AN 11,000-YEAR RECORD OF HYDROCLIMATE VARIABILITY FROM ROCK LAKE, MONTANA

Northwestern Montana has been plagued by persistent drought and water resource availability issues throughout the 20^th century, with notably severe droughts occurring in the 1930’s and 1950’s. A valuable longer-term perspective that can help place recent climate conditions into a greater context lies in lake sediment paleoclimate archives. To that end, we recovered 9-meter long sediment core from the depocenter of Rock Lake to investigate the variability of drought conditions over the Holocene. Rock Lake (48.83° N, 115.01° W; 880 m ASL) is a small (0.16 km²), surficially closed system situated in the semi-arid Tobacco Valley of northwestern Montana. Age control for the core was provided by ²¹⁰Pb, ¹³⁷Cs, 13 terrestrial macrofossil ¹⁴C dates, and the Mt. Mazama tephra layer. Cores were characterized by detailed stratigraphic description, dry bulk density, loss-on-ignition, X-ray diffraction, scanning electron microscopy, and carbonate (<63 μm) stable isotope analyses of δ¹⁸O and δ¹³C (~ 60 years represented per sample). Surface water samples were also collected from regional lakes and streams and analyzed for stable isotopes of hydrogen (δD) and oxygen (δ¹⁸O) to investigate modern hydrologic and water isotope relationships.

Based on our analysis of surface water stable isotopes, we interpret carbonate isotope data to reflect the isotopic composition of lake water at the time of mineral precipitation, which is sensitive to precipitation/evaporation balance. Our δ¹⁸O results indicate relatively wetter conditions (-6.5 ‰; VPDB) at 11,000 years before present (yr BP), with a gradual shift to more arid conditions by 6900 yr BP (1.02 ‰). Thereafter a shift to wetter conditions (-5.16 ‰) takes place and persists until ~500 yr BP. This period is also overprinted by a trend toward drier conditions (i.e. from -5.16 ‰ at 6900 yr BP to 0.26 ‰ by 500 yr BP). Dry conditions also persisted from 500 to 130 yr BP with a maximum δ¹⁸O of 0.99 ‰ at ~330 yr BP. Continuing from 130 yr BP to the present, δ¹⁸O varies between more negative values (from -2.70 ‰ and -4.35 ‰), suggesting that wetter conditions characterized the 20^th century. Our analysis indicates that past Holocene drought conditions in northwest Montana have exceeded those of the 20^th century, which has important implications for the future availability of water resources in the region.