| Climate change is one of the most important manifestations of global change,which is now related to human survival and development.It has been a hot research topic in China and abroad to predict the future climate change by reconstructing the paleoclimate change.Quaternary loess in China is a good carrier for studying paleoclimate change.It records the alternation of cold and warm climate since Quaternary,which is of great significance for reconstructing paleoclimate change in Quaternary.The upper reaches of the Hanjiang River is located on the south side of the Qinling Mountains,north of Daba Mountain,on the edge of the northern subtropical monsoon climate,and is extremely sensitive to climate change.The valleys and basins in the area are distributed alternately.Affected by neotectonic movement,there are asymmetric four-level terraces in the basin.Each level of terraces is covered with aeolian loess of varying thickness,which is undoubtedly a good carrier for studying the geomorphologic evolution of the upper reaches of the Hanjiang River and the climate change since the formation of terraces.The study of terrace chronology can provide a time scale for comparing the sequence of terraces with climate records and tectonic events,thereby providing a basis for exploring the formation of terraces and the response mechanism of rivers to driving factors;and combining chronology with climate alternative indicators of loess can be used to reconstruct the climate change process of the terrace since its formation,reveal the longterm climate change law,and then predict the future climate change trend based on this law.However,in recent years,most of the chronological studies on the terraces of the upper reaches of the Hanjiang River have been concentrated on the lower terraces,revealing mostly the climate change process since the last glacial period.For the chronological research on the higher terraces,due to the lack of sufficient technical dating data support,the research is not mature enough.On the basis of many field investigations,this paper selects the Gaokezhan(GKZ)loess profile on the third terrace of Ankang Basin in the upper reaches of Hanjiang River as the research object.Through paleomagnetic,OSL,cosmogenic nuclides burial dating and stratigraphic correlation methods,the chronological sequence of the overlying eolian loess on the terraces and the formation age and mechanism of the third terrace are determined.Combined with climate alternative indicators such as grain size and chemical elements,the climate change process since the late Mid-Pleistocene in the Ankang section of the upper Hanjiang River is discussed.The main conclusions are as follows:(1)Paleomagnetic dating shows that the bottom of the GKZ section is less than 0.78 Ma(B/M limit),but the section records three geomagnetic polar drift events.Comparing the polar column of GKZ profile with the standard polar column,it is considered that the three polar drift events may be Blake event,Iceland Basin event and CR1 event respectively.Taking the occurrence time of these three events as the node:0.11?0.12 Ma?0.18-0.19 Ma?0.32?0.33 Ma,it is considered that the stratigraphic sequence of GKZ profile from top to bottom is L1,S1,L2,S2,L3,S3,L4,S4,L5.(2)Because the conventional single-chip quartz regeneration dose(SAR)method shows that the luminescence signal of loess samples is saturated,the potassium feldspar two-step method(pIRIR)and quartz thermal transfer luminescence method(TT-OSL)are used to test loess samples.The luminescence signals of the samples tested by the two methods were not saturated,and reliable equivalent doses could be obtained.The luminescence ages of loess samples at 0.6 m,1.5 m,7.9 m,8.4 m and 16.1 m were 53.11±8.60 ka,93.88±9.89 ka,323.07±17.92 ka,379.89±32.49 ka and 486.74±27.71 ka,respectively.By stratigraphic correlation,the stratigraphic sequence of GKZ section from top to bottom is L1,S1,L2,S2,L3,S3,L4,S4,L5,which is consistent with the results of paleomagnetic dating.The bottom age of the profile aeolian loess is 486.74±27.71 ka,indicating that the accumulation time of the terrace aeolian loess is about 490 ka.(3)The results of cosmogenic nuclide dating(10Be/26Al)show that the burial ages of the coarse sand layers at 16.4 m and 16.8 m are 0.93±0.14 Ma and 1.20±0.15 Ma,respectively,indicating that the final accumulation of the river is about 1.0 Ma ago.(4)Combining the age of the bottom of aeolian loess and the burial age of the top of the coarse sand layer of floodplain facies,it is deduced that the third-level terraces of the Ankang section of the upper Hanjiang River began to form at 1.0 Ma,and the loess was accumulated after 490 ka.Around 1.0 Ma,affected by the Kunhuang Movement on the Qinghai-Tibet Plateau,the river began to cut down.During 1.0?0.49 Ma,the Hanjiang River continued to cut down in the Ankang section due to the influence of regional tectonic movement and the extensional revival of the Yuehe fault.The deep-cut meandering streams of the Hanjiang River and its larger tributaries were developed,and the height of the meandering neck and the height of the third terrace were similar,which was evidence.After 490 ka,the structure was stable,the terrace changed from erosion to accumulation,and began to accept loess accumulation stably.(5)The loess deposition in the upper reaches of the Hanjiang River recorded the climate change process since the late Middle Pleistocene.From 490 ka to 384 ka,the climate changed from cold and wet to warm and dry,which was reflected in the decrease of average particle size,the increase of organic matter content,the increase of Rb/Sr ratio,the decrease of magnetic susceptibility but the increase of redness.384?140 ka,the climate as a whole is warmer and wetter than the previous period,but less stable,showing a trend of cold and dry-warm and wet-cold and dry-warm and wet-cold and dry;During the ice age(140?105 ka),the climate is generally warm and humid;after 105 ka,the climate is generally cold and dry. |
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