Geochemical conditions in continental margin sediments: implications for distribution and cycling of phosphorus

Abstract

This thesis investigated the sedimentary phosphorus cycle in different upper continental slope and shelf surface sediments. In this thesis a combined multi-parameter and geochemical approach has been used to improve the speciation of the phosphorus reservoir in selected continental margin surface sediments. In particular the determination of pore water constituents has the potential to examine sediment redox processes associated to organic matter degradation and their impact on phosphorus species distribution in different marine environments. Linked to that, the early diagenetic regeneration of organically bound phosphorus and reductive dissolution of iron-phosphate phases have been identified as important pathways for the release of phosphate in the investigated sediments. Evidence that reducible iron-phosphate minerals are the major reactive pool for phosphorus transformation has been provided by sequential extraction results. Moreover, the variety of phosphorus-containing species that contributed to the total sum of solid-phase phosphorus reservoir has been identified. A model has been applied, which successfully describe and quantify the processes that are responsible for sedimentary phosphorus cycling in Namibian sediments. As well as deduced from conservative mass balance calculations, the presented results revealed that the content in reactive iron phases is not sufficient to produce the quantified release rates of phosphate. Most important in terms of coupling of benthic phosphorus cycling with redox-sensitive iron phases is the influence of non-local particle transport. Such additional transport process has to be considered as determining factor in balancing the sedimentary phosphorus budget.