thesis investigates the use of deep-sea corals as geochemical archives to reconstruct changes in ocean circulation, continental weathering, and seawater oxygenation over the past millennium. Rare earth elements and yttrium (REYs) in modern scleractinian corals record distinct regional patterns across the Atlantic and Southern Oceans. Elevated REY concentrations and lower Y/Ho ratios in Greenland corals reflect enhanced terrestrial input, demonstrating the utility of corals for tracing weathering signals. Fossil corals from Southwest Greenland are then analysed to reconstruct intermediate water mass structure and subpolar Atlantic circulation since ~1400 CE. A multiproxy approach using Li/Mg ratios and REYs reveals a shift from deeper, more stable Subpolar Mode Water (SPMW) during the early Little Ice Age to a shallower, weakened regime by the 18th century. This shoaling coincides with elevated weathering fluxes, suggesting enhanced meltwater discharge from the Greenland Ice Sheet may have contributed to weakening of the Atlantic Meridional Overturning Circulation. Finally, iodine-to-calcium (I/Ca) ratios in modern deep-sea corals are evaluated as a proxy for oxygenation. Aragonitic scleractinian corals exhibit a strong correlation between I/Ca and seawater iodate, with sharply lower values below ~160 μmol/kg O2. In contrast, calcitic bamboo corals show uniformly low I/Ca values, indicating taxon- specific incorporation. Together, these studies demonstrate the power of deep-sea corals and a multiproxy framework to reconstruct physical and chemical changes in the ocean. The findings highlight the influence of continental weathering and cryospheric processes on water mass properties and circulation, offering insight into past and future ocean-weathering-climate feedbacks.
Investigating paleoceanographic proxies in deep-sea corals: Implications for weathering, water mass evolution, and ocean oxygenation
Sun, Y. (Author). 17 Jun 2025
Student thesis: Doctoral Thesis › Doctor of Philosophy (PhD)