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

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

3. Mass-independent fractionation Discovered in 1980s: ozone produced by O( 3 P) + O 2 + M → O 3 + M has equally high δ 17 O and δ 18 O Explanations: –Self-shielding of abundant isotopologue –Greater energy-level density for asymmetric species (e.g. Liang et al 2004)

4. δ 17 O and δ 18 O on Earth

5. Application: Ocean gross O 2 production rate Define Δ 17 O ≡ δ 17 O – 0.521∙δ 18 O: unaffected by respiration & evaporation Δ 17 O of dissolved O 2 at equilibrium with atmosphere: 16 per meg (Δ eq ) Δ 17 O of photosynthetically produced O 2 : 159 per meg (Sea of Galilee); 249 per meg (Ocean) (Δ max ) By isotope mass balance, for the surface ocean I∙Δ eq + GP∙Δ max = (E+R)∙Δ diss where –Δ diss = surface-ocean Δ 17 O –GP = gross O 2 production –I = O 2 invasion –E = O 2 evasion –R = respiratory O 2 consumption equivalently, GP = K∙C 0 ∙(Δ diss – Δ eq )/(Δ max – Δ eq ) where –K = air-sea gas transfer velocity –C 0 = equilibrium water O 2 concentration Largest uncertainty seems to be the accuracy to which K is known, no better than 30%. Luz and Barkan 2000

6. Upper-ocean Δ 17 O Juranek and Quay 2005

7. Net O 2 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. Δ 17 O of CO 2 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 CO 2 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.