Abstract
Bio-essential trace elements, such as iron (Fe) and zinc (Zn), also known as micronutrients, are critical for all life because of their biochemical roles in various metabolic processes. They are also pivotal for marine phytoplankton growth, the base of the marine food web. In the oceans, High-Nutrient Low-Chlorophyll (HNLC) surface
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regions are characterized by sufficient macronutrient (e.g., phosphate, nitrite/nitrate, sillicate) supply but limited primary productivity due to the lack of Fe and light. As the largest HNLC region, the Southern Ocean is suggested to have a great potential to absorb a large amount of atmospheric CO2 if the phytoplankton requirements for Fe and light are fulfilled. Given the importance of the Southern Ocean to the global climate and ocean circulation, it is crucial to understand its marine biogeochemistry, notably the cycling of trace elements in coastal Antarctic regions, and their influence on globally relevant processes. In this thesis, two bio-essential trace elements, Fe and Zn, together with Cd, and their isotopic compositions, are studied in two distinct coastal Antarctic regions – the Amundsen Sea (AS) and the Weddell Sea (WS) to expand our understanding of these crucial bioactive elements in the dynamic Southern Ocean. The AS is characterized by the extensive intrusion of warm modified Circumpolar Deep Water (mCDW) onto the continental shelf through glacial troughs that lead to rapid ice sheet melting as it flows under ice shelves (i.e., Dotson Ice Shelf, DIS). The upwelling of mCDW also accelerates sea ice melting, creating polynyas (i.e., open areas surrounded by sea ice) that harbour productive and long-lasting phytoplankton blooms in spring-summer – the Amundsen Sea Polynya (ASP) shows the highest annual net primary production rate per unit area among Antarctica polynyas. The WS is part of the wind-driven Weddell Gyre, which is a crucial component of the global oceanic circulation as a primary formation region of deep-water masses. In the southern and western parts of the WS, dense, saline, cold shelf water forms through sea ice formation and brine rejection in winter, which subsequently descends along the continental slope, eventually exiting the WS through the Scotia Sea, contributing to AABW and the global ocean conveyor belt. Both the AS and the WS hold the potential to influence or be influenced by the global biogeochemical cycles of trace metals. Chapters 2, 3, and 4 delve into the biogeochemical aspects of these elements in the AS, while Chapters 4 and 5 focus on the WS. These regions hold particular significance due to their potential regional biogeochemical dynamics, which may have global repercussions. I utilized stable isotopic compositions as a tool to unravel the complex dynamics of these elements in these rapid changing environments.
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