Middle- and late-Wisconsin paleobotanic and paleoclimatic records from thesouthern Colorado Plateau, USA

The Colorado Plateau is a distinct physiographic province in western North America, which presently straddles the transition between summer-wet and su mmer-dry climatic regimes to the south and northwest, respectively. In addi tion to climate, the diversity of environments and plant communities on the Colorado Plateau has resulted from extreme topographic diversity. Desert l owlands as low as 360 m elevation are surrounded by forested plateaus, and even higher peaks greater than 3800 m elevation. This environmental diversi ty provides a unique opportunity to study the history of biotic communities in an arid region of North America. Although the Colorado Plateau harbours numerous potential sites, the paleoecological record of the Plateau is poo rly known. Potential deposits for analysis include packrat middens, alluvia l and cave sites at lower elevations, and lake, bog and wetland sites at hi gher elevations. Forty-six sites have been analysed across the nearly 337,0 00 km(2) region, of which 27 contain records that span Marine Oxygen Isotop e Stage (IS) 2 data, with IS 3 information coming from only 12 sites. Most IS 2 and 3 sites are clustered along the lowland regions of the Colorado Ri ver corridor and the uplands of the Mogollon Rim area. We compiled selected data from long paleoecological records to examine patterns of vegetation a nd climate change across the southern Colorado Plateau for the middle and l ate Wisconsin. During the middle Wisconsin, mixed conifers covered middle-e levations presently dominated by ponderosa pine (Pinus ponderosa), and juni per (Juniperus) woodland grew at elevations today covered by blackbrush (Co leogyne) and sagebrush (Artemisia) desert. During the late Wisconsin, borea l conifers, primarily Engelmann spruce (Picea engelmannii), replaced the mi xed conifer association. Estimates of mean annual temperatures (MAT) during IS 3 were at least 3-4 degrees C cooler than today, whereas IS 2 MAT estim ates are at least 5 degrees C colder. Our investigation of millennial-scale climatic variability within the region provided equivocal results. The pac krat midden sequence could not distinguish vegetation changes that might be associated with Heinrich events in the North Atlantic. From the lake recor ds, however, many Heinrich events were associated with generally drier inte rvals, often with elevated sagebrush pollen concentrations. Future paleoeco logical investigations should concentrate on the northern Colorado Plateau, as well as the eastern and western margins. Additional sites, along with c loser-spaced sampling in regions already studied, will be important in dete rmining the history of important climatic phenomena such as the timings of the Arizona monsoon. (C) 2000 Elsevier Science B.V. All rights reserved.