  The origin of the Earth's ocean has been discussed on the basis of
deuterium/hydrogen ratios (D/H) of several sources of water in the solar
system. The average D/H of carbonaceous chondrites (CC's) is known to be close
to the current D/H of the Earth's ocean, while those of comets and the solar
nebula are larger by about a factor of two and smaller by about a factor of
seven, respectively, than that of the Earth's ocean. Thus, the main source of
the Earth's ocean has been thought to be CC's or adequate mixing of comets and
the solar nebula. However, those conclusions are correct only if D/H of water
on the Earth has remained unchanged for the past 4.5 Gyr. In this paper, we
investigate evolution of D/H in the ocean in the case that the early Earth had
a hydrogen-rich atmosphere, the existence of which is predicted by recent
theories of planet formation no matter whether the nebula remains or not. Then
we show that D/H in the ocean increases by a factor of 2-9, which is caused by
the mass fractionation during atmospheric hydrogen loss, followed by deuterium
exchange between hydrogen gas and water vapor during ocean formation. This
result suggests that the apparent similarity in D/H of water between CC's and
the current Earth's ocean does not necessarily support the CC's origin of water
and that the apparent discrepancy in D/H is not a good reason for excluding the
nebular origin of water.
