Optical Dating of Sorted Circles in King George Island, Shetland Islands, West Antarctica as a Potential Time Marker for Local Glacier Retreat

Abstract

During the deglaciation period, the glaciers dump till sediments as they melt away, thus the depositional ages of these sediments provide important information on the timing of glacial retreat(the initiation and the duration of the local deglaciation). However, in many cases, direct dating of till sediments is not readily possible because of a lack of suitable material for conventional dating methods, except where 14C dating is applicable. In addition, optical dating on these sediments has not been always successful because most glacigenic deposits usually have little chances of being exposed to sufficient sunlight, which results in significant overestimation in optical ages. The till sediments, however, sometimes form a diagnostic geomorphic structure, referred to as sorted circle, hrough repeated freeze-thaw cycles on flat areas in polar regions. In the course of its formation, the soil particles in the active layer may move up and down actively, and therefore some of these particles are presumed to have chances to be exposed to sufficient sunlight for the latent OSL signals to be completely bleached, which is one of the most important prerequisite process for reliable optical dating. To test this hypothesis, we collected sediment samples from two sorted circles at the elevation of 41 m and 66 m in King George Island, South Shetland Islands, West Antarctica. Quartz grains extracted from these sediments have various undesirable OSL properties for routine SAR-based optical dating. For instance, the OSL signals of those quartz grains do not have fast OSL component that is the usual signal for optical dating. In dose recovery test using both LM-OSL SAR and conventional SAR procedure, the recovered doses were lower than the given dose by about 10 % or less. The OSL age obtained on the basis of weighted mean De value of 18±1 Gy was 11±1 ka, which implies that the deglaciation near King Sejong station may have occurred before 11±1 ka.