The Formation Model And Implication Of The Climatic Terrace Of The Xilamulun River

The Hunshandake Sandy Land,which is highly sensitive to climate change,is located in North China along the margins of the area influenced by the East Asian monsoon.The study of the paleoenvironment in the area is extremely important for understanding the rules and driving mechanisms of the Quaternary climate.However,based on a large number of studies on lakes and aeolian sands,the understanding of paleo-climate change in the Holocene on sandy land was not consistent,and little is known about paleoclimate changes before the Holocene.The Xilamulun River,which drains the eastern margin of the Hunshandake Sandy Land,has become a canyon river because of its intensification.The multi-level terraces have been developed on the valley slopes of the valley.It provides an opportunity to test the geomorphological processes of rivers and is also an ideal material for studying of the landforms of sandy rivers.By establishing terrace age series of different heights,the erosion process of the riverbed can be obtained.Combined with the analysis of modern surface processes,compared with the existing paleoclimate data,it is hopeful to determine the response pattern of terrace development to regional climate change,and then through the sequence of fluvial terraces reveal the change of regional climate since the last Glaciation.On the basis of remote image maps and topographical translations,investigations have been carried out through geomorphological surveys,including the shape,location,and sedimentation characteristics of the terraces.We investigated terraces at 4 sites along a 38-km-long stretch and performed OSL dating on 26 terraces.Thirty samples of OSL dating were collected from the fluvial alluvi?m and the associated strata that formed the terraces,and sedimentary ages of alluvial deposits and other deposits were measured using a single piece regenerative dose method(SAR).At the same time,the elevation of the terrace is regarded as the height of the ancient river bed,and the volume of the paleo-valley corresponding to the terraces of different heights is calculated.From this,the elevation-age map and valley volume-age map of the terraces of the Xilamulun River are obtained.The results of the study indicate that the age of accumulation of terraces in the Xilamulun River,located inside the Hunshandake Sandy Land,is mainly concentrated in the range of 0.3 ka to 36.2 ka.The observed relationship between the terrace heights and ages is abnormal,i.e.,the terrace age does not increase with the terrace height.the heights of the terraces of the Xilamulun River are not related to the age of accumulation;terraces of older age and lower positions are eroded terraces.The generally distributed gravel layer at the top of the terrace forms the protective layer of the terrace.If there is no protective layer at the top of the terrace,it is difficult to preserve it.This may be the reason why the YD and H1 high-grade terraces cannot be found at present.By analyzing the influencing factors of aeolian sand flux and river sediment discharge in the river,the relationship between precipitation and river bed erosion was determined,and the development pattern of climate terraces was deduced.The development of the terraces in this area is controlled by the regional precipitation,which is the result of the dynamic balance between the output of the sediment and the input of the aeolian sand.Higher terraces formed when the riverbed aggraded due to decreased sediment discharge and increased aeolian sand input during a period of reduced precipitation.In contrast,lower terraces developed when the riverbed eroded during a period of enhanced precipitation.It can be seen from this that the height change of the terraces reflects changes in precipitation,and the higher river bed(lower valley volume)is formed in a relatively cold and dry period,and the lower river bed(higher valley volume)is formed in a relatively warm and humid period.The development pattern of this climate terrace shows that this type of terrace elevation can indicate the amount of precipitation.The Xilamulun River terrace indicates the relative change in precipitation over the last 36 years in the Hunshandake Sandy Land.At 36.2 ka,27.5 ka,23.6 ka,18 ka,11.1 ka,8.4 ka,and 2.7 ka,the precipitationin in the area is less,but in the area of 25.8 ka,21.9 ka,11.3 ka,8.6 ka,7.9 ka,1.1 ka,the precipitation is relatively large.Therefore,the terraces along this Hunshandake Sandy Land river respond to centennial-to millennial-scale precipitation changes even in the absence of background uplift.The variation range of the height of the bed during the Late Glacial Maximum and early Holocene is the largest,among which the river bed height of the early Holocene is the highest.The riverbed since 7.9 ka has been relatively low,indicating that there is more precipitation at this stage,and before 7.9 ka,the river bed height changes greatly,indicating that the precipitation is not stable,and the precipitation in three periods(11.7 ka,21.9 ka,25.8 ka)has been 7.9 ka since This is equivalent,suggesting that in the Late Glacial Maximum and Deglacial periods,the Hunshandake Sandy land may be temporarily affected by the East Asian monsoon.Correspondingly,the precipitation in the two periods(8.2 ka,11.5 ka)is comparable to the Glaciation in the Last Glaciation,which implies that the monsoon has been evacuated from the area.