Provenance Tracing Of Loess In Western Chinese Loess Plateau Evidence From The Iron Mineralogy

The loess-paleosol sequence of the Chinese Loess Plateau(CLP)is favored by paleoclimatologists due to its continuous sedimentation,long chronological sequence and high resolution.Such a good aeolian deposition sequence can completely document the evolution history of Asian ancient dust,inverted the uplift history of the Qinghai-Tibet Plateau,and the process of aridification and desertification in the interior of Asia.The research on paleo-climate of loess-paleosol has been quite mature and has made brilliant achievements.However,there are controversies about the provenance of aeolian sediments in CLP.Identification of the dust sources for the aeolian sediments in CLP is important to better clarify and understand the above hot scientific issues.In this study,desert sediments from arid and semi-arid areas of Northwest(NW)China,including Tarim Basin,Junggar Basin,Qaidam Basin,Alxa Plateau,Ordos Plateau,and the last glacial loess of CLP,were selected.Then we chose three main iron-bearing minerals,magnetite,((Mt)(calculated from ?),hematite(Hm)and goethite(Gt))in different particle-size-fractions(31-63 ?m,8-31 ?m,2-8 ?m)to discussing the concentration variation characteristics.The results show that the iron-bearing mineral contents of particle-size-fractions are closely related to the distribution of mountain ranges and tectonic activity in NW China.The iron-bearing mineral contents of particle-size-fractions are generally low in the Tarim Basin,Qaidam Basin,the southern Alxa Plateau and the Gonghe Basin which all are adjacent to or within the Tibetan Plateau,while the basins controlled by the Central Asian orogenic belt(Junggar Basin,the northern Alxa Plateau and Ordos Plateau),the iron-bearing mineral contents of particle-size-fractions is relatively high.The iron mineral contents of particle-size-fractions in each sedimentary basin showed that Mt and Gt are evidently sensitive to particle size changes.The mean Mt content decreases with decreasing particle size in all the sedimentary basins and CLP,expect for the Ordos Plateau which is part of the western North China Craton.For Gt,the mean content increases with decreasing particle size in all the sedimentary basins and CLP.In contrast to Mt and Gt,the Hm content of the particle-size-fractions is relatively uniform between sedimentary basins.The distinctive iron mineralogy of the surface sediments samples from each arid basin provides an opportunity to analyze their proportional contributions to the CLP loess and the relationship of the sources with particle size.Solver,implemented in Excel,was used to calculate the source contributions from each basin.(i)the 8-31 ?m fractions,the major component of CLP loess,is a mixture of sediment weathering from NE Tibetan Plateau and Central Asian Orogenic Belt(CAOB),with a majority from NE Tibetan Plateau;(ii)S.Alxa Plateau is the dominant source of 31-63 ?m fractions of CLP LG loess;(iii)the Qaidam Basin is the dominant source of 2-8 ?m fractions of CLP LG loess;(iv)the material contribution from Ordos Plateau is limited to CLP LG loess,especially for the loess on the western CLP.In addition,we have investigated the magnetic properties of a large set(541 samples)of surface sediment from NW China.The results of cluster analysis of the magnetic properties reveal three statistically distinct sediment provenance groupings:(a)The largest group mainly comprises samples from the slopes of the Kunlun-Alkin-Qilian Mountains System and the surrounding fluvial plains.These samples are characterized by the lowest concentration of magnetite and by a moderate magnetic grain-size.(b)This group mainly comprises samples from Tien-Altai Mountains System and surrounding fluvial plains and western edge of the Chinese Loess Plateau(CLP).They have a moderate concentration of magnetite and a relatively fine magnetic grain-size.(c)This group mainly comprises samples from the northwestern slopes of the Yin Mountains,Ejina Basin,central Tarim Basin,and Irtysh River Valley.They have the highest concentration of magnetite and the coarsest magnetic grain size.A comparison of the magnetic characteristics of our sample set leads us to propose that the silt material in arid basin is produced from its surrounding high mountains.Furthermore,the content of Mt and Hm is generally low and there is no significant increase in the concentration of 2-8 μm particle size,which indicate that the concentration of the two minerals are controlled by different geological units and pedogenic process effect is weak.The concentration of goethite in desert sediments in 2-8 μm grain size fractions is obviously higher than that in the 31-63 μm grain size fractions,suggesting that goethite concentration is obviously affected by the later pedogenic process.The environment of desert of NW China is not conducive to the formation of goethite.Combined with other geological evidence,we conclude that the low temperature and humid peculiar environment of the high mountains around the basins are propitious to goethite formation and these mountains can continuously produce a large amount of debris with high content goethite.In NW China,a large amount of detrital material produced by the mountain process in inland basins is the material basis for the formation and development of deserts and CLP.Rivers,originated from the high mountains around the basin,is the bond between high mountains and different landforms in the lower altitude regions,and forming a huge fluvial/alluvial fan,by carrying detrital materials produced by the mountain process.These detrital materials went through a series of complex physicochemical weathering processes and then depositional sinks as desert sediments and loess of CLP.The desert is a transfer station for the storage of fine particulate,which can directly and indirectly provide detrital materials for the CLP and even East Asian dust.