The hydrogen isotope composition of seawater and the global water cycle

Long-term evolution of the Earth's water cycle is investigated to predict potential variations in the hydrogen stable isotope composition of seawater. Mass balance calculations are used to estimate the delta D value of the early ocean before storage of water (about 20% of the present-day size) in the biosphere, cryosphere, sediments, and metamorphic rocks. The early ocean was plausibly deuterium-depleted (delta D = -18 +/- 6 parts per thousand) in comparison with the present-day oceans (delta D = 0 parts per thousand). A kinetic treatment of the long-term water cycle suggests that hydrogen isotope variations of the oceans may have occurred at a Ga time-scale in response to the imbalance between fluxes of water trapped at ridges and released along subduction zones. Two limiting cases are observed: (1) the delta D value of the oceans does not exceed a value of + 10 parts per thousand when the oceanic mass decreases by 20%; and (2) the delta D value decreases down to -20 parts per thousand for a 20% mass increase of the oceans. An increase in the delta D value of the planet via an addition of extraterrestrial water is restricted to 7 parts per thousand since 3.5 Ga. The present-day mean D/H ratio of the bulk Earth is calculated to be 149(+/-3) x 10(-6). Since the statistical distribution of the D/H ratios in carbonaceous chondrites exhibits a maximum value around 140 +/- 10 x 10(-6); it is unlikely that the water D/H ratio was significantly fractionated during Earth's accretion relatively to the protosolar water ratio. (C) 1998 Elsevier Science B.V. All rights resented.