Study On Characteristics And Source Provenance Of The Pleistocene Red Earth Sediments In Southern China

Quaternary has been characterized by glacial-interglacial climatic cycling since 2.6Ma,accompanied by the rise and fall of global sea level,the emergence and evolution of human beings,the evolution of modern living things,and the formation of topography,which has a significant impact on the earth's environment.The study about global environmental evolution and local climate responses has been a primary focus in scientific research.During Quaternary,the neotectonic movement dominated by the multi-stage uplift of Tibetan Plateau brings about the three terraces.This uplift has attributed to the enhanced land-sea seasonal thermal contrasts,which results in the formation and intensification of the Asian monsoons.The atmospheric circulation pattern in east Asia has evolved from the early tertiary planetary wind system to the modern monsoon circulation,which is very similar to the Quaternary period.Aridification of the interior of Asia and the monsoon activity potentially subject to accumulation of eolian dust and formation of Chinese Loess Plateau?CLP?.As a typical Quaternary sediment in northwestern China,loess-paleosol is widely spread,with huge thickness and constant deposit and sequence,recording the information of cycling paleoenvironment during Quaternary,which highly conforms to the global climatic and environmental changes interpreted by the oxygen isotope.Loess-paleosol,deep sea sediments,and ice core have been three parallel pillars to investigate the Quaternary climate changes.other climate proxy records from deep-sea sediments and ice cores.The investigation about significant changes of paleoenvironment,such as the formation,source,and distribution of loess-paleosol and neotectonic movement,east Asian monsoon evolution,and aridification of the interior of Asia,providing the key evidence for global Quaternary climate evolution.Corresponding to the northwestern loess-paleosol sediments,the red earth sediment is widely distributed in south China,especially in the Yangtze river and Pearl river basin which are the sensitive area to the Quaternary monsoonal climate.Red earth sediment,a typical deposit resulted by Quaternary monsoonal climate in low-moderate latitude China,whose formation,source,distribution,and physicochemical characteristic potentially matches well with the development of neotectonic movement,east Asian monsoon evolution,the establishment of east Asian monsoonal system and latitude effect of global Quaternary climatic variability.Located in south�monsoonal triangle�,the mid-lower reaches of the Yangtze river are the�confrontational zone�of loess-paleosol in the north and red earth sediment in the south.The research on the coupling mechanism between the variability of the east Asian winter and summer monsoon and the development model of red soils,the invasive boundary of the dust from northwestern China and source of the red earth sediments are not comprehensive.At the same time,the unique hot and humid climate in south China has intensified the chemical weathering effect during the Quaternary period,especially the late Pleistocene.The original sedimentary information of deposits has been greatly changed by strong chemical weathering effect in the process of pedogenesis.The multiple formation and source of the red earth profiles and the uncertainty of depositional model,cause a certain impact on the information contained by the soils.Further the comparison with the recording of northwest loess-paleosol sediments,deep sea sediments and the polar ice core,which hinders the understanding of the Quaternary global and regional climate change.Thus,it is essential to determine the geochemical and source provenance features of the red earth sediments in southern China,comparing with the aeolian deposits,to assess its source and supergene weathering characteristics.Grain sizes of sediments and geochemical compositions of the immobile contents,such as the rare earth elements,are mainly used to determine the source of red earth sediments in previous studies.However,the�double peaks�grain size features can hardly discriminate potential source variation between the northwestern loess-paleosol and southeastern red earth sediments,which may have transported by wind in short distance.The Yangtze river and Pearl river drainage are widely spread.The mixed homogenization