Leg 199 investigates the "Greenhouse" Eocene in the tropical Pacific Ocean

The Eocene epoch from 55 to 33.7 Ma represents a 21-m.y. geologic interval of relatively stable but warm climate conditions very much different than those of the Pleistocene-Recent Earth. The warmest global temperatures in the Cenozoic occurred in the early Eocene (ca. 50 Ma), but conditions sufficiently warm to support Metasequioa forests at paleolatitudes of 80°N occurred even in the middle Eocene, at roughly 40 to 45 Ma (Christie and MacMillan, 1991). The Eocene epoch is sandwiched between two unique boundary events. The base of the Eocene is now defined by a remarkable carbon isotope exursion that may represent a massive gas hydrate dissociation, methane release, and transient greenhouse warming (Dickens et al., 1995). Just above the Eocene-Oligocene boundary is an oxygen isotope event (Oi-1) that marks the beginning of the first permanent ice sheets on Antartica (Miller et al., 1987). Understanding how the Eocene began, how warm climate conditions were maintained for 20 m.y., and how climate changed as Antartica became glaciated are fundamental to understanding basic climate processes on Earth.