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Major benthic foraminiferal changes occurred in the late Eocene at Site 549. A Nuttallides truempyi-dominated assemblage was replaced by a buliminid assemblage (-40-38.5 m.y. ago); this change in faunal abundance was apparently a circum-Atlantic event. The buliminid assemblage was replaced, in turn, by an assemblage dominated by stratigraphically long-ranging and bathymetrically wide-ranging taxa just below the Eocene/Oligocene boundary (~37.5 m.y. ago). A series of late Eocene to earliest Oligocene first and last appearances of taxa accompanied these abundance changes. Similar changes in faunal abundance occurred at Site 548, but the incomplete record recovered at Site 548 prevents a firm dating of the changes. No major benthic foraminiferal changes (denoted by the extinction of Hαntkeninα, Cribrohαntkeninα, and Globorotαliα cerroαzulensis) were associated with the Eocene/Oligocene boundary at these sites; instead, benthic foraminiferal abundance changes, last appearances (both extinctions and local disappearances), and first appearances occurred throughout the late Eocene to earliest Oligocene interval (-40-36 m.y. ago). Benthic foraminiferal δ 1 θ increased ~ l.O‰ in the late Eocene to earliest Oligocene (~ 38-36.5 m.y. ago) at Site 549. Most (~ 0.7‰) of this increase occurred simultaneously with a 0.6‰ increase in benthic foraminiferal δC as a geologically rapid event in the earliest Oligocene (-36.5 m.y. ago). The major benthic foraminiferal abundance change (-40-38 m.y. ago) predates the major isotopic enrichments. A prominent seismic horizon, Reflector R4, has been noted in the Labrador Sea, Rockall, and Biscay regions, and has been dated as latest Eocene to early Oligocene. This horizon marks the onset of increased intensity of abyssal circulation associated with the initial entry of bottom water from the Norwegian-Greenland Sea and/or Arctic Ocean into the North Atlantic. The interval during which Reflector R4 was formed may encompass both the faunal and isotopic changes; the best estimate for the age of this horizon and the associated circulation change suggests that it correlates with the δ 1 θ increase. Our interpretation of these data suggests that a temperature drop, decrease in age (increased O2, lowered CO2, increased pH, hence decreased corrosiveness) of bottom water, and an increase in intensity of abyssal circulation occurred in the late Eocene to earliest Oligocene of the North Atlantic.

Late Paleogene (Eocene to Oligocene) Benthic Foraminiferal Oceanography of the Goban Spur Region, Deep Sea Drilling Project Leg 80