A 6,000-YEAR RECORD OF DROUGHT IN NORTH-CENTRAL WASHINGTON FROM LAMINATED LAKE SEDIMENTS

The Pacific Northwest is prone to multi-decadal droughts that have serious economic impacts on the natural resources of the region including forestry, fisheries and water resources. Although this region is not generally considered to be arid, much of it is, and the population is currently expanding at a very rapid rate and taxing the limited the water resources between a multitude of uses including municipal, agricultural, and hydropower. Drought in this region has been linked with the warm phase of the Pacific Decadal Oscillation (PDO). Knowing the long-term history of drought would give us with an improved ability to predict their timing and severity, but historic records of climate are too brief to provide such an understanding. In order to place current events in the context of the natural variation of the past, we are working to produce a multiproxy study using laminated lake sediments to investigate the complex relationships between large-scale climatic forcing, regional climatic change, and vegetation and lake response at a watershed-scale from multiple sites.

In light of extended ~5-year drought conditions that currently affect much of western North America, substantial effort has been focused on understanding the natural cyclicity and magnitude of such events from the paleoclimate record. Here we present a ~6,000-year record of drought variability with ~5-year resolution based on a multiproxy study including oxygen/carbon isotopes and sediment grayscale analyses from Castor Lake in north-central Washington State. The grayscale record correlates well with the ~1500-year Palmer Drought Severity Index (PDSI) reconstructed from tree-ring data across the west. This serves to validate the legitimacy of both methodologies and allows for confident interpretation of the period for which only lake data are available. Spectral analyses of the ~6,000-year record from Castor Lake reveals a strong and dominant periodicity centered on ~50-years. Such timescales are coincident with those observed in Pacific Ocean variability, and suggest that the PDO may have exerted an influence on regional drought patterns for at least the past ~6,000 years.