Calcium-carbonate accumulation patterns in the deep ocean are determined by the surface water, by dissolution in under-saturated deep water, and by postdepositional addition or removal by currents (Arrhenius, 1952; Berger, 1970; Broecker, 1971). In the eastern equatorial Pacific, downcore variations in Pleistoncene calcium-carbonate concentrations have been shown to be disolution cycles (Arrhenius, 1952, Hays and other, 1969; Saito and others, 1975) which have been correlated to the glacial-interglacial cycles of oxygen-isotope stratigraphy (Emiliani, 1955, Adelseck and Anderson, 1978). Extension of these dissolution cycles back to the Miocene-Pliocene bounday has been attemped (Kaneps, 1973b), but lack of data from Miocene samples has precluded further efforts of this type.
The CENOP project has undertaken the study of the late Miocene ocean, about 7 m.y. before the present, at a time when the Antarctic was glaciated. Based on faunal, floral, and oxygen-isotopic evidence, this was a time of relatively stable oceanographic conditions (Shackleton and Kennett, 1975; Douglas and Savin, 1973; Savin and others, 1975; Savin, 1977). The cores used in this study (Table 1) were chosen because they contain long, continuous sediment records that span this relatively stable late Miocene interval, with good preservation of calcareous and siliceous microfossils. During this interval, these core sites lay under the Pacific high-productivity belt associated with the equatorial Divergence (Arrhenius, 1952; Ewing and others, 1968), in which the carbonate compensation depth (CCD) is depressed due to the excess supply of calcium carbonate (Heath, 1969; van Andel and others, 1975). Because all three piston cores are under the same general surface and deep-water condition changes in the factors that control carbonate sedimentation and preservation should be reflected in a similar way in their sediments. However, the Deep Sea Drilling Project (DSDP) and piston cores used in this study cover a wide range of water depths, from a shallow drilled core (DSDP Site 158) to piston cores that currently lie below the CCD (RC12-66, RC12-65); thus, the carbonate preservation at different sites may vary due to differences in depth as well as corrosiveness of the bottom waters.