MIDDLE WISCONSINAN VEGETATION ON THE COLORADO PLATEAU, UTAH AND ARIZONA, USA: EVIDENCE FOR GLACIAL-AGE MONSOONS?

Packrat (genus Neotoma) middens provide paleoecological information for arid regions of southwestern North America. A rich vegetation history for the late Wisconsinan through Holocene times has been reconstructed for the Colorado Plateau of Utah, Arizona, New Mexico, and Colorado, but midden deposits older than 25 ka yr B.P. are rare. Our review of midden records found that only 7% (n=2243) of middens from the Colorado Plateau exceeded 25 ka ¹⁴C yr B.P. in age. We report the results of paleoenvironmental studies using packrat middens collected from the Colorado Plateau in Canyonlands and Grand Canyon National Parks. Fifteen of sixty middens analyzed yielded a middle Wisconsinan age, dating from 27 ka to 46 ka ¹⁴C yr BP. These are the oldest midden series yet discovered on the Colorado Plateau. The middens from the Grand Canyon contain a substantially mesic assemblage of vegetation for the last 40 ka yrs of the Wisconsinan Glaciation, with some taxa displaced as much as 1200 m vertically downward from their modern distributions. For example fernbush (Chamaebatiaria millefolium), rose (Rosa stellata), and big sagebrush (Artemisia tridentata) were growing on sun-exposed ledges at 1100 m from ~50 ka to 10 ka ¹⁴C yr BP. These species occur between 1300 and 1900 m today. In contrast, the oldest of the middens from Canyonlands (~50 ka to 29 ka ¹⁴C yr BP) record a xeric assemblage of vegetation, including dwarf mountain-mahogany (Cercocarpus intricatus), Mormon tea (Ephedra torreyana), and Utah juniper (Juniperus osteosperma), a grouping almost indistinguishable from modern plant assemblages. Furthermore, some of these middens contain fossils of Arizona single-needle pinyon (Pinus edulis var. fallax), a species that today is more typical of areas in central Arizona receiving abundant monsoon precipitation. This tree does not occur in Canyonlands today, with the closest populations more than 100 km downstream along the Colorado River. The presence of this species in Canyonlands during the middle Wisconsinan, and the absence of montane species, contrasts sharply with the mesic late Wisconsinan vegetation assemblages documented by our work and in the literature. Our data suggest the existence of an interstadial climate regime, and that a weak monsoonal flow may have been present during summer months that possibly influenced the vegetation as far north as central Utah.