Journal of the Geological Society of Korea (지질학회지)

The Geological Society of Korea (대한지질학회)
권호 표지

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

남극 킹조지섬에 분포하는 원형구조토(Sorted Circles)에 대한 OSL 연대측정 적용가능성 연구

Choi, Jeong-Heon;Lim, Hyoun-Soo;Yoon, Ho-Il;Cheong, Chang-Sik;Im, Chang-Bock;Kim, Jong-Wook;Chang, Ho-Wan
최정헌;임현수;윤호일;정창식;임창복;김종욱;장호완

  • Published : 20080800

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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.

빙하퇴적물의 퇴적연대는 빙하후퇴시기 및 극지기후변화 양상을 이해하는 데에 중요한 정보를 제공한다. 하지만, 연대측정 대상 퇴적물로부터 방사성동위원소 연대측정이 가능한 적절한 시료의 채취가 항상 가능한 것은 아니기 때문에, 많은 경우 빙하퇴적물에 대한 직접적인 퇴적연대측정은 한계를 지니고 있다. 더욱이 퇴적물내의 석영을 사용하기 때문에 대상 시료의 제한이 거의 없는 OSL(Optically Stimulated Luminescence) 연대측정법에 있어서도 빙하퇴적물에 포함된 석영입자의 OSL 신호가 퇴적과정 중에 완벽하게 제거되지 않아 OSL 연대측정결과가 실제 퇴적연대보다 오래된 연대를 지시하는 경우가 많다. 극지방의 평탄한 곳에서는 퇴적물 공극내의 수분이 결빙-해동과정을 반복하면서 영구동토층 상부 활동층의 입자가 수직, 수평적으로 순환을 하는 경우가 있으며, 그 결과로 크기가 큰 자갈 혹은 거력들이 내부의 세립질 입자들을 둘러싸는 원형구조토(sorted circle)라는 특징적인 지형으로 나타나기도 한다. 따라서, 원형구조토의 형성과정 동안 빙하퇴적물입자들이 순환을 하며 햇빛에 노출이 되어 기존의 OSL 신호가 제거될 기회가 크기 때문에, 다른 빙하퇴적물에 비해 상대적으로 OSL 연대측정에 적합할 것으로 판단되었다. 이 가정을 확인하기 위하여, 킹조지섬(King George Island) 세종기지 인근에 분포하는 두 개의 원형구조토(해발고도 41 m 와 66 m)에서 시료를 채취한 후, 석영입자를 분리하여 OSL 연대측정 가능성을 검토하였다. 채취한 시료의 석영입자들은 일반적인 단일시료재현법(SAR법)을 적용하여 신뢰도 높은 등가선량(equivalent dose)을 측정하기에 부적합한 OSL 특성을 보인다. 예를 들어, 채취된 석영입자의 OSL 신호에는 OSL 연대측정에 사용되는 fast OSL component가 존재하지 않는다. 또한, LM-OSL과 CW-OSL 신호를 사용한 dose recovery test에서 실험실 조사량이 약 10 % 정도 과소평가되었다. 단일시료재현법으로 측정된 등가선량의 가중평균은 18±1 Gy였고, 이를 바탕으로 한 OSL 연대결과는 11±1 ka로서, 이는 킹조지섬 세종기지 인근의 빙하후퇴가 적어도 11±1 ka 이전에 일어났을 가능성을 의미한다.

Keywords

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