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Ultra-low velocity zones (ULVZs) are localized small-scale patches with extreme physical properties at the core-mantle boundary that often gather at the margins of Large Low Velocity Provinces (LLVPs). Recent studies have discovered several mega-sized ULVZs with a lateral dimension of ~900 km. However, the detailed structures and physical properties of these ULVZs and their relationship to LLVP edges are not well constrained and their formation mechanisms are poorly understood. Here, we break the degeneracy between the size and velocity perturbation of a ULVZ using two orthogonal seismic ray paths, and thereby discover a mega-sized ULVZ at the northern edge of the Pacific LLVP. The ULVZ is almost double the size of a previously imaged ULVZ in this region, but with half of the shear velocity reduction. This mega-sized ULVZ has accumulated due to stable mantle flow converging at the LLVP edge driven by slab-debris in the lower mantle. Such flow also develops the subvertical north-tilting edge of the Pacific LLVP. Ultra-low velocity zones (ULVZs) are localized small-scale patches with extreme physical properties at the core-mantle boundary. Here, the authors discover a mega-sized ULVZ (1,500 × 900 km) at the northern edge of the Pacific Large Low Velocity Province. [ABSTRACT FROM AUTHOR]

Liquid-liquid immiscibility has been widely observed in iron alloy systems at ambient pressure and is important for the structure and dynamics in iron cores of rocky planets. While such previously known liquid immiscibility has been demonstrated to disappear at relatively low pressures, here we report immiscible S(±Si,O)-rich liquid and H(±C)-rich liquid above ~20 GPa, corresponding to conditions of the Martian core. Mars' cosmochemically estimated core composition is likely in the miscibility gap, and the separation of two immiscible liquids could have driven core convection and stable stratification, which explains the formation and termination of the Martian planetary magnetic field. In addition, we observed liquid immiscibility in Fe-S-H(±Si,O,C) at least to 118 GPa, suggesting that it can occur in the Earth's topmost outer core and form a low-velocity layer below the core-mantle boundary. Yokoo et al. find the liquid immiscibility between H-rich and S-rich liquids Fe above 20 GPa. The separation of immiscible liquids could explain the disappearance of Mars' magnetic field and the formation of low-velocity layer atop the Earth's core. [ABSTRACT FROM AUTHOR]

Using density functional theory electronic structure calculations, the equation of state, thermodynamic and elastic properties, and sound wave velocities of Fe3S at pressures up to 250 GPa have been determined. Fe3S is found to be ferromagnetic at ambient conditions but becomes non-magnetic at pressures above 50 GPa. This magnetic transition changes the c/a ratio leading to more isotropic compressibility, and discontinuities in elastic constants and isotropic sound velocities. Thermal expansion, heat capacity, and Grüneisen parameters are calculated at high pressures and elevated temperatures using the quasiharmonic approximation. We estimate Fe-Fe and Fe-S force constants, which vary with Fe environment, as well as the 56Fe/54Fe equilibrium reduced partition function in Fe3S and compare these results with recently reported experimental values. Finally, our calculations under the conditions of the Earth's inner core allow us to estimate a S content of 2.7 wt% S, assuming the only components of the inner core are Fe and Fe3S, a linear variation of elastic properties between end-members Fe and Fe3S, and that Fe3S is kinetically stable. Possible consequences for the core-mantle boundary of Mars are also discussed. [ABSTRACT FROM AUTHOR]

We conducted in situ high‐pressure and ‐temperature X‐ray diffraction measurements of a pyrolitic mantle material up to 4480 K at 122–166 GPa in a laser‐heated diamond anvil cell. Results demonstrate that the phase transition between bridgmanite and post‐perovskite occurs in pyrolite within the lowermost mantle pressure range even at >4000 K. It suggests the ubiquitous occurrence of post‐perovskite above the core‐mantle boundary, which may be consistent with recent high‐quality seismology data that non‐observations of D" reflections are exceptions. Combining with earlier experiments performed at and below the normal lower‐mantle geotherm, our data show that the bridgmanite + post‐perovskite two‐phase region is ∼5 GPa thick and the dP/dT slope of the boundary is +6.5 ± 2.2 MPa/K, slightly smaller than previous theoretical calculations in MgSiO3. The global presence of rheologically weak post‐perovskite at the bottom of the mantle has profound implications in seismology, geodynamics, and heat transfer from the core. Plain Language Summary: (Al, Fe)‐bearing MgSiO3 bridgmanite is a predominant mineral in the lower mantle. While bridgmanite with the MgSiO3 end‐member composition is known to undergo a phase transition to post‐perovskite at lowermost mantle pressures, the pressure, and thickness of the phase boundary in a typical mantle material (pyrolite) have been controversial. The present synchrotron X‐ray diffraction measurements of pyrolite performed up to 4480 K around the core‐mantle boundary (CMB) pressure show that the bridgmanite/post‐perovskite phase transition takes place within the lowermost mantle pressure range even under high temperatures of the CMB region. The bridgmanite + post‐perovskite two‐phase region is found to be about 90 km thick. These results suggest the global presence of post‐perovskite above the CMB, which is consistent with recent seismological observations of D" reflectors not only in the circum‐Pacific high velocity regions but also in many areas away from subduction zones. Post‐perovskite is rheologically weak, and its ubiquitous occurrence in the lowermost mantle has important seismological and geodynamical implications. The large positive dP/dT slope (+6.5 ± 2.2 MPa/K) of the boundary suggests that a phase transition from post‐perovskite to bridgmanite assists upwelling of plumes from hot regions above the CMB. Key Points: We conducted synchrotron X‐ray diffraction measurements of a pyrolitic mantle material up to 4480 K at 122–166 GPa in a laser‐heated diamond anvil cellPhase transition between bridgmanite and post‐perovskite occurs in pyrolite within the lowermost mantle pressure range even at >4000 KUbiquitous occurrence of post‐perovskite above the core‐mantle boundary is consistent with recent high‐quality seismological observations of D" reflections [ABSTRACT FROM AUTHOR]

Light elements in Earth's core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron-electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ~100 to 110 Wm21K21 for liquid Fe-9Si near the topmost outer core. If Earth's core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core-mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core-mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core. [ABSTRACT FROM AUTHOR]

We analyzed the impacts of data span on trend estimates using Earth's long-term polar motion time series, 1846-present, and using methodologies including singular spectrum analysis, and Panteleev's filter to mitigate the time series containing transient signals. Our results show that the fluctuations of the mean rotational pole position, the Markowitz wobble, cannot be fully explained by the oceanic and atmospheric excitations. However, there exists plausible similarity with the variations of amplitudes of the Chandler wobble. To explain the abrupt deviation of the mean pole from the previous state after year 2000, we first compute Earth rotation excitations, using the temporal variations of the second-degree Stokes coefficients, C 21 , S 21 , estimated from GRACE, GRACE Follow-On and Satellite Laser Ranging (SLR), 2002–2021. We then compare their trend estimates with that of the Earth's polar motion, and conclude that the drift of the pole is consistent with the climate-induced mass redistributions within the Earth system during the past two decades. However, the observed trend is not in exact agreement with the prediction values using contemporary glacial isostatic adjustment (GIA) process forward models. The analysis of the J 2 variations since 1976 from SLR and the corresponding length of day (LOD) changes, reveals a clear trend reversal around the year 2000. However, the observed J 2 variations can only explain ∼ 5 % of the long-term LOD changes. The remaining decadal signal in the LOD, usually accounted for by the angular momentum exchange at the core-mantle boundary, is observed to be anti-correlated with the Earth surface temperature anomaly. The geophysical explanation on these relationships remains elusive, and necessitates future studies. [ABSTRACT FROM AUTHOR]

