OSL Dating Of The Quaternary Red Clay Distributed In The Reaches Of The Yangtze River And Paleoclimatic Changes Since The Last Interglacial Period In Mid-subtropical Southeast China

The study of past global climatic changes and regional responses has attractedwordwide attention since recent decades. The Quaternary Red Clay (QRC) in southernChina, a residue of highly weathering under long-term warm and humid climate, has along history and contains much paleoclimatic information, which hence is regarded asan important carrier of past environment for studying paleoclimatic changes insouthern China. Some representative QRC profiles in Xuancheng and Langxi,southern Anhui province, and Jiujiang, Jiangxi province, belonging to the lower andmiddle reaches of the Yangtze River, were selected after many field investigations.Grain-size and geochemical characteristics of the QRC profile were study to revealthe sources and forming processes of the red clay in the study areas. TheYellow-brown Earth (YBE)–Red Clay (RC) sections of the upper QRC profiles weredated by the optically stimulated luminescence (OSL) method, followed by analysesof weathering indexes, to separate and identify possible paleoclimatic events in theYangtze River Valley, Southeast China, during the late Quaternary period. Color ofthe QRC profiles was measured using the L*-a*-b*color system and the correlationbetween the redness (a*) and weathering degree was studied to discuss the feasibilityof using a*as a paleoclimatic indicator. Moreover, soil variations in the study areaswere also widely investigated to study the influence of paleoclimatic change onpedogenic processes and soil distribution during the Last Glacial–Interglacial cycle.The results are as follows:(1) The transfer from the YBE to the Uniform Red Clay (URC) and theReticulate Red Clay (RRC) of the selected QRC profiles, P-GD, P-JJ, P-LX1andP-XZ1, are gradual and continuous, without any depositional hiatuses. Grain sizes ofthe profiles are generally fine and uniform, with no gravel (>2mm) and little sand(>63μm), but a high content of coarse silt (10-63μm), also high contents of fine silt(2-10μm) and clay (<2μm). Moreover, the grain-size characteristics and grain-sizefrequency curves of the representative layers of the QRC profiles are similar to thoseof the typical aeolian loess in northern China and Xiashu loess in the Yangtze RiverValley. The vertical distributions of major elements of the QRC profile are alsouniform, continuous and similar to those of the Xiashu Loess and the loess in northernChina. The trace-element distributing curves of the representative layers of the profiles are almost identical with each other and also similar to the Xiashu Loess andthe typical aeolian loess in northern China in pattern. This suggests that all of the YBE,URC and RRC of the QRC profiles in the study areas have aeolian-dust characteristicsand share the same source provenance with the Xiashu Loess along the Yangtze River.(2) The YBE–RC soil profiles in the study areas were dated with the opticallystimulated luminescence (OSL) method. The results indicated that the YBE wasformed during the Last Glacial, with OSL age less than60ka B. P.; the RRC mainlyduring the Last Interglacial, with the OSL age ranges from80to13460ka B. P.; theURC during the transitional time between the above two, with the OSL age rangingfrom60to80ka B. P.(3) Redness (a*) of the QRC profiles, P-LX1, P-JJ, P-XN and P-YW, increaseswith depth increasing downwards, but oscillates in the RRC layers. a*of the profilesis positively significantly correlated with clay (<2μm) content, free Fe (Fed), Fed/Fetratios and other soil weathering indexes of the QRC (ρ<0.01). a*of the red clay indifferent areas of southern China is negatively significantly correlated with thelatitudes where it is located. Moreover, it is positively significantly correlated with theannual mean temperature and precipitation of its locations, suggesting that a*of thered clay could indicate hydrothermal conditions of the surface and has paleoclimaticimplications. a*actually reflects the content of hematite in the QRC and is a potentialpaleoclimatic indicator as hematite content in the red clay is generally positivelycorrelated with its weathering strength.. a*of the RRC of the profiles, however, isoften significantly reduced due to the dissolution of hematite during long-termintensive water logging, and hence is not correlated with its highly weathering degree.In spite of this, a*is still more promising than magnetic susceptibility to be used as apaleoclimatic indicator, as hematite is more stable than maghemite during thepost-depositional reticulating processes of the RRC.(4) The QRC profiles in the Yangtze River Valley are generally uniform andcontinuous. The weathering degree of the profiles, however, is gradually intensifiedwith the depth increasing downwards. Especially, the transfer from the YBE to RCmight record a great paleoclimatic change in the Yangtze River Valley during the lateQuaternary period, when the humid and warm interglacial climate was replaced by thedry and cold glacial climate and the rubifying processes were terminated and replacedby widespread dust deposition. Combined with the OSL dating and pedogenic studies,some paleoclimatic events could be separated and identified in the YBE–RC profiles..The YBE was formed during the Last Glacial, correlated with the marineoxygen isotopic stage (MIS)2; the URC was formed during the transitional time fromthe Last Interglacial to the Last Glacial, correlated with the marine oxygen isotopicstage (MIS)3; the RRC mainly during the Last Interglacial, correlated with MIS5.Sub-class paleoclimatic events of the RRC cannot be identified possibly due to itsstrong weathering degree and overlapped paleoclimatic information.(5) Pedogenesis and soil classification in the Yangtze River Valley are deeplyinfluenced by the last paleoclimatic cycle during the Last Glacial–Interglacial period.A soil sequence, P-YL1, P-YL2and P-YL3, distributed along a small hill inXuancheng, southern Anhui province, were studied. P-LX1was derived from theYBE, the Xiashu Loess during the Last Glacial; P-YL2from the URC; P-YL3fromthe RRC formed during the Last Interglacial. According to the Chinese traditional soilclassification, they belong to typical zonal soils and are classified as Yellow-brownSoil, Yellow-red Soil and Red Soil, respectively. According to the newly establishedChinese Soil Taxonomy (CST), they could be classified as Acidic–Luvisols, Udic-Luvisols and Ferrisols, respectively. The great climatic change during the last Glacial–Interglacial cycle made soil parent materials diversified and thus led to thecoexistence of different zonal soils in a small scale in the study areas, where some soiltypes are not correlated with the present climatic conditions. This suggests that thepedogenic processes are not only controlled by modern climate, but also significantlyinfluenced by paleoclimatic changes during the late Quaternary, especially inmid-subtropical Southeast China sensitive to climatic change.