Correlating organic carbon concentration and composition with mineralogy in deep-sea pelagic sediments
Thursday, 17 December 2015
Poster Hall (Moscone South)
The majority of organic matter sinking in the ocean is remineralized to carbon dioxide before it reaches the sea floor. Of the organic carbon (OC) that does reach the sediment-water interface, only a small fraction is buried. Organisms residing in the sediment column utilize minerals such as metal oxides as electron donors to degrade a portion of this carbon. However, there is evidence that these metal oxides may also, by adsorbing OC, enhance its preservation and thereby act as a sink for OC on geological timescales. The goal of this research is to examine the potential for mineral-based protection of OC in suboxic and oxic pelagic sediments. Using Elemental Analysis – Isotope Ratio Mass Spectrometry, we characterize the concentration and isotopic composition of OC and nitrogen in sediments collected during the R/V Knorr expedition 223 to the western subtropical North Atlantic in November 2014. Organic carbon concentrations range from 0.05 to 0.25 mol OC/kg sediment while organic nitrogen (ON) varies from 0.01 to 0.026 mol ON/kg sediment. OC concentrations initially decrease with depth but stabilize ~10 meters below core top, suggesting that OC is protected against remineralization since electron acceptors are still present to fuel anaerobic respiration. At the suboxic site, the ratio of OC/ON decreases sharply in the top two meters of the core and then stabilizes at a OC/ON value of approximately 5, illustrating an enrichment in ON relative to Redfield ratios. Oxidized solid-phase manganese (Mn), presumably Mn(III,IV) oxides, decreases with depth indicating active redox cycling. Conversely, solid-phase Fe(III) content as a proportion of total Fe remains relatively constant with depth. X-ray diffraction and density separations to isolate specific mineral classes, such as metal oxides and clays, were also employed to better correlate mineralogy with trends in OC concentrations. The relatively stable OC concentrations at depth suggest that OC is protected against remineralization. The persistence of iron(III) phases and presence of clays such as illite could provide protection of organic carbon at depth. Differences among the sites and with depth point to mineralogical controls on the amount and type of carbon preserved, which may point to mineral-hosted carbon as a dominant control on the subsurface biosphere.