THE UPLIFT OF THE PANAMANIAN ISTHMUS: A PROBLEM SOLVED?

The uplift of the Panamanian isthmus had extensive biological, paleoceanographic and climatic implications. This event separated two oceans and joined two continents. A large body of research done more than a decade ago indicates that the isthmus uplifted around 3.5 ma ago. This dating has been used in wide variety of fields, from molecular biology to paleoceanography. Some have even proposed the rise of the isthmus to be the trigger of the onset of the artic glaciation. Over the past four years, we have had the opportunity to access a vast amount of fresh rocks provided by expansion of the Panama Canal, a multibillion engineering endeavor, that has facilitated the exposure of rocks on a landscape otherwise covered by tropical rainforest. Terrestrial fossils, thermochronology, provenance, geochemistry and structural analysis indicate there are three major exhumation events, during the late Eocene (~38-4 Ma), the earliest Miocene (~20 Ma) and the late Miocene (~10 Ma). There is not evidence of a significant uplift 3.5 Ma ago. Provenance also indicates that by 10 Ma, the Panamian block and South America were so close to each other, that exchange of sediments was already happening. In addition, Neodinium modeling suggests that by the late Miocene (~10 Ma), the deep and intermediate water circulation from the Pacific into the Caribbean ceased, and only shallow waters (<200 m deep) could exchange across the isthmus. Fossil evidence and genetic analysis of a number of clades including snakes, crocodiles, bees, frogs, salamanders, some fresh-water fishes, and plants indicate that frequent migration of terrestrial elements across the isthmus started by 10 Ma ago. In contrast, mammal migration is very limited, and even by the late Pliocene (~1.7 Ma), there is no evidence of North American mammals having migrated into tropical lowlands of northern South America, suggesting than a barrier other than a physical connection did not allow a large scale exchange of mammals across the tropics. Shallow marine molluscan faunas indicated that a fully continuous terrestrial connection started by ~3.5 Ma as earlier research had proposed. The implications on this new geological model on regional and global paleoceanography and paleoclimatology still remain to be explored.