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We made oxygen and carbon isotopic determinations in monospecific samples of benthic foraminifers from over 200 samples from the upper 500 meters of DSDP Site 397. From these measurements, we constructed a history of isotopic variations through the past 7 m.y. (middle-late Miocene to Holocene) with a sampling resolution of better than 20,000 years over much of the interval. The frequency of climatic oscillations during most of this time was similar to that of the Pleistocene (i.e., more than 10 cycles/ m.y.). More constant conditions prevailed between 6 and 5.5 m.y. B.P., and between 4 and 3 m.y.B.P. During both these intervals, oxygen isotopic values are more negative than today; Atlantic Ocean deep waters were probably up to two degrees warmer than they are today. During the last 3 m.y. (late Pliocene to present), the temperature of Atlantic Ocean deep waters has remained rather constant, and the oxygen isotopic record reflects oscillations in the volume of the earth's major ice sheets. Prior to the late Pliocene, we are unable at present to distinguish with certainty between the effect of changes in the size of the Antarctic ice sheet, and changes in the temperature of deep waters, although the fact that the few data available from deep Pacific sites show smaller fluctuations suggests that temperature variations may have been the more important variable. The record of carbon isotopic changes prior to about 3 m.y.B.P. does not show particularly distinctive features, and may not be useful for long-distance stratigraphic correlation purposes. A marked lightening in carbon isotopic composition in this section is similar to that reported from the Equatorial Pacific Ocean, and thus appears to have been of global significance. This event may be a reflection of a drastic reduction in the area of forests similar to those now found in the tropics, an event recognized by Zagwijn in northwestern Europe and identified by him with the Pliocene/Pleistocene boundary.

Oxygen and Carbon Isotope Stratigraphy of Benthic Foraminifers at Site 397: Detailed History of Climatic Change during the Late Neogene