Detection of chemical signatures from the core‐mantle boundary (CMB) could provide an unprecedented glimpse into our planet's deep interior and ancient past. Several isotopic and elemental anomalies in ocean island basalts (OIBs) have been proposed as core tracers. However, the process(es) by which particular chemical signatures from the core are conveyed into the mantle remain uncertain. Here, we propose a hybrid mechanism that results from a collaborative feedback between dynamic topography, porous infiltration of liquid metal into submerged rock, gravitational collapse of weakened metal‐silicate mush, and draw‐down of additional rocks from above in the induced small‐scale mantle circulation. Using a mantle convection model coupled to gravitational spreading of a thin layer, we show that induced mantle circulation due to the gravitational collapse of the layer becomes comparable to buoyancy‐driven mantle flow when the viscosity of the mushy layer is reduced to values ∼105 times smaller than the overlying mantle. Plain Language Summary: The core and mantle may be able to exchange matter by the build up of inverted mountains and valleys at their boundary, and the erosion of this terrain driven by gravity can significantly enhance mantle circulation through this region, allowing metals and rocks to mix more extensively than previously thought. Key Points: Mixing in a metal‐rock mushy layer offers a promising mechanism to explain some geochemical observations linked to core‐mantle interactionA mushy layer produced by core‐mantle boundary topography may become weak and collapse due to gravity, enhancing mantle circulationOur models show that this "soft CMB" mechanism becomes dominant for viscosity contrasts of 105 or more, influencing deep mantle dynamics [ABSTRACT FROM AUTHOR]

Pyrite corona nodules from the ∼2.7 Ga Kapai Slate, a thin, sulfidic carbonaceous shale horizon interbedded with basaltic lava flows in the Yilgarn Craton, Western Australia, have concentric compositional and isotopic zoning with distinctive textural differences between cores and mantles. The sieved-textured cores are enriched in highly compatible trace elements, incl. Ni, As, Ag, Te, Sb, Bi and Pb, and depleted in incompatible Mo and Tl, whereas the radiating-textured mantles are strongly depleted in compatible elements and enriched in incompatible elements, relative to the cores. A striking feature of the data is that both the compatible and incompatible elements are linearly correlated, with correlation coefficients as high as 0.99. A marked drop in the concentration of compatible elements and an increase in incompatible elements at the core-mantle boundary is attributed to a sudden change in the rate of growth of the nodules produced by eruption of the voluminous overlying Paringa Basalt. The weight of the basalt produced sudden compaction of the unconsolidated clays below resulting in upward advection of pore fluid, which thinned the boundary layer around the growing nodules, leading to a marked increase in the rate of pyrite growth. Rapid pyrite growth led to a dramatic depletion in highly compatible elements, and to a build-up in incompatible elements, in the boundary layers around the growing nodule mantles, which resulted in extreme depletion of compatible elements, and enrichment in incompatible elements in the nodule mantles, relative to the cores. The corona nodules are also isotopically zoned with cores that have higher δ34S, with small positive Δ33S values, and mantles that have lower δ34S and higher Δ33S. The increase in Δ33S towards the rims is attributed to S 8 being advected to the growing mantles by upward fluid movement during sudden compaction, and the decrease in δ34S to the lighter S isotope, with its higher reactivity and diffusivity, being preferentially incorporated into the fast growing pyrite mantle. The extreme changes in the growth rates of the Kapai Slate corona pyrite nodules provide a new constraint on the partition coefficients of the trace elements between Archean ocean water and sedimentary pyrite. The compatibility of the analysed trace elements decreases in the order Bi > Te > Sb > Ag > Cu > Pb > Ni ≈ As > (Co, Zn, Se, Cd, Mn, W) > Tl > Mo, which is consistent with the order obtained from modern sedimentary pyrites by Large et al. (2014), except for the redox-sensitive elements Mn, Tl and Mo. These differences are attributed to the lower oxygen content of the Archean atmosphere and oceans. [ABSTRACT FROM AUTHOR]

The spatial relationships of the lower mantle D" layer elements with Phanerozoic Large Igneous Provinces (LIPs), as well as with the young hotspots and kimberlites in the African region, are discussed. Similar relationships are also shown for the Cape Karoo and Parana extended basins, which had developed as inherited structures along the marginal parts of the region since the Early Paleozoic. The specific locations are marked for all these superficial structures and plumes producing them. These locations indicate the connection between these structures origin and the domains where the Earth's core, Large Low-Velocity Provinces (LLSVPs), and slab graveyards contact with each other. The relationships between plume- and plate-tectonic processes in the Earth's evolution are considered. Constant interaction between the Pacific and Indo-Atlantic segments of the Earth is revealed in the course of these processes. The Pacific segment was characterized by ocean formation during the Phanerozoic time and subsequent subduction under the continental landmasses of Indo-Atlantic segment. The constant character of spreading in the Pacific Ocean was obviously determined by the presence of its direct connection with the Pacific LLSVP. Within the D" layer in the areas of contact between the Pacific subducting slabs and African LLSVP, plume generation occurred periodically, subsequently leading to supercontinental breakup. This determined the specific character of the Indo-Atlantic segment development: the alternation of convergence and continental breakup during the Phanerozoic. In general, despite the significant similarities between the antipodal African and Pacific LLSVPs, their role in formation of the superficial largest structural elements appears to be inequable. Perhaps this is what had become one of the main reasons that caused fundamental differences between these two segments and led to the formation of tectonic asymmetry of the Earth. [ABSTRACT FROM AUTHOR]

Seismic observations indicate that the lowermost mantle above the core–mantle boundary (CMB) is strongly heterogeneous. Body waves reveal a variety of ultra-low velocity zones (ULVZs), which extend not more than 100 km above the CMB and have shear velocity reductions of up to 30 per cent. While the nature and origin of these ULVZs remain uncertain, some have suggested they are evidence of partial melting at the base of mantle plumes. Here we use coupled geodynamic/thermodynamic modelling to explore the hypothesis that present-day deep mantle melting creates ULVZs and introduces compositional heterogeneity in the mantle. Our models explore the generation and migration of melt in a deforming and compacting host rock at the base of a plume in the lowermost mantle. We test whether the balance of gravitational and viscous forces can generate partially molten zones that are consistent with the seismic observations. We find that for a wide range of plausible melt densities, permeabilities and viscosities, lower mantle melt is too dense to be stirred into convective flow and instead sinks down to form a completely molten layer, which is inconsistent with observations of ULVZs. Only if melt is less dense or at most ca. 1 per cent more dense than the solid, or if melt pockets are trapped within the solid, can melt remain suspended in the partial melt zone. In these cases, seismic velocities would be reduced in a cone at the base of the plume. Generally, we find partial melt alone does not explain the observed ULVZ morphologies and solid-state compositional variation is required to explain the anomalies. Our findings provide a framework for testing whether seismically observed ULVZ shapes are consistent with a partial melt origin, which is an important step towards constraining the nature of the heterogeneities in the lowermost mantle and their influence on the thermal, compositional and dynamic evolution of the Earth. [ABSTRACT FROM AUTHOR]

The Earth's magnetic field is measured on and above the crust, while the turbulent dynamo in the outer core produces magnetic field values at the core-mantle boundary (CMB). The connection between the two sets of values is usually assumed to be independent of the electrical conductivity in the mantle. However, the turbulent magnetofluid in the Earth's outer core produces a time-varying magnetic field that must induce currents in the lower mantle as it emerges, since the mantle is observed to be electrically conductive. Here, we develop a model to assess the possible effects of mantle electrical conductivity on the magnetic field values at the CMB. This model uses a new method for mapping the geomagnetic field from the Earth's surface to the CMB. Since numerical and theoretical results suggest that the turbulent magnetic field in the outer core as it approaches the CMB is mostly parallel to this boundary, we assume that this property exists and set the normal component of the model magnetic field to zero at the CMB. This leads to a modification of the Mauersberger-Lowes spectrum at the CMB so that it is no longer flat, i.e., the modified spectrum depends on mantle conductance. We examined several cases in which mantle conductance ranges from low to high in order to gauge how CMB magnetic field strength and mantle ohmic heat generation may vary. [ABSTRACT FROM AUTHOR]

