  Observations of 12CO at high redshift indicate rapid metal enrichment in the
nuclear regions of at least some galaxies in the early universe. However, the
enrichment may be limited to nuclei that are synthesized by short-lived massive
stars, excluding classical secondary nuclei like 13CO. Testing this idea, we
tentatively detect the 13CO J=3-2 line at a level of 0.3 Jy km/s toward the
Cloverleaf Quasar at redshift 2.5. This is the first observational evidence for
13CO at high redshift. The 12CO/13CO J=3-2 luminosity ratio is with at least 40
much higher than ratios observed in molecular clouds of the Milky Way and in
the ultraluminous galaxy Arp 220, but may be similar to that observed toward
NGC 6240. Large Velocity Gradient (LVG) models simulating seven 12CO
transitions and the 13CO line yield 12CO/13CO abundance ratios in excess of 100
for the Cloverleaf. It is possible that the measured ratio is affected by a
strong submillimeter radiation field, which reduces the contrast between the
13CO line and the background. It is more likely, however, that the ratio is
caused by a real deficiency of 13CO. A potential conflict with optical data,
indicating high abundances also for secondary nuclei in quasars of high
redshift, may be settled if the bulk of the CO emission is originating
sufficiently far from the active galactic nucleus.
