Oceans on Earth are present as a result of dynamic equilibrium between degassing and regassing through the interaction with Earth’s interior. We review mineral physics, geophysical, and geochemical studies related to the global water circulation and conclude that the water content has a peak in the mantle transition zone (MTZ) with a value of 0.1–1 wt% (with large regional variations). When water-rich MTZ materials are transported out of the MTZ, partial melting occurs. Vertical direction of melt migration is determined by the density contrast between the melts and coexisting minerals. Because a density change associated with a phase transformation occurs sharply for a solid but more gradually for a melt, melts formed above the phase transformation depth are generally heavier than solids, whereas melts formed below the transformation depth are lighter than solids. Consequently, hydrous melts formed either above or below the MTZ return to the MTZ, maintaining its high water content. However, the MTZ water content cannot increase without limit. The melt-solid density contrast above the 410 km depends on the temperature. In cooler regions, melting will occur only in the presence of very water-rich materials. Melts produced in these regions have high water content and hence can be buoyant above the 410 km, removing water from the MTZ. Consequently, cooler regions of melting act as a water valve to maintain the water content of the MTZ near its threshold level (~ 0.1–1.0 wt%). Mass-balance considerations explain the observed near-constant sea-level despite large fluctuations over Earth history. Observations suggesting deep-mantle melting are reviewed including the presence of low-velocity anomalies just above and below the MTZ and geochemical evidence for hydrous melts formed in the MTZ. However, the interpretation of long-term sea-level change and the role of deep mantle melting in the global water circulation are non-unique and alternative models are reviewed. Possible future directions of studies on the global water circulation are proposed including geodynamic modeling, mineral physics and observational studies, and studies integrating results from different disciplines.

地球上的海洋是通过与地球内部相互作用而进行脱气和加气之间动态平衡的结果。我们回顾了与全球水循环有关的矿物物理学，地球物理和地球化学研究，得出的结论是，地幔过渡带（MTZ）中的水含量达到峰值，值为0.1–1 wt％（区域差异很大）。当富水的MTZ材料从MTZ运出时，会发生部分熔化。熔体迁移的垂直方向取决于熔体与共存矿物之间的密度对比。由于与相变相关的密度变化对于固体而言会急剧发生，而对于熔体而言则更为缓慢，因此在相变深度以上形成的熔体通常比固体重，而在相变深度以下形成的熔体比固体轻。因此，在MTZ上方或下方形成的含水熔体返回MTZ，从而保持其高水含量。但是，MTZ的含水量不能无限增加。410 km以上的熔体密度对比取决于温度。在较凉的地区，熔化只会在水含量非常高的材料存在下发生。这些地区产生的熔体水含量高，因此在410 km以上可以漂浮，从MTZ中除去水。因此，较冷的融化区域充当水阀，以保持MTZ的水含量接近其阈值水平（〜0.1–1.0 wt％）。尽管在地球历史上发生了很大的波动，但质量平衡的考虑仍然可以解释观测到的近恒定海平面。回顾了表明深地幔融化的观测结果，包括在MTZ上方和下方存在低速异常以及在MTZ中形成含水熔体的地球化学证据。但是，长期海平面变化的解释以及深层地幔融化在全球水循环中的作用是不唯一的，因此对替代模型进行了综述。提出了关于全球水循环研究的未来可能方向，包括地球动力学模型，矿物物理和观测研究，以及将不同学科的研究成果进行整合的研究。长期海平面变化的解释和深层地幔融化在全球水循环中的作用是不唯一的，并回顾了替代模型。提出了关于全球水循环研究的未来可能方向，包括地球动力学模型，矿物物理和观测研究，以及将不同学科的研究成果进行整合的研究。长期海平面变化的解释以及深层地幔融化在全球水循环中的作用是不唯一的，并回顾了替代模型。提出了关于全球水循环研究的未来可能方向，包括地球动力学模型，矿物物理和观测研究，以及将不同学科的研究成果整合起来的研究。