| The loess-paleosol sequence formed during the Quaternary glacial and interglacial cycles is a good carrier for reconstructing paleoclimatic changes and predicting future environmental changes.The upper reaches of the Hanjiang River,located on the northern fringe of the northern subtropical monsoon climate zone,are quite sensitive to climate change.On the multi-level river terraces developed in this area,thick loess-paleosol sequences are mostly distributed.In recent years,studies of river terraces in the upper reaches of the Hanjiang River have focused on first-level terraces and third-level terraces and above,the age of the loess sediments at the first-level terraces is easily measured by the quartz grains OSL dating technique,the age of sediments at third-level terraces and above can be measured using paleomagnetic dating technology.Because the age of the second-level terraces is much younger than that of the third-level terraces and above,and all of them are in the Brunhes normal polarity ehron,the paleomagnetic dating technology cannot determine its accurate age.In addition,some of the quartz grains in the second-level terraces using conventional OSL technology,their signals have reached saturation,and the dating results will be underestimated.K-feldspar is relatively large in terms of the dating age relative to quartz,but at the same time there is an abnormal bleaching of the OSL signal and it is not easy to obtain reliable age data.Due to the limitations of dating methods,there have been few studies on the formation time and mechanism of the second-level terraces and the climate change after terrace formation in the upper reaches of the Hanjiang River.At present,the age obtained by different scholars using different methods are controversial,and the stratigraphic comparison methods and paleomagnetic dating methods used in most studies are relative dating methods and lack reliable and absolute dating results.After conducting extensive and detailed field surveys along the Hanjiang River valley,the TJZ second-level terrace profile with thick layer loess-paleosol sequence was selected as the study object,and quartz grains and K-feldspar grains were used as dating minerals.The BLSL SAR/Post-IR SAR method and the pIRIR SAR method were used to determine the equivalent dose of the samples,and the degree of bleaching of the samples was analyzed,combined with the dose rate,they were compared and supplemented to determine the OSL age of terraced sedimentary sequences.Combining the chronological results with the climate substitutive indicator,we compared the climatic records of the same period with other regions of the world to analyze the formation time and mechanism of the second-level terraces and the climate evolution of the region on the upper reaches of the Hanjiang River since the Middle Pleistocene.The study of loess-paleosol sequences on the second-level terrace is of great importance to fill the gaps in the paleoclimate change since the Middle Pleistocene in the upper reaches of the Hanjiang River,to improve China’s paleoclimatic recovery research and even to global climate change research.The following conclusions are drawn:(1)The age of the sediment of less than 170 ka can be accurately obtained by quartz grains dating,the upper limit of equivalent dose measured is approximately 550 Gy.Equivalent dose was performed by means of excitation of blue light and infrared and blue light excitation.The final test conditions of pre-heat 260℃ and Cut-heat 220℃ were selected by the preheat plateau,dose recovery and recycle ratio test.The bleaching degree of the OSL signal by three methods:the OSL bleaching curves,the distribution of single-aliquot De versus sensitivity-corrected natural OSL signal and the frequency histogram of equivalent dose distribution.For samples with incomplete bleaching,use a first-order kinetic formula to perform multi-component fitting and splitting of the continuously-excited CW-OSL to obtain fast,medium,and slow components.According to the separation of the fast components to determine the final equivalent dose value.The OSL ages of the three quartz samples obtained by quartz grains dating were 55.89±2.93 ka,116.39±5.56 ka and 166.47±6.66 ka,respectively,with an error of<10%.(2)The age of the sediment of less than 260 ka can be accurately obtained by K-feldspar grains dating,the upper limit of equivalent dose measured is approximately 797 Gy.The equivalent dose of potassium K-feldspar grains isolated from a sample in which the growth curve of OSL signal of the quartz grains has been saturated is determined by pIRIR method.After testing at low temperature of 50℃ and high temperature of 200℃,it is concluded that the effect of the two temperature test conditions on the equivalent dose value of the same sample is almost negligible.Finally,the 200℃ high temperature condition is selected as the equivalent dose of K-feldspar.Since the growth curve of the K-feldspar sample is close to the saturation when using exponential fitting,the equivalent dose value corresponding to line fitting is taken as the minimum value of the sample equivalent dose.The OSL ages of sand loess layer and floodplain sand layer at the bottom of the loess layer obtained by K-feldspar grains dating were>254.44±4.80 ka and>258.67±4.95 ka,respectively.(3)The OSL age>258.67±4.95 ka is the earliest age of terrace formation in the floodplain sand layer of the TJZ section.The OSL age>254.44±4.80 ka of the sand loess layer at the bottom of the loess layer is the latest age of terrace formation.It was finally determined that the time for the formation of the second-level terrace in the upper reaches of the Hanjiang River will be no later than 260 ka.Combined with the tectonic uplift movement and river undercutting history since the Middle Pleistocene in the study area,the climate in the study area was lyrically dry at around 260 ka.The terraced grounds were not affected by the river surface during the flood season,but the structural uplift and undercutting of the river,the height is relatively low.In the period when the wind power of the river valley is relatively large,the order of the ground is mainly filled with sand with a larger particle size,and in the period when the wind is small,fine sand-silt particles with a relatively small particle size are accumulated.In the course of climate fluctuations,an alternating layer of sand and loess layers and aeolian deposits accumulate on the sand layer of floodplain,and then thicker layers of eolian wind loess accumulate as the undercutting effect of the river increases.(4)The terraced sedimentary sequence recorded the climate change process since 260 ka.Based on climate substitutive indicators such as magnetic susceptibility,grain size,chemical element content,quartz micromorphology,and other climate carriers in the same period in other parts of the world.Since the formation of second-level terraces in the upper reaches of the Hanjiang River in the present 260 ka period has begun to receive continuous accumulation of wind accumulations,the terraced grounds have accumulated the loess layer(L2)developed during the penultimate glaciation where the weather is relatively cold and dry,and gradually warm and humid in the climate.There have been many climate fluctuations during the period.Therefore,the sedimentary sequence on the second-order surface is from bottom to top:riverbed gravel layer →floodplain sand layer→sand loess→eolian sand layer→ sand loess layer→ loess layer(L2)→paleosol layer(S→)→Topsoil(MS). |
PDF Full Text Request