Holocene ecological change in relation to hydroclimate variability and post-landslide landscape processes in semi-arid watersheds, Lost River Range, Idaho, USA

Lake sediment cores were collected from two landslide formed lakes in the Lost River Range, central Idaho. Grouse Creek Lake (2 m deep) is in a basin that was formed when a drainage was impounded by a debris slide, likely more than 10,000 yr BP. Under present conditions Grouse Creek Lake is closed to surface water outflow. Since the deposition of the Mazama tephra (c.a., 7,550 yr BP) 2.5 m of sediment containing authigenic carbonate has accumulated in this lake. Modern lake water dD and d18O displays an evaporative signal indicating that oxygen isotopes from Grouse Creek Lake authigenic carbonates should record a signal that is sensitive to past changes in precipitation and evaporation. This core was sampled at 1 cm intervals and sieved at 20 um to remove detrital and biogenic carbonate in preparation for analysis of the fine-grained, authigenic carbonate fraction. The oxygen isotope signal recovered has a range of 5‰ over the length of the record, with several major fluctuations since the deposition of the Mazama tephra. While the d18O generally increases over this period indicating decreasing effective precipitation (P - E), we are modeling the affects of changes in lake hypsometry due to sediment infilling on the isotopic composition of lake water to quantify the hydroclimate signal in the isotopic data. Carlson Lake (10 m deep) formed in the source area of a middle Holocene mudflow. Modern lake water chemistry indicates that Carlson Lake is sensitive to evaporative processes, however sediments from Carlson Lake do not contain enough carbonate for oxygen isotope analysis. A multiproxy effort (C and N isotopes, biogenic silica) is underway to understand how the landscape around Carlson Lake responded to changes in hydroclimate (as determined by the record from Grouse Creek Lake) and post-landslide soil and geomorphic processes. These records are from a region with sparse long-term hydroclimate data, and ultimately will help improve our understanding of past atmospheric circulation in the Northern Rocky Mountains.