Subduction and atmospheric escape of Earth's seawater constrained by hydrogen isotopes

The hydrogen isotopic (D/H) ratio reflects the global cycling and evolution of water on Earth as it fractionates through planetary processes. We model the water cycle taking seafloor hydrothermal alteration, chemical alteration of continental crust, slab subduction, hydrogen escape from the early Earth, and degassing at mid-ocean ridges, hot spots, and arcs into account. The differences in D/H ratios between present-day oceans, oceanic and continental crust, and mantle are thought to reflect isotopic fractionation through seafloor alteration, chemical alteration, and slab dehydration. However, if the speed of plate tectonics has been nearly constant throughout Earth's history, the degassing and regassing rates are too small to reach the present-day D/H ratios. We show that (a) hydrogen escape from reduced early atmosphere, (b) secular net regassing, or (c) faster plate tectonics on early Earth is needed to reproduce the present-day D/H ratios of the water reservoirs. The low D/H ratio of Archean seawater at 3.8 Ga has previously been interpreted as a signature of (a) hydrogen escape, but we find it can also be explained either by (b) secular net degassing or by (c) faster plate tectonics on early Earth. The rates of hydrogen escape from early Earth and secular regassing on present-day Earth are constrained to be lower than 2.1×1011 kg/yr and 3.9×1011 kg/yr. Consequently, the volume of water in the present-day mantle could result entirely from the regassing through Earth's history. In that case, the volume of initial oceans could be 2 to 3 times larger than that of current Earth. We suggest that, in addition to the D/H ratio of Archean seawater, identifying the D/H ratios of both seawater and mantle throughout Earth's history would allow to distinguish these evolutionary scenarios.