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The origin of the Earth's ocean has been discussed on the basis ofdeuterium/hydrogen ratios (D/H) of several sources of water in the solarsystem. The average D/H of carbonaceous chondrites (CC's) is known to be closeto the current D/H of the Earth's ocean, while those of comets and the solarnebula are larger by about a factor of two and smaller by about a factor ofseven, respectively, than that of the Earth's ocean. Thus, the main source ofthe Earth's ocean has been thought to be CC's or adequate mixing of comets andthe solar nebula. However, those conclusions are correct only if D/H of wateron the Earth has remained unchanged for the past 4.5 Gyr. In this paper, weinvestigate evolution of D/H in the ocean in the case that the early Earth hada hydrogen-rich atmosphere, the existence of which is predicted by recenttheories of planet formation no matter whether the nebula remains or not. Thenwe show that D/H in the ocean increases by a factor of 2-9, which is caused bythe mass fractionation during atmospheric hydrogen loss, followed by deuteriumexchange between hydrogen gas and water vapor during ocean formation. Thisresult suggests that the apparent similarity in D/H of water between CC's andthe current Earth's ocean does not necessarily support the CC's origin of waterand that the apparent discrepancy in D/H is not a good reason for excluding thenebular origin of water.

Origin of the Ocean on the Earth: Early Evolution of Water D/H in a Hydrogen-rich Atmosphere