Biogenic silica accumulation in the central equatorial Pacific and its implications for Cenozoic paleoceanography: Summary

Abstract

Since the Deep Sea Drilling Project (DSDP) has made available long cores through Cenozoic deep-sea sediments, geologists have begun to study the history of sedimentation in the world ocean and to infer the oceanographic conditions which have influenced deposition of the sediment. During this time, the central equatorial Pacific Ocean has become one of the most intensively studied geologic provinces in the world ocean. This area lies beneath the biologically productive equatorial current system which supplies great quantities of calcium carbonate and amorphous silica (opal) to the sediment in the form of skeletal tests. This biogenic debris, together with terrigenous, authigenic, and hydrothermal sediment, has produced an unusually thick and complete marine record which was cored by four DSDP Legs (5, 8, 9, and 16). Because the productivity of the equatorial Pacific is a direct consequence of large nutrient concentrations associated with upwelling in the complex equatorial current system, the sedimentary section can be used to interpret the paleoceanography as well as the history of productivity in the equatorial Pacific. The history of carbonate sedimentation in the central equatorial Pacific is well known from the Initial Reports of DSDP Legs 8, 9, and 16 (Tracey and others, 1971; Hays and others, 1972; van Andel and Heath, 1973), as well as from a wealth of published information (Hays and others, 1969; McManus and others, 1972; Berger, 1973; Winterer, 1973; van Andel and others, 1975). There are limitations, however, to the use of carbonate sediments for interpreting the paleoceanography and productivity of surface waters. The patterns of distribution and accumulation of calcareous sediments in the deep ocean are modified by the dissolution of carbonate in undersaturated bottom Waters at rates which vary with depth (Peterson, 1966; Berger, 1973) and with time (van Andel and others, 1975). Because we are, at present, unable to determine past rates of carbonate dissolution in bottom waters unambiguously (van Andel and others, 1975; Heath and others, 1977), we are also unable to assess accurately the rate of supply of carbonate to an area in the past.