We live in an ice age: current sea level is some 70 meters below where it would be if the polar regions were warm. However, we live in a warm interval of this ice age--sea level is 120 meters higher than it was at the last glacial maximum 20,000 years ago. As large continental ice sheets wax and wane in the Northern Hemisphere, sea level fluctuates. Water locked in the ice is depleted in heavy stable isotopes of both hydrogen and oxygen; thus, a buildup of ice enriches the ocean's water with oxygen-18 relative to oxygen-16. The enrichment (and its cancellation during melting) can be measured as changes in oxygen isotope ratios within the calceous shells of marine organisms (as shown by University of Miami paleontologist-physicist Cesare Emiliani in 1955).
Certain planktonic foraminifera are well suited as recorders of isotopic ratios. However, in addition to recording the ice budget, the oxygen-18 to oxygen-16 ratio of their shells reflects changes of surface water temperatures. The best places to obtain unadulterated records of ice mass, therefore, are tropical regions that show little change in temperature from glacial maxima to glacial minima. Such a place is the Ontong Java Plateau in the western equatorial Pacific. The plateau is roughly the size of Texas and rises from the surrounding abyss to 1.6 kilometers below the water surface; it accumulates well-preserved shells of foraminifers.
The Ice Age Record from the Ontong Java Plateau
Five cores collected on ODP Leg 130 (in 1990) at Site 806 provide an excellent record of ice-mass fluctuations over the last two million years (the Quaternary period). We base our interpretation of this record on the theory of the Serbian astronomer Milutin Milankovitch (1879-1958). He proposed that periodic changes in the tilt of Earth's axis and in the eccentricity (deviation from a circle) of Earth's orbit translate into growth and decay of ice mass through changes in summer insolation (the amount of sunlight reaching Earth's surface) in high northern latitudes (say, at 65 |degrees~ N). The formulation and step-wise confirmation of the Milankovitch theory is one of the great scientific success stories of our century (see Nicklas G. Pisias and John Imbrie, Oceanus, 29:4, 1987). In essence, the theory solves the mystery of why ice ages occur in cycles. The study of deep-sea sediments (and especially of oxygen isotopes) was of crucial importance in this context.
There is evidence in the Site 806 oxygen-isotope record for ice-mass control by both eccentricity and obliquity. Three subdivisions regarding climatic state are readily distinguished. The oldest third is dominated by 41,000-year axial-tilt cycles, the youngest third by roughly 100,000-year eccentricity-related cycles. The central third shows the transition from one regime to the other. The three regimes are labeled "Milankovitch" chron, "Croll" chron, and "Laplace" citron after the scientists who introduced the fundamental ideas underlying orbital dating. The Scot James Croll made the first attempt at template-dating of ice ages, while French astronomer Pierre Simon de Laplace's calculations provided a firm base for celestial mechanics, which allow extrapolation of orbital conditions into the distant past. Boundaries between the chrons are set according to the strength of the eccentricity cycle present. For simplicity, they are put precisely at the crests of obliquity-driven cycles 15, 30, and 45. The single most striking feature of the Site 806 ice-mass record (beyond the cyclicity itself) is that the nature of the cyclicity changes at the center of the Quaternary, about 900,000 years ago. We call this the "Mid-Pleistocene Revolution" (MPR).
An Orbital Template for the Ontong Java Plateau
Can simulation of the ice-record from orbital data help us understand the nature of the mid-Pleistocene climate shift? An early attempt to provide a match between target and template using data from the Ontong Java Plateau (by science journalist Nigel Calder, in 1974, with data from Nick J. …