The melting temperature (Tm) of iron at megabar pressures constrains the Earth's core temperature structure and dynamics. Previous experimental studies demonstrated large discrepancies in Tm at high pressures. We used the intrinsic resistance discontinuity across solid‐liquid transition as a melting criterion to study the melting behavior of iron in laser‐heated diamond anvil cells. The resistance jump is sensitive to the incipient melting, capable of detecting the emergence of less than 2 vol.% melts. We found a high melting curve of iron at 30–135 GPa, but a relatively low‐transition temperature of the slow‐fast recrystallization. The determined Tm of iron is 4306(±300) K at the core‐mantle boundary (CMB) pressure in good agreement with the static and shockwave experimental results by Anzellini et al. (2013, https://doi.org/10.1126/science.1233514) and Li et al. (2020, https://doi.org/10.1029/2020gl087758). The high melting point of iron implies a high and steep geothermal gradient and influences a heat flow across the CMB. Plain Language Summary: The melting of iron is fundamental to constrain the thermal structure, solidification, heat flux, and evolution of the Earth's core. Here, we propose a sensitive melting criterion, the resistance discontinuity across the phase transition, which is capable of detecting 2 vol.% melting of the bulk sample. Using this criterion, we determined the melting temperature of iron up to 135 GPa. The new melting curve of iron reconciles with the most recent shockwave and static experimental results, indicating a melting temperature of 4306(±300) K at the Earth's core‐mantle boundary. Large discrepancies existing in the melting of iron are unlikely caused by varying melting criteria, while chromatic aberrations should be one of the major reasons resulting from refractive optical lens in the temperature measurement system. Key Points: The criterion of an electrical resistance jump is sensitive to detect 1%–2% melt fraction of bulk iron to 135 GPa in diamond anvil cellsThe slopes in the temperature‐resistance profile are changed across the solid‐solid and slow‐fast recrystallization transitions of ironThe melting temperature of iron is ∼4306 K at core‐mantle boundary pressure, consistent with latest static and shockwave experiments [ABSTRACT FROM AUTHOR]

The geomagnetic field is generated by a dynamo process in the Earth's core and is characterized by a predominant dipole component that has been steadily decreasing in the last few centuries. The physical drivers behind the fluctuations of the geomagnetic dipole field remain poorly understood. One of the possible explanations rely on the interaction between the dipole mode and other multipole terms of the geomagnetic field. To test this hypothesis, we used two millennial scale models based on spherical harmonic fitting of paleomagnetic data, which allowed to reconstruct the geomagnetic field of the past. By performing causality and information statistical analysis, we found significant interactions between the dipole and smaller scale harmonics (quadrupole and octupole) of the geomagnetic field. In particular, both data sets agree that the spherical harmonic Y 2 2 acts as a source term, whereas the axial dipole term Y 1 0 consists of the term with least information loss. The results suggest a possible control of core–mantle boundary inhomogeneities on the interaction between the components of the geomagnetic field. Our results also show a net information flux from larger to smaller scales, which is compatible with a direct turbulent cascade view of the geodynamo. [ABSTRACT FROM AUTHOR]

Controversy between advocates of 'active' (plume-activated) versus 'passive' (driven by external tectonic stresses) modes of continental rifting and break-up has persisted for decades. To a large extent, inconsistencies between observations and models are rooted in the conceptual model of plumes as voluminous upwellings of hot material sourced from the deep mantle. Such large-scale plumes are expected to induce intensive magmatism and topographic uplift, thereby triggering rifting. In this case of an 'active' rifting-to-break-up system, emplacement of plume-related magmatism should precede the onset of rifting that is not observed in many rifted continental margins, thus providing a primary argument in favour of an antiplume origin for continental break-up and supercontinent fragmentation. However, mantle plumes are not restricted to whole-mantle ('primary') plumes emanating from the mantle-core boundary but also include 'secondary' plumes originating from the upper mantle transition zone or shallower. Over the last decades a number of such 'secondary' plumes with horizontal diameters of only ∼100–200 km (therefore, sometimes also called 'baby' plumes) have been imaged in the upper mantle below Europe and China. The longevity of such small-scale plumes and their impact on geodynamics of continental break-up have so far not been explored. We present results of a systematic parametrical analysis of relatively small thermal anomalies seeded at the base of the lithosphere. In particular, we explore the effects of variations in initial plume temperature (T  = 1500–1700 °C) and size (diameter of 80–116 km), characteristics of the overlying lithosphere (e.g. 'Cratonic', 'Variscan', 'Mesozoic' and oceanic) and intraplate tectonic regimes (neutral or far-field extension of 2–10 mm yr–1). In tectonically neutral regimes, the expected decay time of a seismically detectable 'baby'-plume varies from ∼20 to >200 Myr and is mainly controlled by its initial size and temperature, whereas the effect of variations in the thermotectonic age of the overlying lithosphere is modest. These small but enduring plumes are able to trigger localized rifting and subsequent continental break-up occurring from ∼10 to >300 Myr after the onset of far-field extension. Regardless of the thermomechanical structure of the lithosphere, relatively rapid (tens of Myr) break-up (observed in models with a hot plume and fast extension) favours partial melting of plume material. In contrast, in the case of a long-lasting (a few hundreds of Myr) pre-break-up phase (relatively cold plume, low extension rate), rifting is accompanied by modest decompressional melting of only 'normal' sublithospheric mantle. On the basis of the models presented, we distinguish two additional modes of continental rifting and break-up: (1) 'semi-active' when syn-break-up magmatism is carrying geochemical signatures of the deep mantle with deformation localized above the plume head not anymore connected by its tail to the original source of hot material and (2) 'semi-passive' when the site of final lithospheric rupture is controlled by a thermal anomaly of plume origin but without invoking its syn-break-up melting. These intermediate mechanisms are applicable to several segments of the passive continental margins formed during Pangea fragmentation. [ABSTRACT FROM AUTHOR]

We review the problem of the consistency between the observed values of the lunar recession from Lunar Laser Ranging (LLR) and of the increase of the length-of-the-day (LOD). From observations of lunar occultations completed by recent IERS data, we derive a variation rate of the LOD equal to 1.09 ms/cy from 1680 to 2020, which compares well with McCarthy and Babcock (Phys. Earth Planet. Inter. 44: 281, 1986) and Sidorenkov (Astron. Astrophys. Trans. 24: 425, 2005). This rate is lower than the mean rate of 1.78 ms/cy derived by Stephenson et al. (Proc. R. Soc. A 472: 20160404, 2016) on the basis of eclipses in the Antiquity and Middle Age. The difference in the two observed rates starts at the epoch of a major change in the data accuracy with telescopic observations. The observed lunar recession appears too large when compared to the tidal slowing down of the Earth determined from eclipses in the Antiquity and Middle Age and even much more when determined from lunar occultations and IERS data from 1680 to 2020. With a proper account of the tidal effects and of the detailed studies on the atmospheric effects, the melting from icefields, the changes of the sea level, the glacial isostatic adjustment, and the core-mantle coupling, we conclude that the long-standing problem of the presence or absence of a local cosmological expansion is still an open question. [ABSTRACT FROM AUTHOR]

