The Pleistocene Epoch in Deep-Sea Sediments

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Abstract
Our record of the Pleistocene is the result of piecing together correlating and overlapping sections from 26 cores of deep-sea sediments, ranging in length from 545 to 2190 cm. These cores were selected from more than 3000 cores raised from all the oceans. Climate curves based on the relative numbers of warm-water and cold-water species of planktonic foraminiferans found in each of the 26 cores give a composite climatic record which is correlated with the glacial and interglacial ages of the Pleistocene. Additional evidence for continuity of the Pleistocene record is provided by changes in coiling direction of Globorotalia truncatulinoides. Since periodic glacia-tions most clearly distinguish the Pleistocene from the earlier epochs of the Cenozoic, we consider that the onset of the first ice age, the Nebraskan or Gunz, marks the beginning of the Pleistocene. The Pliocene-Pleistocene boundary is sharply defined by extinction of all dis-coasters and by an abrupt in the planktonic foraminiferans. The most important criteria which distinguish Pleistocene pelagic sediments from those of earlier epochs of the Cenozoic Period are absence of discoasters; the dominance of left-coiling, from 95 to 100%, in Globorotalia menardii; the general occurrence of Globorotalia truncatulinoides in abundance; and the absence of certain species or subspecies closely related to Globorotalia menardii, with consequent reduction of the G. menardii racial complex of the Pliocene to a much more homogeneous group in the Pleistocene. Dates determined by the radiocarbon, the protactinium-ionium, and the protactinium methods provide an absolute time scale from the present back to about 175,000 years ago. These dates indicate that the average rate of accumulation of sediment was on the order of 2.5 cm/1000 years. On the basis of this average rate of accumulation and the fact that we found the thickness of the whole Pleistocene section to be about 38 m, we have, by extrapolation beyond 175,000 years, established a time scale for the entire Pleistocene epoch. Our time scale dates the beginning of the Pleistocene, as defined by the onset of the first ice age, the Nebraskan or Gunz, at about 1.5 million years ago.