Optical dating of the upper 22 m of cored sediments from Daihai Lake, northern China

Daihai Lake is one of the largest lakes in the mid-latitude of northern China. Previous environmental change investigations using the sediments from the lake have been mainly focused on the last 13 ka. In 2006, we drilled an 80-m borehole on the southwest coast of the lake. Here we report the results of initial optical dating of the upper 22-m sediments from the core. Most of the samples allow separation of the 4-11 mu m fine-grained and 45-63 mu m medium-grained quartz, both of which were used for equivalent dose (D-e) determination. Dose recovery test experiments with the single-aliquot regeneration-dose (SAR) protocol indicate that a preheat at 240 degrees C for 10 s combined with a cut-heat of 240 degrees C is suitable for these samples. The D-e values show marked discrepancy between the two grain size fractions with the fine-grained quartz yielding up to over 50% higher D-e values than the medium ones for some samples. We consider the OSL ages for the samples from upper similar to 10 m to be overestimated. The main cause of the overestimation is attributed to the incomplete bleaching of the sediment grains at deposition. The OSL ages for the lower part of the sequence are considered to be reasonable age estimates which provide useful temporal constrains on the deposition of the sediments for the period of 20-40 ka. (C) 2009 Elsevier B.V. All rights reserved.