Carbon And Oxygen Isotope Records And Paleoenvironment Research Since The Pliocene In The Dalangtan Area,Qaidam Basin

Dalangtan is located in the west of Qaidam Basin,and the northeast margin of Qinghai Tibet Plateau.It is an important salt deposit center in Qaidam Basin.The sediment in the basin is controlled by tectonic movement and global climate change.The evolution history of climate environment and the response of uplift of Qinghai Tibet Plateau in this area are recorded in detail.In this study,dry salt lake in Dalangtan area,which entered the salt forming period in the western Qaidam Basin,was selected as the research object.Based on the data of drilling core of Black ZK-01,based on the high resolution dating,the climatic environment evolution information indicated by the carbon and oxygen isotope values of carbonate in the area since the Pliocene was studied,supplemented by other paleoclimatic paleoenvironmental substitution indexes(lithology,pollen,etc.)The comprehensive comparative analysis of the results shows that the evolution law of paleoclimate Paleoenvironment since Pliocene in the study area is reconstructed,and the response of paleoclimate catastrophe events in the region is concerned;and the comparison and analysis is made with other borehole core records and large-scale typical climate change records in Dalangtan area,and the following conclusions are obtained:1.Based on the paleomagnetic dating of sedimentary samples from borehole Black ZK-01,the age scale of borehole Black ZK-01 is established.The B/M boundary depth is 147 m and the M/G boundary depth is 694 m.2.The carbon and oxygen isotopic indices of Lake authigenic carbonates were used in this study.From the results,the overall trend of ?¹⁸O value and ?¹³C value is consistent,and they are gradually biased.Among them,the ?¹⁸O value indicates the change of the ratio of evaporation to precipitation.The value of ?¹³C is affected by many factors,not only related to Lake salinity,but also the exchange degree between total dissolved inorganic carbon(TDIC)and atmospheric CO₂.3.Based on the paleomagnetic age framework,the sedimentary environment evolution of the borehole can be divided into six stages according to the carbon and oxygen isotope analysis of carbonate sediments,lithology and pollen dataDuring the period of 4.553-3.596 Ma B.P.,the climate in Dalangtan area was relatively warm and humid,but after 4 Ma B.P.,it began to change to warm and dry;During the period of 3.596-2.581 Ma B.P.,the climate became drier gradually,and the deposition of evaporite mineral gypsum began before the Quaternary;During the period of 2.581-1.778 Ma B.P.,the climate is warm and wet.From the continuous deposition of evaporite,it can be seen that the sedimentary environment is gradually stable,generally warm,but there are small-scale drought events;During the period of 1.778-1.072 Ma B.P.,the degree of drought was further strengthened,and a large number of evaporite minerals,halite and gypsum were deposited,showing a warm and dry sedimentary environment;During the period of 1.072-0.781 Ma B.P.,the sedimentary environment continued to be arid,the ratio of evaporation to precipitation became higher,the temperature showed a downward trend,and the climate showed obvious alternation of cold and warm,but still dominated by aridity;During the period of 0.781-0.267 Ma B.P.,since the Middle Pleistocene,the degree of climate drought has been further strengthened,the lake has shrunk,and entered the salt lake stage,which is generally characterized by cold and dry climate.4.By comparing the climatic and environmental changes recorded by the core sediments of Black ZK-01 with the core sediments of ZK-336 and Liang ZK-02,it is found that there is a good comparability between them,and the response of Qinghai Tibet Plateau Uplift in this region is discussed.5.Compared with the Luo Chuan loess paleosol climate curve and LR04 ?¹⁸O standard curve,the identified multiple oxygen isotope characteristic events have their own particularity,which indicates that the basin climate change not only reflects the global climate change,but also reflects the impact of the uplift of the Qinghai Tibet Plateau on the basin climate change and has its regional characteristics.