Mid Holocene Environments

From 7800 to 3000 yr b.p., insect fossil assemblages from La Poudre Pass and Lake Isabelle show a gradual summer cooling trend. The 7800 yr b.p. assemblage from Lake Isabelle yielded a calibrated MCR estimate of mean July temperature 2.1°C warmer than modern temperatures. The 5250 yr b.p. assemblage from Sky Pond yielded a mean July temperature estimate 0.4°C warmer than modern levels. This is the oldest Holocene assemblage that yielded a TMAX range that dipped near the

• Calibrated estimates Mutual climatic range of assem

Radiocarbon Years Before Present X 1000

Figure 18.3 (Top) Insolation curve for 45° N latitude during the last 15,000 years. Data from Berger (1978). (Bottom) MCR reconstruction of late Pinedale and Holocene mean July temperatures, as indicated by fossil beetle assemblages from the Colorado Front Range region, shown as departures from modern mean temperatures at the study sites.

• Calibrated estimates Mutual climatic range of assem

Radiocarbon Years Before Present X 1000

Figure 18.3 (Top) Insolation curve for 45° N latitude during the last 15,000 years. Data from Berger (1978). (Bottom) MCR reconstruction of late Pinedale and Holocene mean July temperatures, as indicated by fossil beetle assemblages from the Colorado Front Range region, shown as departures from modern mean temperatures at the study sites.

modern baseline level. However, most assemblages that date between 7000 and 3000 yr b.p. yielded TMAX estimates that were warmer than modern values by 2-3°C. According to the MCR reconstructions of TMIN values, winter temperatures remained below modern levels throughout the mid-Holocene. Winter temperatures below modern mean values persisted in the study region until the last 1000 years. Again, this is in agreement with the insolation curve for midlatitudes in the Northern Hemisphere.

The fossil insect MCR reconstructions shed new light on the question of a mid-Holocene thermal event. The concept of a hot, dry, "altithermal" climatic regime from 7500 to 4000 yr b.p. was first invoked by Antevs (1948), based on archaeological evidence from the northern Great Basin. Benedict (1979) called for a mid-Holocene altithermal period from 7500-5000 yr b.p., based on shifting land-use patterns in the Archaic cultural period. Human adaptations may have been driven more by drought cycles than by changes in temperature. The fossil insect record of the Rockies does not provide estimates of past moisture regimes, but it provides pa-leotemperature estimates that contradict the theory of a mid-Holocene thermal maximum. In terms of summer temperatures, the thermal maximum for the postglacial period in the Rocky Mountains of Colorado took place between 11,000 and 9000 yr b.p.. There was a second peak in summer temperatures between about 7000 and 5000 yr b.p., but on a slightly lesser scale than the earlier peak.

Pollen evidence from the eastern slope of the Colorado Rockies suggests that the tree line shifted upslope to elevations beyond its modern limit during the interval from 7000 to 3500 yr b.p. (Short 1985; Fall 1985). Evidence from the western slope suggests that trees migrated above modern tree line there from 9000-4000 yr b.p. (Fall 1997). These upward shifts in tree line have generally been interpreted as being driven by climatic warming. The conflicting interpretations of insect, pollen, and archaeological data during the mid-Holocene interval are puzzling, but highlight the need for additional regional studies to clarify the climatic reconstruction of the mid-Holocene.

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