Outburst floods of the Maly Yenisei. Part II – new age constraints from Darhad basin

Some of the largest cataclysmic floods of the Quaternary followed multiple breaches of glaciers damming the headwaters of the Maly Yenisei river in southern Siberia. The shorelines of the impounded lake in Darhad basin suggest at least four depths of 290, 175, 145, and 65 m. Fossil evidence, together with previous 14C and luminescence dating, indicates the existence of a deep lake during MIS 3; the eroded character of the highest shoreline suggests that the deepest lake was older. 10Be dating of moraines in the surrounding mountains has documented major glacial advances during MIS 2, although no published direct dating has confirmed a highstand of the lake then. To address this problem, we extracted lake sediments from a 92.6 m deep borehole, sampled beach sands from the nearby basin edge, and dated them both using luminescence methods. We also dated, with 10Be, the eroded remnants of the end moraine deposited by the last glacier that dammed Darhad basin, as well as other moraines in the mountains surrounding the basin. These numerical ages confirm that a deep lake existed in Darhad basin at ~20 ka and that a large glacier crossed the Maly Yenisei and dammed Darhad basin at ~21 ka. The deep lake persisted episodically until ~14 ka. The 10Be dating in the surrounding mountains shows that the MIS 2 glaciers subsequently retreated but stalled or re-advanced at ~12, 10, and 1.5 ka. 10Be dating from the central massif of Mongolia is consistent with this chronology and confirms that MIS 3 equilibrium-line altitudes were slightly (~75 m) lower or approximately the same as those of the MIS 2. The temporal and spatial patterns of glacial advances in southern Siberia and central Mongolia coincided with those of glacial advances in similar climate conditions of the Altai mountains.

第四纪时期规模最大的灾难性洪水之一，多形成于西伯利亚南部小叶尼塞河（Maly Yenisei）河源冰川堰塞体多次溃决之后。达尔哈德盆地（Darhad basin）内堰塞湖的岸线记录显示，其至少存在4级水深，分别为290米、175米、145米及65米。结合此前的碳十四（14C）与光释光测年结果，化石证据表明，海洋同位素阶段3（Marine Isotope Stage 3, MIS 3）期间该区域曾存在深水湖泊；最高岸线的侵蚀特征显示，最深的那期湖泊形成年代更早。
对周边山地冰碛垄的铍10（10Be）测年结果证实，海洋同位素阶段2（MIS 2）期间曾发生大规模冰川前进，但尚无已发表的直接测年数据证实当时湖泊曾处于高水位阶段。为解决这一科学问题，本研究从92.6米深的钻孔中提取了湖泊沉积物，并在盆地附近的岸缘采集了滨岸砂，二者均采用光释光方法进行年代测定。此外，本研究还利用铍10测年技术，对封堵达尔哈德盆地的末次冰川遗留的侵蚀性终碛垄残体，以及盆地周边山地的其他冰碛垄开展了年代测试。
这些数值年代数据证实，约20 ka时达尔哈德盆地内曾存在深水湖泊，而约21 ka时，大型冰川曾越过小叶尼塞河并封堵了达尔哈德盆地。该深水湖泊间歇性存续至约14 ka。对周边山地的铍10测年结果显示，MIS 2期间的冰川随后发生退缩，但在约12 ka、10 ka及1.5 ka时出现停滞或再次前进。蒙古国中部地块的铍10测年结果与这一年代序列相符，并证实MIS 3的冰川平衡线海拔较MIS 2略低（约低75米）或大致相当。西伯利亚南部与蒙古国中部的冰川前进时空格局，与气候条件相似的阿尔泰山脉（Altai mountains）的冰川前进模式相一致。