Discovery and characterization of two methane seep fields with unique fluid discharge features on the US Atlantic continental margin

Analysis of over 94,000 km² of multibeam echosounder water column backscatter data collected by NOAA ship Okeanos Explorer between 2011 and 2013 revealed widespread methane seepage at over 570 locations on the northern US Atlantic margin. Subsequent analysis of 85,000 km² of more recently collected water column data revealed over 100 additional methane seeps on the margin. Exploration of two of these more recently discovered seep fields in 2019, with the NOAA ROV Deep Discoverer, confirmed the presence of gas seepage and documented unique seafloor fluid discharge patterns and structures, which have not previously been observed at methane seeps on the northern US Atlantic margin. Exploration of the Bodie Island seep field revealed a sharp contrast in seafloor characteristics between seeps in close proximity that was coincident with variation in seafloor morphology. Seeps on the south side of a continental slope ridge at a depth of 415-360 m were characterized by exceptionally thick authigenic carbonate rock outcrops (<1 m) and robust chemosynthetic ecosystems. Conversely, proximal (within 50 m) seeps on the north side of the ridge exhibited a pronounced absence of authigenic carbonate rock outcrop and very limited chemosynthetic communities. Exploration of newly discovered seeps adjacent to the known Norfolk seep field revealed unique seafloor fluid discharge mound structures. The surface of a 1 m tall mound observed at a depth 1590 m was covered in numerous pores that emitted small jets (~2 cm tall) of sediment laden fluid. The fluid ran downslope and suspension of some of the discharged sediment resulted in an area of elevated turbidity immediately around the feature. Several smaller and lower relief mounds were observed nearby, with some discharging fluid in a similar manner. Here, we evaluate multiple physical and geochemical processes that may be responsible for these unique fluid discharge patterns and structures.