Upper Pleistocene deposits from the Cauca Valley

Abstract

The rise of the Isthmus of Panama is one of the major biogeographical events of the Cenozoic. It is a massive natural experiment in biological migrations, as lands formerly separated - South America and Central/North America - became connected. There is, however, a difference in timing between the final closure of the Isthmus (4.2-3.5 Ma) and the onset of the massive mammal migrations (i.e., Great American Biotic Interchange, GABI, 2.7 Ma). This time lag has been brought up to suggest that other factors rather than a land connection were major drivers of the GABI. A large biome change from forest to savanna in Central America and Northwestern South America could have provided the conduit for accelerating the biotic interchange between both continents. This hypothesis has been rarely tested as the Plio-Pleistocene fossil record of those regions has not been sufficiently studied. Gheny K. Shuster discovered a brand new Upper Pleistocene fossil deposit in and under the Cauca River (Southwestern Colombia), one of such possible migration corridors. We conducted a field geophysical and sedimentological study to understand the geometry of the fossil-rich deposits, dated them using carbon isotopes (C14), and assessed the type of biome (savanna versus forest) that dominated the region by looking at pollen samples. We found that the conglomerates accumulated in lenticular bodies, perpendicular to the modern course of the Cauca River during the Late Pleistocene in alluvial fan settings. The vegetation corresponds to a tropical dry forest and there is no evidence of extensive savannas. The Cauca River deposits could become a relevant site for the Neotropical Pleistocene.