Elucidating Geochemical Controls on the Concentration and Composition of Organic Carbon in Deep Pelagic Sediments

Thursday, 17 December 2015
Poster Hall (Moscone South)
Emily R Estes1, Colleen M Hansel1, Chloe H Anderson2, Richard W Murray2, Melinda Darby Dyar3, Dennis Nordlund4, Scott D Wankel5, Dana Johnson6, Arthur J Spivack7, Justine Sauvage8, Claire Cecelia McKinley9, Kira Homola10, Theodore Michael Present11, Robert A Pockalny12 and Steven D'Hondt13, (1)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (2)Boston University, Boston, MA, United States, (3)Mount Holyoke College, South Hadley, MA, United States, (4)Stanford Synchrotron Radiation Lightsource, Menlo Park, CA, United States, (5)Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, MA, United States, (6)Northwestern University, Evanston, IL, United States, (7)University of Rhode Island - GSO, Oceanography, Narragansett, RI, United States, (8)University of Rhode Island - GSO, West Warwick, RI, United States, (9)Texas A & M University College Station, College Station, TX, United States, (10)University of Rhode Island, Graduate School of Oceanography, Narragansett, RI, United States, (11)California Institute of Technology, Pasadena, CA, United States, (12)Univ Rhode Island, Narragansett, RI, United States, (13)University of Rhode Island, Narragansett, RI, United States
In marine sediments, total organic carbon (OC) content correlates strongly with mineral surface area as well as the abundance of specific mineral classes such as clays and metal oxides. Adsorption to mineral surfaces and the formation of mineral-organic matter aggregates are thought to provide protection against remineralization, yet the extent and mechanism(s) of this protection are unknown. Accordingly, the goal of this research is to elucidate the role of minerals in preserving carbon and the potential for this reservoir of mineral-hosted carbon to support heterotrophic metabolisms in the otherwise carbon-poor subseafloor. Here, we characterize the composition of OC in oxic and suboxic sediments collected during R/V Knorr expedition 223 to the subtropical western North Atlantic in November 2014. We find that OC concentrations decrease linearly over ~25 meters burial depth, from ~0.15 to 0.075 mol OC/kg solid. Organic C/N varies but is consistently less than Redfield values of ~6. Relative contributions of functional groups quantified using bulk-scale Fourier transform infrared (FTIR) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy change with depth and site lithology/geochemistry. We further observe microscale heterogeneity, including discrete carbonate particles amid disperse aromatic and amide/carboxylic-rich organic carbon, using scanning transmission X-ray microscopy (STXM) coupled to NEXAFS. In the suboxic sediments, there is a transition from Mn(III/IV) phases toward more reduced phases shown by X-ray absorption spectroscopy between ~3-11 meters below core top, approximately between the interstitial water nitrate and nitrite maxima. Conversely, Fe(III)-bearing minerals are present throughout the core and may contribute to stabilization of OC. By further coupling micro- and macro-scale analysis, the role of minerals in OC sequestration in the marine subsurface will come to light.