Abstract
The presence of liquid water makes our planet unique. Its budget over geological timescales, i.e., the long-term global sea level, depends on the balance of water exchanges between the Earth's mantle and the surface through both volcanism (mantle degassing) and subduction of hydrous minerals (mantle regassing). Current estimates of subduction water fluxes predict that regassing exceeds degassing by 50%, thereby suggesting a sea-level drop of several hundred meters in the last 540 Ma. The models further suggest that the subducting crust is the main supplier of water to the deep mantle. In contrast, various observations advocate for a near-steady state long term sea level and report voluminous water subduction via the hydrated lithospheric mantle. We have revised the subduction water flux calculations using constraints from recent experimental data on natural peridotites under high-pressure and high-temperature conditions. Our novel thermopetrological models show that the present-day global water retention in subducting plates beyond mid-upper mantle depths barely exceeds the estimations of mantle degassing, and thus quantitatively support the steady-state sea level scenario over geological times. Furthermore, the limited mantle regassing is solely driven by the lithospheric mantle of the coldest subduction zones.