HR: 1330h
AN: V42B-0344    [PDF]
TI: Late Quaternary Tephrochronology From Three Lakes, Kenai Peninsula, Alaska
AU: * de Fontaine, C S
EM: csd3@dana.ucc.nau.edu
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86011 United States
AU: Kaufman, D S
EM: darrell.kaufman@nau.edu
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86011 United States
AU: Anderson, R
EM: scott.anderson@nau.edu
AF: CESE/Quaternary Sciences Program, Northern Arizona University, Flagstaff, AZ 86011 United States
AU: Werner, A
EM: awerner@mtholyoke.edu
AF: Department of Earth and Environment, Mount Holyoke College, South Hadley, MA 01075 United States
AU: Waythomas, C F
EM: chris@usgs.gov
AF: U.S. Geological Survey and Alaska Volcano Observatory, 4230 University Drive, Suite 201, Anchorage, AK 99508 United States
AU: Wallace, K L
EM: kwallace@usgs.gov
AF: U.S. Geological Survey and Alaska Volcano Observatory, 4230 University Drive, Suite 201, Anchorage, AK 99508 United States
AB: Tustumena, Paradox, and Jigsaw Lakes, located 60-120 km southwest of Anchorage, Alaska, on the Kenai Peninsula, were cored to evaluate the conditions best suited for the preservation of tephra, and to determine the ages and frequency of ash fall associated with regional volcanic eruptions. The lakes are located downwind of the eastern Aleutian Arc, where they are likely to receive tephra fallout, especially from Augustine, Iliamna, Redoubt, Spurr and Hayes Volcanoes. The lakes are close together ($<$60 km), so differences among their tephra records should primarily reflect lake-dependent processes and features, although variations in individual tephra plumes may preclude deposition in all the lakes. Multiple cores from each lake were analyzed to locate tephra using magnetic susceptibility (MS) and petrographic techniques (all tephra were found to have high MS). Tephra beds are typically light-gray ash, 1-2 mm thick, and are composed of glass shards and pumice (commonly 0.06-0.3 mm diameter), and varying proportions of glass-coated phenocrysts. Most are relatively pure, suggesting primary deposition with minor subsequent reworking. Core depth-age models were derived based on 28 radiocarbon ages on plant macrofossils preserved in the stratigraphy. Of the three lakes, sediment from Paradox Lake recorded the greatest frequency of tephra-fall events: 6.1 tephra/ka (81 over the 13,200-cal-yr record). Sediment from Jigsaw and Tustumena Lakes contains only 1.1 and 1.9 tephra/ka over their 9700-cal-yr records. Paradox Lake is better suited for tephra preservation compared to Tustumena and Jigsaw Lakes because, (1) it is small (0.11 km$^{2}$) yet has a relatively large drainage-basin-to-lake area ratio of more that 5 times the other lakes for accumulating tephra; (2) it is situated in a narrow (ca. 0.6 km), steep-sided (10-18$\deg$) valley that focuses tephra deposition and shelters the lake from wind-driven mixing (the other lakes are more exposed and susceptible to wind-driven mixing); (3) it is deep enough (16 m) to maintain an anoxic bottom layer to inhibit bioturbation, compared to Jigsaw Lake, with its better-oxygenated, shallower subbasins (most $<$12m); (4) it has a higher sedimentation rate (0.7 mm/yr; compared to $\leq$0.5 mm/yr at the other lakes) to resolve tephra layers. At Paradox Lake, the frequency of tephra fall ranges from 0 to 17 tephra/ka, and the number of tephra dating to the last 1000 yr (7) is about average for the last 13 millennia. No tephra layers are preserved during the intervals between 1000-2000 and 4000-5000 cal yr B.P., despite continuously laminated lacustrine sediment. In contrast, between 5000-9000 cal yr B.P., the frequency averages 11.8 tephra/ka. This may reflect changes in the predominant wind direction, or more likely the increased eruptive activity of the Cook Inlet volcanoes. Because the upper several centimeters of all cores were disturbed during coring, evidence of historical eruptions (last ca. 70 yr) was not preserved. Excluding this recent portion of the record, clusters of 3-5 tephra deposited within a 100-year interval occur nine times, most recently during the century leading to the 1912 A.D. eruption of Mount Katmai. The different number of tephra preserved in the three lakes demonstrates the importance of careful lake selection for obtaining the most complete record of tephra fall.
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting