Carbon And Oxygen Isotope Characteristics Of Cenozoic Carbonate Rocks In Qaidam Basin And Their Paleoclimate Significance

The Qaidam Basin is a Cenozoic inland sedimentary basin located in the northeast of the Tibet Plateau.About 12,000 m thick Cenozoic sediments have been deposited to date,which record the history of uplift,erosion and climate change of surrounding mountains.The carbonate rocks in the basin have a wide geographical distribution and a long geological history.They are widely distributed in brackish water,salt water,fresh water and salt lake environments.They contain valuable paleoclimate and paleenvironment information and are important data for the study of paleoclimate change.The analysis of carbon and oxygen isotope characteristics of carbonate rocks in the Qaidam Basin can provide continuous and long-term geological evidence for the restoration of paleoclimatic evolution since Cenozoic.This paper takes Honggou section of Qaidam Basin as the research object,By collecting 85 carbonate samples from Xiaganchaigou Formation to lower strata of Shangyoushashan Formation and 600 chroma samples from Xiaganchaigou Formation to Shangyoushashan Formation,In this paper,the paleoclimate evolution of Qaidam Basin since Cenozoic is reconstructed based on the petrological characteristics,carbon and oxygen isotope analysis and chroma measurement.The main conclusions and understandings are as follows:(1)According to the characteristics of mineral composition,grain type and microstructure,the carbonate rocks exposed in Honggou section are mainly bioclastic limestone,bioclastic micritic limestone,clastic micritic limestone,clastic micritic limestone and micritic limestone.Among them,Xiaganchaigou Formation is bioclastic limestone and bioclastic micritic limestone;Shangganchaigou Formation is calcarenite,micritic limestone and arenaceous limestone;Xiayoushashan Formation consists of arenaceous limestone and arenaceous micritic limestone.Shangyoushashan Formation is micritic limestone.(2)The evolution process of paleoclimate environment recorded by carbon and oxygen isotopes in carbonate rocks is roughly as follows:The average?¹³C and?¹⁸O values are-5.52‰and-10.82‰,respectively.The?¹³C and?¹⁸O of the carbonate rocks at 17?14 Ma are negatively biased,which corresponds to the global Middle Miocene warm period;During 14?9 Ma,the climate in the basin gradually becomes dry,The climatic environment of 9?8 Ma basin continued to deteriorate,and the degree of aridification intensified further.(3)The?¹³C and?¹⁸O values of the carbonate rock show a large positive deviation at 14 Ma,with the?¹³C values increasing from-5.52‰to-1.65‰,and the?¹⁸O values increasing from-11.26‰to-6.84‰.The?¹³C and?¹⁸O values of the carbonate rock show a small positive deviation at 9 Ma.The?¹³C value increased from-4.36‰to-1.81‰,and the?¹⁸O value increased from-8.32‰to-5.16‰.The positive deviation is probably caused by global climate change and the uplift of Qilian Mountains and Tibet Plateau during 14?8 Ma.(4)The sedimentary chromaticity parameters reflect the climate evolution process of the Qaidam Basin as follows:The 23.5?14 Ma basin climate is humid,which is related to global climate change;The climate of the basin became cold and dry during 14?8 Ma,which was related to the decrease of global temperature and the uplift of qilian Mountains.The aridity degree of 8?6 Ma basin increased and became aridity,which was the result of global continuous cooling and uplift of Tibet Plateau in late Miocene.