Morphology and composition of submarine barchan dunes on the Scotian Shelf, Canadian Atlantic margin

Submarine barchan dunes have been mapped on Browns Bank, Scotian Shelf, using multibeam sonar bathymetry and backscatter strength. The morphology of subaerial barchans has been studied for almost a century but the advent of multibeam sonar technology now enables a quantitative investigation of their submarine counterparts. The Browns Bank submarine barchans occur at a depth of 60-70 m and are crescentic in planform, reaching almost 700 m in horn width and 5 m in height. The barchans are convex to the SE with steep lee faces to the NW, indicating a dominant NW-flowing current. The barchans overlie a widespread gravel lag covered elsewhere with little or no sand. Obstacle marks emanate from the lee faces of the barchans and represent a lack of sand deposition and exposure of gravel lag on the sea floor. The Browns Bank submarine barchan sediment texture is gravelly sand or sandy gravel and is primarily composed of subrounded to well-rounded quartz grains. The allometric relationship between submarine barchan slip face height and distance between horns is markedly different from that of subaerial barchans. For the same dune height, barchan horn width is about ten times greater in the marine environment. The superimposition of megaripples on the stoss slopes of the submarine barchans suggests that the barchans are active and therefore represent an engineering risk to sea floor infrastructure. Current observations and models indicate that seasonal mean current strength is less than the critical velocity required for barchan migration. However, the barchans may be active under higher-velocity, storm-induced currents. Repetitive multibeam sonar mapping is required to detect if barchan migration is occurring over longer time scales.