The Carbon Isotope Fluctuations And Its Paleoenvironmental Significance Of The Upper Jurassic Bulk Carbonate From Amdo Area,Tibet

The global carbon cycle constitutes one of the most fundamental biogeochemical systems affecting all surface reservoirs on the earth,with complex biosphere-atmosphere-hydrosphere-lithosphere interactions that modulate and drive climate change on both short and long timescales.The Jurassic period was an important episode of palaeogeographic evolution,and also constituted a key period with regards to global palaeogeographic and palaeoclimatic Mesozoic evolution.The stable?carbon,oxygen?isotope record has provided convinced evidence in measuring and understanding global climate changes through earth's history.This paper focuses on the Upper Jurassic black rock series in Amdo area,south Qiangtang Basin,Tibet.Based on the multidisciplinary approaches including petrological,sedimentological,stable isotope geochemical and element geochemical observations,the alteration information of the samples,transcontinental comparisons of carbon isotope trends from the world and paleo-environment parameters have been studied to reconstruct the paleoceanographic and paleoclimatic conditions for the Late Jurassic eastern Tethys.The main results and observations are reported here as follows:?1?Paired high-resolution carbonate and organic matter carbon isotopes have been measured from the Upper Jurassic whole-rock carbonates from Amdo area,Tibet,through intensive sampling,thus establishing the carbon isotope curves for the Late Jurassic Tibetan Tethys.The?¹³C curve from the Amdo area shows consistent carbon isotope excursions?CIE?with the worldwide.Additionally,transcontinental comparisons of coeval carbon isotope curves show significant supraregional disparities in long-term?¹³C trends in different regions,which relate to the varied organic carbon burial flux and thus changed carbon isotope compositions in seawater resulting from the opening of the Atlantic Ocean and the forming of the Atlantic basin.?2?The atmospheric p CO₂ levels in the Tibetan Tethys have been reconstructed quantificationally based on the carbon isotopic fractionation between the concentration of the dissolved carbon dioxide in ocean and the carbon isotope composition of marine phytoplankton(?¹³C_P)and[CO_2?aq?].The reconstructed results are coincided with the traditional stomatal index method and paleosol carbonate paleobarometry method.Tese equations have provided an new method for reconstructing past p CO₂ levels.?3?The carbon isotope values of four different inorganic species(including CO_2?g?,CO_2?aq?,[HCO₃^-]and DIC)involved in the ocean-atmosphere system calculated from the fractionation formula show the?¹³C_CO2?g?values range from-9.4‰to-4.3‰with the average of-7.3‰during the Late Jurassic.The sea-level changes indicated by whole-rock cerium anomalies??Ce?manifest an overall trend towards decline.Such trend is related to the Lhasa-Qiangtang collision and the resulting closure of the Bangong-Nujiang Tethyan Ocean during the Middle Jurassic to the Early Cretaceous.?4?The ?¹³C_DIC values in surface seawater are mainly affected by two important factors,the temperature-dependent equilibrium fractionation and the biological fractionation.According to the temperature fractionation formula and the isotopic mass balance,firstly reported in current study,between DIC_P sequestrated by biology and DIC_S in seawater,it has quantitatively evaluated the dominant influence of biological effect on carbon isotope excursions?CIE?in Amdo area.It will lead to an excrusion of 5.2‰in?¹³C values when the DIC_P/DIC_S ratio is 0.19.?5?Detailed comparisons among stable isotope values,climate and organic matter burial rate indicate highly correlations,showing high?¹³C and f_org values correspond to relative warm climate,while low?¹³C and f_org values correspond to cold climate.During the warming intervals,the amplified hydrologic cycle and increased continental weathering enhance delivery of terrestrial nutrients to the ocean,marine productivity and organic carbon burial,leading to increased?¹³C values in dissolved inorganic carbons.The decrease of primary productivity and burial rate of organic matter relating to the reduced nutrient supply during cooling periods,in turn,are especially coupled with low?¹³C values.this coupled correlation between the carbon isotope and climate variations apparently supports the speculation that palaeoclimatic changes could be inferred from marine organic carbon burial and isotopic trends.