Origin of longitudinal triangular ripples on the Nova Scotian continental rise

Abstract

Since the recognition of current-produced bedforms in the deep ocean, attempts have been made to predict direction, intensity and variability of abyssal flow from the nature of sea floor irregularities (see refs 1–3). Longitudinal triangular ripples (LTRs) are inter mediate-scale bedforms (<20 cm high, metres long) that are becoming recognized as important and geographically widespread predictors of bottom-current flow. I describe here recent sea floor photographs of LTR fields on the lower continental rise off Nova Scotia which show that the bedforms develop in fine-grained, cohesive sediment with an alignment parallel to the regional contours and to mean current direction. An observational model for LTR formation is proposed, wherein LTRs are deposited initially as ‘tails’ behind large sea floor biological mounds and are subsequently constructed and propagate in the direction of mean current by sediment transport oblique to the bedform axis. This transport is induced by short-term currents that deviate from mean current direction and cause formation of a separation bubble on one side or the other of the LTR crest.