Atmospheric CO₂ Concentration Using Carbonate Isotopes From The Tianshui Basin,Northern China

Cenozoic CO₂ records are critical for understanding forcing mechanisms of paleoclimate variations.Thus,establishing reliable CO₂ records in geological times is a hot topic in paleoclimatic studies.CO₂ concentrations during the Pleistocene have been well studied by ice core records?Lüthi et al.,2008?.But the longest ice core records can only go back to 800 ka.Thus,researchers have been developing multiple methods(?¹¹B,stomata,B/Ca,carbonate)to explore the CO₂concentrations in deep times.Those methods provide more and more reliable CO₂ records by improving the mothod itself and also the CO₂ resolution.However,despite that the Pliocene CO₂records based on different methods are close to be similar,situation for Miocene CO₂reconstruction are more complicated.Miocene warm periods,an epoch where earth's climate was 3°C warmer than modern during the past 23 Ma and the pCO₂ concentration similar to modern⁴00 ppm,has been regard as one of the best analogs for future climate and thus has been recently targeted as a high priority for atmospheric pCO₂ research.Thus,the main purpose of this work is to generate more mid-late Miocene atmospheric pCO₂records to compare with existing data.We plan to achieve following goals:1)Generating more CO₂ data covering the middle to late Miocene using paleosol carbonate CO₂ barometer.2)Exploring the relationship between CO₂ concentration and paleoclimate.But two important parameters are needed when reconstructing CO₂ history:soil respirated CO₂?S?z??and temperature when carbonate formed.Thus,we carried out following studies in order to resolve these two issues:1)monitoring soil gas profile to quantify S?z?.2)Clumped isotope based temperature reconstruction.The main results and conclusions are listed as follows:1)Monitoring soil gas profiles?CO₂ and CH₄?on the Loess Plateau for two years.Our results show that the S?z?is 572±273 ppm,which is within the range of other proxy-based soil respiration CO₂ results.In addition,we also got the CH₄ concentration when measure the CO₂concentration.We calculated the average depth of soil respiration and CH₄ oxidation.The average depth of soil respiration varies from 10 to 70 cm below soil surface and is typically 20 to 40 cm;the average depth of CH₄ oxidation varies from 3 to 10 cm.2)Reconstructing the temperature using clumped isotope method.The clumped isotope based soil temperature varied from 22 to 13°C?SE:2.23°C?.During the middle Miocene Optimum?MCO?,average soil temperature was 22°C,which decreased to an average value of 17°C during the late Miocene.3)Based on the S?z?and temperature in our work,we reconstructed the middle Miocene CO₂concentration.Our results show that middel Miocene CO₂ concentrations varied between 250-400ppm,and late Miocene CO₂ concentrations varied between 100-300 ppm.