Silicate liquids are important agents of thermal evolution, yet their thermal conductivity is largely unknown. Here, we determine the thermal conductivity of a silicate liquid by combining the Green‐Kubo method with a machine learning potential of ab initio quality over the entire pressure regime of the mantle. We find that the thermal conductivity of MgSiO3 liquid is 1.1 W m−1 K−1 at the 1 bar melting point, and 4.0 W m−1 K−1 at core‐mantle boundary conditions. The thermal conductivity increases with compression, while remaining nearly constant on isochoric heating. The pressure dependence arises from the increasing bulk modulus on compression, and the weak temperature dependence arises from the saturation of the phonon mean free path due to structural disorder. The thermal conductivity of silicate liquids is less than that of ambient mantle, a contrast that may be important for understanding melt generation, and heat flux from the core. Plain Language Summary: Silicate liquids, the major constituents of lavas and magmas, are present not only at Earth's surface, but also in the deep interior. If present at the core‐mantle boundary at a depth of 2,891 km, silicate liquids may affect the heat that flows from the core to the mantle. The material property that describes the ability to transport heat is the thermal conductivity. The thermal conductivity of silicate liquids, however, is largely unknown. In this study, we calculate the thermal conductivity of silicate liquids across Earth's mantle conditions, which is made possible by the potential developed based on machine learning methods. The results show that the thermal conductivity of silicate liquids at the core‐mantle boundary is around half of that of the ambient mantle. This thermally insulating silicate liquid would reduce the core‐mantle heat flux, and has fundamental impacts on many deep Earth processes. Key Points: A machine learning based potential with ab initio quality is developed for MgSiO3 liquid across Earth's mantle conditionsThermal conductivity of MgSiO3 melts increases with compression, while remaining nearly constant on isochoric heatingSilicate melt is thermally insulating at the core‐mantle boundary and may enhance the heterogeneity of core‐mantle heat flux [ABSTRACT FROM AUTHOR]

We use magnetic field data collected in orbit around Mercury by the MErcury Surface, Space ENvironment, GEochemistry and Ranging satellite, to detect a regional magnetic field anomaly that is spatially associated with Mercury's Northern Rise topographic signature. Regional spectral analysis indicates a source depth at or below the core‐mantle boundary, and hence the anomaly is of core, not crustal, origin. This observation supports previous studies linking the Northern Rise to a deep‐seated gravity anomaly and reveals connections among core, mantle, and crustal dynamics, likely enabled by Mercury's thin mantle. Plain Language Summary: From magnetic measurements made by the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) satellite that orbited the planet Mercury from 2011 to 2015, we discovered a feature in Mercury's magnetic field that coincides with a 1,000 km‐wide and few km‐high bulge on Mercury's surface called the Northern Rise. We calculated that the source of this magnetic field pattern is about 400 km below the planet's surface, at or below the boundary between Mercury's rocky mantle and its metallic core. Previous studies by other researchers have shown that the elevated Northern Rise may be linked to the structure and dynamics of the deepest part of Mercury's mantle. The magnetic pattern that we discovered, our finding that it comes from Mercury's core and the association with the Northern Rise suggest connections between Mercury's mantle and the magnetic field generated in the core. Key Points: We discovered a magnetic anomaly that coincides with Mercury's Northern Rise topographic featureThe source depth of this magnetic anomaly is close to Mercury's core‐mantle boundaryTogether with previous research, this indicates links among core, mantle, and crustal dynamics [ABSTRACT FROM AUTHOR]

The core‐mantle boundary (CMB) is the most extreme boundary within the Earth where the liquid, iron‐rich outer core interacts with the rocky, silicate mantle. The nature of the lowermost mantle atop the CMB, and its role in mantle dynamics, is not completely understood. Various regional studies have documented significant heterogeneities at different spatial scales. While there is a consensus on the long scale length structure of the inferred S‐wave speed tomograms, there are also notable differences stemming from different imaging methods and datasets. Here we aim to overcome over‐smoothing and avoid over‐fitting data for the case where the spatial coverage is sparse and the inverse problem ill‐posed. We present an S‐wave tomography model at a global scale for the Lowermost Mantle (LM) using the Hierarchical Trans‐Dimensional Bayesian Inversion (HTDBI) framework, LM‐HTDBI. Our LM‐HTDBI analysis of ScS‐S travel times includes uncertainty, and the complexity of the model is deduced from the data itself through an implicit parameterization of the model space. Our comprehensive resolution estimates indicate that short‐scale anomalies are significant and resolvable features of the lowermost mantle regardless of the chosen mantle‐model reference to correct the travel times above the D" layer. The recovered morphology of the Large‐Low‐Shear‐wave Velocity Provinces (LLSVPs) is complex, featuring small high‐velocity patches among low‐velocity domains. Instead of two large, unified, and smooth LLSVPs, the newly obtained images suggest that their margins are not uniformly flat. Plain Language Summary: The lowermost mantle sits atop the core‐mantle boundary, the most dramatic boundary within our planet, with contrasts in physical properties that exceed those that exist at the surface. Despite significant progress in recent years, this part of the Earth is not well understood, and various tomographic studies on a global scale, along with regional studies that focus on seismic waveform modeling, pave the path towards higher resolution and new understanding. Important questions to answer are on the distribution, shape, size and composition of inhomogeneities in the lowermost mantle, and their critical role in the mantle and core dynamics. While there is a general consensus on the long‐scale length structures inferred from long‐period shear waves, there are notable differences in details of the tomograms of the lowermost mantle, stemming from the use of different imaging methods and datasets. Here, we utilize a large travel time data set of ScS and S waves with a significant addition of new measurements sensitive to the lowermost mantle to perform a probabilistic shear‐wave tomography, and we retrieve a high‐resolution image of the lowermost mantle. The new shear‐wave speed tomogram and comprehensive resolution‐estimations indicate that short and medium scale inhomogeneities are omnipresent features of the lowermost mantle. Key Points: S‐wave tomography of the lowermost mantle using state‐of‐the‐art Bayesian approach with 2D spherical Voronoi cellsThe inversion technique treats the model complexity and the data noise as free parameters and avoids damping and smoothingThis study provides an important bridge between long‐scale features at a global scale and short‐scale features of regional models [ABSTRACT FROM AUTHOR]

Following the observed pattern of a new geomagnetic jerk every 3–4 years, certain predictions suggested that a new event should occur around 2020 after the one observed around 2017.5. In this work, we explore this scenario by analysing the secular variation of the East geomagnetic field component in both ground and satellite geomagnetic data. At ground, we use the available data from 2015 to 2021 in 10 observatories worldwide distributed. This analysis shows the occurrence of the mentioned jerk in mid-2017 at observatories located in the Pacific region, but also reveals a new jerk between mid-2019 and early 2020 with a clear global character. Swarm satellite data also corroborate these findings by means of the secular variation estimated using virtual observatories at 440 km altitude. In addition, a general view using the most recent CHAOS geomagnetic model confirms the global character of the 2020-jerk with V-shaped secular variation changes in meridional sectors covering the Eastern Pacific, America, Asia and the Indian Ocean; and Λ-shapes in Europe, Africa and Western Pacific. The radial geomagnetic field at the core–mantle boundary is investigated as the origin of the new jerk. Results show that the global-average secular acceleration of the radial field exhibits a new pulse at mid-2018, establishing the starting epoch of the 2020-jerk. Highlights: Observatory and Swarm data reveal a new geomagnetic jerk around 2019–2020. A very recent geomagnetic field model indicates the global character of the new jerk. A new pulse at the CMB around mid-2018 seems to be the starting point of this jerk. [ABSTRACT FROM AUTHOR]

