Chapter One Deep-Sea Sediment Deposits and Properties Controlled by Currents

Abstract

This chapter focuses on the deep-sea sediment deposits and properties controlled by currents. Initial recognition of the importance of deep currents in redistributing sediments came from the combination of: (1) photographic evidence for bedforms under known currents, (2) photographic evidence for bedforms under known currents, (3) acoustic profiler data from echo-sounder and 3.5 kHz, and (4) seismic reflection profiles documenting large sediment bodies under current systems. Deep-sea sediments normally show few structures other than biological disturbance, therefore, grain size parameters have been used as the best indicator of relative flow speed. The deep circulation does not deliver sediment to the ocean, but reworks it once it has arrived. Sediment delivery is mainly by gravity-driven processes. In the open ocean far from land, pelagic sinking flux is rapid—on a timescale of several tens of days — because of particle aggregation and biological packaging. Close to continental margins, sediment is carried downslope in copious quantities by turbidity currents and debris flows that are responsible for construction of much of the continental rise. Gravity flows do not deliver sediment bearing a pure signature characteristic of conditions in the source area at the time they were triggered. Strong deep-sea currents resuspend this sediment, forming nepheloid layers, and move it to areas of spatial decrease in flow speed, causing deposition. The chapter discusses the biological indicators of flow speed, global ocean flow patterns, sediment transport and deposition by deep-sea currents, and current problems and prospects.