US Atlantic Margin Methane Plumes Identified From Water Column Backscatter Data Acquired by NOAA Ship Okeanos Explorer

The NOAA Office of Ocean Exploration and Research routinely uses NOAA Ship Okeanos Explorer to collect EM302 (30 kHz) multibeam bathymetric data and water column backscatter imagery. These backscatter data have been used to identify gas plumes associated with seafloor methane seeps as part of previous investigations in the Gulf of Mexico and at Blake Ridge. Here, we use QPS Fledermaus Midwater software to analyze over 200,000 km2 of multibeam data acquired on the continental slope and outer shelf of the US Atlantic margin in 2011, 2012, and 2013. Preliminary application of this analytical methodology in late 2012 revealed the first deepwater (> 1000 m water depth) cold seeps found on the US Atlantic margin north of Cape Hatteras as well as 47 new upper slope seeps (http://www.noaanews.noaa.gov/stories2012/20121219_gas_seeps.html). In this new analysis, we identify over 500 water column backscatter anomalies (WCA) originating at the seafloor and extending to various heights in the water column between Cape Hatteras and the Nantucket margin. Data set quality control was achieved through secondary independent analysis of all WCA backscatter records by a highly experienced researcher who assigned a quality factor to each anomaly. Additionally, a subset of the data was analyzed using a Matlab code designed to automatically detect WCA in backscatter data. These quality-control and WCA comparison procedures provide confidence that several hundred of the WCA are robust picks. The observed WCA are structurally consistent with previously confirmed gas bubble plumes, being vertically elongate, rooted at the seafloor, and deflected by currents. They are not structurally consistent with other common WCA such as schooling or swarming organisms. Additionally, the bases of selected WCA that were identified in this analysis have recently been visually and acoustically confirmed to be associated with emission of gas bubbles from the seafloor by the NOAA remotely operated vehicle Deep Discoverer. The physical characteristics and location of the WCA suggest that they are likely methane plumes, although this has yet to be confirmed by direct gas sampling. The WCA occur both in isolation and in clusters, and repeated observation of select seep fields indicated intermittent WCA identifications that could not be explained by uncertainties in the spatial resolution of the data. Thus, some of the WCA appear to exhibit ephemerality on time scales of hours to days. This research was undertaken while the lead author was a NOAA Hollings Scholar intern with the NOAA Office of Ocean Exploration and Research.