ALBERT

Description: he forty-first cruise of Glomar Challenger was devoted to the study of the evolution of the eastern basins of the North Atlantic, off the continental margin of West Africa. As the available data a the time was showing that most litho-stratigraphic units in the deep basins of the Atlantic had enough lateral extension, the drilling of a limited number of sites in key areas would allow for large-scale regional interpretation. One of the sites was loacted in the Cape Verde deep Basin (Site 367) while others were located in shallower waters such as, the Sierra Leone Rise (Site 366), the Cape Verde Rise (Site 368) or the Continental Slope off Spanish Sahara (Site 369).

Description: Pliocene and Pleistocene planktonic foraminiferal biogeography and paleoceanography have been examined in Deep Sea Drilling Project (DSDP) sites of the Panama Basin (Pacific Ocean) and Colombian and Venezuelan Basins (Atlantic Ocean) to determine the timing of the isolation of Atlantic and Pacific tropical planktonic faunas resulting from the development of the Central American isthmus. Previous studies have suggested a late Miocene to middle Pliocene occurrence of this event. The Panama Basin (DSDP site 157) and the Colombian Basin (DSDP site 154A) share two early Pliocene biogeographic events: (1) great abundance of sinistral coiling Neogloboquadrina pachyderma at 4.3 m.y. ago at site 157 and 0.7 m.y. later at site 154A, and (2) a sinistral-to-dextral change in the coiling-direction preference in Pulleniatina 3.5 m.y. ago at both locations. Identification of these events farther to the east in the Venezuelan Basin (DSDP site 148) is complicated by insufficient lower Pliocene core recovery, but abundant sinistral N. pachydcrma appear to have extended far to the east in the Caribbean 3.6 m.y. ago; perhaps the early Pliocene abundance of this form is not indicative of cool water. The coiling-direction history and stratigraphic ranges of Pulleniatina became different in the Atlantic and Pacific Oceans during the early Pliocene; this is inferred to result from geographic isolation of the assemblages. Saito (1976) used the temporary disappearance of this genus from Atlantic waters at 3.5 m.y. ago to mark the closure of the Isthmus of Panama, but I show that in the Colombian Basin (site 154A) its disappearance was closer to 3.1 m.y. ago. This suggests the possibility of surface-water communication between the Atlantic and Pacific until that time.