Hydrologic Variability in the Pacific Northwest During the Past 13,000 Years from High-Resolution Studies of Finely Laminated Lake Sediments

Collaborative Research

Bruce Finney, Institute of Marine Science, University of Alaska, Fairbanks (finney@ims.alaska.edu)

Jon Riedel, North Cascades National Park, National Park Service (Jon_Riedel@nps.gov)

Students Involvement

Byron Steinman, Pitt, Masters/Ph.D student Aug 2005 to present, bas68@pitt.edu

Daniel Nelson, Pitt, Masters Dec 2004, danielbnelson@gmail.com

Erica Hill, Pitt, undergraduate research project Sept 2003 - Nov 2005

Images from the North Cascades

Images from the North Cascades II

Project Summary

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 droughts 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 undertaking a multiproxy study using finely 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. We are analyzing lake cores from sites we determined to be laminated at ~5-year resolution to provide detailed paleoclimate records over the past 13,000 years. This project together with a synthesis of regional paleorecords and our ongoing research in Alaska and the Yukon Territory will allow us to investigate large-scale changes in climate in the northeast Pacific region during the Holocene and latest Pleistocene. 

The main goal of our proposed research is to produce high-resolution multiproxy records documenting changes in effective moisture for the last 13,000 years from lakes with high sedimentation rates and annual laminations (Castor and Lime Blue lakes). We will test hypotheses, outlined in the project description, concerning the nature and causes of shifts in the regional precipitation-evaporation (P-E) balance and connections with the PDO. We selected lakes that record climatic information on an annual basis in varves characterized by couplets of authigenic CaCO3, precipitated from the water column, draped by clay caps. These hydrologically closed-basin lakes respond rapidly to drought conditions as illustrated by the d18O and dD characteristics of their water. We will assess this variability using statistical and time-series analyses, and synthesis our results with other records to better understand modes of atmospheric circulation in the North Pacific. The region is highly relevant to hydrologic issues in the Pacific Northwest, and has been sensitive to changes in precipitation resulting from historic PDO and ENSO shifts.

Our preliminary work in the region demonstrated: (1) the proposed lakes have continuous, high-resolution archives spanning the period from deglaciation until present (~13,000 years), (2) chronologies can be well-constrained by using tephrochronology, 14C on charcoal and terrestrial macrofossils, 210Pb, 137Cs, and geomagnetic correlation, and (3) we are working with a set of reliable paleohydrological proxies including elemental and stable isotope geochemistry, pollen, charcoal, sedimentology, and thin-section analysis. Our sampling strategy will yield paleoclimate records with 5 year resolution for 2 sites and 2 additional sites will be studied at lower resolution to clearly document long-term regional patterns. We will use historical records and modern calibration studies across the region to constrain our down-core interpretations of the stable isotopes. Hydrologic fluctuations will be determined by both lake-level and stable oxygen isotope studies on authigenic CaCO3, and constrained by hydrologic and isotope mass balance models. Other climatic indicators will be studied, including fire frequency through charcoal analysis and biogenic silica analyses to identify productivity changes. The data we collect will enable an evolutive spectrum of drought variability to be developed, placing 20th century in a longer-term context.

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