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