644 SEDIMENTARY PROCESSES/Deep Water Processes and Deposits parallels the continental margin Thus, for example, on the western side of the North Atlantic Ocean a powerful surface current (the Labrador Current) strongly influences the seafloor to water depths of 1500 m, and the deep Western Boundary Undercurrent of cold saline water derived from the Norwegian and Greenland Seas flows southwards at water depths of 3500–5000 m off the coast of Canada and the USA Constricting topography results in the intensification of currents, for example around and over seamounts, through major fracture zones in the mid-ocean ridge system, and where microcontinental fragments such as Rockall Bank or the Campbell Plateau create constrictions The mean flow in both surface currents and the thermohaline circulation is generally a few centimetres per second, but locally the flow can be complex and energetic, for example in the large eddies in the Gulf Stream and Kuroshio, the former creating the ‘abyssal storms’ measured at the Hebble site in the Western Boundary Undercurrent of the North Atlantic, where velocities exceed 0.2 m s Similarly high velocities are found in topographical constrictions, for example in the Gulf of Cadiz, where Mediterranean Overflow Water exits the Straits of Gibraltar Such flows are capable of eroding finegrained seafloor sediment, may leave lag sands and gravels, and produce a variety of abyssal bedforms Suspended sediment derived from the continental margin or suspended by ocean currents may be advected long distances as nepheloid layers – zones of higher amounts of particulate matter in the ocean circulation In lower energy areas, this suspended matter settles to form sediment drifts Boundaries between different thermohaline water masses are commonly marked by zones of higher turbulence and internal waves, which lead to higher amounts of suspended particulate matter and, in some cases, to erosion where they interact with the continental slope or with seamounts Organic matter sinking from the surface waters of the ocean is progressively oxidized, thereby depleting mid-level waters in oxygen and at the same time returning many nutrients to seawater These nutrients become available in areas of upwelling Deeper waters therefore have higher concentrations of dissolved carbon dioxide, with the highest concentrations being found in the older deep water of the Pacific Ocean Deep waters of the Pacific Ocean are consequently undersaturated in calcium carbonate, leading to the dissolution of biogenic carbonate Most dissolution of opaline silica occurs during oxidation of the protoplasm, but further dissolution takes place in undersaturated deep waters The wind-driven surface circulation is sedimentologically important in dispersing surface plumes derived from rivers and glacial margins In addition, icebergs are transported in the surface circulation and, as they reach warmer waters and melt, they deposit ice-rafted detritus on the seabed During Pleistocene Heinrich events in the North Atlantic Ocean, surface melt-water plumes were dispersed 2500 km south-eastwards from Hudson Strait and icebergs were dispersed even farther by the surface circulation Atmospheric circulation is a lesser agent of sediment transport to the deep ocean Volcanic eruptions contribute fine volcanic ash through both troposphere plume transport, which may extend for thousands of kilometres, and worldwide distribution of fine ash that reaches the stratosphere Dust derived from deserts is the principal source of terrigenous material in much of the mid-latitude pelagic realm Infrequent gravity-driven processes, including turbidity currents, debris flows, and submarine landslides, transport the largest volumes of sediment from the shallow continental margins to the deep sea (see Sedimentary Processes: Particle-Driven Subaqueous Gravity Processes) A turbidity current is a density current in which the denser fluid is a sediment suspension Turbidity currents may be initiated by many processes: all that are needed are a mechanism to put sediment into suspension and a steep slope to maintain the flow Thus, hyperpycnal flow of rivers, glacial outburst floods (joă kulhlaups), storm resuspension of shelf or beach sediment (see Sedimentary Environments: Storms and Storm Deposits), and in-mixing of ambient water into a debris flow will all initiate turbidity currents on steep slopes Potential energy is converted to kinetic energy as the flow accelerates down the continental slope, commonly eroding the seafloor and eventually depositing sediment on the deep ocean floor Processes within an individual turbidity current may range from hyperconcentrated flow at the head and base of the flow to normal turbulent sediment suspension in the upper part and tail of the flow, ultimately producing a low-density suspension, which contributes to nepheloid-layer dispersion by the thermohaline circulation Turbidity currents occur every 1–10 years in steep river-fed basins with narrow shelves and off large shelf-crossing rivers, but only every 100–1000 years on most continental margins and every 1000–10 000 years on distal abyssal plains The largest submarine landslides involve debris avalanches of blocks tens to hundreds of metres in size, which flow in a laminar fashion and scour deep grooves into the seafloor Slide blocks overlying sands liquefied by seismic shaking and muddy rotational slumps may break up and in-mix water to evolve into debris flows Large coherent slides may plough