Geochemical Controls on the Distribution And Composition of Biogenic and Sedimentary Carbon
Emily Estes, Ph.D., 2017
Colleen Hansel, Advisor
Organic carbon (OC) in marine sediments acts as a reduced carbon sink that balances the global carbon cycle. Understanding the mechanisms underpinning the balance between preservation and degradation is critical to quantifying this reservoir and estimating the extent of life in the deep subsurface. This work characterizes OC content and composition of various environmental systems in order to identify the role of minerals and geochemistry in OC preservation. Manganese oxides rapidly associate with OC following precipitation; this association is stable despite mineral structural ripening. OC associated with manganese oxides is proteinaceous, including proteins involved in Mn oxidation. Pelagic sediments underlying the South Pacific and North Atlantic gyres and spanning a gradient of sediment redox state were analyzed in order to contrast the roles of oxygen exposure, recalcitrance, and mineral-based protection in preservation. OC and nitrogen concentrations scale with sediment oxygenation to a first order. Observations by bulk and spatially-resolved X-ray absorption spectroscopy reveals that OC consists of primarily amide and carboxylic carbon in a scaffolding of aliphatic and O-alkyl moieties, corroborating the low C/N values observed. This work documents how interactions with mineral surfaces and exposure to oxygen generate a reservoir of OC stabilized on 25-million year time scales.