Holocene Lake-Level Changes in Northwest Montana Indicate Substantially Drier than Modern Conditions in the Mid-Holocene

Abstract:

Lake-level reconstructions have provided an important record of hydroclimate change during the late-Quaternary in the arid western U.S. Fewer records of water level change have been developed from lakes in the high-elevation, snow-dominated portions of the Rocky Mountains. Such records can, however, offer important insight into the past climatic conditions of the dominant water-producing areas of the western US. Lakes in these humid areas today contain submerged paleoshorelines, indicating of periods of drought in the past. Here, we focus on Foy Lake, a deep carbonate-producing lake located at the eastern edge of the Salish Mountains in Montana. The 35-m deep lake contains two basins, which combined have a surface area of 1.1 km².The lake is closed at the surface and sensitive to changes in the local water table. Sediment cores collected in 2018 from water depths of 16 and 8 m contain sandy paleoshoreline deposits representing low water episodes from 10.2-9.6, 8.8-7.9, and 6.9-4.8 ka. Sediment accumulation rates during the later event, when sand concentrations were greatest, dropped to less than 10 cm/ka, consistent with near-shore erosion. After 2.9 ka, sandy near-shore deposits were overlain by varves in both cores, consistent with a substantial rise in water levels and a major change in hydroclimate of the region, possibly marking the onset of the modern winter precipitation regime and its connection of the precipitation in the Pacific Northwest.