EMERGENCE OF THE ISTHMUS OF PANAMA

The Miocene to Pliocene emergence of the southern Central American isthmus from the sea was a long-term event. Most evidence for the timing of emergence has come from several sources that reflect its effects: the open-ocean paleoceanographic effects of the rising sill that progressively cut off waters between the tropical Eastern Pacific and Caribbean, the evolutionary and biogeographic divergence of Pacific-Caribbean marine faunas, and the history of terrestrial vertebrate migration between North and South America. This study traces the history of emergence with paleobathymetry determined from foraminifera preserved in the rocks of southern Central America. Foraminiferal species have environmental ranges that are correlated with water depth; their assemblages were used to interpret the water depths at which they accumulated, and rates of isthmian emergence over time were calculated.

The paleobathymetry of the proto-isthmus was interpreted from benthic foraminifera contained within marine sediments of: the Chucunaque and Sambu river basins of Darien (Panama), the Panama Canal, Bocas del Toro (Panama), Limon (Costa Rica), and the Pacific Burica Peninsula. Shallowing from early to middle Miocene occurred in the Limon and Bocas del Toro basins, although sedimentation remained deep (bathyal) open ocean. A bathyal connection existed between Central and South America until at least late Miocene. By early late Miocene, all areas of Darien and northwestern Panama had shallowed from lower-middle bathyal to neritic (< 200m) depths. About 6 Ma, deepening in the proto-Panama Canal to bathyal depths caused an inflow of Pacific surface water to the Caribbean, but by early Pliocene, emergence of the Isthmus had closed all Pacific-Caribbean straits.

Rates of isthmian emergence (m/Ma) were calculated from paleobathymetric changes within 38 geologic sections. Emergence rates generally decreased after 9 Ma and between 9-6 Ma most were negative (submergence), although Burica Peninsula's Plio-Pleistocene rates were extreme due to subduction of the Cocos Ridge. Similar rates occurred along the northern coast (Panama Canal, Bocas del Toro, Limon), but Darien experienced higher emergence rates, most likely because of its proximity to the Panama arc – South America plate collision.