Abstracts Chemistry

Late Quaternary Depositional History, Sedimentary Geochemistry, and Organic Carbon Burial at Mississippi Canyon 118: A Deep-sea Site on the Northern Gulf of Mexico Slope Containing a Gas-Hydrate and Cold-Seep Field

by Wesley Clint Ingram

Dissertation Abstract Wesley Ingram Late Quaternary Depositional History, Sedimentary Geochemistry, and Organic Carbon Burial at Mississippi Canyon 118: A Deep-sea Site on the Northern Gulf of Mexico Slope Containing a Gas-Hydrate and Cold-Seep Field (Under the direction of Dr. Stephen Meyers) This dissertation investigates late Quaternary sediments surrounding a gas-hydrate and cold-seep field situated on the northern Gulf of Mexico slope, to reconstruct depositional history, document sedimentary geochemistry, and evaluate organic carbon burial. The study locale encompasses the first Gas Hydrate Seafloor Observatory in the Gulf of Mexico, located within offshore federal lease block Mississippi Canyon 118 (MC118). Sediments recovered in 10 gravity cores surround the MC118 gas-hydrate and cold-seep field and yield a record spanning 20 ka-present. Three related studies document the depositional history at MC118, through assessment of: (1) lithostratigraphy/biostratigraphy/chronostratigraphy, (2) sedimentary geochemistry and organic carbon burial, and (3) redox processes, with a focus on manganese cycling. A central objective is evaluation of the impact of global (deglacial sea level rise), regional (delta lobe switching), and local (gas hydrates, seeps, salt diapirism) influences on sedimentation surrounding the MC118 field. A litho-bio-chronostratigraphic framework is developed using X-ray fluorescence (XRF) core scanning (Ca/Ti), biostratigraphy, and radiocarbon dating. Four stratigraphic units are identified; within each mapped stratigraphic unit, sedimentation rates generally increase with distance from the MC118 field and with age at each core site. This framework is integrated with a chemical stratigraphy (inorganic and organic geochemical data) to assess terrigenous versus pelagic sediment burial fluxes, and to evaluate the controls on organic carbon burial. Results indicate that terrigenous sediment burial flux is the primary control on temporal changes in sedimentation rate, bulk sediment composition and organic carbon burial, linked to factors such as deglacial sea level rise and delta lobe switching. All geochemical burial fluxes generally increase with distance from the MC118 field, suggesting a stable spatial pattern of deposition throughout the late Quaternary. The final study evaluates authigenic manganese layers, which vary in frequency and concentration with distance from the field. The manganese record is interpreted as indicating intermittent periods of steady state and non-steady state conditions during formation within dysoxic sediments, providing a novel means to assess stability of sedimentation. The results from this dissertation research provide an important paleoenvironmental context for ongoing geochemical and geophysical monitoring at the MC118 field.