Critical Intervals | Table of Contents


Early Paleogene Warm Climates
We know that the climate of the very early Paleogene was markedly different from that of the rest of the Cenozoic. The very warm temperatures (~12°C) estimated for high latitudes and deep waters, as well as the relatively stable temperatures of the Eocene tropical regions, have led us to confront the single greatest paradox of paleoclimate studies: if warmer high-latitude climates depend on enhanced wind-driven ocean currents or wind-carried heat and moisture to transport heat to the poles, how can this transport have been maintained under the weaker pole to-equator thermal gradients? Such a scenario should give rise to weaker winds and diminished wind-driven transport. It is a paradox that has defeated most mathematical models of global climate. If the dynamics of Eocene climate can be understood, we will gain a fundamental understanding of the physics of Earth's climate.

New data from the tropical oceans are necessary to define the climatic and oceanographic processes that caused early Paleogene warmth. Measurement of tropical sea-surface temperatures, for example, is an important way to distinguish between greenhouse-induced warming of the poles and warming by either atmospheric or oceanic heat transport. Data on winds and currents are needed to partition heat transport between atmosphere and oceans. Finally, the pattern of tropical wind and ocean circulation is a key element of global circulation. There are clear indications that these patterns may have been markedly different in the early Paleogene.

The Paleogene Equatorial Transect (Leg 199) will drill an early Paleogene transect across the world's most long-lived wind-driven current system, a system that contains the confluence of the Northern and Southern Hemispheric winds, and a system whose pattern, strength, and biogenic productivity is linked to global climate patterns.

The drilling of an equatorial transect will provide better and more continuous records of sea surface and abyssal temperatures with which to assess stability of the water column and the magnitude of heat transfer out of the tropics. Changes in sea-surface temperature, plankton communities, and paleoproductivity across the transect will also provide important data concerning ocean circulation and the location and strength of the trade wind belts and ITCZ. The composition and rates of dust deposition will be used to locate both the ITCZ and the transition to the westerlies, whereas mass accumulation rates of biogenic debris will be used to assess the position and the strength of upwelling zones. Stable carbon isotope data will be used to assess nutrient flows in the water column and to constrain the global carbon cycle.

Critical Intervals | Table of Contents