Seismic modelling and paleoceanography at DSDP Site 574

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 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 1 3 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 1 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 1 8 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 1 8 θ , and a major North Atlantic reflector interpreted as representing an intensification of North Atlantic bottom-water circulation. The Brown reflector is roughly associated with a small carbonate minimum, an enrichment in δ 1 8 θ , the late Miocene δ 1 C depletion, a drop in the relative sea-level curve, and major faunal changes. The Green reflector is associated with a large carbonate minimum, an enrichment in δ 1 8 θ , a major western North Atlantic erosional event, and a widespread eastern Atlantic seismic reflector. The bulk of evidence supports correlation with the onset of Northern Hemisphere glaciation, but detailed isotopic analyses indicate that this isotopic event may be linked to the establishment of colder bottom waters without major ice-sheet development. Several types of reflectors have been identified. The reflectors in the older section result from diagenetic effects; the regionally correctable reflectors are associated with global events. In the younger (post-18 Ma) section, local reflectors are characterized by velocity contrasts, whereas regional reflectors are associated with density contrasts caused by carbonate minima. Two modes of generation of carbonate minima (and thus of reflectors) spanning the equatorial Pacific are (1) the intensification of AABW without the concurrent intensification of NADW and so without fractionation of silica between the Atlantic and the Pacific; this mode results in the less extreme carbonate minima; and (2) the intensification of AABW in response to the intensification of NADW; this mode results in extreme carbonate minima and a correlation of equatorial Pacific reflectors with North Atlantic events. Mayer, L. , Theyer, F , et al., Init. Repts. DSDP, 85: Washington (U.S. Govt. Printing Office). This work was supported by ONR contract N00014-82-K-0625 to L.M. and by the Natural Sciences and Engineering Council of Canada. F.T. 's research was supported by NSF grants OCE 81-17997 and OCE 85-04146. T.S. was supported by N00014-81-K-0728. 2 Addresses: (Mayer) Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1; (Shipley) University of Texas, Institute of Geophysics, Austin, TX 78712; (Theyer, present address) Department of Geological Sciences, University of Southern California, University Park, Los Angeles, CA 90089; (Wilkens) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; (Winterer) Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093. INTRODUCTION The fertile waters of the central equatorial Pacific have produced a thick section of biogenic sediment that is an extremely sensitive indicator of the interplay among tectonism, circulation, productivity, and diagenesis. Studies of equatorial Pacific piston cores (e.g., Arrhenius, 1952; Berger, 1973; CLIMAP, 1976; Hays et al., 1969;