James C. Chatters

Response of the Columbia River fluvial system to Holocene climatic change

Abstract An understanding of the response of a fluvial system to past climatic changes is useful for predicting its response to future shifts in temperature and precipitation. To determine the response of the Columbia River system to previous climatic conditions and transitions, a well-dated sequence of floodplain development in the Wells Reservoir region was compared with the paleoenvironmental history of the Columbia River Basin. Results of this comparison indicate that aggradation episodes, occurring approximately 9000-8000, 7000-6500, 4400-3900, and 2400-1800 yr B.P., coincided with climatic transitions that share certain characteristics. The inferred climates associated with aggradation had at least moderate rates of precipitation that occurred mainly in winter coupled with moderate winter temperatures. Such conditions would have resulted in the buildup of snowpacks and a high frequency of rain-on-snow events. The warming and precipitation increases predicted for the Pacific Northwest under most CO2-doubling scenarios are likely to repeat these conditions, which could increase the frequency of severe, sediment-laden floods in the Columbia River Basin.

A new genus of megalonychid ground sloth (Mammalia, Xenarthra) from the late Pleistocene of Quintana Roo, Mexico

ABSTRACT A new genus and species of late Pleistocene megalonychid sloth, Nohochichak xibalbahkah, gen. et sp. nov., is described from Hoyo Negro, a chamber in the Sac Actun cave system, Quintana Roo, Mexico. Phylogenetic analysis indicates that this new sloth is most closely related to Meizonyx salvadorensis from the middle Pleistocene of El Salvador, and that these two genera in turn are the sister clade to Megistonyx and Ahytherium in South America and not the other North American megalonychids, Pliometanastes and Megalonyx. This new sloth indicates that the number of sloth taxa involved in the Great American Biotic Interchange is greater than previously understood, and that a significant part of the Interchange biodiversity, as represented by taxa confined to the semitropical and tropical portions of Central and North America, remains to be discovered.