STABILITY OF HOLOCENE CLIMATE REGIMES IN THE YELLOWSTONE REGION

A 12,500-yr pollen record from Loon Lake, Wyoming provides information on the climate history of the southwestern margin of Yellowstone Nati onal Park. The environmental reconstruction was used to evaluate hypot heses that address spatial variations in the Holocene climate of mount ainous regions. Loon Lake lies within the summer-dry/winter-wet climat e regime. An increase in xerophytic pollen taxa suggests drier-than-pr esent conditions between ca. 9500 and 5500 C-14 yr B.P. This response is consistent with the hypothesis that increased summer radiation and the expansion of the east Pacific subtropical high-pressure system in the early Holocene intensified summer drought at locations within the summer-dry/winter-wet regime. This climate history contrasts with that of nearby sites in the summer-wet/winter-dry region, which were under the influence of stronger summer monsoonal circulation in the early H olocene. The Loon Lake record implies that the location of contrasting climate regimes did not change in the Yellowstone region during the H olocene. The amplitude of the regimes, however, was determined by the intensity of circulation features and these varied with temporal chang es in the seasonal distribution of solar radiation. (C) 1995 Universit y of Washington.