Abyssal circulation change In the equatorial Atlantic: Evidence from Cenozoic sedimentary drifts off West Africa

摘要

Seismic profiling in the equatorial Atlantic reveals deep-water (>4500 m) sediment bodies formed by current-controlled deposition near the intersection of large-offset fracture zones with the African margin. A 600 km-long drift accumulation, the Ivory Coast Rise, lies north of the St Paul Transform near 3 deg N. A smaller drift deposit has been identified along the northern side of the Guinea Transform at 10 deg N. Antarctic Bottom Water (AABW), which presently enters the eastern Atlantic basins through the Romanche and Vema Fracture Zones, probably played an important role in the development of these features. Proto-AABW may have reached the equatorial region as early as mid-Eocene time, before the establishment of permanent ice sheets in Antarctica. The Ivory Coast Rise existed as a distinct sedimentary drift by the mid-Eocene as a result of deposition from bottom water moving southwards along the African margin and westwards parallel to the St Paul Fracture Zone. This early flow pattern is in the opposite sense to the present movement of deep water in the Sierra Leone Basin. A reversal in abyssal circulation may have been caused by the northward passage of the region across the paleoequator during the Cenozoic.