Benthic foraminifera from sediments of Holocene age exhibit a consistent bathymetric zonation along the continental slope and rise from the Grand Banks to Cape Hatteras. Slumping, gravity flow, and contour currents have not destroyed this pattern, although displaced benthic faunas can occur locally. Consistently recognizable faunas are dominated by Bulimina (less than 1,000 m), Uvigerina (1,000 to 2,600 m), Hoeglundina (2,600 to 3,800 m), and Osangularia (deeper than 3,800 m). The Holocene faunal pattern may reflect the position and intensity of contour-following currents, the physical and chemical nature of the bottom water, and the sediment type; all are interrelated.
In cores from the slope and rise (2,200 to 3,900 m), uppermost Pleistocene faunas are dominated by Uvigerina, while Hoeglundina is dominant or subdominant in the Holocene. This faunal transition is dated by radiocarbon. At 3,000 m, it occurs at about 12,000 yr B.P., and at 4,000 m it occurs at about 8,000 yr B.P. Below 4,000 m, the glacial to modern faunal shift is subtle, but it clearly occurs later than on the upper rise. The faunal turnover seems to coincide with the initiation of the modern North Atlantic circulation pattern. If so, the North Atlantic became an increasingly important source of deep water around 12,000 yr ago, and this dense water came to dominate the pattern of Atlantic circulation over a period of some 4,000 to 5,000 yr.