Scholz, Florian (2018) Sedimentary iron and trace metal cycling in oxygen minimum zones : implications for marine productivity and redox evolution. (Professorial dissertation), Christian-Albrechts-Universität, Kiel, Germany, 208 pp.

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Abstract

Because of anthropogenic global warming, the world ocean is currently losing oxygen. This trend called ocean deoxygenation is particularly pronounced in low-latitude upwelling-related oxygen minimum zones (OMZs). In these areas, the temperature-related oxygen drawdown is additionally modulated by biogeochemical feedback mechanisms between sedimentary iron and phosphorus release, water column nitrogen cycling and primary productivity. Similar feedbacks were likely active during past periods of global warming and ocean deoxygenation. However, their integrated role in amplifying or mitigating climate change-driven ocean anoxia has not been evaluated in a systematic fashion. Moreover, many studies on past (de)oxygenation events emphasize anoxic-sulfidic (i.e., euxinic) basins such as the Black Sea rather than upwelling-related OMZs as modern analogue systems. This habilitation thesis consists of nine published stand-alone articles dealing with the biogeochemical cycling of iron and other redox-sensitive metals (and their isotopes) in the sediments and water columns of modern and ancient OMZs. The findings of these independent chapters are summarized and synthesized in a review article entitled “Identifying oxygen minimum-type biogeochemical cycling in Earth history using inorganic geochemical proxies”. The goal of this synthesis chapter is to provide a general framework of paleo-redox proxies that can be used to identify OMZ-type biogeochemical cycling and the associated biogeochemical feedbacks in the geological record. Nitrate-reducing (i.e., nitrogenous) conditions in the water column and iron-reducing (i.e., ferruginous) to sulfidic conditions in the surface sediment are identified as key-features of anoxic OMZs in the modern ocean. Because of the elevated iron solubility under anoxic conditions in OMZs, sedimentary Fe can be mobilized into the water column and redistributed across the continental margin. By using a generalized model of sedimentary iron release and trapping, it is demonstrated that the extent of iron mobilization and transport in modern OMZs is comparable to that inferred for the euxinic Black Sea and ferruginous water columns in Earth history. Based on this result, it is suggested that many sedimentary iron enrichments in the geological record are broadly consistent with OMZ-type biogeochemical cycling, especially if enhanced chemical weathering and reactive iron supply to the ocean during past periods of global warming are taken into account. Future studies on paleo-(de)oxygenation events with a combined focus on iron, sulfur and nitrogen cycling may reveal that nitrogenous OMZs were an important feature of the ocean through Earth’s history.

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