The analysis of high-resolution watergun seismic profiles collected in support of DSDP Leg 85 drilling reveals several major, regionally traceable reflectors that can be correlated over more than 360,000 km 2 in the central equatorial Pacific. Synthetic seismograms generated from shipboard physical property measurements (carefully corrected to in situ values) for DSDP Site 574 show excellent agreement with the field records; the agreement suggests that the traveltimeto-depth conversion is accurate and permits the precise (± 5 m) location of reflectors in the cored section. The reflectors can be dated (±0.5 Ma) as follows: Orange, 21.5 to 22.5 Ma; Yellow, 20.5 to 21.5 Ma; Lavender, 16 to 17 Ma; Red, 13.5 to 14.5 Ma; Purple, 11 to 12 Ma; Brown, 7 to 8 Ma; and Green, 3 to 4 Ma. Similar analyses at the other Leg 85 sites result in identical ages. The reflectors are thus time surfaces; this chapter relates them to major paleoceanographic events and changes in the relative sea-level curve. The Orange and Yellow reflectors are associated with a marked increase in δ 13 C, a major change in planktonic foraminiferal assemblages, the development of the deep Circum-Antarctic Current, and the establishment of steep thermal gradients between tropical and polar regions. This reorganization of the oceanic circulation system was probably a response to the opening of the Drake Passage, and it resulted in changes in the chemistry of tropical Pacific waters that caused the induration (and thus impedance contrasts) associated with these reflectors.The Lavender reflector is associated with a large carbonate minimum, the "Chron 16 carbon shift," a widespread hiatus (NH2), major eustatic sea-level fluctuations, and a significant increase in silica deposition in the Pacific. It is not associated with 18 O enrichment or climatic cooling. We conclude that this event represents an intensification in Antarctic Bottom Water (AABW) circulation and the partitioning of silica between the Atlantic and the Pacific, caused by the introduction of North Atlantic Deep Water (NADW) in response to paleobathymetric and tectonic events. The Red reflector is associated with a subdued carbonate minimum, a widespread hiatus (NH3), a sea-level drop, significant changes in microfossil assemblages, and a major increase in δ 18 θ that has been linked with the buildup of Antarctic ice. Detailed isotopic analyses reveal that this isotopic shift occurred within an interval of 30,000 yr. and precisely at the depth of the Red reflector.The Purple reflector is associated with an extremely large carbonate minimum, a change in the style of carbonate deposition in the Pacific, a major lithologic boundary, a widespread hiatus (NH4), an increase in the provincialism between low and high latitudes in all planktonic microfossil assemblages, an apparent fall in eustatic sea level, an enrichment in δ 18 θ, and a major North Atlantic reflector interpreted as representing an intensification of North Atlantic bottom-water circulation.The Brown reflector is roughly associated wi...

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Biostratigraphic studies using planktonic and benthic foraminifers, calcareous nannofossils, radiolarians, diatoms, and silicoflagellates are summarized for the four main sequences (Sites 572 to 575) collected during DSDP Leg 85 in the central equatorial Pacific. Upper Eocene through Quaternary reference sections were recovered at Sites 573 and 574, and middle Miocene through Quaternary and lower Miocene through Quaternary reference sections were cored at Sites 572 and 575, respectively. Detailed graphic correlation plots suggest that Site 573 may contain as many as nine short hiatuses. Site 572 in the fertile eastern equatorial Pacific is apparently free of hiatuses, and Sites 574 and 575 have hiatuses (two each) only in the compressed upper sections.Few inconsistencies exist in the various microfossil stratigraphies. Diachronous species occurrences are identified, and age estimates for 66 late Eocene through Quaternary microfossil datums have been extrapolated from age-vs.-depth plots constructed from 63 datums with direct and/or indirect ties to paleomagnetic stratigraphy.An addendum presents an alternative biostratigraphic time scale for the middle and late Miocene based on a correlation of paleomagnetic Anomaly 5 with paleomagnetic Chron 11.

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