Late-Holocene Drought Variation in Northwestern Montana, Based on Limnological and Fire Records

Records of past environmental change are critical for understanding the natural variability of the Earth's hydrologic system, including the frequency, duration, and spatial extent of drought and its ecological consequences. In the northern Rockies, periods of drought are controlled by the strength and position of the Aleutian low in winter and the northeastern Pacific subtropical high in summer. We present a 2000-year-long history of drought based on records of δ¹⁸O of authigenic carbonates, diatoms, charcoal, and pollen from Foy Lake (N 48°10', W 114°21', 1006 masl), located at lower treeline. The site preserves annually laminated sediments, which were analyzed in contiguous six-year intervals. The oxygen-isotope and diatom stratigraphy provide information on variations in effective moisture and consequent water-level change. Macroscopic charcoal records indicate times of high fire occurrence associated with summer drought, and pollen ratios of climatically sensitive taxa register vegetation changes associated with available moisture. The duration of drought at Foy Lake has varied over the last 2000 years, but the frequency has remained relatively constant. From AD 0 to 800, isotopic data suggest multi-decadal drought intervals, spaced about 50-60 years apart. Fire frequency was relatively low, fires were associated with dry intervals, and the vegetation was dominated by closed-forest taxa. Between AD 800 and 1250, fire frequency and vegetation were relatively unchanged, although drought intervals were shorter (<10 years), as evidenced by the isotopic and diatom records. The last 750 years (AD 1250 to 2000) marks a shift towards open xeric vegetation and protracted (multi-decadal) dry intervals, spaced 50-60 years apart. Extreme droughts associated with pronounced lowering of lake level and multiple fire events occurred at AD 1350, 1660, and 1930 and coincided with times of recorded solar minima. The shift in drought regime at AD 1250 is coherent with shifts documented elsewhere in western North America and the Midwest. Thus, the data suggest a regional response to multi-decadal variability in the atmospheric circulation patterns across North America, associated with changes in the strength and location of the Aleutian low.