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INTRABASINAL VARIABILITY OF HOLOCENE TEPHRA IN A SMALL KETTLE LAKE, LORRAINE LAKE, UPPER COOK INLET, ALASKA

KATHAN, KASEY  Mount Holyoke College.
Werner, Alan  Mount Holyoke College.
Kaufman, Darrell  Northern Arizona University.
de Fontaine, Christian  Northern Arizona University.
Kingsbury, Esther  Mount Holyoke College.

The record of volcanic ash fall record preserved in Holocene lake sediments of Lorrane Lake was used to investigate the intrabasin variability of individual tephra units. Six sediment cores between 3.2 and 5.8 m long were recovered from Lorraine Lake, a small (0.53 km2) shallow (ca. 8 m) kettle located on the Elmendorf Moraine, of late Pleistocene age, 11 km northwest of Anchorage, Alaska. Situated in a region of low relief (49 m), the basin has a limited drainage basin (1.3 km2), no inflow and remains ice covered for approximately six months of the year.

Cores were recovered from the north, south, east, and west parts of the lake, which is divided into two (north and south) sub-basins. A total of 21 AMS 14C ages were obtained on terrestrial macrofossils. Radiocarbon ages from the base of three of the cores average ca. 12,300 14C yr BP, confirming that the cores contain the entire postglacial sedimentary record. Eleven tephra deposits, ranging from non-visual to several centimeters in thickness, have been identified and can be correlated among the cores based on their relative depths and spacing, color, texture, and thickness, high magnetic susceptibility (MS), low loss-on-ignition, X-ray gray scale value, and abundance of magnetic minerals. Although other less prominent tephra units may occur, these 11 clearly defined units are used to compare tephra deposition within the lake. Several physical characteristics were compared including: stratigraphic thickness, MS peak area, X-ray density stratigraphy and a numerical classification scheme ranking visual and stratigraphic prominence based on thickness, purity of ash and nature (sharpness and continuity) of stratigraphic contacts.

Despite sedimentation rate variations (0.75 to 0.32 mm/yr) the thickness of the tephra units does not appear to vary between cores recovered from different parts of the basin or different water depths. The stratigraphic prominence of the tephra layers and their similarity between core sites implies that probable depositional complexities (e.g., wind skimming, waves, lake ice, blowing snow) and post-depositional processes (e.g., bioturbation, bathymetric focusing) have a minimal impact on the preservation of tephra units. Open basins having larger runoff or higher relief areas, however, likely have more complex intra-basin stratigraphies.


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