during the transport process of the sediments lead to the trace elements patterns,where the red earth sediments are sourced from the average value of the upper crust.Thus,it is not appropriate to compare the profiles in a horizontal view,and cannot distinguish the characteristic of different red earth profiles.To obtain more indicative criteria for sediment identification,it is essential to take the weathering pedogenesis characteristic into consideration.Effective indexes to determine the source region,including immobile major and trace element ratios,and the Sm/Nd-Rb/Sr isotopic features,which are stable during weathering processes and widely applied in the provenance researches.Zircons are resistant to external influence due to its well-closure,therefore the U-Pb ages and trace elements features of zircons are considered as reliable methods for source investigation.The comprehensive application of the above discriminant methods is expected to determine the provenance relationship between Pleistocene net-like red sediments in southern China and the aeolian deposits in northwestern China.The XRD patterns of the bulk samples show that the red earth sediments of Jiujiang,Shengli,and Xiaomei profiles contain similar mineral components.Quartz and clay minerals are ubiquitous,minor K-feldspar,plagioclase and goethite are also detected.With the development of white veins,the contents of K-feldspar,plagioclase and goethite decrease gradually.Minor calcites and mica are present in the Luochuan sediments,indicating that its leaching degree is lower than the red soils.Clay mineral assemblage of Pleistocene net-like red sediments mainly consist of kaolinite,illite,and¹4?clay minerals?smectite and vermiculite groups?.A certain part of the profile consists of mixed layer clay minerals.The differences in content and type of clay minerals shows the intense chemical weathering with a warm and humid paleoclimatic fearutes during Pleistocene.However,the presence of chlorite in loess-paleosol sediments,Luochuan profile,northwestern China,reflecting the supergenesis dominated by physical weathering.The characteristic of mineral variation is consistent with the glacial-interglacial cycling.The XRF results show that Jiujiang,Shengli and Xiaomei net-like rea earth sediments mainly consist of SiO₂,Al₂O₃,and Fe₂O₃,which corresponds to high quantities of quartz,clay minerals and iron minerals.UCC-normalized plots show the enrichment of TiO₂,Fe₂O₃ and depletion of MnO,CaO,Na₂O,K₂O and P₂O₅ in net-like red earth sediments.The concentration of SiO₂ and Al₂O₃ are similar to that of UCC.The chemical index of alteration?CIA?indicates more intensive weathering of red earth sediments in south China,compared to Luochuan loess-paleosols and aeolian deposits in northwestern China.And the chemical weathering degree gradually increased from the bottom to the top in Jiuajing and Shengli profiles,but decreased in Xiaomei profile,respectively.Compared to the aeolian sediments in the northwest,the A-CN-K ternary diagram shows a stronger chemical weathering with kalium depletion and aluminum enrichment of red earth sediments.Besides,the degree of chemical weathering is negatively related to the latitude of profile.The distribution patterns of major and trace elements are similar with those of the upper continental crust?UCC?,post-Archean Australian shale and loess-paleosols,indicating that red earth sediments are probably consist of the the well-mixed weathering residues from the upper continental crustIn respective of geochemistry,the ratio of major elements?SiO₂/TiO₂-TiO₂/Al₂O₃,SiO₂/Al₂O₃-TiO₂/Al₂O₃?,trace elements(La/Nb-Th/Nb?Zr/Nb-Hf/Nb,Y/Nb-Zr/Hf?Ti/Nb-Al/Nb,Zr/Ti*10^-2-Zr/Al*10^-3,Y/Al*10^-3-Zr/Al*10^-3)and the?Eu-La_N/Yb_N,Gd_N/Yb_N-La_N/Sm_N of red earth profiles are obviously different with those of aeolian deposits in Shangbaishuan,Luochuan and Xifeng profiles,northwest China.The discrimination functions?DF?of rare elements show that the correlation between red earth sediments and the Yangtze drainage is closer than that of the northwest loess-paleosols.The isotopic features show that the ¹⁴³Nd/¹⁴⁴Nd does not change within the chemical weathering processes.The ⁸⁷Sr/⁸⁶Sr may be enriched by the adsorption of the clay-sized compositions under high CIA.The variation coefficient?CV?of isotopic compositions of Jiujiang,Shengli and Xiaomei net-like red earth profiles is tiny,reflecting the same source for all