| As a geological process,Chemical Weathering plays an important role in all spheres of the Earth,which is a "bridge" or a link among various spheres.Chemical and mechanical denudation of the silicate rocks on the surface of the Earth may affect the global carbon cycle and even the global climate.Cretaceous is a typical "greenhouse" period for its high CO2 concentration and temperature close to present day,so it is of great significance to research on its climate change for understanding the today’s pattern of climate and environment.The Lower Cretaceous good continental strata exposed in northwest of China,Liupanshan Group,Liupan Area,is the ideal carrier for the study of Cretaceous climate and environment change.The proxies,such as Magnetic susceptibility,color and pollen et al.,have been used to reconstruct the Lower Cretaceous palaeoclimate in Liupanshan Group,but not yet to study the intensity of chemical weathering recorded in the sediments and even the palaeoclimatic signicifance.In this article,the geochemical elements of Lower Cretaceous samples in Liwaxia and lower part of Madongshan Formation,Liupanshan group,are analyed for calculating the Chemical Index of Alteration(CIA)to show the intensity of chemical weathering.Combining with climatic proxies that are CaC03(wt%),(CaO + Na2O + K2O)/Al2O3 and SiO2/Al2O3,Sr/Ba,Sr/Cu ratio,climate and environment are discussed.The study indicates that compared with the elements of Average Shale,the sediments in Liupanshan middle segment enrich Mg,Ca and poor Si,Al,Fe,Na,P and Ti,with the K equivalent in the major elements.In addition,in trace elements,the sediments enrich in Sc,Co,Sr,Cs,Hf,W,As,Bi and poor Cr,V,Mn,Rb,Y,Zr,Nb,Th,Tl,Cu,Pb,Zn,S,Ni,Mo and Nd,with the Ba,Ce,U,Ga equivalent.The sediments are not the recycled deposition and are suitable for the research of chemical weathering.The ratio of plagioclase and K-feldspar is 4:1 in the original rocks which may be the Hercynian granite.Sediments during the diagenesis generally subjects to potassium metasomatism.The values of Chemical Index of Alteration(CIA)corrected are located between 54.41 and 76.08%with an average of 64.57%,reflecting the weathering degree of the original rocks is approximately in low-intermediate level.In addition,both the CIA(molar)values and Plagioclase Index of Alteration(PIA)also show that the weathering degree of the original rocks is in low-intermediate level and still in the primary weathering stage of de-sodium,calcium.Both of the Index of Weathering and climate proxies reflect a warm and dry palaeoclimate during 124.97Ma to 112.96Ma with the warm and humid climate in the short-scale.Specifically,there are three stages.The first segment is that the warm and dry palaeoclimate during 124.97Ma to 121.14 Ma,including a stage of temperature droping between 124.97Ma and 123.09 and another stage of temperature rising between 123.09Ma and 121.14Ma.The second segment is that the warm and humid to warm and dry climate between 121.14 and 115.04 Ma,including a stage of warm and humid during 121.14Ma to 119.46Ma and another stage of warm and dry during 119.46Ma to 115.04Ma.The third segment is that the warm and dry to hot and dry stage during 115.04 to 112.96 Ma,including a stage of warm and dry between 115.04 and 113.92Ma and another stage of hot and dry between 113.92Ma and 112.96Ma.Palaeoclimatic conditions reflected by CIA are compared with that by other proxies which are colors,magnetic susceptibility,clay minerals and pollen.The result indicates that the palaeoclimate reflected by clay minerals,pollen and color is also warm and dry,which is similar to that by CIA,but some differences on the segmentation.The magnetic susceptibility indicator reflects the warm and humid climate,which is different from the chemical weathering indicators.The reasons may be that Susceptibility reflects the relative climate and environment,whereas weathering index is due to quantitative characteristics. |
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