We have compiled all previous ultralow velocity zone (ULVZ) studies, and digitized their core‐mantle boundary (CMB) sampling locations. For studies that presented sampling locations based on infinite frequency ray theory, we approximated Fresnel zones onto a 0.5° × 0.5° grid. Results for these studies were separated according to wave type: (1) core‐reflected phases, which have a single location of ULVZ sampling (ScS, ScP, PcP), (2) core waves that can sample ULVZs at the core entrance and exit locations of the wave (e.g., SPdKS, PKKP, and PKP), and (3) waves which have uncertainties of ULVZ location due to long CMB sampling paths, e.g., diffracted energy sampling over a broad region (Pdiff, Sdiff). For studies that presented specific modeled ULVZ geographical shapes or PKP scatter probability maps, we digitized the regions. We present summary maps of the ULVZ coverage, as well as published locations arguing against ULVZ presence. A key finding is that there is not a simple mapping between lowermost mantle reduced tomographic velocities and observed ULVZ locations, especially given the presence of ULVZs outside of lowermost mantle large low velocity provinces (LLVPs). Significant location uncertainty exists for some of the ULVZ imaging wave types. Nonetheless, this compilation supports a compositionally distinct origin for at least some ULVZs. ULVZs are more likely to be found near LLVP boundaries, however, their relationship to overlying surface locations of hot spots are less obvious. The new digital ULVZ database is freely available for download.

The Earth’s lowermost mantle large low velocity provinces are accompanied by small-scale ultralow velocity zones in localized regions on the core-mantle boundary. Large low velocity provinces are hypothesized to be caused by large-scale compositional heterogeneity (i.e., thermochemical piles). The origin of ultralow velocity zones, however, remains elusive. Here we perform three-dimensional geodynamical calculations to show that the current locations and shapes of ultralow velocity zones are related to their cause. We find that the hottest lowermost mantle regions are commonly located well within the interiors of thermochemical piles. In contrast, accumulations of ultradense compositionally distinct material occur as discontinuous patches along the margins of thermochemical piles and have asymmetrical cross-sectional shape. Furthermore, the lateral morphology of these patches provides insight into mantle flow directions and long-term stability. The global distribution and large variations of morphology of ultralow velocity zones validate a compositionally distinct origin for most ultralow velocity zones.

Thin patches with ultralow velocities have been proposed to exist at the core‐mantle boundary (CMB). The detection and mapping of ultralow velocity zones (ULVZs) are difficult, in part, because of limited source‐receiver geometries of seismic phases used in ULVZ modeling. Here we develop a new approach that simultaneously utilizes ScS precursor and postcursor energies to investigate the CMB region for ULVZ structure. We stacked source‐deconvolved ScS waveforms within 1.5° geographic bins to extract ScS precursor and postcursor energies, if present, with ScS effectively removed from waveforms. We investigate the CMB beneath the central Pacific Ocean, and evidence for ULVZs is clearly apparent. Geographic bin stacks possessing similar ScS precursor‐plus‐postcursor behavior are grouped by using cluster analysis to produce more robust waveforms by enhancing the signal‐to‐noise ratios. Synthetic seismograms that demonstrate the amplitude and timing of the ULVZ arrivals are sensitive to ULVZ thickness and internal velocities. To pursue local ULVZ properties we processed 13,850 1‐D synthetic models with various ULVZ thicknesses and internal properties, using the identical ScS‐stripping method as with the data. A best fitting model was found for each geographical bin cluster by using an amplitude‐sensitive cross‐correlation algorithm. While limitations exist due to 1‐D modeling, strong lateral variations are clearly apparent in ULVZ thickness and properties across the large low shear velocity province (LLSVP) margin in our study area. Inside hypothesized LLSVP edges, ULVZs appear to distribute unevenly, suggesting 3‐D variations of convection currents.

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The carbon and water cycles in the Earth's interior are linked to key planetary processes, such as mantle melting, degassing, chemical differentiation, and advection. However, the role of water in the carbon exchange between the mantle and core is not well known. Here, we show experimental results of a reaction between Fe3C and H2O at pressures and temperatures of the deep mantle and core‐mantle boundary (CMB). The reaction produces diamond, FeO, and FeHx, suggesting that water can liberate carbon from the core in the form of diamond (“core carbon extraction”) while the core gains hydrogen, if subducted water reaches to the CMB. Therefore, Earth's deep water and carbon cycles can be linked. The extracted core carbon can explain a significant amount of the present‐day mantle carbon. Also, if diamond can be collected by mantle flow in the region, it can result in unusually high seismic‐velocity structures.