  We compute the ozone distribution for a model terrestrial paleoatmosphere in
which the present oxygen abundance is largely replaced by carbon dioxide, which
we argue is a reasonable working assumption. In principle, the presence of
carbon dioxide might supplement the ozone shield as compared with models based
on nitrogen without high carbon dioxide abundance so that early life need not
have been as UV-resistant as often assumed. An extrasolar planet with a
high-CO2 atmosphere might contain enough O3 to be a source of false positive
biomarkers. We find that the globally averaged O3 column density can be the
same, or nearly four times higher (depending upon the O2 partial pressure) when
CO2 is used in place of N2 as the replacement component for lowered O2 in a
1-atm terrestrial planet with solar radiation. The effect is important for
making quantitative deductions from future data, but does not invalidate the
use of O3 as a biomarker for free oxygen. These results make prospects for
detection of extrasolar planetary O3 absorption somewhat better than before.
