Three Holocene Tephras Identified in Lacustrine Sediment Cores from the Wonder Lake Area, Denali National Park and Preserve, Alaska, U.S.A.

Abstract

Eighteen percussion piston cores were recovered from Wonder Lake and three nearby kettle ponds in Denali National Park and Preserve, Alaska. Three prominent tephra deposits, two felsic and one mafic, have been recognized in the upper 1 to 3 m of the cores. Because of the relatively low magnetic susceptibility (MS) of the lacustrine sediment, tephras appear as prominent MS peaks allowing confident correlation of core stratigraphies. These MS-based correlations are supported by microprobe geochemical analyses of 11 tephra samples from six of the cores. Five accelerator mass spectrometer (AMS) radiocarbon ages, determined from terrestrial plant remains, closely constrain the timing of these ashfall events. Microprobe results indicate that the youngest tephra is correlative with the Jarvis Ash, which has an accepted age of 3660 ± 125 14C yr BP. The next older felsic tephra is correlative with the middle Holocene Oshetna tephra. New 14C ages reported in this study suggest that this regionally extensive tephra was deposited about 6000 14C yr BP. Neither of these tephras has previously been identified this far west, indicating that both are significantly more extensive than previously thought. A fine-grained basaltic ashfall deposit, identified in several of the Wonder Lake cores and dated to ca. 10,000 14C yr BP, is a newly discovered tephra that provides an important stratigraphic marker horizon at the Pleistocene-Holocene boundary in central Alaska.