利用宇生核素^(10)Be暴露测年技术重建冰川漂砾运动历史

Reconstructing Movement History of Glacial Boulders by Using ^(10)Be Exposure Dating Method

20世纪80年代兴起的宇生核素暴露测年技术是目前冰川地貌研究中最为成功的测年方法之一,多数学者通常选择冰川漂砾进行暴露测年研究。然而,冰川漂砾在出露后期可能经过再次搬运造成"翻转"现象,从而影响测年结果。重建这类翻转漂砾的运动历史,对准确测定冰川地貌年代具有重要意义。青藏高原东南部的稻城古冰帽区保存了大量的古冰川遗迹,是宇生核素暴露测年研究的天然实验室。冰帽区有一被人为劈开的巨型漂砾,其内部垂直剖面上底部样品的宇生核素^(10)Be浓度高于顶部样品,推断该漂砾自初次出露之后可能发生过翻转。本文利用该漂砾顶部、底部、左右侧面的样品通过宇生核素^(10)Be暴露测年技术来重建该漂砾的运动历史。研究结果表明:该漂砾可能是由于深海氧同位素3阶段(Marine Isotope Stage,简称MIS 3)冰川作用暴露于地表,随后经过翻转直至目前状态。

Cosmogenic nuclide^10Be exposure dating is one of the most successful dating methods used to study glacial geomorphology since 1980 s. In most studies,glacial erratics have been preferred for exposure dating material. However,glacial erratics may have undergone re-transportation and roll-over during the post-exposure stage,which can affect the dating results. Reconstructing the movement history of these boulders is therefore of crucial importance for obtaining reliable chronology of glacial landform by exposure dating of glacial erratics. The paleo-Daocheng ice cap in the southeastern Tibetan Plateau exhibits abundant glacial remnant and provides an ideal natural laboratory for cosmogenic nuclides exposure dating. A big boulder,found in the Daocheng ice cap,was split because of road construction. Concentrations of^10Be in the bottom sample were more than the top sample,which indicated that the boulder might have rolled over during the post-exposure period. In this study,samples from the top,bottom,left and right side of the boulder were selected and used to reconstruct the movement history by using^10Be exposure dating method. The result demonstrated that the boulder might have been exposed to the surface due to the glacial advance during Marine Isotope Stage（ MIS 3）,and thereafter undergone several times retransportation and roll-over events.