A ~4000 YEAR RECORD OF HYDROLOGIC VARIABILITY FROM THE OLYMPIC MOUNTAINS, WASHINGTON

Sedimentological and geochemical analyses of gravity and piston cores retrieved from Lake Quinault, a 15 km2 and 70 m deep moraine-dammed lake located on the western front of the Olympic Mountains in Washington, USA, reveal a record of deposition over the past ~4000 years that is detrital-dominated and driven by large flood events. Individual historic flood layers preserved in sediment cores contain a coarse-grained basal deposit that is enriched in terrigenous organic matter. These same flood layers display peaks in the ratio of incoherent to coherently scattered X-ray radiation, or inc/coh, from μXRF core scans, a parameter which traces the concentration of coarse-grained, organic-rich sediment throughout each core. Given this relationship, we utilize the inc/coh ratio as a proxy for individual flood event layers that allows for an assessment of the periodicity of overall hydrologic variability and the distribution of extreme events (paleofloods) throughout the late Holocene. Despite the lack of a long-term trend, notable peaks in event number occurred during ~2350 to ~2450 cal BP and ~1910 to ~2010 CE. The period ~2350-2450 cal BP is unremarkable within the context of existing Pacific Northwest paleoclimate records, but the spike in extreme events over the last century suggests that an increase in the frequency of large storms tracking over the Olympic Peninsula is connected to recent climate change. This connection, coupled with high spectral power of the inc/coh time series at multi-decadal and multi-centennial periodicities, leads us to hypothesize that the record of hydrologic variability as preserved in in Lake Quinault sediments reflects trends in climate conditions favorable for the formation of north Pacific atmospheric rivers. Since atmospheric rivers are modulated by fluctuations in large scale atmospheric and oceanic teleconnections such as the Pacific Decadal Oscillation, and are predicted to increase in frequency and severity during the course of the 21st century, understanding past hydrologic variability has important implications for the landscape and ecosystem response of Olympic Mountain catchments to future climate warming.