黄河中游响应气候变化和地表相对抬升发育阶地研究

揭示河流系统响应气候变化和地表抬升的机制是理解流域地貌演化以及水系发育过程的基础,其核心难题是如何充分认识它们在阶地发育中扮演的角色.以往的研究倾向于分开讨论气候变化和地表抬升在河流阶地发育中的作用,认为河流堆积/侧蚀和下切行为分别与冰期和间冰期气候对应,或者将阶地作为地表抬升的直接证据.首先,从上下游河段对比的视角初...

The study on the terrace development by the middle reaches of Yellow River as a response to the climate change and relative surface uplift

The challenge to understanding fluvial response to climate change and surface uplift is a thorough distinguishing between their roles within terrace formation， which can provide an excellent insight to the landform and drainage evolution in catchment scale. Previous studies tend to correlate terrace development with climate change or surface uplift separately， attributing deposition-lateral erosion and incision to glacial-interglacial climate， or regarding fluvial terrace even as a direct evidence of surface uplift. A preliminary explanation for the terrace development of the middle reaches of Yellow River is proposed by linking its upper and lower reaches. In comparison with the past work， river terrace here is considered as a combination result of fluvial response to climate change and relative surface uplift. Intermittent downcutting by the middle reaches of Yellow River on the uplifted Ordos Block and Emei Platform relative to the subsiding Fenwei Basin has created 7 terrace staircases and a sequence of 6 aggradational terraces， respectively，during the past 1.2 Ma. The analysis of loess stratigraphy combined with geochronology revealed that these terraces were overlaid directly by a basal paleosol layer on the fluvial sediments， indicating that the abandonment of these treads due to incision by the Yellow River occurred at the transition from glacial to interglacial climates. This suggests that the glacial-interglacial climate cycle probably has a temporal control on the terrace generation by determining the fluvial behavior alternation between deposition-lateral erosion and incision， even though the Yellow River develops in the subsiding Fenwei Basin. Remarkably， terrace records however are absent at some transitions from glacial to interglacial climate， indicating that climate cycle may not be the only decisive factor for terrace occurrence. The terrace generation may be sporadic and in the form of unusual stacked pattern before the Emei Platform was uplifted within the eastern Fenwei Basin. And when the Ordos block were uplifted slowly relative to the Fenwei basin， terraces seem to only be created by the middle reaches of Yellow River at these transitions from extremely dry glacial to interglacial. In contrast， the terrace staircases almost occurred in synchrony with glacial-interglacial climate cycles as they were uplifted fast enough relative to the Fenwei Basin. This contrastive pattern indicates that surface uplift may be necessary in large terrace staircase genesis. It can force river to downcut deeply enough during climatic transitions to separate terrace levels adequately， favoring the generation and subsequent preservation of the terrace tread. Based on above analysis， the terrace sequences created by the middle reaches of Yellow River can therefore be regarded as a combined archive of climate change and relative surface uplift.