Paleobotany
and paleoclimate of the southern Colorado Plateau (page 2 of 2)
Adapted by R. Scott Anderson from R. Scott Anderson, Julio Betancourt, Jim Mead, Richard Hevly, and David Adam. In Press. Middle- and Late-Wisconsin paleobotanic and paleoclimatic records from the southern Colorado Plateau, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 154.
End of the Last Glaciation: the Pleistocene-Holocene transition
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Paunsaugunt Plateau, Utah. Photograph courtesy of Cline Library Special Collections, Northern Arizona University. |
The end of the cold climate of the last glaciation brought a major reorganization of vegetation on the southern Colorado Plateau. On the Markagunt Plateau, sagebrush and spruce forest was converted to spruce forest, and species common to today's mixed-conifer forests retreated upslope to near their present-day elevational range. The warmer climate may have also allowed the widespread establishment of ponderosa pine across middle elevations on the plateau. Though common today, it appears this pine is a recent colonizer in the area, at least in terms of geologic time. Pinyon-juniper woodlands began to dominate elevations between 1600 and 2100 meters, and desert-scrub communities succeeded juniper woodlands in some areas such as the Grand Canyon.
Modern climate and paleoclimate of the southern Colorado Plateau
Today, much of the Colorado Plateau is considered arid or semi-arid with annual precipitation amounts less than 10 inches. The high plateaus and small mountain ranges, however, receive considerably more precipitation than the more widespread middle and lower elevations on the plateau due to orographic lifting and cooler temperatures. Most areas above 8000 feet receive 20-25 inches annually, while areas above 11,000 feet often receive about 35 inches per year. Precipitation is low to moderate in the early winter, increasing in February and March, and then dropping off quickly into April. May through June are very dry throughout the region. Precipitation increases in July with the advent of the Arizona summer monsoon, particularly on the southern plateau, but then drops off again with the onset of autumn in early September. Conditions remain dry until the winter cyclonic storms develop and periodically pass through the region beginning roughly in November in the north and December to the south.
During the late Wisconsin, the climate of the plateau was significantly different in terms of mean annual temperatures and amount and seasonal distribution of the region's precipitation. Mean annual temperatures were probably at least 5 degrees Celsius cooler than modern times, with an increase in winter precipitation estimated to be 35-65% greater than that observed today. Warm-season temperatures may have been even cooler, perhaps 6.3 degrees Celsius cooler. The much cooler, and likely wetter, conditions of the middle and late Wisconsin period led to heavy snowfall and the development of glaciers on the highest areas of the plateau, including the La Sal Mountains, the Aquarius Plateau, and the San Francisco Peaks. A heavy spring snowpack over the plateau and southern Rockies may have prevented summertime heating and thermally-induced low pressure, and this, combined with a southerly jet stream and lower sea levels in the Gulfs of Mexico and California, may have inhibited the development of the Arizona monsoon during the Wisconsin period. The absence of ponderosa pine on the Colorado Plateau during the Wisconsin may have been due to the suppressed monsoonal flow. The rapid migration of the tree into the region during the Pleistocene-Holocene transition may be directly related to the development of the Arizona monsoon at this time.