Paleohydrological Information Of Carbon And Oxygen Isotopes And Major Chemical Elements In Hardpan Calcrete Of Loess Deposits

Loess deposition is a kind of continental sediments,whose complete and continuous sedimentary sequence effectively preserves the paleoenvironmental information.Carbonate is often used as an ideal carrier for the study of palaeoenvironment and palaeoclimate.O ne is that carbonate rocks account for a large proportion in sedimentary rocks,and sedimentary rocks are most widely distributed on the surface of the earth,so carbonate rocks are also widely distributed.The other is that carbonate rocks are composed of calcite,dolomite and other carbonate minerals,which record the evolution process of many environmental factors such as paleotemperature,Paleoprecipitation and paleosalinity in detail and accurately.Calcrete is a special form of carbonate aggregate in loess deposition,which has many forms such as nodular,tubercular,pedotubule and hardpan.The hardpan calcrete layer has the characteristics of continuous and layered distribution,which is more convenient for sampling,testing and analysis.Taking the hardpan calcrete layer at the bottom of S11 and S12 of folklore village section of BAILU tableland as the research object,this paper further analyzes the characteristics and causes of the hardpan calcrete layer through the test and analysis of the major elements and carbon and oxygen stab le isotopes,and then discusses the reduction reliability of the climate environment and paleohydrological information during the formation period of the hardpan calcrete layer.Through analysis and discussion,the conclusions are as follows:(1)In the hardpan calcrete layer of the folk village profile,compared with the loess-paleo,the content of Ca O increased significantly,while the content of Si O ₂ and Al₂O₃ decreased significantly.This change trend is consistent with the correlation of mineral composition.The content of calcite in the hardpan calcrete layer is significantly higher than that of loess-paleo soil,and the content of quartz,feldspar and clay minerals is reduced.(2)By excluding the influence of temperature and pore-filling calcite,it can be concluded that in the internal structure of hardpan calcrete,from weak calcification to strong calcification,Si and Al elements decrease,Ca element increases,that is,quartz and clay minerals decrease,calcite increases.(3)Compared with the loess-pale,the hardpan calcrete has larger potassium and sodium,the calcium nodule has smaller potassium and sodium.The high potassium-sodium ratio also reflects the change of clay minerals in the hardpan calcrete layer,with more potassium-containing il ite and less sodium-containing montmorillonite.(4)The isotopic mean values of carbon and oxygen isotopes in different parts of the hardpan calcrete layer under the S11 lower part of the ancient soil in the Folk Village are similar to those of the"calcium plate"layer under the S10.The hardpan calcrete layers at the bottom of S10 and S11 have a similar diagenetic environment.The percentage of reduced C4vegetation is 25.85%?33.16%,and the ancient temperature of reduction is 10.77??12.08?.(5)According to the characteristics of carbon and oxygen isotopes in the diagenetic environment of carbonate rocks,it is determined that the diagenetic environment of hardpan calcrete layer is the atmosphere fresh water environme nt,and the characteristics of hardpan calcrete carbon and oxygen isotopes complex diagenetic carbonate rocks.(6)The ?¹³C value of the hardpan calcrete is more negative than the?¹³C value of the soil-formed carbonate in loess.The value of?¹³C of atmospheric CO₂ is about-7‰,and the high negative value of ¹³C of the"hardpan calcrete"layer may be more affected by atmospheric CO₂.(7)The causes of calcrete are divided into soil-forming type and groundwater type.The hardpan calcrete is not a soil-forming calcrete,and the formation environment is related to groundwater.The?¹³C value of groundwater with atmospheric CO₂ as the carbon source is about-7‰,which is similar to the"hardpa n calcrete".The ancient hydrology when the hardpan calcrete was formed may be the groundwater environment affected by atmospheric CO₂.