"Varve" counting vs. tephrochronology and 137Cs and 210Pb dating: A comparative test at Skilak Lake, Alaska

Abstract

The age of recently deposited sediments in Skilak Lake has previously been estimated only by counting [open quotes]varves[close quotes]. We measured the sedimentation rate of Skilak Lake using two radionuclides, [sup 137]Cs and [sup 210]Pb, and found it to be about an order of magnitude lower than previous estimates based on varve counts. We also identified several tephras through a combination of visual inspection, core X-radiographs, observation of variations of the magnetic susceptibility, intensity of magnetization of the unconsolidated sediments, and microprobe analyses of volcanic glass shards. Tephrochronologic dates using matches with the Katmai 1912 tephra and an Augustine tephra from 500-550 yr B.P. are in good agreement with the sedimentation rate estimates based on the radioisotope data, and indicate that the rhythmite layers at Skilak Lake are not annual varves. These new estimates of sedimentation rate reaffirm that care is needed in varve dating and require that earlier work on sunspot and climate changes thought to have been recorded in the Skilak Lake sediments be reevaluated. 16 refs., 6 figs., 1 tab.