Detailed near-bottom investigation of a series of giant, kilometer scale, elongate
pockmarks along the edge of the mid-Atlantic continental shelf confirms that methane is
actively venting at the site. Dissolved methane concentrations, which were measured
with a commercially available methane sensor (METS) designed by Franatech GmbH
mounted on an autonomous underwater vehicle (AUV), are as high as 100 nM. These
values are well above expected background levels (1-4 nM) for the open ocean.
Sediment pore water geochemistry gives further evidence of methane advection through
the seafloor. Isotopically light carbon in the dissolved methane samples indicates a
primarily biogenic source. The spatial distribution of the near-bottom methane anomalies
(concentrations above open ocean background), combined with water column salinity and
temperature vertical profiles, indicate that methane-rich water is not present across the
entire width of the pockmarks, but is laterally restricted to their edges. We suggest that
venting is primarily along the top of the pockmark walls with some advection and
dispersion due to local currents. The highest methane concentrations observed with the
METS sensor occur at a small, circular pockmark at the southern end of the study area.
This observation is compatible with a scenario where the larger, elongate pockmarks
evolve through coalescing smaller pockmarks.