Interaction of the Western Boundary Undercurrent with the continental margin off Newfoundland

Abstract

ABSTRACTSedimentation on the Newfoundland rise is strongly influenced by the Western Boundary Undercurrent (WBU). The upper rise (2600‐2800 m) is swept by a rapid (Ū= 8·5 cm sec−1), south‐flowing core of the WBU which has generated a sandy contourite facies characterized by coarse gravelly, sandy sediments; current‐induced bedforms such as scour moats, lineations and lee drifts; ferro‐manganese‐stained gravel clasts; a high proportion of broken foraminiferal tests and a diagnostic benthic foraminiferal assemblage. The overlying nepheloid layer, when compared to adjacent waters, is thickest (800 m), most sediment laden (80 μg 1−1), contains the highest proportion of terrigenous sediment and exhibits the best developed bottom mixed layer (∼ 15 m thick). Comparisons with earlier data from the same area imply the dimensions and sediment load of the nepheloid layer vary with time. Empirical considerations, based on near‐bottom current meter records from Labrador and Newfoundland, suggest the WBU is capable of transporting bedload with threshold friction velocities (u*) of around 0·87‐1·14 cm sec−1 for between 1 and 15% of the time. The prevailing transport direction is southwards along the rise, but this may be punctuated periodically by brief incursions to the north.The erosional regime of the upper rise is bordered by a regime of fine‐grained deposition typified by muddy contourites. Both the lower slope and lower rise are mantled by bioturbated muds, the former zone having terrigenous mud and the latter, biogenic calcareous mud. The accompanying nepheloid layer is thin, biogenic‐rich and devoid of an identifiable mixed layer.