A 5000-year record of fire and sedimentary response for the Uinta mountains, northeastern Utah

MUNROE, Jeffrey S., and LAIDLAW, Jamie, Geology Department, Middlebury College, Bicentennial Hall, Middlebury, VT 05753

The history of forest fire in the Uinta Mountains was investigated using charred plant remains isolated from lake sediment as a proxy for paleo-fire, and percent loss-on-ignition (%LOI) and sediment grain size as proxies for energy of the depositional environment. Three AMS dates on charcoal and wood fragments provide age control (corrected to cal yr BP). From 5500 to 4700 BP, charcoal accumulation rates (CHAR) averaged 29 fragments/cm²/year, significantly higher (P < 0.001) than in the remainder of the record. Also, both the mean size of charcoal fragments (2.5 mm²) and % LOI (mean 61%) were significantly greater (P = 0.029 and P < 0.001 respectively) than for other periods, and the mean sediment grain size (20 μm) was greater than in all but the most recent period, indicating that fires were common. From 4700 to 2900 BP, fires were still common (mean CHAR 22 frag/cm²/yr) but CHAR values were considerably more variable (coefficient of variation 2.1, all other periods < 0.9). Mean charcoal fragment size (2.0 mm²) was twice the value post-2900 BP indicating a complex fire regime in which large fires punctuated relatively fire-free intervals. In contrast, fires were essentially absent from 2900 to 1500 BP, with only one CHAR peak (1.8 frag/cm²/yr at ~2100 BP) reaching above background levels of ~0.3 frag/cm²/yr. %LOI values for this period (mean of 30%) are significantly lower (P < 0.001) than in the rest of the record. Finally, four discrete CHAR pulses (> 0.9 frag/cm²/yr), spaced ~200 years apart, occurred between 1500 and 850 BP. This final interval corresponds temporally to the Medieval Warm Period and may indicate a return to periodic low-intensity fires after a lull during the Neoglaciation.