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Solid-state C NMR analysis of size and density fractions of marine sediments: Insight into organic carbon sources and preservation mechanisms |
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| Authors: | Angela F Dickens Jeffrey A Baldock Stuart G Wakeham Yves Gélinas |
| Institution: | a School of Oceanography, University of Washington, Box 355351, Seattle, WA 98195-5351, USA b Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195-1700, USA c CSIRO Land and Water, PMB #2, Glen Osmond, SA 5064, Australia d Soil and Land Systems, School of Earth and Environmental Sciences, The University of Adelaide, Waite Campus, Glen Osmond, SA 5064, Australia e Skidaway Institute of Oceanography, 10 Ocean Sciences Circle, Savannah, GA 31411, USA f Chemistry and Biochemistry Department, Concordia University, 1455, de Maisonneuve Blvd West, Montreal, Que., Canada H3G 1M8 |
| Abstract: | Burial of organic carbon (OC) in ocean sediments acts as the ultimate long-term sink for both terrestrial and marine carbon, however, the mechanisms controlling the preservation of this carbon are poorly understood. To better understand these mechanisms, we applied solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, along with elemental, stable carbon isotopic (δ13C) and lignin phenol analyses, to size and density fractions of sediments influenced by either mixed terrestrial and marine OC inputs (Washington Coast slope) or dominantly marine inputs (Mexican Margin). Elemental, isotopic and lignin analyses all reveal that within the Washington Coast sediment, the OC mixes linearly between nitrogen-poor and 13C-depleted, lignin-rich OC in the large and light fractions and nitrogen-rich and 13C-enriched, lignin-poor OC in the small and dense fractions, suggesting that this sediment contains a two-component mixture of terrestrial vascular plant- and marine-derived OC. The integral areas of each of seven NMR spectral regions in the different samples trend linearly when plotted versus δ13C signature, with most R2 values of 0.78 or greater, demonstrating that the NMR spectra of the two sources of carbon also mix linearly between the two endmembers. The terrestrial endmember in this sediment appears to be dominated by lignin and black carbon whereas the source of the marine endmember is less clear from the NMR spectra. In contrast, all of the analyses indicate that OC in the Mexican Margin sediment fractions is homogenous and derives almost exclusively from marine sources. It appears that selective preservation of (bio)chemically recalcitrant lignin and black carbon is the primary mechanism of preservation of terrestrial OC, whereas mineral-protection is the dominant mechanism preserving marine OC in the Washington coast sediment. There is little evidence showing that either preservation mechanism functions in the Mexican Margin sediments. |
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