Critical Intervals | Table of Contents
THE SCIENTIFIC PROBLEM
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
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.