The biogeochemical cycles of organic carbon, nutrients, oxygen, and sulfur in the oceans have been suggested to dominantly occur across the shelf–ocean transition over the continental margin, although this zone represents only a small percentage of the global ocean area. Coastal upwelling zones in eastern boundary upwelling systems is an example of the most productive ocean waters over continental margins where intense supply of nutrients occur from deeper ocean waters.
Interesting questions arise related to the biogeochemical cycles in such upwelling systems; such as 1) how the recently observed active but cryptic sulfur cycle possibly is coupled to the nitrogen cycle in an oxygen-minimum-zone (OMZ), 2) what is the relation between the shelf–ocean exchange, continental shelf width and development of the observed bottom water anoxia/euxinia associated with different configurations of continental margin bathymetry, and 3) what processes determine the observed variability of total organic carbon (TOC) content in shelf sediments underlying the upwelling system, with implications for the formation of petroleum source rocks. Here, a numerical ocean modeling approach is used in this thesis to explore these questions centering on shelf–ocean exchange and biogeochemical cycle in the coastal upwelling systems under oxic and anoxic conditions.
Firstly, I developed a new biogeochemical model which resolves coupling between cycles of the elements nitrogen, oxygen, phosphate, and sulfur by considering several key reactions and biological respiration of marine organic matter (remineralization) under oxic and anoxic conditions.
The developed model was coupled into a three-dimensional physical circulation model called the Regional Ocean Modeling System (ROMS). Then, the coupled model was employed and calibrated to reproduce the observed coupling between nitrogen and sulfur cycles in OMZ of the northern Chile upwelling systems. The model results show that sulfate reduction contribute significantly to organic matter remineralization in the OMZ water depths, along with nitrate reduction. In the model, anaerobic ammonium oxidation (anammox) has the major role in removing the fixed nitrogen relative to canonical denitrification (organic remineralization by nitrite reduction plus sulfide-driven denitrification). The model implies that the source of ammonium for the anammox is obtained from organic remineralization associated with nitrate reduction to nitrite. Secondly, the coupled model was used to simulate upwelling and iogeochemical cycles associated with different continental shelf geometries. In particular the generalized effect of shelf width was investigated. Anoxic/euxinic conditions in bottom waters are found to be related to relatively wide shelf areas because more sinking organic matter reach the shelf sea-floor and remineralize there, enhancing the nutrient trapping effect of the shelf circulation system. These results highlight the important role of the continental shelf bathymetry in modulating the shelf–ocean exchange processes and the development of anoxia/euxinia under the present day or past geological conditions.
Thirdly and last, processes controlling distribution of total organic carbon (TOC) content in sediments across the continental margin is evaluated by application of the model to the Benguela upwelling system. In the model, biological primary production and shelf bottom-water anoxia result in enhanced sedimentary TOC concentrations on the mid shelf and upper slope. The simulated TOCs implicate that bottom lateral transport only has a significant effect on increasing the deposition of the organic carbon on the mid slope and deeper depths. The coupled model may potentially serve as a robust tool in investigation of the dynamics of oceanic biogeochemical cycle throughout Earth history as well as a practical method to quantified storage of carbon flux into the ocean across the continental margins under present day, future, and geological past conditions.