CONTROLS ON SEDIMENTARY GEOCHEMISTRY AND ORGANIC CARBON BURIAL AT A LARGE GAS-HYDRATE AND COLD-SEEP FIELD ON THE NORTHERN GULF OF MEXICO SLOPE

This study evaluates late Pleistocene-Holocene sedimentation, sedimentary geochemistry, and organic-carbon burial at the Mississippi Canyon 118 (MC118) gas-hydrate and cold-seep field - the first National Gas Hydrate Seafloor Observatory - to provide a paleoenvironmental context for ongoing geochemical and geophysical monitoring. A chemical stratigraphy is established from sediments that were recovered in 10 shallow gravity cores surrounding the MC118 field. Geochemical data (e.g., X-ray fluorescence core scanning, CO₂ coulometry, and inductively coupled plasma emission spectrometry) are evaluated within the context of a detailed geochronology, to map geochemical (e.g., Al, Ca, Ti, Ba, Fe, CaCO₃ and TOC) mass accumulation rates across the MC118 site during specific intervals that represent substantial changes in depositional environment.

This unique dataset provides a means to assess quantitatively the temporal evolution of spatial patterns in geochemistry across the seafloor, and its relation to the position of the modern MC118 gas-hydrate and cold-seep field. The results of this mapping indicate that terrigenous inputs are the primary control on temporal changes in sedimentation and organic carbon burial, linked to sea level change and Mississippi delta lobe switching. Lithogenic inputs dramatically increase at each core site during the latest Pleistocene; (15,000 to 10,000 years BP) while conversely, CaCO₃ accumulation (presumably pelagic sourced) is more uniform. The gas-hydrate and cold-seep field itself appears to provide a secondary control on temporal changes in sedimentation and sediment geochemistry, by modulating the geomorphology of the seabed in concert with seafloor warping by salt deposits. In general, the analyses indicate reduced accumulations of all sedimentary components in close proximity to the field. This result suggests a stable spatial heterogeneity of sedimentation associated with the field and the dynamics of sedimentation within its proximity.