Phosphorus cycling in the deep sedimentary subseafloor environment
The deep subseafloor environment is dominantly a prokaryotic habitat, with active microbial communities extending into the upper oceanic crust and affecting global biogeochemical cycles. The focus of this thesis is on phosphorus (P) biogeochemical cycling in marine sediments, particularly carbon-depleted open ocean sediments, which represent approximately 70% of the seafloor. The sediment samples used in each study are from different ocean basins and span a wide range of redox conditions, sediment types, basement ages, hydrothermal settings and P sources to the seabed, and offer valuable insights into how these parameters affect sedimentary P composition, as well as biotic and abiotic processes altering solid-phase P. Each study in this thesis provides new insights into sediment P biogeochemistry by using an array of interdisciplinary tools. In the first study, a new method for preparing sediment samples for solution 31P nuclear magnetic resonance spectroscopy (31P NMR) is presented that enhances the use of this tool with organic matter–depleted sediments by amplifying the signal of low abundance P forms, which include organic P compounds. This is achieved with a two–step pretreatment prior to the solution 31P NMR extraction that quantitatively removes orthophosphate, thus increasing the signal of low abundance P forms relative to orthophosphate without the use of extremely long experiments. This new method allows the use of this tool on open-ocean sediments and substantially improves our understanding of the dynamics of organic P in this environment. Results from the second study shed light on sedimentary P geochemistry of ridge-flank systems, which are regions far from the magmatic influence of seafloor spreading, including areas that are coming under the influence of (but not yet undergoing) subduction. This study focuses on two ridge-flanks, the Mid-Atlantic Ridge flank and the Juan de Fuca Ridge flank, where active hydrothermal circulation in young oceanic crust affects not only microorganisms in sediments and upper basaltic crust, but also chemical fluxes in this setting (especially at the Mid-Atlantic Ridge flank). Sequential extractions of sediments from these two well-studied sites reveal that P is mainly present in mineral phases, which vary in abundance based on sediment redox conditions and P source to the seafloor. This study also highlights the persistence of organic P throughout the sediment column in these open ocean sediments. The third study uses a combined approach of metagenomic analyses and solution 31P NMR to explore potential microbial P uptake mechanisms and P substrates for the deep biosphere. Results from this study show that organic P forms in oligotrophic sediments beneath the North Atlantic Gyre are diverse, and decrease in relative abundance with depth and changing redox conditions, thus supporting the putative P uptake pathways identified with the metagenome.