Abstract

Marine sediments constitute the major long-term sink for organic carbon (OC) on Earth, with <0.3% of the organic matter (OM) photosynthesized yearly in the global ocean ultimately becoming buried (Hedges and Keil, 1995). Burial of this small fraction of OM plays an important role in the global O2 and CO2 cycles, making elucidation of the mechanism controlling OM preservation of great importance. Three major OM fractions with contrasting chemical reactivities have been identified within sediments: labile (degraded in oxic/anoxic conditions), non-hydrolyzable (degraded in oxic conditions), and refractory (preserved in oxic/anoxic conditions). Identification of the reactivity classes, however, remains operationally defined; >75% of sedimentary OM remains molecularly uncharacterized. Bulk, spectroscopic and molecular techniques were used to investigate the mechanistic and chemical effects of physical protection and composition on OM preservation. A chemical fractionation procedure was implemented to separate and quantify OM from estuarine and lacustrine sediments with differing OM sources. Bulk (elemental and isotopic) analysis of the isolated fractions revealed the overall contribution of each class to total OM and any mechanistic relationships that may contribute to preservation. The project then focused on one fraction of particular interest, non-hydrolyzable organic matter (NHOM). Spectroscopic (FTIR and NMR) analysis was used to determine the chemical nature of NHOM. An optimized chemical oxidation utilizing ruthenium tetroxide (RuO4) was used to investigate the molecular structure, revealing a composition of cross-linked aliphatics regardless of the OM source. Compound specific isotope analysis of oxidation products suggested a common isotopic makeup of the preserved OM.