布朗大学同位素地球化学-Carbonisotope2.ppt

Carbon isotopePart II Topics for this section Carbon isotope variation during photosynthesis of algae Application of carbon isotopes in Groundwater studies Origin of methane paleoclimate and paleoenvironmental research Pollutant studies using carbon isotope data Objectives Understand the systematic of carbon isotopic fractionation in photosynthesis in aquatic systems Know the current range of applications of carbon isotopes in environmental and groundwater studies Carbon isotope variations in living organisms Carbon isotopic fractionation in aquatic organisms Diffusion rate of CO2 in water is 1000 times slower than air: carbon is far more limited for aquatic organisms than for land plants, often leading to higher d13C values (organisms less selective of 12C/13C) Blooms of phytoplankton can significantly draw down the [CO2]aq, thus algae grow later in the season or during blooms have higher d13C values C isotope Fractionation by Algae Two End-Member scenarios Marine planktonic algae d13C≈ -20 ‰ d13Cair ≈ -7.5 ‰ D13C = -20 – (-7.5) = -12.5 ‰ -12.5 = -4.4 + Ci/Ce * (-29+4.4) Ci/Ce (algae) = 0.33 compare with, C3 land plants Ci/Ce ≈ 0.64 Environmental factors affecting the d13C values of algae Temperature: CO2 more soluble in water at lower temperature. d13C↓ at lower temperature pH level: at higher pH level, CO2 concentration decrease due to carbonate speciation changes (more HCO3-). d13C↑ at higher pH Light: increased light intensity increase photosynthetic carbon demand. If transport of C into cells cannot catch up, d13C↑ Water velocity and turbulence: There is a so called “boundary layer (BL)” surrounding the algal cells, which limits the carbon transport rate to inner cells. Greater water velocity/turbulence reduce the thickness of BL, d13C↓ CO2 concentrating mechanism Many algae possess ability to actively transport CO2 into their cells (cost energy, ATPs). The ability is used when CO2-limited. The process is unselective of 12C/13C, d13Calgae↑