RECONSTRUCTING THE ASHFALL HISTORY OF THE ANCHORAGE ALASKA AREA

Anchorage, Alaska is located within a few hundred kilometers of some of the most active volcanoes in North America, yet, the history of ashfall for the Anchorage area is not well documented. Ash clouds from historical eruptions of volcanoes in the eastern Aleutian arc have reached Anchorage, threatened air traffic and caused significant, but temporary, problems for life and property. However, deposits of volcanic ash can also provide important marker beds and can provide a means for determining ashfall frequency. Small, shallow, lake basins are natural repositories for volcanic ash, and the host sediments can provide a datable stratigraphic context.

During the summer of 2000 we recovered a 2.5 m core from Little Campbell Lake, located within a kilometer of Ted Stevens Anchorage International Airport. The lake is a kettle basin on a moraine that formed 12-15 Ka. The drainage basin that includes the lake is forested, has low relief (2-10 m), and contributes only small amounts of clastic sediment to the lake. Using visual stratigraphy, magnetic susceptibility, and loss on ignition values, at least seven tephra layers were identified. The base of the core reached a hard, compact tephra layer that may be the 3.5 Ka Hayes tephra.

The abundance of terrestrial organic matter for radiocarbon dating and the lack of bioturbation in the core stratigraphy demonstrate that this lake basin, and possibly others like it, may be useful for determining the history of ashfall in the area. Other lakes in the Anchorage area will be cored during the summer of 2001 to evaluate the lateral extent and stratigraphic significance of specific volcanic ash layers. Geochemical "fingerprinting" of tephras and radiometric dating of associated organic matter may allow us to identify source volcanoes, and determine the timing and frequency of Holocene eruptions.