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Equatorial Atlantic and Pacific paleoceanography: Late Miocene to Pleistocene

William Paul Chaisson, University of Massachusetts Amherst


The record of the changing composition of planktonic foraminifer assemblages constitutes a record of changes in the hydrography of the upper water column. A priori knowledge of the ecology and paleoecology of prominent taxa is used to interpret past changes in the hydrography of the upper water column, particularly the position of the seasonal thermocline. In this study planktonic foraminifers have been counted at five sites in the equatorial Pacific (ODP Site 806, 807, 847 and 852) and Atlantic (ODP Site 925). The initial sampling interval at all sites is $\sim$9.5 m ($\sim$250 kyr) through approximately the last 6 m.y. In the first paper the results of low-resolution faunal counts of four sites are presented in the depth domain. In the upper Miocene both eastern and western samples contained 60% thermocline dwellers and 40% mixed-layer dwellers. At the western sites the mixed-layer dwellers increase until they constitute 80% of the assemblage in the Pleistocene. At ODP Site 847 thermocline dwellers increase in the Pleistocene to comprise $>$80% of the assemblage. In the second paper data from ODP Sites 806 and 847 was placed in the time domain and higher-resolution sampling ($\sim$20 cm = $\sim$10 kyr) was carried out through two intervals (4.35-4.10 Ma and 3.15-2.90 Ma). The disappearance of thermocline-dwelling Neogene globoturborotalitids at ODP Site 806 coincides with the reduction of late Miocene Antarctic ice volume and southeast tradewind strength, the end of the biogenic bloom in the Pacific and the beginning of the closing of the Central American Seaway. The expansion of thermocline dweller Neogloboquadrina dutertrei at ODP Site 847 follows the onset of large-scale Northern Hemisphere glaciaton. The third paper explores the relationship between isotopic data for three species (Globigerinoides sacculifer, Neogloboquadrina dutertrei and Globorotalia crassaformis) and faunal count data. Assemblage composition at ODP Site 925 is principally affected by the intensity of downwelling associated with the ITCZ, which moves steadily equatorward from the early Pliocene to the early Pleistocene due to the cooling of the Northern relative to the Southern Hemisphere.

Subject Area


Recommended Citation

Chaisson, William Paul, "Equatorial Atlantic and Pacific paleoceanography: Late Miocene to Pleistocene" (1996). Doctoral Dissertations Available from Proquest. AAI9619378.