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Seismic stratigraphic evidence from the western and northern North Atlantic indicates that a major change in abyssal circulation occurred in the latest Eocene to earliest Oligocene. In the northern North Atlantic, the widely distributed reflector R4 correlates with an unconformity that can be traced to its correlative conformity near the top of the Eocene. This horizon reflects a change from weakly (Eocene) to vigorously (early Oligocene) circulating bottom water. Sediment distribution patterns provide evidence for strong contour-following bottom water flow beginning at reflector R4 time; this suggests a northern source for this bottom water, probably from the Arctic via the Norwegian-Greenland Sea and Faeroe-Shetland Channel. Erosion and current-controlled sedimentation ontinued through the Oligocene; however, above reflector R3 (middle to upper Oligocene), the intensity of abyssal currents decreased. Above reflector R2 (upper lower Miocene), current-controlled sedimentation became more coherent and a major phase of sedimentary drift development began. This resulted from further reduction in speeds and stabilization of abyssal currents.
Late Paleogene paleontological and stable isotopic data support these interpretations. In the Bay of Biscay/Goban Spur regions, a major ^dgr18O increase began at ~38 Ma (late Eocene), culminating in a rapid (< 0.5 m.y.) increase in ^dgr18O just above the Eocene/Oligocene boundary (~36.5 Ma). A rapid ^dgr13C increase also occurred at ~36.5 Ma in these sites. Major changes in benthic foraminiferal assemblages also occurred between the middle Eocene and the earliest Oligocene: (1) In the Labrador Sea, a predominantly agglutinated assemblage was replaced by a calcareous assemblage between the middle Eocene and early Oligocene, (2) In the abyssal (> 3 km, 10,000 ft paleodepth) Bay of Biscay, an indigenous Eocene calcareous fauna including Nuttallides ruempyi, Clinapertina spp., Abyssammina spp., Aragonia spp., and Alabamina dissonata became extinct between the middle Eocene and earliest Oligocene, (3) In shallower sites (< 3 km, 10,000 ft paleodepth) throughout the Atlantic, a Nuttallides truempyi-dominated assemblage was replaced by a Globocassidulina subglobosa-Gyroidinoides-Cibicidoides ungerianus-Oridorsalis assemblage in the early late Eocene (~40 to 38.5 Ma). These faunal and isotopic changes represent the transition from warm, old, corrosive Eocene bottom waters to colder, younger (lower CO2 and higher pH, hence less corrosive) early Oligocene bottom waters.
A 18O enrichment noted previously in the Southern and Indian Oceans is synchronous with the enrichment in the North Atlantic. The enrichment probably cannot be attributed only to initial entry of Arctic/Norwegian-Greenland Sea sources of cold bottom water. There is evidence that initial formation of cold, vigorously circulating bottom water from both northern sources (as denoted by reflector R4 and Horizon Au) and southern sources (as denoted by erosion of widespread unconformities and other changes previously described from the Southern and Pacific Oceans) began near the end of the Eocene. These events also were reflected by a major 18O enrichment. High-salinity water provided by North Atlantic deep water is important in the formation of Antarctic bot om water today. Such linkages or "teleconnections" might be invoked to explain the formation of southern bottom-water sources following the tectonically-controlled entry of northern sources of bottom water into the North Atlantic.
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