Geochemical controls on the distribution and composition of biogenic and sedimentary carbon

Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2017.Cataloged from PDF version of thesis.Includes bibliographical references.Organic carbon (OC) preserved in marine sediments acts as a reduced carbon sink that balances the global carbon cycle. Understanding the biogeochemical mechanisms underpinning the balance between OC preservation and degradation is thus critical both to quantifying this carbon reservoir and to estimating the extent of life in the deep subsurface biosphere. This work utilizes bulk and spatially-resolved X-ray absorption spectroscopy to characterize the OC content and composition of various environmental systems in order to identify the role of minerals and surrounding geochemistry in organic carbon preservation in sediments. Biogenic manganese (Mn) oxides formed either in pure cultures of Mn-oxidizing microorganisms, in incubations of brackish estuarine waters, or as ferromanganese deposits in karstic cave systems rapidly associate with OC following precipitation. This association is stable despite Mn oxide structural ripening, suggesting that mineral-associated OC could persist during early diagenetic reactions. OC associated with bacteriogenic Mn oxides is primarily proteinaceous, including intact proteins involved in Mn oxidation and likely oxide nucleation and aggregation. Pelagic sediments from 16 sites underlying the South Pacific and North Atlantic gyres and spanning a gradient of sediment age and redox state were analyzed in order to contrast the roles of oxygen exposure, OC recalcitrance, and mineral-based protection of OC as preservation mechanisms. OC and nitrogen concentrations measured at these sites are among the lowest globally (