黄河源玛曲段末次冰消期以来古洪水事件与冻融褶皱现象研究

通过对黄河源玛曲段开展广泛细致的野外考察，在黄河左岸第二级阶地前缘达尔琼东（DEQ-E）地点发现了两组漫滩相古洪水泛滥沉积层（OFD1和OFD2）。剖面厚448.5 cm，层次分明，未经人为扰动，沉积学特征显著。结合野外宏观特征与室内磁化率和粒度指标分析结果，确定这两组OFD（245.0~448.5 cm）为典型的黄河大洪水溢出河槽形成的泛滥沉积物，共记录了两期11次古洪水事件。基于光释光（OSL）测年数据，确定了这些古洪水事件的发生时段以及OFD1上部（270.0~330.5 cm）冻融褶皱现象的发育年代。这两期古洪水事件分别发生在14900±1010~12800±1130 a和11060±880~9820±390 a，在时间坐标上分别对应于末次冰消期中的Bølling-Allerød暖期和早全新世气候逐渐转暖阶段。由于青藏高原冰川对气温变化响应敏感，在Bølling-Allerød暖期和早全新世气候逐渐转暖的大背景下，此时黄河源区发育的大规模冰川积雪会加速消融，抑或叠加区域暴雨的影响，进而导致了该河段大洪水事件的频发。另外，根据沉积地层的先后顺序及其发育特征，确定OFD1上部冻融褶皱发育年代为12500~11500 a，在时间坐标上与末次冰消期中的新仙女木寒冷事件（YD）对应。由于此时青藏高原整体以急剧降温为特征，使得该研究河段的沉积地层受到显著的冻融作用，进而形成了典型的冻融褶皱/冻融泥流等冰缘地貌类型。该研究成果有助于深入理解黄河源流域水文系统和地貌发育过程对全球气候变化的响应规律。

Palaeoflood events and periglacial involutions during the last deglaciation in the Maqu reach in the Yellow River source area

Palaeoflood hydrological studies were carried out in the Maqu Reach in the Yellow River source area, two units of overbank flood deposits(OFD) was found at the DEQ-E site(34°00'13″N, 101°54'04″E; 3430 m a. s. l.). The DEQ-E profile is 448.5 cm thick, with complete outcropping, clear layers and remarkable sedimentological features. The pedostratigraphy of the DEQ-E profile from the top to the bottom is listed as modern soil(MS, 0.0~60.0 cm) → mid-Holocene paleosol(S0, 60.0~180.0 cm) →Early Holocene transitional loess(Lt, 180.0~245.0 cm) → overbank flood deposits(OFD, 245.0~448.5 cm). In total, 94 samples were collected for sedimentary analysis and 8 samples were obtained from the DEQ-E profile to determine the chronology. The low magnetic susceptibility(χlflf) values in the DEQ-E profile vary from 10.4×10-8 to 96.6×10-8 m3/kg, and lower χlflf values(10.4×10-8~24.1×10-8 m3/kg) were found in the OFD units. This implies that the OFD were not affected by weathering pedogenesis after deposition. The grain-size distribution curves in the profile show that the OFD units are dominated by sand(>63 μm). The loess-soil deposits(modern soil, transitional loess and palaeosol) at the DEQ-E site are dominated by silt(2~63 μm). In addition, grain-size indices in the profile, such as skewness(SkI), kurtosis(KG), and sorting coefficients (S) also differentiate the OFD and aeolian loess-soil deposits well. According to the optically stimulated luminescence(OSL) dating method, the first episode of overbank flooding was dated to 14900±1010~12800±1130 a and correlated with the Bølling-Allerød warm interval of the last deglaciation. The second episode occurred occurred at 11060±880~9820±390 a, that is, the Early Holocene. As for the mountain glaciers are sensitive to temperature and precipitation. Under the warm and humid climate during the Bølling-Allerød warm interval and the Early Holocene, and thus the mountain glaciers in the Yellow River source area melted more quickly. The accelerated melting of glacier snow, or the superimposition of regional torrential rains, may eventually lead to the occurrence of major flood events. In addition, by using the OSL dating techniques in combination with stratigraphic sequence and development characteristics, the age of involution were dated to 12500~11500 a and coincided with the Younger Dryas cold spell of the last deglaciation. During the Younger Dryas period, the Tibetan Plateau is in an extremely cold environment. The sediments in the Maqu Reach in the Yellow River source area were freeze-thaw action obviously. Consequently, a typical periglacial geomorphology such as periglacial involutions and solifluctions were formed. These results are of great significance in further understanding the response of watershed hydrologic system and geomorphic developmental process in the Yellow River source area to global climate change.