HR: 1340h
AN: V23A-0619    [Abstracts]
TI: Using Kettle Lake Records to Date and Interpret Holocene Ash Deposition in Upper Cook Inlet, Anchorage, AK
AU: * Werner, A
EM: awerner@mtholyoke.edu
AF: Department of Earth and Environment Mount Holyoke College, 321 Clapp Lab, South Hadley, MA 01075 United States
AU: Kathan, K M
EM: kmk238@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: Hancock, J R
EM: juliettehancock@yahoo.com
AF: Department of Earth and Environment Mount Holyoke College, 321 Clapp Lab, South Hadley, MA 01075 United States
AU: Waythomas, C F
EM: chris@usgs.gov
AF: U.S. Geological Survey 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 Alaska Volcano Observatory, 4230 University Drive, Suite 201, Anchorage, AK 99508 United States
AB: Fourteen sediment cores recovered from three kettle lakes (Goose, Little Campbell and Lorraine) near Anchorage, AK were used to document and date Holocene volcanic ash deposition in the upper Cook Inlet area. Small lakes ($<$0.5 km$^2$) with small ($<$1.5 km$^2$), low relief ($<$50 m), and well-vegetated drainage areas were selected in order to minimize ash remobilization by mass wasting and fluvial processes. The resulting stratigraphic records are interpreted as primary terpha-fall stratigraphies. Relative to the surrounding lacustrine sediments, the ash layers exhibit low organic-matter content (as determined by loss-on-ignition, LOI), high magnetic susceptibility (MS), increased density (X-radiographs), and bubble-wall glass shards. Some ash layers are up to 1 cm thick (macrotephra) consisting of pure glass, some occur as light bands, while others (microtephra) can only be located using non-visual techniques (MS, LOI and X-radiography). The thinnest microtephras observed occur either as discrete (1 mm) layers or diffuse laminations composed of tephra mixed with ambient lake sediment. Forty-five AMS C-14 dates on terrestrial macro fossils were used to constrain sedimentation-rate models for the cores, and to assign absolute ages to ash units. Comparison of inferred tephra ages corroborates our intra and inter basin stratigraphic correlations (+/- 200 yrs) based on physical and MS stratigraphy. Ten out of 12 macrotephras can be confidently correlated among all three lakes, whereas, two of the prominent tephras occur in one basin but not in the others. This suggests subtle differences in ash plume extents or differences in tephra preservation between lakes. A total of 24 Holocene ash units (12 macro and 12 micro) have been recognized and dated in the Anchorage area, suggesting an ash-fall frequency of about 2.4/1000 yrs. By comparison, historical records suggest more frequent ash-fall events (120/1000 yrs). Our data indicate that, either the ash layers are not consistently preserved in the kettle basins, or more likely, these records lack the resolution to differentiate closely spaced ash-fall events. Core top stratigraphies support the latter interpretation: The 10-12 historically observed ash-fall events are represented by two diffuse zones in the upper 15 cm of the cores. As such, ash records from small kettle lakes should be regarded as conservative statements of ash deposition. Further, ash plumes can have narrow geographic distributions and ash-fall thicknesses can change markedly over short distances. Therefore distal ash-fall stratigraphies underestimate eruption frequencies.
DE: 0370 Volcanic effects (8409)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting