AUTHIGENIC MANGANESE DEPOSITS SURROUNDING THE MISSISSIPPI CANYON 118 GAS HYDRATE AND COLD-SEEP FIELD: THEIR DOCUMENTATION, INTERPRETATION, AND UTILITY FOR EVALUATING REDOX STATE AND STABILITY OF SEDIMENTATION

This study investigates a unique deep-sea manganese (Mn) record obtained from ten sediment cores surrounding a large gas-hydrate and cold-seep field situated on the northern Gulf of Mexico Slope (Mississippi Canyon 118; MC118), the first National Gas Hydrate Seafloor Observatory. Mn deposits in late Quaternary sediments surrounding the field are well documented by high-resolution (1-cm scale) X-ray fluorescence (XRF) core scanning, and occur as enriched layers (up to 2800 ppm) ranging from 1-10 cm thick (henceforth “Mn-layers”). Evaluation of the Mn record within the context of the established chronostratigraphy demonstrates that the Mn-layers are generally not stratigraphically correlative, and they show no consistent relationship to XRF proxies for biogenic or lithogenic deposition. Thus, these Mn-layers are interpreted as authigenic deposits.

Evaluation of the Mn-layers using reductive dissolution and acidification procedures indicates they are primarily Mn carbonate, suggesting dysoxic deposition associated with a “Mn-pump”. Spatially, the individual Mn-layers vary in concentration and frequency with respect to the position of seafloor gas hydrates and cold seeps, occurring more frequently and with lower concentrations of Mn at core sites positioned adjacent to MC118 field. This study investigates this key observation and a possible connection to dynamic sedimentation, which is in turn linked to the geomorphology of the field itself (related to salt diapirism), and importantly, variable redox conditions through changes in the duration and frequency of steady state versus non-steady state conditions. These highly-resolved Mn-deposits may provide important insight into the consistency of sedimentation at MC118, and by inference, related interactions between seafloor gas hydrates, cold seeps and associated salt diapirism at the site during the late Pleistocene and Holocene.