HOLOCENE PALEOENVIRONMENTAL ANALYSES FROM MULTIPLE LAKE SEDIMENT CORES IN NORTH-CENTRAL WASHINGTON STATE

Concurrent analyses of sediment cores from five lakes in north-central Washington provides the basis for detailed Holocene paleoenvironmental analysis. The use of multiple lakes allows for the removal of localized influences in individual basins, increasing the level of confidence in the conclusions that may be drawn from a cumulative record. The region was selected for study because of its sensitivity to minor changes in the Pacific climate that dominates the coastal region and its location amongst the thickest section of marine sedimentary bedrock in the state. Chronological control is constrained by both AMS radiocarbon dates from terrestrial plant remains and charcoal found in all cores, as well as the presence of six distinct tephra deposits from proximal Cascade Range volcanism that are dispersed throughout the entire Holocene record in all lakes. Three tephra layers have been traced to Glacier Peak, two to Mount Saint Helens, and one to the Mazama climactic eruption. These tephra layers provide age markers of 13155, 7590, 5700, 3790, 2140, and 470 cal yrs BP, and are present in all uninterrupted sediment cores used in this study. Lab analyses performed on sediment cores include general sedimentology, magnetic susceptibility, wet and dry bulk density, loss on ignition (LOI) at 550° and 1000°, as well as d¹³C and d¹⁸O on authigenic carbonate precipitated from dissolved bedrock in the lake water. Interpretations attempt to balance changing precipitation – evaporation (P-E) balance as reflected in the lake sediments, with the gradually increasing elevation of the lake bottom through time as a result of sediment deposition.

Broad trends in all lakes show evidence for cooler and wetter conditions in the early and late Holocene compared to extensive drought conditions in the middle Holocene. Evidence for peak drought conditions between approximately 5000 and 6000 cal yr BP comes from a variety of sedimentary and geochemical indicators from Big Twin, Little Twin, and Davis Lakes. Understanding of these broad climate trends facilitates high-resolution analysis of sections of the records, such as middle Holocene drought, which may serve as an analog for regional impacts of projected greenhouse gas warming.