In response to Susan Waldron's request, & for any other interested
parties...

You might like to look at:

R. F. Keeling (1995):  "The atmospheric oxygen cycle: The oxygen isotopes of
atmospheric CO2 and O2 and the O2/N2 cycle."  In: Reviews of Geophysics,
Supplement, pages 1253-1262. U.S. National Report to International Union of
Geodesy and Geophysics 1991-1994.

This paper states that isotopic measurements of tropospheric O2 made more
recently than by Kroopnick and Craig (1972) indicate that the delta-18 O is
constant to at least 0.03 per mil world-wide. (This is a pers. comm.
citation, attributable to M. Thiemens.)  The same paper suggests that known
sources of variability are expected to produce changes of about an order of
magnitude smaller than this. The authors quote the example that the delta-18
O value would be expected to be lower in summer than winter, in both
northern and southern hemispheres, but by about only 0.002 per mil.
Difficult to detect!

With regard to variations in delta-18 O of atmospheric O2 on a larger
timescale, it would seem that knowledge is limited to variations over recent
glacial cycles and that these variations are largely consistent with a
constant Dole effect over this period. In this context, you could look at:

M. Bender and T. Sowers (1994):  "The Dole effect and its variations during
the last 130,000 years as measured in the Vostok ice core."  Global
Biogeochemical Cycles, Vol. 8, No. 3, pages 363-376.

Hope that this helps!

Regards,
Martin

> ===========================================
Martin F Miller
Planetary Sciences Research Institute,
The Open University,
Walton Hall,
Milton Keynes,
MK7 6AA
UK

Tel:  (direct) +44 1908 652011
Fax: (departmental) +44 1908 655910
E-mail:  [log in to unmask]
> ===========================================
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