the samples in different profiles.The red earth sediments in southern China have higher ratios of ⁸⁷Sr/⁸⁶Sr and ¹⁴⁷Sm/¹⁴⁴Nd in comparison with Chinese loess.Meanwhile,the isotopic features of Shengli and Jiujiang are similar to Yangtze drainage basin,while¹⁴³Nd/¹⁴⁴Nd ratio of Xiaomei profiles is similar to Pearl River sediments.Thses suggested that the parent materials for red earth profiles in southern China are different from the aeolian deposits in the northwest but originated from the fine grained materials of well-mixed weathered continental crust in the drainage basin.The age populations of zircon grains of Jiujiang red earth sedimentsrange from³9-174 Ma?Eocene to middle Jurassic?,¹74-363 Ma?middle Jurassic to Carboniferous?,³89-530 Ma?middle Devonian to Cambrian?,⁶76-1000Ma?Neoproterozoic?and¹818-2248 Ma?Paleoproterozoic?.The ages of Shengli grains range from¹74-233 Ma?early Jurassic to middle Triassic?,²34-351 Ma?middle Triassic to Carboniferous?,³90-508 Ma?middle Devonian to middle Cambrian?,⁷00-860 Ma?Cryogenian to Tonian?,⁸66-1000 Ma?Tonian?,1750-2000 Ma to¹750-2500 Ma?Paleoproterozoic?.The Xiaomei U-Pb ages range from²30-300 Ma?middle Triassic to Permian?,³20-400 Ma?early Carboniferous to middle Devonian?,⁴00-500 Ma?middle Devonian-Cambrian?,⁵00-3200Ma?middle Cambrian to Precambrian?.The Luochuan U-Pb ages range from⁴2-180 Ma?Eocene to late early Jurassic?,¹80-320 Ma?early Jurassic to late Carboniferous?,³80-510 Ma?late Devonian to middle Cambrian?,⁵20-2890 Ma?early Cambrian to Mesoarchean?.Most zircon grains show a similar magmatic original rare elements patterns with a within-plate/anorogenic genesis.The age population groups in Luochuan loess are consistent with the paleosol,though there exists slight difference in probability densities.It is consistent with the frontier,that the materials of Luochuan doposits were originated from the northern Tibet Plateau and Qaidam Basin composite.However,it is greatly different with the age probability-density distributions of Shengli,Jiujiang,and Xiaomei,which indicates that the sources are different.Probability-density distributions of U-Pb ages of Shengli and Jiujiang are similar and complex,while that of Xiaomei is relatively simple.By compared with the frontier zircon U-Pb ages of the outcropped rocks and the sediments within the Yangtze drainage,the Shengli profile in Chengdu Basin may be supplied by the weathered materials from Qamdo Block,Songpan-Ganzi fold belt and the western margin of Yangtze block.With the development of Yangtze river from west to east,the materials on the Yangtze Block,Qinling-Dabie orogenic belt and Cathaysia Block were added to the fluvial composite by the convergence of tributaries and short distance transport by wind.These could lead to the differences in age probability density of Jiuajing profile inherited from Shengli sediments in upper reaches.As the final source region,the Emeishan Large Igneous Province?ELIP?has provided the²60 Ma zircon grains to the upper Permain sandstone/mudstone,which are widely in Bose Basin,Youjiang drainage.Further,the Indosinian mountain building made the detrital zircon grains within³60-500 Ma and²00-270 Ma ages from southeastern South China Carton were added to the upper lower Triassic to middle Triassic sandstone.The consistency of detrital zircons U-Pb age distributions and zircon rare elements features between Xiaomei samples and late Permian to middle Triassic clastic building suggested that Xiaomei red earth sediments were originated from the well-mixed weathered clastic rocks on upper continental crust,Youjiang drainage.Net-like red earth sediments in southern China are the regional response to Pleistocene paleoclimatic evolution,which controlled by the East Asian monsoon.The mineralogical and geochemical characteristics reveal the pedogenesis processes which is dominated by the intensive chemical weathering in southern China,during Pleistocene period.The source tracers show that the provenance of Jiujiang,Shengli and Xiaomei red earth sediments are different from the aeolian deposits in northwestern China.They were originated from the fine materials of well-mixed weathered upper continental crust in the drainage basins and transported by water or wind in the short distance.The role of the East Asian winter monsoon in transporting the dust from inner Asia to southern China since middle Pleistocene may have been overestimated in previous studies.