A POSTGLACIAL PALEOECOLOGICAL RECORD FROM THE SAN JUAN MOUNTAINS OF COLORADO: FIRE CLIMATE AND VEGETATION HISTORY

TONEY, J.L.¹, AND ANDERSON, R.S.^1,2

¹Quaternary Sciences Program, Box 5644, Northern Arizona University, Flagstaff, AZ 86011, ²Center for Environmental Sciences & Education, Box 5694, Northern Arizona University, Flagstaff, AZ 86011

Continuous sediment, charcoal, and pollen records were developed from a ~4.5 m sediment core from Little Molas Lake (LML), 3370 m elevation, San Juan County, CO. LML was formed by 11,200 cal BP subsequent to glacial retreat. Turbated clay and gyttja was derived from in-lake productivity and outwash sediments from the drainage basin from ~11,100 cal BP until ~10,200 cal BP. Cessation of glacial input correlates with the termination of the YD. Tundra vegetation was replaced by Picea forest by ca. 11,000 cal BP, which indicates warming and new habitat availability. An increase in P. ponderosa pollen, probably from lower elevations, reflects the influence of the southwestern monsoon ca. 10,160 cal BP. Pollen ratios indicate that Picea and other conifers persisted near the lake for the remainer of the Holocene. The driest Holocene period occurs ca. 6200 to 5900 cal BP when lake levels were the lowest. Wetter conditions during the last ca. 2600 cal BP favored the expansion of P. edulis and P. ponderosa. Late glacial fire events occurred every 65 years with a doubling of the fire return interval in the early Holocene. This may reflect an increase in biomass for burning during a period of rapid vegetation turnover. The lowest fire event frequency occurs during the Neoglacial (after ca. 4100) and indicates moister and cooler climate. A latest Holocene pronounced peak in charcoal coincides with the historically documented 1879 AD-Lime Creek Burn.