5,000-YEAR LAKE CARBONATE AND TREE-RING BASED SNOWPACK RECONSTRUCTIONS REVEAL LONG-TERM TRENDS IN SOUTHWEST MT HYDROCLIMATE

Declines in Northern Rocky Mountain (NRM) snowpack over the past 50 years have prompted discussion on the history of hydroclimate. Expanding the temporal reaches of hydroclimate records will contextualize recent drought and shifting baselines. Southwestern Montana contains headwaters to the Upper Missouri River which depends on snowpack as its primary method of surface water storage. Considering the spatial, environmental and societal influence of this watershed, it is important to understand fluctuations in snowpack of the region. This study aims to provide a millennial scale record using carbonate sediments from Morrison Lake, MT and NRM tree ring chronologies.

Morrison is a relatively small, topographically closed-basin lake with a large contribution of winter snowmelt, continual groundwater recharge and precipitation of authigenic carbonate. Based on 4 years of repeat collections, seasonal variation of lake water δ¹⁸O is ~2 ‰ with an average of -12.4 ‰, indicating sensitivity to precipitation. These samples sit midway on the regional evaporation line, representing equal influence of precipitation and evaporation on the lake system. A high sedimentation rate allows lake sediment carbonate to record decadal to millennial signals.

Sediment cores were collected from Morrison Lake in 2018, the longest reaching 2.6 m and sampled at 0.5 cm resolution. Loss on ignition was used to determine carbonate concentration. Samples were sieved at 32 µm to fully remove detrital carbonate grains. A depth-age model was created with bulk-sediment radiocarbon ages and Mazama tephra using BACON software. Pending results from IRMS measurements of carbonate δ¹³C and δ¹⁸O, we will determine relationships between lake-system dynamics and climatic variables like precipitation, evaporation and temperature. Lake carbonate δ¹⁸O will be compared to ~800 year tree-ring based NRM climate reconstructions (Pederson, 2011). Mutual large amplitude signals between the two datasets will be correlated visually using tie-points and validated using the MATCH algorithm. Based on the lake’s modern sensitivity to annual snowpack variations and lake basin dynamics we anticipate a high-resolution winter-season climate record. Our preliminary sediment record spans the past ~5,000 years offering a long-term perspective of hydroclimate.