ISOTOPE SCLEROCHRONOLOGY OF MIOCENE MOLLUSCS FROM THE GUAJIRA PENINSULA, COLOMBIA

The Neogene is a time of profound changes in climate and major biota exchange within the Americas. Unfortunately, the fossil and climate record of the Neotropics during this time interval is very scarce. The Guajira Peninsula in northern Colombia is characterized by a dry climate with xerophitic vegetation, 300 mm of mean annual precipitation and less of 30 days of precipitation per year. A new set of geological localities in the Guajira Peninsula contains a well-preserved and abundant record of plants, vertebrates, and invertebrate from the early Miocene to late Pliocene. The fossil record suggests the presence of extensive swamps and lakes, in a delta complex with moderate to high rates of rainfall. Here we present carbon and oxygen stable isotopes of biogenic carbonates (Turritella spp.) from the Jimol and Castilletes formations (17.9-14.2 Ma). These units provide a nearly four million year record of environmental conditions on the Guajira Peninsula that are coincident with the Middle Miocene Climatic Optimum. Turritella were especially abundant and diverse (>9 species) in these shallow marine sediments. Oxygen isotopic sclerochronology on several Turritella show large isotopic variations (1.5 to 2.0 per mil). These changes cannot be explained by ocean temperature variations alone. Salinity changes caused by riverine inputs, precipitation and/or upwelling are an important factor controlling these oxygen isotopic variations. These results together with the well preserved vertebrate and invertebrate fauna indicates that the region has suffered a drastic climate change over the last 2-3 million years.