THE HYDROGEN ISOTOPE COMPOSITION OF SEAWATER AND THE GLOBAL WATER CYCLE

Long-term evolution of the Earth's water cycle is investigated to pred ict 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 pre sent-day size) in the biosphere, cryosphere, sediments, and metamorphi c 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-ter m water cycle suggests that hydrogen isotope variations of the oceans may have occurred at a Ga time-scale in response to the imbalance betw een fluxes of water trapped at ridges and released along subduction zo nes. Two limiting cases are observed: (1) the delta D value of the oce ans does not exceed a value of + 10 parts per thousand when the oceani c 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 increa se in the delta D value of the planet via an addition of extraterrestr ial water is restricted to 7 parts per thousand since 3.5 Ga. The pres ent-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 car bonaceous chondrites exhibits a maximum value around 140 +/- 10 x 10(- 6); it is unlikely that the water D/H ratio was significantly fraction ated during Earth's accretion relatively to the protosolar water ratio . (C) 1998 Elsevier Science B.V. All rights resented.