Surficial sediment stability on Georges Bank, in the Great South Channel and on eastern Nantucket Shoals

Surficial sediment stability was estimated on Georges Bank, in the Great South Channel and on eastern Nantucket Shoals (36,699 km2) by determining where benthic shear stresses derived from an ocean model matched or exceeded the critical shear stress of the observed surficial sediments. The shear stress resulting from M2 and S2 semi-diurnal tides was estimated with the Finite-Volume Community Ocean Model. Mixed-sediment critical shear stress levels were calculated for sediment compositions ranging from sand to boulder-dominated using 67,400 underwater video quadrats sampled from 1999 to 2010. Stresses matched or exceeded the sediment critical levels in 16,926 km2 (46%) of the study area, and were inversely related to water depth (r2=69.1%). In depths >50 m (10,953 km2) all sediments were stable due to weak flow (≤0.4 N m−2). In the shallower higher flow areas (>0.4 N m−2, 25,716 km2) only sediments containing gravel remained stable. The largest stresses occurred on Nantucket Shoals and central and northeastern Georges Bank (≥2 N m−2); in these areas only sand with cobbles or sediments dominated by gravel remained stable. Outcrops of these stable sediments were surrounded by highly unstable areas with stresses 2 to 9 times higher than the sediment critical levels. This analysis identifies the locations which likely remain stable even under the high shear stresses typical of Georges Bank, the Great South Channel and eastern Nantucket Shoals. Further, we provide the map products needed to begin investigating the influences of natural sediment disturbance on the spatial and temporal patterns of the benthos including the resilience of stable versus unstable areas to anthropogenic disturbances.

► First comprehensive sediment stability assessment for the Georges Bank region.
► Sediment composition based on 67,400 underwater video quadrats.
► 16,926 km2 (46%) of sediments unstable on bi-weekly basis.
► Stress inversely related to depth.
► Glacial deposits create a patchwork of stable outcrops in highly unstable sands.