Oceans on Earth are present as a result of equilibrium between degassing and regassing through the interaction of the Earth’s interior. Because of mantle convection and resultant partial melting and melt transfer, water may circulate deep into the mantle and return through Earth’s surface into the oceans.
The total water-storage capacity of the mantle is about 10× the present ocean mass. Particularly interesting is the high water-storage capacity of the Mantle Transition Zone, roughly 6× Earth’s ocean mass.
An important clue to the history of ocean mass can be obtained from sea-level history. For the time scale of 1000 – 10,000 years, the largest contribution is ice-sheet formation/melting and the resultant vertical crustal motion due to isostasy. This contributed to sea-level change by approximately 200 meters. For time scales longer than 100 million years, the influence of ice sheet formation/melting cancels when averaged over time. Over these longer periods, sea-level history is controlled by tectonic factors and/or changes in ocean mass.
The most substantial degassing process to add water to the oceans occurs at mid-ocean ridges where basaltic magmas are transported to the ocean bottom, and upon cooling some water is added to the oceans. In a new type of degassing proposed in this study, water-rich materials are transported to near-surface regions in the continental regions. It is not clear how water transported to near surface regions in the continents adds water to the oceans.
https://progearthplanetsci.springeropen.com/articles/10.1186/s40645-020-00379-3