Late Pleistocene and Holocene Seasonal Temperatures Reconstructed from Fossil Beetle Assemblages in the Rocky Mountains

Abstract

Mutual Climatic Range (MCR) analysis was applied to 20 fossil beetle assemblages from 11 sites dating from 14,500 to 400 yr B.P. The fossil sites represent a transect of the Rocky Mountain region from northern Montana to central Colorado. The analyses yielded estimates of mean July and mean January temperatures. The oldest assemblage (14,500 yr B.P.) yielded mean July values of 10–11°C colder than present and mean January values 26–30°C colder than present. Postglacial summer warming was rapid, as indicated by an assemblage dating 13,200 yr B.P., with mean July values only 3–4°C cooler than modern. By 10,000 yr B.P., several assemblages indicate warmer-than-modern mean summer and winter values. By 9000 yr B.P., MCR reconstructions indicate that both summer and winter temperatures were already declining from an early Holocene peak. Mean July values remained above modern levels and mean January values remained below modern levels until 3000 yr B.P. A series of small-scale oscillations followed.