Investigation of redox-dependent benthic nutrient and metal feedbacks under anoxia using early diagenetic modeling

Biçe, Kadir
Loss of oxygen in marine systems is a major management challenge that requires an integrated combination of watershed, water column and seafloor benthic (sediment) biogeochemical models. This challenge has not been successfully confronted yet. One of the primary reasons is the lack of understanding on the complex biogeochemical consequences of redox processes ocurring under anoxia. This thesis focuses on uncovering the complex effects of hypoxia/anoxia on sediment biogeochemistry and its feedbacks to the water column low-oxygen marine environments, taking the Black Sea as a model system. To do this, we have used a synthesis of early diagenetic modeling approaches from literature which are constructed upon reaction-transport modeling framework. Results about our simulations show that, with decreasing oxygen in bottom water, sediment becomes dominated with sulfate reduction and methanogenesis. Therefore, production of hydrogen sulfide and methane critically increases which represents the sulfidic conditions of the Black Sea. Results of metal ions show that, considerable amounts of Fe(II) and Mn(II) are also produced due to high organic matter rain and less available oxygen. Flux estimates show that, deficiency of oxygen leads to lower phosphate and higher ammonia fluxes to the water column, thereby modifying the N/P ratio of the benthic-released nutrients. Under these conditions, if we assume that the mixing does not slow down as a result of warming, this feedback mechanism cause production of organic matter to increase further which leads to a more critical future deoxygenation.