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Presented By: Earth and Environmental Sciences

Smith Lecture: Tidal Tomography: What an Often-neglected Phenomenon Known as Earth Tides Can Tell Us About Buoyancy in the Deepest Part of the Mantle

Harriet Lau, Harvard

Earth’s mantle is a key component of the Earth system: its circulation drives plate tectonics, the long-term recycling of Earth’s volatiles, and as such, holds fundamental implications for the Earth’s surface environment. In order to understand this evolution, a key parameter of the mantle must be known, namely its buoyancy. In this talk, I will discuss how Earth’s body tide can provide fresh and independent constraints on deep mantle buoyancy through a newly developed technique called Tidal Tomography. This comes at a time when other interesting and exciting data sets sensitive to deep mantle buoyancy, e.g., Stoneley modes, have been brought to bear, and we will explore our conclusions in the context of other recent finds.

In particular, we will focus on two regions of the deep mantle known as the Large Low Shear Velocity Provinces (LLSVPs), the buoyancy of which has attracted much debate over the past few decades. Using a global GPS data set of high precision measurements of the Earth’s body tide, we perform a tomographic inversion to constrain the integrated buoyancy of these LLSVPs at the base of the mantle. As a consequence of the long-wavelength and low frequency nature of the Earth’s body tide, these observations are particularly sensitivity to LLSVP buoyancy. Using a probabilistic approach we find that the data are best fit when the bottom two thirds (~700 km) of the LLSVPs have an integrated excess density of ~0.60%.
The detailed distribution of this buoyancy, for example whether it primarily resides in a thin layer at the base of the mantle, will require further testing and the augmentation of the inversions to include independent data sets (e.g., seismic observations). Nevertheless, our inference of excess density requires the preservation of chemical heterogeneity associated with the enrichment of high-density chemical components, possibly linked to subducted oceanic plates and/or primordial material, in the deep mantle. This conclusion has important implications for the stability of these structures and, in turn, the history and ongoing evolution of the Earth system.

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