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

The distal volcanic ash-fall record preserved in Holocene lake sediments 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 km ²) shallow (ca. 7.9 m) kettle located on the Elmendorf Moraine, 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.

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, including basal ages of ca. 12,300 ¹⁴C yr BP in three of the cores, confirming that the cores contain the entire postglacial sedimentary record. Eleven tephra units, 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. The degree of core-to-core variation, recognized by several physical characteristics, was assessed for each tephra. These characteristics include: tephra stratigraphic thickness, MS peak area and a numerical classification scheme ranking visual and stratigraphic prominence based on thickness, purity of ash and nature (sharpness and continuity) of 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 the units. However, it remains likely that open basins having larger runoff or higher relief areas may have more complex tephra stratigraphies.