Laurentide ice sheets, the Isthmus of Panamá, & the Great American Biotic Interchange: A tectonic red herring inserted between climatic cause and biological consequence by Dr. Peter Molnar
Geological Sciences & CIRES, University of Colorado Boulder
Two events occurred virtually simultaneously at ~2.5 Ma: the first big ice sheet of Northern Hemisphere Ice Ages reached as far south as St. Louis, and largely large mammals crossed through the Isthmus of Panamá from North to South America and vice versa in the Great American Biotic Interchange. An emergent isthmus is necessary for such animals to change hemispheres. In a popular myth, recited by textbooks, the isthmus emerged near ~2.5 Ma or perhaps slightly earlier, and by doing so, it altered Atlantic Circulation sufficiently to enable ice sheets grow on Canada. I contend that such logic is inverted. Large Laurentide ice sheets pushed the intertropical convergence zone (ITCZ) southward so as to cool and aridify eastern Panamá, and create a savannah-like environment. Most of the animals involved in the Great American Biotic Interchange seem to have been savannah-dwellers, and I presume that they would have avoided the swamps, jungles, snakes, crocodiles, mosquitoes, and other nasty predators in eastern Panamá before savannah-like environments drew them in and facilitated safe passage. So, the Great American Biotic Interchange is not a symptom of a tectonic change that enabled Ice Ages, but instead a consequence of those Ice Ages.
Coffee and social hour @ 11:45 am.