GEOLOGICAL CHARACTERIZATION OF AN ACTIVE SHALLOW WATER COLD SEEP OFFSHORE FROM THE NORTHERN CHANNEL ISLANDS, CALIFORNIA, USA

We identified an active shallow water cold seep, the shallowest known in southern California, offshore from Anacapa Island, the easternmost of the northern Channel Islands. This seep has been given the Sʰamala (Chumashan language) name Ma saputiwaxmu’ (the place where it seeps through) by the Cultural Department at the Santa Ynez Band of Chumash Indians. This tribe is one band of the Chumash people, the original caretakers of the land where this research was conducted. We collected multibeam bathymetry and Chirp subbottom data and conducted ROV dives to visually and geomorphologically characterize the seep. The seep is located in ~90 m water depth on the shelf-platform and is characterized at the seafloor by a pockmark field ~2.2 x 0.6 km in size. The pockmark field is elongated in the NW-SE direction. The majority of the pockmarks are 20-35 m diameter, but a single larger pockmark is ~55 m diameter. This larger pockmark was visited by ROV dives and authigenic carbonate was observed on the seafloor. Additionally, white bacterial mats were observed on ROV dives throughout the pockmark field. In subbottom data, the large pockmark is located in what appears to be an erosional low in underlying laminated and folded stratigraphy. The low is filled with an acoustically transparent unit and is ~800 m wide and ~14 m deep. Water column noise is also observed in the subbottom data above the seep, suggesting active gas venting. Additionally, when seafloor bacterial mats were disturbed by the ROV, visible gas bubbles were released into the water. The shallow and nearshore location of this active cold seep presents an opportunity to understand seep characteristics and ecological communities in shallow, oxygen-rich continental shelf environments.