Impact of variations in bottom water oxygen and organic matter input on phosphorus dynamics in continental margin sediments

Abstract

Phosphorus (P) is a key limiting nutrient in marine environments. Input of P into marine environments occurs primarily through terrestrial runoff and the only long-term removal from the marine environment is burial within sediments. Early diagenetic pathways are key in determining P speciation within sediments and are influenced by environmental conditions such as bottom water oxygen (O2) concentrations and organic matter input to the sediments. The main objective of this study is to investigate the sediment geochemistry, with a specific focus on P dynamics, of episodically oxygenated semi-enclosed basins on the highly productive Californian and Mexican continental shelves. Bottom water O2 concentrations and organic matter input to the sediments vary across basins resulting in different redox conditions within the sediments. Primary focus is directed to the impact of redox conditions on P speciation and burial. Results show that reducing conditions promote effective recycling of P with Fe-oxides at all hypoxic and episodically anoxic sites while sink-switching of labile P forms to authigenic P minerals occur only at one site potentially due to the presence of sulphur-oxidising bacteria from the Thioploca spp. Secondly, the P speciation and burial efficiency observed in the Californian and Mexican shelf basins sediments are compared to similar studies done in sediments of the Black Sea (including the oxic shelf and deep sulfidic basin), the Arabian Sea (including its pronounced oxygen minimum zone) and the Gulf of Mexico (including its eutrophic coastal zone). The comparison of datasets suggests: 1) A positive correlation between authigenic P content and P retention efficiency of sediments across different environmental settings. 2) Reduced P retention efficiency under low bottom water O2 conditions in the studied areas. Local factors such as atmospheric dust deposition containing a significant fraction of calcium bound P, sorption of dissolved P on Fe-oxide facies and promoted P authigenesis through sulphur-oxidising bacterial activity are shown to potentially breakdown the observed correlation.