BIOTIC RESPONSE TO THE DEMISE OF THE CORDILLERAN ICE SHEET

Mutual Climatic Range (MCR) analysis of fossil beetle assemblages from a transect of sites in the Rocky Mountain region (northern Montana to Colorado and Utah) indicate that regional summer temperatures (TMAX) were depressed by 10-11 °C and winter temperatures (TMIN) were 25-30 °C colder than today, towards the end of MIS 2 (17,200 cal yr BP). Postglacial summer warming was rapid and intense, with TMAX only 3-4 °C below modern levels at 15,900 cal yr BP. By 11,600 cal yr BP the beetle assemblages indicate warmer-than-modern TMAX and TMIN values. By 10,100 cal yr BP, regional beetle MCR reconstructions indicate both summer and winter temperatures were declining from their early postglacial maxima. These reconstructions correspond with the timing insolation peaks in the high latitudes of the Northern Hemisphere. The timing of maximal postglacial warming also agrees well with the timing of deglaciation in the Rockies. Deglaciation in northern Montana was well underway before 13,900 cal yr BP (Carrara, 1995). The Yellowstone Plateau was apparently deglaciated before 15,800 cal yr BP (Porter et al., 1983; Pierce, 2004); the Park Range of northern Colorado was deglaciated prior to 16,700 cal yr BP (Porter et al., 1983).

Pollen and plant macrofossil reconstructions of paleoclimate in the Rockies are mostly limited to postglacial records. Whitlock (1993) documented the establishment of alpine tundra vegetation on the Yellowstone Plateau from the time of deglaciation until about 13,400 cal yr BP, followed by a succession of plant communities culminating in the modern forest cover by about 12,400 cal yr BP. Ecologically, it makes sense that the bare mineral soils exposed at the end of the last glaciation could not support coniferous forest, even if early postglacial climates were quite warm. In contrast, some thermophilous beetles are able to become established in new regions as soon as pioneering vegetation (mosses and herbs) becomes established. The process of forest establishment must progress through a series of stages in ecological succession. In the case of the Yellowstone region, this process took one millennium. The same millennial time lag between thermophilous beetle and subalpine forest establishment in the Colorado Front Range (Elias, 1996).