PALEOENVIRONMENTAL CHANGES IN WESTERN NORTH AMERICA DURING THE PLEISTOCENE-HOLOCENE TRANSITION

The period from the Last Glacial Maximum (~21,000 yr cal. B.P.) through the early Holocene (~7000 yr cal. B.P.) was a time of great changes in global climate, insolation, continental ice masses, sealevel, mountain glaciers, hydrology, and atmospheric carbon dioxide. Ecosystems of the world were reshaped, sometimes at a very rapid pace. The environment of Western North America was strongly influenced by these changes, as the continental ice sheet retreated into Canada and mountain glaciers and ice caps melted back. The large pluvial lakes of the intermountain region were still rising as the continental and montane ice ablated, but these lakes experienced an apparently rapid decline as the Pleistocene waned. Marsh and wetland complexes across the West also retracted at the end of the Pleistocene, and plant species adjusted their range boundaries by moving generally northward and upslope as the climate warmed. Faunal populations underwent profound changes as a large portion of the large mammal fauna became extinct and surviving animal populations changed their geographic and elevational distributions. Although the general trend toward warmer and drier conditions described above pertained through most of the Pleistocene to Holocene transition, there were reversals that apparently lasted for hundreds of years. Many of these reversals occurred within the period where the calendric calibration of radiocarbon dates is imprecise, and it is challenging to demonstrate synchroneity (and thus geographic extent) of reversals as recorded in different paleoenvironmental data sets.