Oxygen triple isotope composition for estimating photosynthesis rates

1. Oxygen triple isotope composition for estimating photosynthesis rates Nir Krakauer niryk@caltech.edu June, 2006

2. O isotopes • 17O/16O ≈ 3.8 ∙ 10−4 • 18O/16O ≈ 2.0 ∙ 10−3 • Isotope mass ratio, (m17O − m16O) / (m18O − m16O) = 0.5010 • Different standard materials: • Standard mean ocean water • PDB carbonate • Atmospheric O2 Coplen et al 2002

3. Mass-independent fractionation • Discovered in 1980s: ozone produced by O(3P) + O2 + M → O3 + M has equally high δ17O and δ18O • Explanations: • Self-shielding of abundant isotopologue • Greater energy-level density for asymmetric species (e.g. Liang et al 2004)

4. δ17O and δ18O on Earth

5. Application: Ocean gross O2 production rate • Define Δ17O ≡δ17O – 0.521∙δ18O: unaffected by respiration & evaporation • Δ17O of dissolved O2 at equilibrium with atmosphere: 16 per meg (Δeq) • Δ17O of photosynthetically produced O2: 159 per meg (Sea of Galilee); 249 per meg (Ocean) (Δmax) • By isotope mass balance, for the surface ocean I∙Δeq + GP∙Δmax = (E+R)∙Δdiss where • Δdiss = surface-ocean Δ17O • GP = gross O2 production • I = O2 invasion • E = O2 evasion • R = respiratory O2 consumption • equivalently, GP = K∙C0∙(Δdiss– Δeq)/(Δmax– Δeq) where • K = air-sea gas transfer velocity • C0 = equilibrium water O2 concentration • Largest uncertainty seems to be the accuracy to which K is known, no better than 30%. Luz and Barkan 2000

6. Upper-ocean Δ17O Juranek and Quay 2005

7. Net O2 release / photosynthesis ratio • Independent of gas transfer velocity • Export ratio for productive ocean should be around 0.1 Juranek and Quay 2005

8. More ratios (Southern Ocean) Hendricks et al 2004

9. Δ17O of CO2 • Increases in the stratosphere because of O exchange with photolyzed ozone, decreases toward 0 from O exchange with water in plants and the ocean • If the stratospheric contribution is known, high-precision measurements could provide an estimate of gross CO2 flux through land plants, otherwise hard to measure Hoag et al 2005

10. References Coplen, T. B., J. K. Bohlke, P. De Bievre, et al. (2002), Isotope-abundance variations of selected elements - (IUPAC Technical Report), Pure and Applied Chemistry, 74(10), 1987-2017. Hendricks, M. B., M. L. Bender and B. A. Barnett (2004), Net and gross O-2 production in the Southern Ocean from measurements of biological O-2 saturation and its triple isotope composition, Deep-Sea Res. Part I-Oceanogr. Res. Pap., 51(11), 1541-1561. Hoag, K. J., C. J. Still, I. Y. Fung, et al. (2005), Triple oxygen isotope composition of tropospheric carbon dioxide as a tracer of terrestrial gross carbon fluxes, Geophys. Res. Lett., 32(2). Juranek, L. W. and P. D. Quay (2005), In vitro and in situ gross primary and net community production in the North Pacific Subtropical Gyre using labeled and natural abundance isotopes of dissolved O-2, Global Biogeochem. Cycles, 19(3). Luz, B. and E. Barkan (2000), Assessment of oceanic productivity with the triple-isotope composition of dissolved oxygen, Science, 288(5473), 2028-2031. Thiemens, M. H. (2006), History and applications of mass-independent isotope effects, Annu. Rev. Earth Planet. Sci., 34, 217-262.