Deep-Sea Bottom Currents: Their Nature and Distribution

The oceans are stratified into distinct layers with different physical and chemical properties. One of the principal divisions is between the
upper warm-water sphere and the deep cold-water sphere, separated by the thermocline. The thermocline is the zone of marked temperature
change from about 200 to 1000 m water depth. For physical oceanographers, deep-sea bottom currents are generally defined as the flow of
water masses in the cold-water sphere beneath the base of the thermocline (Zenk, 2008).
There are at least three different bottom current types that can be recognized as operating in deep water settings (Shanmugam, 2008;
Rebesco et al., 2014; Esentia et al., 2018) including: (a) wind-driven bottom currents, (b) thermohaline bottom currents, and (c) deepwater
tidal bottom currents, both barotropic and baroclinic. Bottom currents are also affected by intermittent processes, such as giant eddies,
benthic storms, flow cascading, and tsunamis. All of these currents and processes are capable of affecting seafloor sediment through their
erosion, transport and deposition.
Early work by the German physical oceanographer Wüst (1933) initially proposed that bottom currents driven by thermohaline
circulation might be sufficiently strong to influence sediment flux in the deep ocean basins. But his work was largely ignored until the early
1960s when Bruce Heezen of Woods Hole Oceanographic Institute took up the challenge from a marine geological perspective. In their now
seminal paper of 1966, Heezen, et al. demonstrated the very significant effects of contour following bottom currents or contour currents
in shaping sedimentation on the deep continental rise off eastern North America. The deposits of these semipermanent alongslope currents
soon became known as contourites, clearly distinguishing them from the deposits of downslope event processes known as turbidites. The
ensuing decade saw a profusion of research on contourites and bottom currents in and beneath the present-day oceans, and the demarcation
of slope-parallel, elongate, mounded sediment bodies made up largely of contourites that became known as contourite drifts (Hollister and
Heezen, 1972; McCave and Tucholke, 1986).
For the most part, physical oceanographers have worked independently of geologists on the nature and variability of bottom currents,
so that much integration is still required between these disciplines. Important contributions that to some extent bridge this divide have
come from the HEBBLE project on the Nova Scotian Rise (Hollister and McCave, 1984), work along the Brazilian continental margin
(Viana et al., 1998), and an extensive program of research in the Gulf of Cadiz, culminating in IODP Expedition 339 (Stow et al., 2013;
Hernandez-Molina et al. 2014). Prior to this latest mission, the international deep-sea drilling program in its various guises (DSDP, IPOD,
ODP, IODP) has contributed enormously to contourite research—the paleoceanographic context and study of oceanic gateways remain
primary targets at present. A topical synthesis of ocean currents can be found in Stow (2017).
This contribution in the Encyclopedia of Ocean Sciences is one of three on deep-sea bottom currents and their deposits. The focus
here is on the nature and variability of bottom currents, based largely on physical oceanographers’ observations of modern oceans, their
water mass structure and patterns of circulation. The other two contributions outline the contourite drifts, erosion surfaces and bedform
morphology caused by bottom current interaction with the seafloor, and the nature of bottom current deposits, known as contourites.