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The earliest phase of intense abyssal circulation in the North Atlantic occurred near the Eocene-Oligocene boundary about 36 to 38 Ma. The abyssal currents eroded a prominent unconformity that is best developed along and near basin margins and at paleodepths below about 3 km (9,800 ft). The unconformity forms a strong reflecting interface in both the northern North Atlantic (reflector R4) and the western North Atlantic (horizon Au). We estimate from the geometric relations of the unconformity that about 0.5 × 106 km3 of sediment were eroded from the margins of the northern and western North Atlantic basins. Below the unconformity, bedding relations in seismic reflection profiles show little evidence for significant abyssal circulation however, deep-sea boreholes have recovered abundant biosiliceous Eocene sediments that may be related to increased upwelling that was stimulated by a weaker, precursory abyssal circulation phase.
Erosional and depositional patterns indicate that the bottom water source for the early Oligocene abyssal circulation system was in the northern Atlantic. The Greenland Sea opened about this time and may have provided a passage for dense, intermediate or deep Arctic water to enter first the Norwegian Sea and then the North Atlantic via passages in the Greenland-Scotland Ridge (Denmark Straits and Faeroe-Shetland Channel). This water presumably penetrated into the South Atlantic and beyond, transporting its suspended sediment load to depositional locales peripheral to and away from basins margins. Penetration of the water mass to circum-Antarctic areas may have provided a reinforcing teleconnection that aided formation of deep and bottom water around Antarctica, which apparently was pr requisite to initial development of the global psycrosphere at this time.
There is a suggestion in the distribution of the unconformity in North Atlantic borehole and seismic data that with time the abyssal currents progressively affected shallower sea floor (< 3 km; < 9,800 ft) and became more restricted to proximal areas of basin margins. Along the eastern United States continental slope and probably elsewhere, the unconformity locally was reexcavated by erosion (turbidity currents and sediment mass movements) associated with a mid-Oligocene sea-level lowstand. Flow intensity of the abyssal current system decreased throughout the Oligocene. By late Oligocene to early Miocene time, current-controlled deposition, as opposed to erosion and/or transport, was widespread. Most of the major sedimentary ridges first began to nucleate at this time and sedime t waves became a common and persistent feature in the sedimentary record.
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