Studies of deep-sea sedimentary microtopography in the North Atlantic Ocean

Abstract

Many of the small-scale topographic features (dimensions of centimeters to kilometers) found on the Blake-Bahama Outer Ridge (western North Atiantic, water depth greater than 4000 m) and in the Rockall Trough (northeastern North Atlantic, water depth greater than 2000 m) have been formed as bed forms of deep currents. These bed forms, all developed in cohesive sediments, include current ripples (spacings of tens of centimeters, formed transverse to the flow), longitudinal triangular ripples (spacings of meters, formed in sandy muds and parallel to the flow), furrows (spacings of tens to 100's of meters, formed parallel to the flow and presently either erosional or depositional), and regular sediment waves (spacings of a few kilometers, now found oblique to the flow and migrating either upstream or downstream). The local distribution of any given bed form is influenced by the presence of larger features. Bed forms are often found in zones which strike parallel to the regional contours. Debris flows, affecting areas of 1000's to 10,000's of square kilometers, are also present in these areas. A debris flow studied in the Rockall Trough is erosional at its shallowest depths and depositional at greater depths. Gravitational flows strike perpendicular to the contours. Pockmarks (tens of meters in diameter, marking fluid seeps) are also found on the Blake-Bahama Outer Ridge. The larger topographic features (greater than several meters) with steep slopes (greater than about 20°) can be observed on surface echo-sounding profiles either as fields of regular hyperbolic echoes (e.g., echoes from regularly spaced furrows), fields of irregularly spaced, dissimilar hyperbolae (e.g., echoes from blocks, ridges, and folds in debris flows), or as regular features whose structure is often obscured by side echoes (e.g., echoes from sediment waves). Although near-bottom investigations are required to describe the features, the nature of the sea floor can often be inferred from the character of the echo-sounding profile. Similar echo-sounding records in different areas of the ocean indicate the presence of similar sea-floor features. The morphology of the bed forms studied and the current and temperature structure of the overlying water column lead to conclusions about bed form origin and present-day interactions with deep currents. Furrows form as erosional bed forms during high-velocity (>20? cm/sec) current events by large, helical secondary circulations in the bottom boundary layer. Once formed, furrows may develop into depositional features, or they may continue as erosional ones, depending on the local currents and the sediment supply. Large, regular sediment waves may be formed at current speeds of 5 to 10 cm/sec by lee waves generated by topographic irregularities on the sea floor, such as submarine canyons, or by instabilities in the flow of deep, contour-following currents. Sediment waves develop where there is an abundant supply of sediment and steady mean currents. Waves appear to migrate upstream where tidal current fluctuations are smaller than the mean velocity, and downstream where they are larger. Near-bottom currents appear to be faster on the downstream side of upstream-migrating sediment waves than on their upstream side. The resulting variations in bed shear stress lead to higher sedimentation rates on the upstream side and bed form migration in that direction.