Deterministic Versus Stochastic Explanations

Oxygen isotope data for a Pliocene interval from 3.6 to 2.8 million years ago show a mean increase (0.5 per mil) of benthic 6180 at about 3.2 million years ago, whereas planktic 6180 does not increase. This lack of covariance indicates that the event at 3.2 million years did not result in a permanent increase in the ice budget of either the Northern or the Southern Hemisphere. The history of continental ice sheets and their effect on the composition and climates of the ocean is a pervasive theme in paleoceanography. For example, within the Neogene, much of the evidence for the magnitude and timing of the initiation of major ice sheets in the Northern Hemisphere has come from analysis of the stratigraphy of ice-rafted deep-sea sediments (1, 2) and increased 6'" of benthic foraminifera (3, 4). Although the first major glaciation in the Northern Hemisphere has traditionally been placed in the Pliocene ( 3 , some investigators have suggested that ice sheets were present as early as the late Miocene (6). In this report I examine the oxygen isotope evidence for major glaciation at 3.2 million years ago (Ma) a date often cited for the initiation of Northern Hemisphere glaciation (1-5). Prior to the availability of Deep Sea Drilling Project (DSDP) hydraulic piston cores. the highest quality oxygen isotopic record relevant to the question of Pliocene glaciation was that of core V28179 from the equatorial Pacific (7). At approximately 3.2 Ma, this record contains a steplike increase of about 0.4 per mil in the average 6180 of benthic foraminifera. This baseline shift separates an interval of low variability and relativeiy low isotopic values, called the "preglacia]" Pliocene, from an interval of higher and variable isotopic values (the "glacial" Pliocene) ( 7 ) . The increased 6180 values were interpreted to indicate increased 6180 of the ocean due to the presumed initiation of Northern Hemisphere ice volume. With one apparent 692 exception (2), this pattern of benthic 6180 is widely observed in all oceans (3, 4). A subsequent increase in the mean 6180 at about 2.4 Ma has recently been