Portrait of an Axial Volcanic Ridge: sidescan sonar and bathymetry at three resolutions

We present bathymetric and sidescan sonar results from the Mid-Atlantic Ridge between 45°20’N and 45°44’N, where the axial valley is host to a ~4km wide and ~500m high axial volcanic ridge (AVR). The AVR and surrounding seafloor was surveyed concurrently using: a) the ship-mounted Simrad EM-120 echosounder, yielding bathymetry gridded at a resolution of 50m per pixel, and b) TOBI (Towed Ocean Bottom Instrument), which collected both sidescan sonar data (resolution 3m/pixel) and phase data, which were processed to provide bathymetry with a resolution of 10m/pixel. The TOBI survey entailed two ~45km-long north-south tracks and twenty-two ~15km-long east-west tracks; thus, much of the area was surveyed from up to four orthogonal and reciprocal directions, eliminating most regions of shadow and poor image quality. Phase bathymetry was used to generate a synthetic sidescan image, which can be compared to the original sidescan image to subtract angle-of-incidence effects from the image brightness, yielding an image of seafloor reflectivity, a proxy for age. Two areas on the AVR (~4km2 and ~6km2, respectively) were surveyed using the MS2000 sonar mounted on the ROV Isis. These microbathymetry data have a resolution of ~0.5m and provide an independent check of the TOBI phase bathymetry processing techniques. Preliminary study of sidescan and bathymetry from the region suggests that the AVR consists of over 8000 individual monogenetic volcanic cones, and our seafloor reflectivity data allow us to investigate temporal variations in volcanic eruptions during the development of the AVR. The combination of comprehensive data coverage and hierarchy of resolutions of these datasets allow for an unprecedented study of the development of this AVR, which in turn allows for the development of new insights into the timing and distribution of the construction of the volcanic seafloor.