Spectral Features Of Late Miocene Sediments From The Jianzha Basin In Northeastern Margin Of The Tibetan Plateau And Their Paleoclimatic Significance

Global climate change is a hot area of current geological research.The information of paleoclimate change in the natural environment evolution is usually recorded by biosphere,hydrosphere,atmosphere and lithosphere.Loess,ice cores and deep-sea sediments are known as the three major carriers for studying global climate change,and global change is studied by extracting climate indicators from the carriers.Finding simple and effective paleoclimate proxy indicators is of great importance in the field of climate research.Reflection spectroscopy have the advantages of being fast,convenient,inexpensive and non-destructive,and can provide important information about sedimentary mineralogy,which in turn reflects the climate change at the time of sediment formation,while reflection spectroscopy is less applied in paleoclimate studies of red clay.In this paper,the Miocene sediments from the Jiarang section of the northeastern margin of the Tibetan Plateau in the Jianzha Basin,which is located in a climate-sensitive area,is used as the object.The reflection spectral data of visible near-infrared and short-wave infrared were obtained by reflectance spectroscopy testing of eolian red clay samples.By extracting the climate spectral parameters and comparing them with traditional paleoclimate indicators,the following conclusions were drawn:(1)The spectral curve of the red clay in the Jiarang section has absorption peaks around ～500 nm,～900 nm,～1400 nm,～1900 nm,～2200 nm,and ～2340 nm.The absorption peaks at ～500 nm and ～900 nm in visible near-infrared band are mainly caused by the electron energy leap of cations,indicating the Fe-bearing minerals such as magnetite,goethite and hematite in red clay.The absorption peaks at ～1400 nm,～1900 nm,～2200 nm and ～2340 nm in the short-wave infrared are mainly caused by the bending vibration of the anion,indicating the clay minerals such as montmorillonite and illite in red clay.The principal component analysis of the original spectral curves showed that the red clay samples had more obvious group divisions in the 6.55-7.2 Ma,7.2-8.5 Ma and 8.5-8.95 Ma,indicating that the red clay in three stages have relatively obvious spectroscopic behaviors and characteristics.(2)The spectral parameters in the visible near-infrared band were compared with the magnetic susceptibility and frequency-dependent magnetic susceptibility,and the results of four parameters,R500,R900,D500 and D900(R and D represent the reflectance and depth of the corresponding absorption peaks,respectively)have a good correlation correspondence.The four parameters can reflect the variation of iron-bearing mineral content,in which the content of iron-bearing minerals is negatively correlated with R500 and R900,and positively correlated with D500 and D900.When D500 and D900 increase,it indicated that more iron-bearing minerals are formed in the red clay,indicating a warm climate environment,and when it decreases,it indicated that the iron-bearing minerals content decreases and the climate environment is colder.In addition,the spectral parameters and oxygen isotope curves can also correspond well,and the comprehensive analysis suggests that the spectral parameters in the visible near-infrared band are more sensitive to temperature,and they can reflect the climate evolution on the orbital scale to a certain extent.(3)The short-wave infrared band spectral parameters were compared with the traditional paleoclimate indicators of illite,montmorillonite and Rb/Sr ratio.The results revealed that the three parameters AS1400,D1400/D1900 and D1900(AS represents the asymmetry of the corresponding absorption peaks)can be well corresponded to each other,respectively.The analysis found that the spectral parameters are related to the relative content of illite and montmorillonite,and they are more sensitive to the intensity of weathering,and the Rb/Sr ratio can correspond well.Increasing AS1400 and D1400/D1900 indicate enhanced weathering and vice versa,while increasing D1900 indicates weaker weathering.The reflectance of the absorption peak near ～2340 nm(R2340)is related to the illite content,which shows an opposite trend,with the reflectance decreasing when its content increases.(4)In order to verify the applicability of the obtained spectral parameters to the climatic study of Quaternary loess-paleosol,this article extracted the spectral parameters of the loess-paleosol sequence in the Chaona section of the central Chinese Loess Plateau.The results show that there is a strong correlation between the spectral parameters of the visible near-infrared band and the magnetic susceptibility,frequency-dependent magnetic susceptibility and oxygen isotope record in the loess-paleosol sequence.The spectral parameters are more sensitive to the temperature,and the study results are similar to the Jiarang section,which indicates that the spectral parameters in the visible near-infrared band are also applicable in loess-paleosol.