なぜ地球の海水量は安定なのか？：マントル深部での部分融解の効果

抄録

In order for life to emerge and evolve, ocean must be present on a planet for billions of years. Observed sea-level change shows that the sea-level fluctuates a lot, but on average (over several 100 Mys) it is stable: after a rapid reduction (<100 Mys),the sea-level recovered quickly leading to the stable ocean mass. However, the cause for the stable ocean mass has been unclear because ocean mass is controlled by dynamic equilibrium rather than static equilibrium. A model is developed to explain the stability of the ocean mass considering both degassing and regassing. It is shown that degassing at mid-ocean ridges is too inefficient to stabilize the ocean mass after the reduction of the ocean mass. It is suggested that the deep mantle melting (across the transition zone) provides a key role (i) to maintain the high water content in the transition zone, and (ii) to stabilize the ocean mass via the water leak from transition zone in cold regions. The area of cold regions needed to stabilize the ocean mass is estimated to be ～2×10¹⁰ m² that corresponds to a few volcanoes such as Chanbaishan (this review is based on Karato et al., 2020).