Biogeochemical cycling of trace metals in shelf regions: The importance of cross-shelf exchange
Marine phytoplankton form the base of the marine food web and are fundamental to the efficiency of the ocean’s ‘biological pump’. The biological pump is especially important in the homeostasis of atmospheric CO2 levels through the sequestration of carbon into the deep ocean via organic matter production in the surface ocean that utilizes metalloprotein enzymes or intermediates. Some of these metals are present at very low concentrations in the oceans, potentially limiting phytoplankton productivity, which has a significant effect on global carbon and climate cycles. The primary processes controlling trace metal concentrations and distributions in the ocean are a combination of external inputs and removal processes, as well as internal recycling processes. The extent of these processes varies in different oceanic regions but generally they are all prominent in shelf locations. In particular, the recent increase in meltwater input from declining sea-ice and glaciers supplies the Southern Ocean with freshwaters containing additional sources of some trace metals. During the austral spring in the Western Antarctic Peninsula region, a substantial input of Fe into shelf surface waters occurred by meltwater input and sea-ice formation, and into shelf bottom waters by non-reductive sedimentary processes. However, the characterization of distinct meltwater sources is not well constrained to differentiate the sources of Fe-enriched meltwater. High precision isotopic analysis of the conservative element uranium (U) resolved subtle variations in the 234U/238U isotope ratio (reformulated into delta-notation as δ234U) and a linear correlation between δ234U and salinity in the Dotson-Getz Trough region, West Antarctica. A three-component mixing model using δ234U and salinity of possible endmembers predicts that glacial meltwater and Antarctic stream waters are the most likely freshwater sources that, in combination with sea-ice meltwater, control the observed δ234U – salinity variations in the Dotson-Getz Trough region, potentially in other shelf regions of Antarctica as well. The importance of intruding open ocean waters and interaction with ice shelves as a source of trace metals to high-latitude shelf regions is apparent, but the influence of shelf processes on trace metal distributions in other oceanic regions remains limited, especially in low-nutrient, stratified waters, such as those across the northeastern continental shelf of New Zealand. This continental shelf, overlain by waters of the Hauraki Gulf, is relatively narrow and temporal variation in these processes, such as localized upwelling and the extent of subsurface water intrusion, results in the observed inter-annual variability in the concentrations and distributions of trace metals across the northeast continental margin of New Zealand. These studies demonstrate the importance of cross-shelf exchange in connecting shelf and open ocean waters, and the profound influence this has on the regional and global concentrations and distributions of trace metal micronutrients. If the current trend of climate warming continues, it will result in increased rates of ice melt and elevated water temperatures. This will likely lead to stronger stratification, weakening vertical diffusivity between the surface water layer and the water beneath the pycnocline, which could drive surface water microorganisms into (micro-) nutrient limitation, resulting in a decreased efficiency of the global biological pump.
Advisor: Stirling, Claudine; Middag, Rob; Reid, Malcolm
Degree Name: Doctor of Philosophy
Degree Discipline: Chemistry
Publisher: University of Otago
Keywords: trace metals; spring Fe; U isotopic composition; Western Antarctic Peninsula; Dotson-Getz Trough; Hauraki Gulf
Research Type: Thesis