EVALUATING THE INTERCONTINENTAL SYNCHRONEITY OF MAMMALIAN TURNOVER; NEW RESULTS FROM THE PALEOCENE OF THE CHIJIANG BASIN, CHINA

To understand the role of global climate change in mediating mammalian turnover in the geological past, it is necessary to generate high-resolution stratigraphic correlations between fossiliferous sections on different continents. In recent years, we have been working in China to correlate the Paleocene Asian Land Mammal Age (ALMA) framework to the geological time scale to understand whether these ALMA boundaries are coincident in time with land mammal age boundaries on other continents (e.g. North America). If the boundaries are coincident, then these Paleocene mammalian turnovers may have been triggered by climatically controlled dispersal like that known to have occurred at the Paleocene-Eocene boundary. If they are not coincident, then these mammalian turnovers are more likely to have been controlled by regional factors. Here we report results from the Chijiang Basin in Jiangxi Province of south China.

The Chijiang basin is an elongate graben containing ~1200 meters of Paleogene red beds that represent 3 formations: the Shizikou Formation, the Chijiang Formation, and the Pinghu Formation. All three formations are characterized by highly oxidized fine- to coarse-grained clastic deposits of fluvial and lacustrine origin. Fossil mammals have been recovered from 21 localities throughout the Chijiang sequence and represent all three of the Paleocene ALMAs (Shanghuan, Nongshanian, and Gashatan). In order to correlate the Chijiang mammal biostratigraphy to the geological time scale, we collected samples for paleomagnetic and stable isotope analysis from three overlapping stratigraphic sections. Results suggest that the Shanghuan-Nongshanian ALMA boundary is near a normal-to-reverse polarity transition and slightly precedes a minimum in carbon isotopic values as measured in dispersed organic matter. The Torrejonian-Tiffanian North American Land Mammal Age (NALMA) boundary also falls near a normal-to-reverse polarity transition (Chron C27N to Chron C26R) and slightly precedes a long-term carbon isotopic minimum in the marine record. These results are consistent with the Shanghuan-Nongshanian ALMA boundary being coincident with the Torrejonian-Tiffanian NALMA boundary and open the possibility that these geographically distinct mammalian turnovers were caused by the same external driver.