Early Holocene Environments

During the Holocene, the Colorado Front Range experienced a series of climatic fluctuations. Insect assemblages from several sites are indicative of warmer-than-present summer temperatures and colder-than-present winter temperatures. The earliest Holocene records in the insect fossil study transect come from Sky Pond and La Poudre Pass. Sky Pond is an alpine pond in Rocky Mountain National Park. A fossil beetle assemblage from 10,000 yr b.p. yielded a calibrated MCR estimate of mean July temperature that is approximately 3°C warmer than modern. The La Poudre Pass site is a peat bog situated near tree line near Cameron Pass, just north of Rocky Mountain National Park. Here, an assemblage dated 9850 yr b.p. yielded a calibrated MCR estimate of mean July temperature that is about 5°C warmer than modern. This assemblage represents the greatest degree of summer warming of the entire 14,000-year record in the Rocky Mountain region. Winter temperatures were as much as 10°C colder than modern temperatures, however, so the degree of con-tinentality also reached a peak at this time. These predictions based on fossil beetle data agree well with Berger's (1978) reconstruction of incoming solar radiation (insolation), based on the Milankovitch insolation model, which predicts a summer insolation maximum and winter insolation minimum in the midlatitudes of the Northern Hemisphere from about 9000-12,200 14C yr b.p. (10,000-14,200 calendar yr b.p.) (figure 18.3). This peak in summer insolation coincides precisely with the MCR estimates of the postglacial warming in the Colorado Rockies. The fossil insect record is the only fossil data source from the Rocky Mountain region to register this degree of warming in early postglacial times, consistent with glaciologi-cal data that suggest rapid melting of regional glaciers before 12,000 yr b.p. (Madole and Shroba 1979). Evidence from the San Juan Mountains indicates that the major glaciers in that region had melted as early as 15,000 yr b.p. (Carrara et al. 1984).

By 9000 yr b.p., the fossil insect data indicate that summer temperatures were already declining from an early Holocene peak, though they were still above modern values (figure 18.3). A fossil insect assemblage from Lake Isabelle, a subalpine lake in the Indian Peaks Wilderness area, yielded calibrated MCR reconstructions indicating mean July temperatures 3°C warmer than modern temperatures and mean January temperatures well below modern levels.

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