Surface exposure constraints on net regassing of the deep water cycle

Abstract:

The present-day net regassing flux of water into the mantle has been estimated to be between 10^11-12 kg/yr. These values could increase the amount of water in the mantle by up to 3 oceans worth over the age of the Earth (4.6 Gyrs). Taking this much water from the oceans means, however, that the Earth’s surface must have been flooded for much of its history. This is inconsistent with the strong geological evidence for sub-aerial land from at least 1 Gyrs after Earth formation.

Here we use a parametrised convection model to assess how much water can be taken into the mantle while still satisfying surface exposure constraints. We compare our results to the present-day hypsometry and show that to match geological evidence, less than half an ocean (< 220 ppm) can be returned to the mantle. This is similar to upper mantle estimates of water content and limits average net regassing estimates to 1.5 - 2.7 x 10¹¹ kg/yr. This suggests mantle water contents in excess of half an ocean’s worth would be primordial in nature and have been isolated from the convecting mantle for most of Earth history.