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

TL;DR

This paper explores how early Earth's hydrogen-rich atmosphere could have altered the D/H ratio in water, challenging previous assumptions about the ocean's origin based on static D/H values.

Contribution

It demonstrates that atmospheric hydrogen loss and exchange processes can significantly increase D/H ratios, affecting interpretations of Earth's water sources.

Findings

D/H in Earth's water can increase by a factor of 2-9 due to atmospheric processes.

The similarity in D/H ratios does not necessarily indicate a common origin of water.

The nebular origin of Earth's water remains plausible despite D/H ratio differences.

Abstract

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.