The Distribution And Transportation Characteristics Of Carbon Isotope And The Preliminary Application In Paleoenvironment In Karst Areas Of Chongqing And Guizhou

As an important carrier of reconstruction of paleoclimate, spelothems are increasingly used to provide high resolution, accurately dated paleoclimate information to reconstruct paleoclimate and paleoenvironment. Among the abundant proxies of the spelothems, δ¹³C record has the diversity for the indication of climate and environment, due to the complex influencing factors, coupled with the regional differences and different sedimentary environments, but its significance for the paleoclimate and paleoenvironment can not be ignored. In order to accurately interpret the environmental significance of carbon isotope, we need to carry out comprehensive multi-site studies and reproducibility studies, and systematically study the vertical transportation characteristics and main influencing factors of carbon isotope in different caves, thus obtaining reliable information of surface ecological and environmental changes.The δ¹³C average value of twenty-four plant subsamples overlying Jinfo Mountain is-28.91‰, they all belong to the modern C3 photosynthesis vegetation. The δ¹³C average value of plant subsamples overlying Jinfo Mountain is about 3‰ heavier than that overlying Furong cave, because of the influences of biological species and the higher altitude of Jinfo mountain. The Pi/Pa values will decrease with the increase in altitude, correspondingly, the δ¹³C values of plant become heavier. The δ¹³C values of organic matter for the soil profile JF3 become lighter suddenly in the depth of 55 cm, while the δ¹³C values of organic matter for the soil profiles JF1 and JF2 become heavier with the depth of profiles and stabilize to a certain depth, this is because of the carbon isotope fractionation during the decomposition processes of organic matter in soils.There are both C3 and C4 photosynthesis vegetations among the plant subsamples collected in Liuzhi, Guanling, Panxian of Guizhou. Due to the biological species and arid soil environment of Guizhou, the δ¹³C values of plant subsamples in the desertification region of Guizhou are significantly heavier than that of Chongqing without desertification. Additionally, because of the harsh environment and higher altitude of Panxian, the δ¹³C values of plant subsamples are heavier generally, and the variation ranges of soil organic carbon content are bigger than that of other soil profiles. The soil organic carbon contents of seven profiles in Guizhou decrease dramatically with the depth of soil profiles. The δ¹³C values of organic matter for the soil profiles ND、LP1、LP2、LP3 become heaver with the increasing depth, after reaching the maximum they gradually become lighter and stabilize, showing significant correspondence with their soil organic carbon content. While the changes of δ¹³C values of organic matter for the soil profiles LZ1、LZ2、LZ3 with depth have large differences.The DIC-δ¹³C of the drip water and pool water in Furong cave show the seasonal characteristic of heavier in summer and autumn, while lighter in winter and spring, they have a significant lag period for the response of the climate change. The DIC-δ¹³C of cave water are affected by the soil CO₂, Prior Calcite Precipitation, bedrock dissolution and the opening degree of vadose zone, among them the soil CO₂ is dominant. The DIC-δ¹³C values of drip water in Yangkou cave are generally heavy, never showing the significant seasonal variation. Speculating the strong cave degassing, the exchange between the DIC-δ¹³C of soil water and the soil CO₂ is insufficient, relatively weak soil biological activity and bedrock dissolution are the main reasons for the heavy DIC-δ¹³C values. The DIC-δ¹³C values of cave water in Naduo are also generally heavy, never showing the significant seasonal variation, this may be related to the insufficient exchange between the DIC-δ¹³C of soil water and the soil CO₂. The fractionation degree of carbon isotope in the process of migration from the overlying plants, soil, bedrock, drip water and modern speleothems are significantly different, which is related to the geographical environment, the thickness of overlying soil, cave ventilation, the fractionation and so on.Use the δ¹³C and δ18O of the stalagmite FR5, we reconstruct the Asia monsoon intensity and the changes of vegetation biomass and surface biological processes in the period of 0³8 ka BP. The δ¹³C and δ18O show similar changes in the overall trend, but are negative correlated with each other at certain times, the carbon isotope reflects the effective humidity changes. The stalagmite JFYK7 meet the Hendy test, we can use the δ¹³C and δ18O records of JFYK7 to reconstruct the paleoclimate. The responses of the δ¹³C and δ18O of JFYK7 for the climate change are almost synchronous, instructing the Asia monsoon intensity and the changes of vegetation biomass in the period of 37.8⁷8 ka BP. The δ¹³C records of the two stalagmites are consistent with the NGRIP record and other stalagmite records in East Asian monsoon region, recording a series of H events and D/O events.Studying the driving mechanism of the δ¹³C records of FR5 and JFYK7, we find, on the orbital scale, the δ¹³C records respond to the change of solar radiation energy sensitively, while on centennial and millennial scales, the temperature change in the North Atlantic region changes the circulation pattern of ocean and gas, resulting in the climate change of East Asian monsoon, which is the main factor influencing the δ¹³C changs of FR5 and JFYK7. When the North Atlantic region become cold, westerly passes the climate signals of North Atlantic region to the eastern Siberia, the Siberian high pressure becomes enhanced, leading the temperature of Northern Hemisphere descend and the ITCZ move southwards, then the summer monsoon circulation weakens, responding to the happening of H events. When the H events have happened, the summer monsoon circulation weakens and the climate becomes dry and cold, then the vegetation biomass may decrease and soil biological activity weaken, eventually the δ¹³C of stalagmite become heaver; When the D/O events happene, the summer monsoon circulation strengthens and the climate becomes warm and humid, then the δ¹³C become lighter. On annual scale, the flow rate of karst groundwater, water-rock interaction, CO₂ degassing, Prior Calcite Precipitation and the dissolution of soil CO₂ and so on may also effect the δ¹³C of stalagmite.The δ¹³C and δ18O of JFYK7 have recorded the summer monsoon weakening and vegetation biomass reducing during 72⁷5 ka BP, this may be caused by the global cooling induced by the Toba volcanic eruption. The comparison between the δ¹³C of stalagmite JFYK7 and the δ18O、δ¹³C of stalagmite XY2 illustrates that the δ¹³C of stalagmite is the result of the interaction between the local climate and environment, having significant regional differences. Therefore, when we use the δ¹³C of stalagmite to reconstruct paleoclimate, we need to combine the δ18O records of the stalagmite and the δ¹³C and δ18O records of other stalagmites, finally ensure the accuracy of the paleovegetation recovery.