Earth's deep mantle hosts continental-scale enigmatic structures whose origin, characteristics, and fate are uncovered by advances in tomographic modeling, geochemical analysis, measurements of mantle mineral properties under extreme conditions, and novel methods in mantle convection modeling. Probabilistic tomography revealed that these deep mantle structures, dubbed large low shear velocity provinces (LLSVPs), are likely enriched in iron-bearing bridgmanite, making them denser than the surrounding ambient mantle (establishing a stabilizing negative buoyancy). A distinct geochemical signature observed in ocean-island basalts is thought to be sourced from the deep mantle, which indicates that LLSVPs may be candidates for a primordial reservoir enriched in heat-producing elements (promoting a destabilizing positive buoyancy). Using conductivity measurements from upper and lower mantle minerals, we derived a novel parametrized depth-, temperature-, and composition-dependent thermal conductivity model. With thermochemical mantle convection models, we demonstrated how the thermal state and total buoyancy of primordial reservoirs (as an analogue for LLSVPs) are influenced by a dynamically responsive and (spatially, thermally, and compositionally) heterogeneous thermal conductivity field. We examined the influence of moderate to high temperature-dependent conductivity models on the evolution of primordial reservoirs characterized by intrinsic chemical density contrast between 1.5 and 5% and heat-producing element (HPE) enrichment factors between 1 and 20. The lowermost mantle attains mean conductivities of ∼6 W/m/K within reservoirs and ∼9 W/m/K outside for moderate temperature dependence. For a high temperature dependence, mean conductivities are reduced to ∼2 W/m/K and ∼5 W/m/K, respectively. We found that heterogeneous thermal conductivity exacerbates the thermally driven effect of HPE enrichment, and the strength of the thermal contribution to the total conductivity is decisive in determining the long-term stability (> 10 Gyr of simulation time) of primordial reservoirs. For the parameter space we consider, the stability of primordial reservoirs with a particular HPE factor improves with greater density contrast. However, with a particular density contrast, stability decreases with a greater HPE factor. Thus, pile instability occurs for a combination of parameters that promote the domination of thermal buoyancy over chemical buoyancy. For a moderate HPE enrichment (factor of 10) and a highly temperature-dependent conductivity, reservoirs characterized by a density contrast ∼ 4% attain a mean density ∼ 1.4% and achieve long-term stability. For a moderately temperature-dependent conductivity, reservoirs characterized by a density contrast ∼ 3% attain a mean density ∼ 1.0%, consistent with tomographic estimates of LLSVP density and achieving long-term stability. When HPE enrichment is present, we found that a greater thermal buoyancy is more likely when temperature dependence is stronger due to the piles' larger temperature. Thus, a greater intrinsic chemical density contrast is required to maintain stable and long-lived primordial reservoirs. • Instability is more likely when conditions promote thermal over chemical buoyancy • Heterogeneous thermal conductivity intensifies piles' HPE-induced thermal buoyancy • Conductivity's thermal dependence determines the onset of piles' instability • For 10x HPE enrichment, piles with chemical density contrast > 3% remain stable • Low density contrast piles remain stable if lower mantle conductivity is large enough. [ABSTRACT FROM AUTHOR]

Study of the paleosecular variation of the Earth's Magnetic Field is an almost unique way to estimate the critical conditions at the core-mantle boundary. The continuous and quasi-periodic changes in the magnitude and directions (declination and inclination) could be detected from direct instrumental measurements or through the remanent magnetization of volcanic and sedimentary rocks. Burned archaeological artifacts are another reliable source of geomagnetic information. Besides the great efforts in Central and North America to retrieve the fine characteristics of the geomagnetic field during the last centuries, no homogeneous data distribution has still been achieved in Mexico and the surrounding areas. To improve the analysis of the geomagnetic full vector in Mexico, we compiled and updated available data for the last 200 years exclusively based on full geomagnetic vectors obtained from direct historical measurements, magnetic repeat stations, geomagnetic observatories and maritime exploratory campaigns. A relocation error procedure allowed the selection of 658 reliable data, discarding those belonging to the sites located in the most northern part of the analyzed area. The spectral analysis through the second derivatives reveals relatively fast fluctuations in both intensity and directions between 1890 and 1960. Moreover, a marked difference is observed between the Mexican record and global geomagnetic models between 1840 and 1940. • Updated available data for the last 200 years exclusively based on full geomagnetic vectors. • A relocation error procedure allowed the selection of 658 reliable data. • Homogeneously distribution across Mexico and the southern United States. • Marked differences with previously obtained records and main geomagnetic global models. • Extreme variation episodes between 1840 and 1940, some of which were observed in Europe. [ABSTRACT FROM AUTHOR]

Large lateral variations in the depth, sharpness, and magnitude of the D″ discontinuity are observed beneath Central America and the Caribbean using a new innovative grid search modeling method. The strong correlation between the D″ topography and the underlying seismic velocities suggests a large lateral gradient in the thermal condition in the lowermost mantle. Low‐velocity patches were identified among the high‐velocity areas, coinciding with regions showing a disruption in the D″ discontinuity. The low‐velocity zones may be attributed to the heat trapped in the middle of the Farallon slab remnant while those farther to the west are likely to be associated with the upwelling generated at the edge of the ancient slab. A broad range in the sharpness of the D″ discontinuity was also observed, suggesting the existence of different chemical components in the region, as a result of a disparate mixture of basalt, harzburgite, and pyrolite. Plain Language Summary: The D″ region is the lowermost few hundred kilometers of the mantle and regarded as a critical thermal and chemical boundary layer above the core‐mantle boundary. Characterizing the structure of this region is crucial for a better understanding of the mantle's thermochemical evolution and the nature of core‐mantle interactions. In order to understand the origin of the D″ region and its role in mantle dynamics, we use an innovative modeling method to constrain the velocity structure. Using this method, we found a large variation in the seismic features in the lowermost mantle beneath Central America. We explain this by inferring a diverse thermal and chemical environment associated with the Farallon slab remnants. Some fine‐scale features of the D″ region shown here have not been previously identified, which are important for a better understanding of the origin of the D″ and mantle dynamics beneath Central America. Future efforts on explaining the dynamics and chemical compositions of the lowermost mantle should take these results into account. Key Points: D″ layer beneath Central America is characterized by a complicated structure with high‐velocity areas and intermittent low‐velocity zonesStrong correlation between the D″ topography and the underlying velocity suggests a large lateral temperature variation in the D″ regionThe D″ beneath Central America is suggested to be a mixture of different chemical compositions associated with slab remnants [ABSTRACT FROM AUTHOR]

The origin of ultralow‐velocity zones (ULVZs) remains an open question despite recent advances in mineral physics and seismology. Here, we examine the hypothesis that FeSi formed from core‐mantle chemical reactions is a plausible source of ULVZs at the core‐mantle boundary (CMB). The sound velocities of B2‐structured FeSi were measured up to 115(2) GPa and 1600(200) K by nuclear inelastic scattering (NIS) in laser‐heated diamond anvil cells (LH‐DACs). Within uncertainties, the sound velocities of B2‐FeSi display negligible anharmonicity, hence validating the extrapolation of velocity‐density relations (Birch's law) to P‐T conditions of the CMB. The sound velocities of B2‐FeSi are significantly lower compared to other candidate phases in a lowermost mantle assemblage, and the Preliminary Reference Earth Model at CMB conditions. Less than 8.4 vol% of FeSi in the aggregate is thus sufficient to explain both the velocity decrements and the high density anomaly observed in a wide range of ULVZs. Plain Language Summary: Seismic studies have revealed patches on the Earth's core‐mantle boundary (CMB), which have extremely low seismic velocities. However, the origin of these so‐called ultralow‐velocity zones (ULVZs) remains unclear. One plausible source could be FeSi, which is assumed to form from chemical reactions between material from the core and the mantle. Here, we test this hypothesis in an experimental study, where the elastic behavior of FeSi is measured while the sample is exposed to very high pressures and temperatures comparable to those in the Earth's lower mantle. From these measurements we derived the sound velocities of the high‐pressure phase of FeSi at the CMB. We found that the sound velocities of FeSi are significantly lower than those of the average lowermost mantle measured in seismic studies, and also lower than those of other mineral phases in this region, which are also considered as potential sources of ULVZs. FeSi is thus a plausible candidate to explain the origin of ULVZs. Our results show that less than 8.4% of FeSi by volume in the aggregate are enough to explain both the amplitude of the velocity reduction and the high density anomaly observed in a wide range of ULVZs. Key Points: Very low sound velocities in B2‐FeSi, a candidate phase at the CMB, measured up to 115 GPa and 1600 K by NISNegligible anharmonic contributions to sound velocities validate the extrapolation of Birch's law to CMB conditionsLess than 8.4 vol% B2‐FeSi produced by core‐mantle reactions are enough to explain the seismic anomalies associated with the ULVZs [ABSTRACT FROM AUTHOR]

