Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA.
Lamont-Doherty Earth Observatory, Columbia University, New York, USA.
Nature. 2017 Nov 15;551(7680):321-326. doi: 10.1038/nature24452.
Earth's body tide-also known as the solid Earth tide, the displacement of the solid Earth's surface caused by gravitational forces from the Moon and the Sun-is sensitive to the density of the two Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific. These massive regions extend approximately 1,000 kilometres upward from the base of the mantle and their buoyancy remains actively debated within the geophysical community. Here we use tidal tomography to constrain Earth's deep-mantle buoyancy derived from Global Positioning System (GPS)-based measurements of semi-diurnal body tide deformation. Using a probabilistic approach, we show that across the bottom two-thirds of the two LLSVPs the mean density is about 0.5 per cent higher than the average mantle density across this depth range (that is, its mean buoyancy is minus 0.5 per cent), although this anomaly may be concentrated towards the very base of the mantle. We conclude that the buoyancy of these structures is dominated by the enrichment of high-density chemical components, probably related to subducted oceanic plates or primordial material associated with Earth's formation. Because the dynamics of the mantle is driven by density variations, our result has important dynamical implications for the stability of the LLSVPs and the long-term evolution of the Earth system.
地球体潮——也称为固体地球潮，是由月球和太阳的引力引起的固体地球表面的位移——对非洲和太平洋下方的两个大型低速剪切波区（LLSVPs）的密度敏感。这些大规模区域从地幔底部向上延伸约 1000 公里，其浮力在地球物理学界仍存在争议。在这里，我们使用潮汐层析成像来约束地球深部地幔的浮力，该浮力来自基于全球定位系统（GPS）的半日体潮变形的测量。使用概率方法，我们表明，在两个 LLSVPs 的底部三分之二处，平均密度比该深度范围内平均地幔密度高约 0.5%（即其平均浮力为负 0.5%），尽管这种异常可能集中在地幔的最底部。我们的结论是，这些结构的浮力主要由高密度化学成分的富集所驱动，可能与俯冲的海洋板块或与地球形成相关的原始物质有关。由于地幔的动力学是由密度变化驱动的，因此我们的结果对 LLSVPs 的稳定性和地球系统的长期演化具有重要的动力学意义。
