Outburst floods of the Maly Yenisei. Part I

The Yenisei, the largest river flowing to the Arctic Ocean, repeatedly hosted cataclysmic floods during the Pleistocene. The largest of the known floods likely originated from palaeolakes in northern Mongolia, at the headwaters of the Little, or Maly, Yenisei. These ancient floods are among the greatest known globally. They left giant gravel dunes and wide abandoned channels in the Kyzyl basin, and high terraces in the gorges upstream. However, few detailed field studies of the flood deposits and no measurements of their ages have been made thus far. The largest palaeolakes were impounded during major glaciations by outlet glaciers from the East Sayan ice field in southern Siberia. The shorelines suggest four distinct palaeolake depths of 290, 175, 145, and 65 m. The timing and location of the glacier impounding the deepest lakes are uncertain but at its maximum, the Tengis outlet glacier was likely capable of impounding the 175 m lake. The dating of glacial deposits in and around the basin reveals that the maximum late Pleistocene glaciers were during Marine Oxygen Isotope Stage (MIS) 3. The ages for deep-lake sediments exposed in the basin behind the dam’s location support this finding. During MIS 2 the Tengis glacier was large enough to impound at least the 145 m lake. However, the existence of a deep MIS 2 palaeolake in the basin has been challenged, because no evidence has been published of MIS 2 lake sediments from the cutbank outcrops and deep drilling cores. Additionally, the end moraines of the Tengis glacier, separated from the deeply eroded lateral moraines by the Maly Yenisei, remain undated; therefore it is uncertain exactly when this glacier crossed the river. This review is part I of a two-part article; Part II presents new age data to constrain the ages of the glacial dam.

叶尼塞河（Yenisei）是注入北冰洋的最大河流，在更新世时期曾反复暴发灾难性洪水。已知规模最大的此类洪水极有可能源自蒙古北部小叶尼塞河（Little Yenisei，又称Maly Yenisei）源头处的古湖。这类古洪水属于全球已知规模最为宏大的洪水之列，它们在克孜勒盆地（Kyzyl basin）塑造了巨型砾石沙丘与宽阔的废弃河道，并在上游峡谷中留下了高阶地遗迹。然而迄今为止，针对这些洪水沉积的详细实地研究仍十分匮乏，且尚未开展任何相关年代测定工作。
规模最大的古湖是在大规模冰期阶段，由西伯利亚南部东萨彦冰原（East Sayan ice field）的出口冰川封堵形成。湖岸线遗迹表明该区域曾存在4个不同水深的古湖，分别为290米、175米、145米与65米。封堵最深古湖的冰川的形成时间与位置目前尚无定论，但当该冰川规模达到鼎盛时，滕吉斯出口冰川（Tengis outlet glacier）完全具备封堵形成水深175米古湖的能力。对该盆地及周边区域冰碛物的年代测定结果显示，晚更新世的最大冰川活动发生于海洋氧同位素第3阶段（Marine Oxygen Isotope Stage，简称MIS 3），坝址后方盆地内出露的深湖沉积年代数据也佐证了这一结论。
在MIS 2阶段，滕吉斯冰川的规模足以封堵形成至少水深145米的古湖。但学界对该盆地中是否存在MIS 2阶段的深古湖仍存在争议，原因是目前尚未有公开资料显示在侵蚀崖露头与深钻井岩心中存在MIS 2阶段的湖相沉积。此外，被小叶尼塞河与强烈侵蚀的侧碛垄分隔开的滕吉斯冰川终碛垄，其形成年代仍未被测定，因此仍无法确定该冰川具体是在何时跨越了该河道。本综述为两篇系列论文的第一篇，第二篇将提供新的年代数据以限定该冰碛坝的形成年代。