Extending the geochronometry of arctic lake cores beyond the radiocarbon limit by using thermoluminescence

Abstract

Reconstructing detailed paleoclimatic/paleoenvironmental histories from Pleistocene sediments in the Arctic is an ongoing challenge because of the difficulty of measuring accurate timescales beyond the range of radiocarbon (14C) dating. Important archives of such histories are arctic lakes that exist outside the known extent of the last major glaciations. To develop reliable geochronometry of sediment cores from arctic and other lakes, we applied thermoluminescence (TL) sediment‐dating methods to 24 samples (8 reported earlier but some reanalyzed here) selected from cores from Squirrel and Joe Lakes in northwestern Alaska. A sequence of 23 14C ages from the upper parts of these cores permitted tests of the zeroing assumption of TL dating: that all fine‐silt grains were exposed to daylight before final burial. We performed multiple TL tests on most samples to isolate stable TL signals and measured TL emissions spectra to help characterize samples. TL ages generally agree with the ∼30 kyr age‐depth trend derived from 14C dating and extend the timescale apparently to ∼180 ka for Squirrel Lake and to at least ∼60 ka for Joe Lake. An anomalously old TL result from Squirrel Lake is attributed to failure of the TL zeroing assumption within that sediment horizon. In the lower Joe Lake core we assign a timing of 45 ka to 50 ka for the beginning of an apparently climatic warming interval. These results identify and quantify the main TL sources of TL age inaccuracy and the preferred TL protocols for accurate TL sediment dating in northern high‐latitude lake deposits. They also provide the first direct chronology beyond the 14C dating range for these lakes.