The Canary and Madeira provinces in the Central‐East Atlantic Ocean are characterized by an irregular spatio‐temporal distribution of volcanism along the hotspot tracks, and several alternative scenarios have been suggested to explain it. Here, we combine results from seismic tomography, shear‐wave splitting and gravity along with plate reconstruction constraints to investigate the mantle structure and dynamics beneath those provinces. We find that the Central‐East Atlantic Anomaly (CEAA), which rises from the core‐mantle boundary and stalls in the topmost lower mantle, is the deep source of distinct upper‐mantle upwellings beneath the region. The upwellings detach intermittently from the top of the CEAA and appear to be at different evolutionary stages. We argue that the accumulation of plume material in the topmost lower mantle can play a key role in governing the first‐order spatio‐temporal irregularities in the distribution of hotspot volcanism. Plain Language Summary: The Canary and Madeira provinces, located in the Central‐East Atlantic Ocean, show lineaments of volcanic islands and seamounts, known as hotspot tracks, which differ from most other tracks for their irregular distribution. These lineaments cannot be easily explained by the African plate movement over a fixed, narrow plume of hot mantle material rising from the deep Earth and alternative mechanisms may be required. Here, we integrate observations from seismology and gravity to demonstrate that some first‐order spatio‐temporal irregularities of volcanism in both provinces are due to small‐scale upper‐mantle plumes ("plumelets"), which sporadically rise from the top of a wide lower‐mantle low‐velocity structure, here named "Central‐East Atlantic Anomaly" (CEAA). The CEAA extends vertically from the base of the African large low‐shear‐velocity province (LLSVP), a structure in the lowermost mantle situated under Africa and adjacent oceans and characterized by low‐shear seismic velocities. According to the interpretation of global and regional tomography models, the CEAA material stalls in the topmost lower mantle, between ∼700 and 1,200 km depth, intermittently generating plumelets under the Central‐East Atlantic. Plate reconstructions from Cenozoic to present confirm that the CEAA is underlying these and other volcanic provinces (e.g., Western Iberia and NW Morocco) since at least 90 Ma. Key Points: The Canary and Madeira hotspots are underlain by distinct upwellings sourced from the lower‐mantle Central‐East Atlantic Anomaly (CEAA)A "vote" analysis of 34 tomography models shows that the CEAA extends vertically from the African LLSVP up to the topmost lower mantleThe plumelets seem at different stages of evolution and rise sporadically from mantle material accumulated below the 660‐km discontinuity [ABSTRACT FROM AUTHOR]

We analyzed 4,754 broadband seismic recordings of the SKS, SKKS, and SPdKS wavefield from 13 high quality events sampling the Samoa ultralow‐velocity zone (ULVZ). We measured differential travel‐times and amplitudes between the SKKS and SKS arrivals, which are highly sensitive to the emergence of the SPdKS seismic phase, which is in turn highly sensitive to lowermost mantle velocity perturbations such as generated by ULVZs. We modeled these data using a 2‐D axi‐symmetric waveform modeling approach and are able to explain these data with a single ULVZ. In order to predict both travel‐time and amplitude perturbations we found that a large ULVZ length in the great circle arc direction on the order of 10° or larger is required. The large ULVZ length limits acceptable ULVZ elastic parameters. Here we find that δVS and δVP reductions from 20% to 22% and 15% to 17% respectively gives us the best fit, with a thickness of 26 km. Initial 3‐D modeling efforts do not recover the extremes in the differential measurements, demonstrating that 3‐D effects are important and must be considered in the future. However, the 3‐D modeling is generally consistent with the velocity reductions recovered with the 2‐D modeling. These velocity reductions are compatible with a compositional component to the ULVZ. Furthermore, geodynamic predictions for a compositional ULVZ that is moving predict a long linear shape similar to the shape of the Samoa ULVZ we confirm in this study. Plain Language Summary: Ultralow‐velocity zones (ULVZs) on the core‐mantle boundary (CMB) are some of the most extreme features discovered in Earth's mantle. What ULVZs physically represent and how they have arisen in the Earth are still open questions. Important clues about the origin and makeup of ULVZs may come from studying the Samoa ULVZ which, with a length of approximately 1,600 km along the CMB, is the largest ULVZ yet discovered. We used a new collection of seismic data with greater sensitivity to the Samoa ULVZ than has been used in previous studies to determine its physical properties. We determine ULVZ properties by comparing computations of what seismic waveforms look like for various ULVZ models to the data we collected. Our analysis suggests that the Samoa ULVZ is likely a compositional anomaly. Recent geodynamic modeling suggests that a compositional anomaly that is being advected along the CMB will take on a long linear shape similar to the shape we observe for the Samoa ULVZ. If some of the Samoa ULVZ material is being entrained into recent hot spot volcanism in the Samoa hot spot chain, then geochemical evidence from the volcanism may imply that the ULVZ is derived from ancient subducted slab material. Key Points: Waveform modeling of SKKS, SKS, and SPdKS amplitudes and travel‐times put greater constraints on ultralow‐velocity zone (ULVZ) size and elastic parametersData are fit with a 1.3 δVS to δVP ratio which may be indicative of a compositional originThe long linear shape of the Samoa ULVZ may indicates this ULVZ is currently moving across the core‐mantle boundary [ABSTRACT FROM AUTHOR]

We use one‐dimensional full‐waveform inversion to simultaneously infer the S‐velocity (VS) and anelastic (QS) structures of the lowermost 480 km of the mantle in 11 contiguous corridors located beneath Northern South America and Central America. Previous studies in this region have reported the presence of slabs at the core‐mantle boundary (CMB) and large variations in the depth of the D′′ discontinuity, suggesting local temperature variations. We use transverse component waveform data cut around the ScS phase, including the Scd phase due to the D′′ discontinuity, from 31 events beneath South America recorded at USArray stations. The minimum period used in this study is 8 s, allowing us to constrain the fine‐scale structure of the D′′ region and the depth of the D′′ discontinuity. We correct the data for the effect of amplitude focusing due to lateral velocity anomalies (i.e., out‐of‐plane focusing effect) in D′′ on the inferred QS structure using three‐dimensional waveform calculations. We show that, in the explored region, corrections for out‐of‐plane focusing effects are necessary to obtain QS models that have a reasonable interpretation in terms of temperature and fraction of post‐perovskite (pPv). Our final models show regions with high VS, high QS, and an elevated D′′ discontinuity, surrounding a low‐velocity corridor with lower QS and a depressed D′′ discontinuity that can be explained by local temperature variations. However, differences between temperature perturbations inferred from VS and QS suggest lateral variations in the fraction of pPv. Plain Language Summary: Because the Earth's mantle is not perfectly elastic, seismic waves are slightly attenuated as they travel through it. Attenuation is mostly sensitive to temperature, with larger temperature leading to stronger attenuation, and can be measured from the analysis of seismic waveforms. More specifically, attenuation is parameterized with the quality factor, Q, which is inversely proportional to the attenuation, that is, lower Q indicates stronger attenuation. Together with the velocity of seismic shear waves (VS), Q provides key information on the deep mantle thermo‐chemical structure. Here, we measure radial profiles of VS and Q in the mantle lowermost 480 km in 11 corridors located beneath Central America, where a subducted slab is believed to be present. Our results show that VS and Q are overall larger than their average values at these depths, and that the D″ discontinuity, which is believed to be the signature of the phase transition from bridgmanite to post‐perovskite (pPv), is elevated. In the central region, however, VS is slower, attenuation larger, and the D″ discontinuity is deeper than in surrounding corridors, suggesting a higher temperature. A careful examination of our results suggest that this temperature increase is accompanied by a thinning of the pPv lens. Key Points: One‐dimensional full‐waveform inversion jointly constrains VS and QS structures of lowermost mantleCorrections for focusing effects in D′′ improve the geophysical interpretationOur VS and QS models suggest variations in temperature and in fraction of post‐perovskite [ABSTRACT FROM AUTHOR]

