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

Abstract

During the period of deglaciation, the glaciers leave moraine sediments behind as they retreat, thus the depostional ages of moraine sediments provide important information on the timing of glacier retreat, and the beginning and the duration of local deglaciation. However, in many cases, direct dating of moraine sediments is not readily possible because of a lack of suitable materials for conventional dating methods, except where 14C dating is applicable. In addition, optical dating on these deposits has not been successful because moraine sediments tend to have little chance of being exposed to sufficient sunlight, which results in incomplete bleaching of sediments and siginificant age overestimation. The moraine sediments, however, sometimes form a distinctive geomorhpic structure, called sorted circle, through repeated freeze-thaw cycles on flat areas in polar region. This structure has a sorted appearance commonly due to a border of stones surrounding finer materials. In the course its formation, the soil particles (or sand sized grains) above permafrost layer actively move up and down as the stone-soil profile repeats freezing and thawing, and therefore some of these particles are presumed to have had chances to be exposed to sunlight for long time enough to be completely bleached, which is the most important prerequisite process for reliable optical dating. To test our hypothesis, we collected 10 soil samples from 5 sorted circles at the elevations of 37 m, 41 m, 66 m, 120 m in King George Island, South Shetland Islands, West Antarctica. In the laboratory, the samples were sieved and etched to recover 90-250 &micro

m fractions of pure quartz grains. These quartz extracts were then used for further investigations related to optical dating. In this paper, we describe and characterise the luminescence properties of quartz, investigate equivalent dose distribution based on single grain/small aliquot SAR (single-aliquot regenerative-dose) procedure, and propose opt