Al-Hashem, Ali (2023) The Biogeochemical Cycling of Particulate Trace Metals off the Southwest African and Peruvian Shelves. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany.

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Abstract

Marine particles in the ocean play an important role in the marine biogeochemical cycling of trace metals (TMs), yet our understanding of the marine biogeochemical cycling of TMs remains incomplete. Additionally, particulate TM (pTM) datasets from key dynamic regions of the global ocean, such as continental shelves, upwelling sites, and oxygen minimum zones (OMZs), are lacking, impeding progress toward fully understanding the processes which impact pTM cycling and distributions.

This work studies the biogeochemical cycling of particulate iron (Fe), manganese (Mn), cobalt (Co), aluminum (Al), copper (Cu), nickel (Ni), zinc (Zn), cadmium (Cd), lead (Pb), titanium (Ti), vanadium (V), and phosphorus (P) on the SW African and Peruvian shelves, employing a chemical leach to marine particles collected on two separate research expeditions. The distributions and biogeochemical cycling of biogeochemical tracers, molybdenum (Mo), barium (Ba), chromium (Cr), tungsten (W), thorium (Th), and uranium (U) are explored only on the SW African shelf. Transects from these sites traversed through contrasting biogeochemical regimes with diverse particle sources, including the Benguela and Humboldt upwelling regions, OMZs, and the Congo River plume, offering unique gradients under which pTM cycling can be studied. Differentiating between labile and refractory pTM fractions, provided a more accurate assessment of the biogeochemical behavior and fate of pTMs within the different marine environments. Elemental ratios and particle type proxies were also utilized to determine redox and scavenging processes, and biological uptake of TMs in the water column. This approach allowed for the characterization of the principal carrier phases of pTMs in the water column, which is a critical step toward constraining the mechanisms that control their fate and interactions with dissolved phases in the water column.

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