A model for the simultaneous cooling of the Earth's core and mantle in the time interval from accretion, 4.5 Gyr ago, to 1.5 Gyr into the future is considered. The model allows one to obtain the present-day radius of the solid core, the heat flux at the planet's surface close to the observed one, and to satisfy the constraints on the temperature and viscosity of the mantle, and the parameters of the layer and the lithosphere. The Earth's evolution scenarios whereby the heat flux at the core–mantle boundary underwent no significant changes have been studied. Consequences for the geodynamo are given. [ABSTRACT FROM AUTHOR]

We propose a parametric approach to the topographic (TOP) coupling between the mantle and outer core for refinement of the latest triaxial three-layered Earth rotation theory. Based on three models of the core–mantle boundary (CMB) topography, we obtain the axial components of the TOP torque as −2.08 × 1019, −2.72 × 1018 and −1.97 × 1017 N m, respectively. Under the frame of the triaxial three-layered Earth rotation theory, we solve the corresponding periods of free core nutation as −(329.83 ± 28.12), −(457.54 ± ∼) and −(428.23 ± 1.09) mean solar days (d), respectively. The other three normal modes, namely, Chandler wobble, inner core wobble and free inner core nutation, are almost not affected by the TOP coupling of the CMB, their period values being 433.24, 2718.69 and 934.02 d, respectively. Calculations show that the TOP torque is highly sensitive to the adopted model of the topography, which is known to be robust. Taking into account the normal modes of the triaxial three-layered Earth rotation, the results of the CMB topography obtained by seismic tomography can be constrained in the future to a certain extent. In this study, considering the TOP coupling with the appropriate topography model, the estimates for the dynamic ellipticity e f of the fluid core lie between 0.0026340 and 0.0026430, values that are 3.56 % higher than the hydrostatic equilibrium value. [ABSTRACT FROM AUTHOR]

Mantle convection induces dynamic topography, the lithosphere's surface deflections driven by the vertical stresses from sublithospheric mantle convection. Dynamic topography has important influences on a range of geophysical and geological observations. Here, we studied controls on the Earth's dynamic topography through 3-D spherical models of mantle convection, which use reconstructed past 410 Myr global plate motion history as time-dependent surface mechanical boundary condition. The numerical model assumes the extended-Boussinesq approximation and includes strongly depth- and temperature-dependent viscosity and phase changes in the mantle. Our results show that removing the chemical layer above the core–mantle boundary (CMB) and including depth-dependent thermal expansivity have both a limited influence on the predicted present-day dynamic topography. Considering phase transitions in our models increases the predicted amplitude of dynamic topography, which is mainly influenced by the 410 km exothermic phase transition. The predicted dynamic topography is very sensitive to shallow temperature-induced lateral viscosity variations (LVVs) and Rayleigh number. The preservation of LVVs significantly increases the negative dynamic topography at subduction zones. A decrease (or increase) of Rayleigh number increases (or decreases) the predicted present-day dynamic topography considerably. The dynamic topography predicted from the model considering LVVs and with a Rayleigh number of 6 × 108 is most compatible with residual topography models. This Rayleigh number is consistent with the convective vigor of the Earth as supported by generating more realistic lower mantle structure, slab sinking rate and surface and CMB heat fluxes. The evolution of the surface heat flux pattern is similar to the long-term eustatic sea level change. Before the formation of Pangea, large negative dynamic topography formed between the plate convergence region of Gondwana and Laurussia. The predicted dynamic topography similar to that of present-day has already emerged by about 262 Ma. Powers for degrees 1–3 dynamic topography at 337 and 104 Ma which correspond to times of higher plate velocities and higher surface heat fluxes are larger. [ABSTRACT FROM AUTHOR]

We present first principles predictions of the absolute entropy of a silicate liquid (CaSiO 3) over a wide pressure-temperature range encompassing the Earth's mantle (2000–6000 K, 0–270 GPa). The results are derived from molecular dynamics simulations based on density functional theory and the two-phase thermodynamic (2PT) method, which divides the vibrational density of states into solid-like and gas-like parts, and which we describe in detail. The heat capacity derived from the absolute entropy agrees well with experimental measurements at low pressure. We find that the vibrational contribution accounts for more than 75% of the total entropy over the range of our study. We find that the two-body approximation to the excess entropy (relative to the ideal gas), which is computed with knowledge only of the radial distribution function, is excellent over most of the range considered, except at the lowest pressures and temperatures. We also compute the entropy of CaSiO 3 perovskite and use the entropy of liquid and solid phases to determine their absolute free energies and the melting curve. The melting curve agrees well with experiment and independent theoretical determinations based on Clausius-Clapeyron integration and the ZW method, showing a melting temperature of 5600 K at the Earth's core-mantle boundary. We find a nearly universal scaling of the self-diffusion coefficients with the excess entropy D ∗ = 0.9 exp 1.2 S ex , where D* is the self-diffusion coefficient suitably non-dimensionalized using macroscopic thermodynamic properties, and S ex is the excess entropy in units of the Boltzmann constant per atom. We use our results to estimate the temperature distribution in an isentropic, molten silicate Earth, finding 4.4 kJ/kg/K to be the lowest entropy that is completely molten, and producing a temperature at the core-mantle boundary of 6000 K. [ABSTRACT FROM AUTHOR]

The solar system's outer planets, and many of their moons, are dominated by matter from the H-C-N-O chemical space, based on solar system abundances of hydrogen and the planetary ices H2O, CH4, and NH3. In the planetary interiors, these ices will experience extreme pressure conditions, around 5 Mbar at the Neptune mantle-core boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. While temperature will dictate the formation of compounds, groundstate density functional theory allows us to probe the chemical effects resulting from pressure alone. These structural developments in turn determine the planets' interior structures, thermal evolution, and magnetic field generation, among others. Despite its importance, the H-C-N-O system has not been surveyed systematically to explore which compounds emerge at high-pressure conditions, and what governs their stability. Here, we report on and analyze an unbiased crystal structure search among H-C-N-O compounds between 1 and 5 Mbar. We demonstrate that simple chemical rules drive stability in this composition space, which explains why the simplest possible quaternary mixture HCNO--isoelectronic to diamond--emerges as a stable compound and discuss dominant decomposition products of planetary ice mixtures. [ABSTRACT FROM AUTHOR]

We determine a new 3-D shear wave velocity (V s) model down to 400 km depth beneath the Cape Verde hotspot that is far from plate boundaries. This V s model is obtained by using a new method of jointly inverting P- and S -wave receiver functions, Rayleigh-wave phase-velocity data and S -wave arrival times of teleseismic events. Two V s discontinuities at ∼15 and ∼60 km depths are revealed beneath volcanic islands, which are interpreted as the Moho discontinuity and the Gutenberg (G) discontinuity. Between the north and south islands, obvious high- V s anomalies exist in the uppermost mantle down to a depth of ∼100–150 km beneath the Atlantic Ocean, whereas obvious low- V s anomalies exist in the uppermost mantle beneath the volcanic islands including the active Fogo volcano. These low- V s anomalies merge into a significant column-like low- V s zone at depths of ∼150–400 km beneath the Cape Verde swell. We propose that these features in the upper mantle reflect a plume-modified oceanic lithosphere–asthenosphere system beneath the Cape Verde hotspot. [ABSTRACT FROM AUTHOR]

