Carbon And Oxygen Isotopic Compositions Of Carbon Concretion In The Strata In The Lanzhou And Liupanshan Basin,China,and The Early Cretaceous And Oligocene Atmospheric CO₂ Concentration

The Cretaceous-Paleogene is an important period of global climate change.It is characterized by the transition from the Cretaceous-Eocene "Greenhouse Earth" through the Oligocene-Miocene transform to the Quaternary period "glacial earth".The atmospheric CO₂ concentration from extreme high-value during the Cretaceous-Eocene started to decrease over the Oligocene-Miocene interval,then fell to the minimum in the Quaternary.Reconstruction of the atmospheric CO₂ concentration is an important way to understand the mechanism of climate shift in the past.Nevertheless,the records of the Cretaceous-Paleogene are still limited to a certain extent,which restricts our understanding to the mechanism of the global climate change and carbon cycle.In the Lanzhou basin and Liupanshan basin,northwestern China,some Mesozoic-Cenozoic continental sedimentary strata holds the paleosol layers or the carbon concretion.This provides an opportunity to study the atmospheric CO₂ concentration.Based on the petrological characteristic and the carbon and oxygen isotope of the carbon concretion of the Lower Cretaceous Hekou Group in the Lanzhou Basin,and of the Lower Cretaceous Liupanshan Group and Oligocene Sikouzi Formation in the Liupanshan Basin,combined with the existing magnetostratigraphy data,the paleo-atmospheric CO₂ concentrations during the Early Cretaceous and Oligocene were estimated using thd pedogenic carbonate method.The conclusions are as following.1.The Early Cretaceous Carbon concretion,four layers in the Lower Cretaceous Hekou group of the Lanzhou basin,and five layers in the Lower part of Lower Cretaceous Liupanshan group of Liupanshan basin,are found in this study.The carbon concretion in two the basins is formed in the mud sandstone of fluvial or the floodplain mudstones in the alluvial facies.The Oligoence carbon concretion is a total of 12 layers within the Oligocene Sikouzi Formation in the Sikouzi and Shi Xianzi section,which formed in the delta front sandstone.Carbon concretion is mainly distributed in the paleosol layer or soiled silt sandstone.The carbon concretion is irregular the shape.The concretion is made of clastic?quartz and feldspar?and the cerement?clay and calcite?.Calcite is micro-and fine crystalline and formed in the process of the soil forming process.2.The ?13C values of carbon concretion of the lower Cretaceous Hekou group in Lanzhou Basin is between-5.923‰ and-1.558‰?averaged at-3.504‰?,?18O value was-9.467‰ to-3.012‰?average value:-4.649‰?.The ?13C value of carbon concretion in the lower strata of the Liupanshan group is between-4.140‰ and-2.501‰?average value:-3.493‰?,?18O value is-10.727‰ ^-7.865‰?average value:-9.195‰?.The ?13C value of carbon concretion of the Oligocene Sikouzi Formation is between-6.857‰ and-3.655‰?average value:-4.295‰?,?18O value is between-13.868 ‰ and-3.065‰?average value:-6.011‰?in Sikouzi profile,and the ?13C value is between-8.615‰ and-4.095‰?average value:-6.507‰?,?18O value is-8.036‰ ^-5.269‰?average value:-6.846‰?in the Shixianzi section.3.The estimated atmospheric CO₂ concentration in Lanzhou Basin and Liupanshan Basin of the Early Cretaceous and Late Oligocene ranges from 507 to 1743 ppmv,122 to 1002 ppmv,respectively.In the Early Cretaceous the atmospheric CO₂ concentration is 507¹244ppmv in the Berriasian-Valanginian,and 527⁹69ppmv in the early Barremian is,and 1015¹743ppmv in the Early Albian,with an increasing trend.The estimated atmospheric CO₂ concentration is strongly fluctuation in the late Oligocene,but the average is exceedingly close to that obtained by other methods.