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Deep Sea Drilling Project Sites 502 (Colombian Basin, western Atlantic Ocean) and 503 (eastern equatorial Pacific Ocean) are well located for comparison of the late Neogene paleoceanographic history of the tropical Atlantic and Pacific oceans. Comparing the faunal and floral history at each location should reveal changes arising from the late Neogene shoaling of the Isthmus of Panama and the separation of the tropical oceans. At each site we recovered an apparently continuous, largely undisturbed sedimentary sequence from Holocene to about 8 m.y. in age. Tropical and subtropical planktonic foraminifers occur throughout each sequence, although diversity is lower and preservation poorer in the Pacific. At each location the Miocene/Pliocene boundary is defined by the first appearance pf Globorotalia tumida, the early-late Pliocene boundary is defined by the extinction of Sphaeroidinellopsis, and the Pliocene/Pleistocene boundary is defined by the first appearance of G. truncatulinoides. Planktonic foraminiferal assemblages at Sites 502 and 503 are generally similar until the early Pliocene when the faunal histories diverge. Two important exceptions are the delayed first occurrence of Pulleniatina at the Atlantic site (4.4 Ma) relative to the Pacific site (5.5 Ma) and the common presence of sinistral Neogloboquadrina acostaensis and sinistral N. pachyderma at the Atlantic site. Sinistral N. acostaensis is rare at the Pacific site, and sinistral N. pachyderma is absent entirely. The significance of sinistral N. pachyderma at Site 502 is unclear. If this phenotype is due to cool water at this location, it might reflect seasonal upwelling because the fauna is otherwise tropical-subtropical. As noted by other workers, the first planktonic foraminiferal evidence for increasing Atlantic-Pacific provinciality occurs in the early Pliocene at 4 Ma with the appearance of Pulleniatina spectabilis in the Pacific and its exclusion from the Atlantic. Subsequent to that, Pulleniatina disappeared from the Atlantic (3.3 Ma) and did not reappear until about 2.1 Ma, whereas it ranged continuously in the Pacific. The strongest biogeographic evidence based on planktonic foraminifers for the separation of the two tropical oceans by the Panama Isthmus has been the development of an endemic lineage of Globorotalia in the Atlantic. Members of this group become numerically important in the late Pliocene and give tropical Atlantic faunas of that age a distinctive appearance. Until recently the more advanced members of this lineage have not been recorded from Pacific sediments. At Site 503 the distinctive species G. pertenius first appears at 3.3 Ma and last appears at 3.2 Ma, whereas at the Atlantic site it extends to 2.5 Ma. The short range of this species in the Pacific and the complete absence of G. miocenica is further evidence that by about 3 Ma the emergent Panama Isthmus was an effective barrier to the exchange of tropical surface waters between the Atlantic and Pacific.

Neogene Planktonic Foraminifers from Deep Sea Drilling Project Sites 502 and 503