Introduction

The deep sea is that part of the ocean beyond the shelf break and remains one of the least explored areas of the Earth. The topography varies depending on the geological setting. Passive margins such as those around most of the Atlantic have a continental slope which passes oceanwards into a continental rise beyond which there is abyssal plain. Active margins may have more complex topography with perched sedimentary basins and terraces on the slope and a trench at the base. Slopes may be cut by valleys, known as submarine canyons, which act as conduits for the transport of sediment by turbidity currents or mass flow into deeper water. In the centre of the Atlantic and Indian oceans there is a mid-ocean ridge system; in the Pacific it is in the east. Between the ocean ridge and the continental rises there are abyssal plains. There is no universal agreement on the terminology of topographic divisions of the oceans. In this book the term bathyal refers to the continental slope and rise from the shelf break to ∼4 km, abyssal to areas > 4 km and hadal is used for trenches where depths reach as much as 11 km.

Oceanography

Good general overviews of oceanic circulation are given in Kennett (1982) and Gage and Tyler (1991). Each water mass has its own temperature–salinity characteristics, which define its density. Where water masses are layered, the densest is at the bottom (thermohaline stratification).