Pliocene-Pleistocene Sediments of the Equatorial Pacific: Their Paleomagnetic, Biostratigraphic, and Climatic Record

Abstract

Magnetic stratigraphy of 15 oriented cores from the equatorial Pacific was determined as far back as the Gilbert reversed-polarity epoch. Ranges of selected species of four major microfossil groups (diatoms, silicoflagellates, foraminifers and Radiolaria) are compared with the record of geomagnetic reversals during the last 4.5 m. y. in eastern equatorial Pacific deep-sea cores. Characteristics of the fossil assemblages are used as criteria for recognition of most of the paleomagnetic reversals that occurred during this interval. Two zones of major paleontological change occur characterized by extinctions of several species and coiling direction changes in some foraminifers. The first change comes in the middle of the Gauss normal magnetic series (about 3 m.y. B.P.) and the second near the Olduvai magnetic event (about 2.0 m.y. B.P.). Seven equatorial foraminiferal species, two radiolarian species, and two diatom species become extinct near reversals. The establishment of the true chronostratigraphic relationships of these selected microfossil species allows us to date zonations of previous authors and provides absolute dates that can be used in worldwide correlation of marine sediments. The percentage of calcium carbonate was determined throughout the lengths of four cores. Eight distinct carbonate cycles are present in the Brunhes series, having periodicities of about 75,000 years in the upper Brunhes to over 100,000 years in the lower Brunhes. It is possible to correlate these carbonate cycles among our cores and also to correlate them with the previous work of Arrhenius who equated the carbonate peaks with glacial stages and the troughs with interglacial stages. This interpretation is supported by paleomagnetic and C14 dating of the last carbonate high which is synchronous with the Wisconsin glaciation (80,000 to 11,500 years B.P.). It, therefore, is probable that there were eight major glacial fluctuations during the last 700,000 years. During the last 400,000 years there is good correlation between the carbonate cycles of the Pacific and evidence of climatic fluctuations in the Atlantic established by Ericson and Wollin (1968) and Emiliani (1966) based on fossil abundances and oxygen isotope ratios, respectively. The rates of sedimentation during the Brunhes series range between 3.5 mm/1000 years for siliceous ooze to 17.5 mm/1000 years for highly calcareous sediment.