TIMING OF THE ISOLATION OF THE CARIBBEAN FROM THE TROPICAL EASTERN PACIFIC

Recent fieldwork has shown that geological uplift along the Central American Arc began in the Middle Eocene, rekindling biological interest in when the Caribbean was isolated from the eastern Pacific. However, three independent lines of evidence strongly support the existence of abundant seawater exchange between the oceans until about 3 Ma. (1) Data for carbon and oxygen isotopes and the carbonate sand fraction from deep ocean cores demonstrate that divergence in surface salinity and deep carbonate accumulation did not begin until 4.7 to 4.2 Ma, and did not reach recent values until the Early Pleistocene. Likewise, strong Caribbean upwelling in shelf environments comparable to that in the eastern Pacific persisted until 4 to 3 Ma; whereupon collapse in planktonic productivity initiated profound shifts in benthic ecosystems, carbonate accumulation, and extinction rates. (2) Fossil and molecular data demonstrate that North and South American terrestrial mammals and tropical forest bird faunas remained overwhelmingly isolated until about 2.7 Ma, whereas some generalist birds, fishes, amphibians, and plants began mixing earlier. The rarity and ambiguity of reported exceptions after a century of intensive sampling strengthen the general pattern. (3) Bathyal foraminifera first diverged across the developing Isthmus 12 Ma but numerous shelf depth mollusks and cheilostomes are known from both oceans until the Middle Pliocene. Some of these species now restricted to the Pacific persisted in the Caribbean until the Pleistocene, but no species that originated after the Middle Pliocene occurs in both oceans except for human introductions. Molecular clock analyses for numerous other taxa strongly support these patterns, with estimated times of divergence ranging from 3 Ma or less for near intertidal species to 5 to 7 Ma for deeper water taxa. Isolation of the Caribbean from the Pacific about 3 Ma stands secure as a model system for the study of vicariance in the oceans.