布朗大学同位素地球化学-Carbonisotope1.ppt

Carbon isotopePart I Topics for this section Carbon isotopic fractionation in natural water systems Carbon isotope variation during photosynthesis in land plants (C3, C4, CAM plants) Objectives Understand the systematics of carbon isotopic fractionation in inorganic and organic systems Know the basis of carbon isotope application in study of carbon cycling and plant ecology Characteristics of Carbon Carbon has two stable isotopes: 12C=98.89%; 13C=1.11% Carbon plays a key role in the biosphere, inorganic/organic carbon also exists in a diversity of compounds with different oxidation states CH4: -4 CH3-CH3: -3 Diamond/graphite: 0 CO: +2 CO2, H2CO3, HCO3- +4 CO32-, CaCO3, CaMg(CO3)2 The d13C values often show similar sequence as the oxidation states: d13Ccarbonate d13Cgraphite d13CCH4 Carbon Isotope standards Natural variations in d13C values The Global Carbon Cycle Carbonate system CO2(g) ? CO2(aq)?H2CO3* H2CO2(aq) ? H++HCO3-(aq) HCO3- ? H++ CO32-(aq) Dissolved inorganic carbon (DIC) In equilibrium with atmospheric CO2, d13C values of DIC vary with pH, as carbonate speciation changes, because different carbonate species (CO2(g), CO2(aq), HCO3-, CO32-) have different d13C values Carbonate 13C fractionation at different temperature Kinetic C isotope fractionation during photosynthesis CO2+H2O → CH2O+O2 (large kinetic fractionation) Two key enzymes Ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) for C3 plants (trees, aquatic plants and algae): “Rubisco” for short Phospoenol pyrurate carboxylase (PEP Carboxylase) for C4 plants (tropical grasses, sedges etc) Key isotopic effects of steps leading to CO2 fixation in C3 plants Stomata Photosynthetic discrimination (D) in C3 plants (Farquhar equation) Inside/outside CO2 ratio and D Ci/Ca in different tree species and their d13C value Mean d13C value of C3 plants -27 ‰ Air CO2 d13C = -7.8 ‰ 13D = 27 – 7.8 ≈ 19 ‰ 19 =4.4 + (27-4.4)×Ci/Ca Ci/Ca = 0.64 (this is th