Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity, College of Urban and Environmental Sciences, Northwest University, Xi'an, China.
Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity, College of Urban and Environmental Sciences, Northwest University, Xi'an, China; State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China.
Sci Total Environ. 2022 Dec 20;853:158242. doi: 10.1016/j.scitotenv.2022.158242. Epub 2022 Aug 22.
Paleochannel sedimentary sequences can provide abundant information on regional environmental changes. A typical paleochannel (paleo-oxbow lake type) section of the Yellow River was identified within the Zoige Basin on the NE Tibetan Plateau. A multi-index approach was used to accurately identify sediments of different genetic types, such as riverbed deposits of the Yellow River, paleo-oxbow lake deposits, and overbank flood deposits (OFD) in the section. Based on optically stimulated luminescence (OSL) and AMS C dates, we examined the environmental evolution recorded by the section. The results show that: (1) The section is a record of environmental change since 4.17 ± 0.49 ka. During 4.17 ± 0.49 to 3.24 ± 0.26 ka, the ancient Yellow River occupied the channel. At 3.24 ± 0.26 ka, the paleochannel experienced a neck cutoff, and the fluvial environment began to change into the oxbow lake environment. After 2.45 ± 0.11 ka, the paleo-oxbow lake gradually disappeared. Subalpine meadow soil has developed at this site since 1.31 ± 0.05 ka. (2) Paleoenvironmental proxies indicate that the Zoige Basin was warmer and wetter before ~3.00 ka, and became drier and colder after ~3.00 ka, which may be mainly related to the weakening of the East Asian summer monsoon (EASM) and the strengthening of the Westerlies. (3) Two episodes of extreme overbank flooding occurred at 2.96 ± 0.24 to 2.87 ± 0.27 ka and 1.84 ± 0.20 to 1.70 ± 0.16 ka, correlated with climate shift period from the mid-Holocene climatic optimum to the late Holocene and the Dark Age Cold Period (DACP), respectively. Due to the relatively cold and dry climate in these periods, glaciers generally advanced on the Tibetan Plateau, and the contribution of snow and ice meltwater weakened. Therefore, the strong rainfall caused by the abnormal atmospheric circulation may be the main cause of these extreme overbank flooding.
古河道沉积序列可以提供有关区域环境变化的丰富信息。在青藏高原东北部的若尔盖盆地中，识别出了一段典型的黄河古河道（古牛轭湖型）剖面。采用多指标方法准确识别了不同成因类型的沉积物，如黄河河床沉积物、古牛轭湖沉积物和剖面上的漫滩洪水沉积物（OFD）。基于光释光（OSL）和 AMS C 年代测定，我们研究了该剖面记录的环境演变。结果表明：（1）该剖面是自 4.17 ± 0.49 ka 以来环境变化的记录。在 4.17 ± 0.49 至 3.24 ± 0.26 ka 期间，古黄河占据了河道。在 3.24 ± 0.26 ka，古河道经历了颈部截流，河流环境开始转变为牛轭湖环境。在 2.45 ± 0.11 ka 之后，古牛轭湖逐渐消失。自 1.31 ± 0.05 ka 以来，这里已经发育了亚高山草甸土壤。（2）古环境代用指标表明，若尔盖盆地在 ~3.00 ka 之前更温暖湿润，在 ~3.00 ka 之后更干燥寒冷，这可能主要与东亚夏季风（EASM）减弱和西风增强有关。（3）两次极端漫滩洪水事件发生在 2.96 ± 0.24 至 2.87 ± 0.27 ka 和 1.84 ± 0.20 至 1.70 ± 0.16 ka，分别与从中全新世气候最适宜期到全新世晚期和黑暗时代冷期（DACP）的气候转变期相关。由于这些时期相对寒冷干燥，青藏高原上的冰川普遍前进，冰雪融水的贡献减弱。因此，异常大气环流引起的强降雨可能是这些极端漫滩洪水的主要原因。
