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CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula


The abundances of oxygen isotopes in the most refractory mineral phases (calcium-aluminium-rich inclusions, CAIs) in meteorites1 have hitherto defied explanation. Most processes fractionate isotopes by nuclear mass; that is, 18O is twice as fractionated as 17O, relative to 16O. In CAIs 17O and 18O are nearly equally fractionated, implying a fundamentally different mechanism. The CAI data were originally interpreted as evidence for supernova input of pure 16O into the solar nebula1, but the lack of a similar isotope trend in other elements argues against this explanation2. A symmetry-dependent fractionation mechanism3,4 may have occurred in the inner solar nebula5, but experimental evidence is lacking. Isotope-selective photodissociation of CO in the innermost solar nebula6 might explain the CAI data, but the high temperatures in this region would have rapidly erased the signature7. Here we report time-dependent calculations of CO photodissociation in the cooler surface region of a turbulent nebula. If the surface were irradiated by a far-ultraviolet flux 103 times that of the local interstellar medium (for example, owing to an O or B star within 1 pc of the protosun), then substantial fractionation of the oxygen isotopes was possible on a timescale of 105 years. We predict that similarly irradiated protoplanetary disks will have H2O enriched in 17O and 18O by several tens of per cent relative to CO.

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Figure 1: Model results for the time evolution of molecular abundances and isotope ratios at the nebula midplane at a heliocentric distance of 30  au.
Figure 2: The time evolution of the non-mass-dependent oxygen isotope component of total nebular H 2 O for a range of FUV flux enhancement factors.
Figure 3: Three-isotope plot (δ 17 O SMOW versus δ 18 O SMOW ) of total nebular H 2 O at the midplane, with time labelled along the trajectory.
Figure 4: Three-isotope plot of total nebular H2O in the model, including an estimate of wavelength-dependent absorption by H2.


  1. Clayton, R. N., Grossman, L. & Mayeda, T. K. Component of primitive nuclear composition in carbonaceous meteorites. Science 182, 485–488 (1973)

    Article  ADS  CAS  Google Scholar 

  2. Clayton, R. N. Oxygen isotopes in meteorites. Annu. Rev. Earth Planet. Sci. 21, 115–149 (1993)

    Article  ADS  CAS  Google Scholar 

  3. Thiemens, M. H. & Heidenreich, H. E. Mass independent fractionation of oxygen: a novel isotope effect and its possible cosmochemical implications. Science 219, 1073–1075 (1983)

    Article  ADS  CAS  Google Scholar 

  4. Gao, Y. Q. & Marcus, R. A. Strange and unconventional isotope effects in ozone formation. Science 293, 259–263 (2001)

    Article  ADS  CAS  Google Scholar 

  5. Marcus, R. A. Mass-independent isotope effect in the earliest processed solids in the solar system: A possible chemical mechanism. J. Chem. Phys. 121, 8201–8211 (2004)

    Article  ADS  CAS  Google Scholar 

  6. Clayton, R. N. Self-shielding in the solar nebula. Nature 415, 860–861 (2002)

    Article  ADS  CAS  Google Scholar 

  7. Lyons, J. R. & Young, E. D. Towards an evaluation of self-shielding at the X-point as the origin of the oxygen isotope anomaly in CAIs. Lunar Planet. Sci. Conf. XXXIV, abstr. 1981 (2003)

  8. Bally, J. & Langer, W. D. Isotope-selective photodissociation of carbon monoxide. Astrophys. J. 255, 143–148 (1982)

    Article  ADS  CAS  Google Scholar 

  9. Federman, S. R. et al. Further evidence for chemical fractionation from ultraviolet observations of carbon monoxide. Astrophys. J. 591, 986–999 (2003)

    Article  ADS  CAS  Google Scholar 

  10. van Dishoeck, E. F. & Black, J. H. The photodissociation and chemistry of interstellar CO. Astrophys. J. 334, 771–802 (1988)

    Article  ADS  CAS  Google Scholar 

  11. Yurimoto, H. & Kuramoto, K. Molecular cloud origin for the oxygen isotope heterogeneity in the solar system. Science 305, 1763–1766 (2004)

    Article  ADS  CAS  Google Scholar 

  12. Lyons, J. R. & Young, E. D. Evolution of oxygen isotopes in the solar nebula. Lunar Planet. Sci. Conf. XXXV, abstr. 1970 (2004)

  13. Aikawa, Y. & Herbst, E. Two-dimensional distributions and column densities of gaseous molecules in protoplanetary disks. Astron. Astrophys. 371, 1107–1117 (2001)

    Article  ADS  CAS  Google Scholar 

  14. Allende Prieto, C., Lambert, D. L. & Asplund, M. A reappraisal of the solar photospheric C/O ratio. Astrophys. J. 573, L137–L140 (2002)

    Article  ADS  CAS  Google Scholar 

  15. Le Teuff, Y. H., Millar, T. J. & Markwick, A. J. The UMIST database for astrochemistry 1999. Astron. Astrophys. Suppl. Ser. 146, 157–168 (2000)

