The dynamics of pore-water dissolved organic carbon and the fate of organic matter in anoxic marine sediments: An assessment using carbon isotopes and ultrafiltration

Pore-water DOC plays a significant role in the microbial degradation of organic matter in anoxic environments, the preservation of organic matter in marine sediment, and may contribute to the global oceanic DOC reservoir. The primary focus of this work was to elucidate the role pore-water DOC plays in the biogeochemical cycling of organic matter in marine sediments.; Four experiments were conducted. In the first, fresh organic matter was added to anoxic sediment; methanogenic and sulfate-reducing treatments were established; and the transformation of carbon from the particulate form, to dissolved organic intermediates and inorganic end products was traced. This experiment showed that the source of organic matter, not the terminal metabolic community, was the primary determinant of remineralization efficiency; and, through measurements of the isotopic signature of the degradation intermediates, that a fraction of the organic matter which was unreactive in situ, was degraded as a result of the addition of fresh substrate.; In the second experiment, methanogenic sediment was amended with fresh phytoplankton, and the concentration of DOC was measured as a function of time. Additionally, DOC was size fractionated using stirred-cell ultrafiltration. Designed to elucidate the relationship between reactivity and size, this experiment showed that net production of DOC occurred in the 10–100 kDa size range.; The third and fourth experiments were field experiments. 13C-labelled diatoms were placed on the seafloor, and samples were collected at t = 0, 1.5 days and 14 months. 13C-enriched POC at depths >10 cm indicated that the activities of macrobenthic organisms subducted labeled substrate in 1.5 days, and enriched DOC and dissolved inorganic carbon (DIC) isotopic signatures at depth 1.5 d after the carbon addition was evidence of rapid remineralization. After 14 months, 13C-labeled carbon remained at both sites, providing evidence of the importance of internal recycling of carbon and the profound effect that infauna can have on sediment geochemistry.; The fourth experiment utilized bomb 14C as a tracer for modern carbon in continental slope sediment. Unfortunately, beyond successful methods-development, this experiment failed to produce meaningful results due to a contamination problem.