    Article  ADS  CAS  Google Scholar 

  16. Hasegawa, T. I., Herbst, E. & Leung, C. M. Models of gas-grain chemistry in dense interstellar clouds with complex organic molecules. Astrophys. J. 82, 167–195 (1992)

    Article  ADS  CAS  Google Scholar 

  17. Willacy, K., Klahr, H. H., Millar, T. J. & Henning, Th. Gas and grain chemistry in a protoplanetary disk. Astron. Astrophys. 338, 995–1005 (1998)

    ADS  CAS  Google Scholar 

  18. van Zadelhoff, G. J., Aikawa, Y., Hogerheijde, M. R. & van Dishoeck, E. F. Axi-symmetric models of ultraviolet radiative transfer with applications to circumstellar disk chemistry. Astron. Astrophys. 397, 789–802 (2003)

    Article  ADS  CAS  Google Scholar 

  19. Markwick, A. J., Ilgner, M., Millar, T. J. & Henning, Th. Molecular distributions in the inner regions of protostellar disks. Astron. Astrophys. 385, 632–646 (2002)

    Article  ADS  CAS  Google Scholar 

  20. Stone, J. M., Gammie, C. F., Balbus, S. A. & Hawley, J. F. in Protostars and Planets IV (eds Manning, V., Boss, A. P. & Russell, S. S.) 589–611 (Univ. Arizona Press, Tucson, 2000)

    Google Scholar 

  21. Lee, H. H., Herbst, E., Pineau des Forêts, G., Roueff, E. & Le Bourlot, J. Photodissociation of H2 and CO and time dependent chemistry in inhomogeneous interstellar clouds. Astron. Astrophys. 311, 690–707 (1996)

    ADS  CAS  Google Scholar 

  22. Adams, F. C., Hollenbach, D., Laughlin, G. & Gorti, U. Photoevaporation of circumstellar disks due to external far-ultraviolet radiation in stellar aggregates. Astrophys. J. 611, 360–379 (2004)

    Article  ADS  CAS  Google Scholar 

  23. Young, E. D. & Russell, S. S. Oxygen reservoirs in the early solar nebula inferred from an Allende CAI. Science 282, 452–455 (1998)

    Article  ADS  CAS  Google Scholar 

  24. Cuzzi, J. N. & Zahnle, K. J. Material enhancement in protoplanetary nebulae by particle drift through evaporation fronts. Astrophys. J. 614, 490–496 (2004)

    Article  ADS  Google Scholar 

  25. Gammie, C. F. Layered accretion in T Tauri disks. Astrophys. J. 457, 355–362 (1996)

    Article  ADS  Google Scholar 

  26. Cuzzi, J. N., Dobrovolskis, A. R. & Hogan, R. C. in Chondrules and the Protoplanetary Disk (eds Hewins, R. H., Jones, R. H. & Scott, E. R. D.) 35–43 (Cambridge Univ. Press, Cambridge, 1996)

    Google Scholar 

  27. Clayton, R. N. & Mayeda, T. K. The oxygen isotope record in Murchison and other carbonaceous chondrites. Earth Planet. Sci. Lett. 67, 151–161 (1984)

    Article  ADS  CAS  Google Scholar 

  28. Choi, B. G., Krot, A. N., McKeegan, K. D. & Wasson, J. T. Extreme oxygen-isotope compositions in magnetite from unequilibrated ordinary chondrites. Nature 392, 577–579 (1998)

    Article  ADS  CAS  Google Scholar 

  29. Young, E. D. The hydrology of carbonaceous chondrite parent bodies and the evolution of planet progenitors. Phil. Trans. R. Soc. Lond. A 359, 2095–2110 (2001)

    Article  ADS  CAS  Google Scholar 

  30. Navon, O. & Wasserburg, G. J. Self-shielding in O2: a possible explanation for oxygen isotope anomalies in meteorites? Earth Planet. Sci. Lett. 73, 1–16 (1985)

    Article  ADS  CAS  Google Scholar 

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J.R.L. thanks K. McKeegan, J. Cuzzi and A. Boss for discussions, and P. Plavchan for assistance with IDL. J.R.L. acknowledges funding from the NASA Origins Program and from the UCLA Center for Astrobiology. E.D.Y. acknowledges support from the UCLA Center for Astrobiology.Author Contributions J.R.L. conceived and carried out the calculations presented here, using a modified version of a photochemical code provided by J. Kasting. The paper was written by J.R.L. and E.D.Y.

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Supplementary Methods

A detailed description of the mathematics used in this work, including the following: the 1-D continuity equation for chemical volume fractions; the photodissociation loss rates for CO isotopologues, including the shielding functions; the power law expressions for the CO shielding functions including the correction terms to account for H2 absorption; the calculation of oxygen isotope δ-values. (DOC 85 kb)

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Lyons, J., Young, E. CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula. Nature 435, 317–320 (2005).

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