Quantifying the density contrasts of the Earth's inner core boundary (ICB) is crucial to understand core-mantle coupling and the generation of the geodynamo. The PKiKP/PcP amplitude ratio is commonly used to obtain the density contrast at the ICB, but its applications are limited by scattered observed data. In this study, we selected the PKiKP and PcP phases reflected at the same region of inner-core and core-mantle boundaries beneath Northeast Asia from different earthquakes for the first time, and the observations suggested that the PKiKP/PcP amplitude ratio is widely scattered. We also compared the PKiKP and PcP amplitudes, which demonstrated that the scatter cannot be attributed only to ICB anomalies but might also arise from raypath differences and heterogeneities throughout the crust and mantle. By fitting the observed PKiKP/PcP amplitude ratio, we obtained a density contrast of approximately 0.65 g cm−3 and a compressional velocity contrast of approximately 0.87 km s−1 at the ICB beneath Northeast Asia. The larger contrast values indicate the possible occurrence of local crystallization occurring at the inner core surface. [ABSTRACT FROM AUTHOR]

If the hydrous melts released from the slab mantleinthetransitionzoneorlowermantle migrate into slab crust the water would dissolve into the undersaturated dense SiO 2 phase (Fig. Once a slab makes it to the core- mantle boundary region, water held in the slab crust or the slab mantle may be releasedduetothehighgeothermalgradient. Waterisaddedtothebasalticoceanic crust and peridotitic mantle in lithospheric plates (hereafter, slab crust and slab mantle, respectively) at mid-ocean ridges, at transform faults, and in bendingfaultsformedattheouterrisepriorto subduction[3]. [Extracted from the article]

As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D" layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+δ (0 < δ < 1, denoted as 'OE-phase'). It forms at pressures greater than 40 gigapascal when (Mg, Fe)-bearing hydrous materials are heated over 1500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2H x which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1000 km in the Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial or subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry. [ABSTRACT FROM AUTHOR]

Computational models of mantle convection must accurately represent curved boundaries and the associated boundary conditions of a 3-D spherical shell, bounded by Earth's surface and the core–mantle boundary. This is also true for comparable models in a simplified 2-D cylindrical geometry. It is of fundamental importance that the codes underlying these models are carefully verified prior to their application in a geodynamical context, for which comparisons against analytical solutions are an indispensable tool. However, analytical solutions for the Stokes equations in these geometries, based upon simple source terms that adhere to physically realistic boundary conditions, are often complex and difficult to derive. In this paper, we present the analytical solutions for a smooth polynomial source and a delta-function forcing, in combination with free-slip and zero-slip boundary conditions, for both 2-D cylindrical- and 3-D spherical-shell domains. We study the convergence of the Taylor–Hood (P2–P1) discretisation with respect to these solutions, within the finite element computational modelling framework Fluidity, and discuss an issue of suboptimal convergence in the presence of discontinuities. To facilitate the verification of numerical codes across the wider community, we provide a Python package, Assess, that evaluates the analytical solutions at arbitrary points of the domain. [ABSTRACT FROM AUTHOR]

We use 20 years of continuous magnetic field measurements from the Ørsted, CHAMP and Swarm satellite missions, supplemented by calibrated platform magnetometer data from the CryoSat-2 satellite, to study time variations of the Earth's core field at satellite altitude and at the core–mantle boundary (CMB). From the satellite data we derive composite time series of the core field secular variation (SV) with 4-month cadence, at 300 globally distributed Geomagnetic Virtual Observatories (GVO). A previous gap in the GVO series between 2010 and 2014 is successfully filled using CryoSat-2, and sub-decadal variations are identified during this period. Tests showed that similar sub-decadal SV patterns were obtained from the CryoSat-2 data regardless of whether IGRF-13 or CHAOS-6x9 was used in their calibration. Cryosat-2 radial field SV series at non-polar latitudes have a mean standard deviation level compared to smoothing spline fits of 3.5 nT/yr compared to 1.8 nT/yr for CHAMP and 0.9 nT/yr for Swarm. GVO radial SV series display regional fluctuations with 5–10 years duration and amplitudes reaching 20 nT/yr, most notably at low latitudes over Indonesia (2014), over South America and the South Atlantic (2007, 2011 and 2014), and over the central Pacific (2017). Applying the Subtractive Optimally Localized Averages (SOLA) method, we also map the radial SV at the CMB as a collection of locally averaged SV estimates. We demonstrate that using 2-year windows of CryoSat-2 data, it is possible to reliably estimate the SV and its time derivative, the secular acceleration (SA), at the CMB, with a spatial resolution, corresponding to spherical harmonic degree 10. Along the CMB geographic equator, we find strong SA features with amplitude ± 2.5 μ T / yr 2 under Indonesia from 2011–2014, under central America from 2015 to 2019, and sequences of SA with alternating sign under the Atlantic during 2004–2019. We find that platform magnetometer data from CryoSat-2 make a valuable contribution to the emerging picture of sub-decadal core field variations. Using 1-year windows of data from the Swarm satellites, we show that it is possible to study SA changes at low latitudes on timescales down to 1 year, with spatial resolution corresponding to spherical harmonic degree 10. We find strong positive and negative SA features appearing side-by-side in the Pacific in 2017, and thereafter drift westward. [ABSTRACT FROM AUTHOR]

Numerical geodynamo simulations capture several features of the spatial and temporal geomagnetic field variability on historical and Holocene timescales. However, a recent analysis questioned the ability of these numerical models to comply with long‐term paleomagnetic field behavior. Analyzing a suite of 50 geodynamo models, we present here the first numerical simulations known to reproduce the salient aspects of the paleosecular variation and time‐averaged field behavior since 10 Ma. We find that the simulated field characteristics covary with the relative dipole field strength at the core‐mantle boundary (dipolarity). Only models dominantly driven by compositional convection, with an Ekman number (ratio of viscous to Coriolis forces) lower than 10−3 and a dipolarity in the range 0.34–0.56 can capture the observed paleomagnetic field behavior. This dipolarity range agrees well with state‐of‐the‐art statistical field models and represents a testable prediction for next generation global paleomagnetic field model reconstructions. Plain Language Summary: Earth's magnetic field varies on a wide range of timescales, from less than a year to hundreds of millions of years and longer. Such variations are produced by the complex fluid dynamic processes in the liquid iron core, which are generally studied using three‐dimensional computer simulations. While these simulations reproduce several features of the geomagnetic field on relatively short timescales (less than 10 kyr), their compliance with the field characteristics on longer timescales has been recently questioned. Here, we present the first simulations known to reproduce the salient features of the geomagnetic field behavior over the last 10 Myr. Analyzing a large suite of simulations, we demonstrate that the most Earth‐like ones employ buoyancy sources modeling the release of light elements from the inner core, have a low enough viscosity and a magnetic field morphology which is sufficiently, but not too strongly, dipolar. Our estimates of the degree of dipole dominance agree well with those obtained from observational field models. Our findings can be employed by future studies to reliably simulate long‐term geomagnetic field behavior, hence improving our understanding of the Earth's core and its evolution. Key Points: We present the first numerical geodynamo simulations known to reproduce the main features of paleomagnetic field variability since 10 MaAll simulated characteristics of paleomagnetic behavior covary with the degree of dipole dominance (dipolarity)Only chemically driven dynamos at sufficiently low Ekman numbers in a specific dipolarity range capture paleomagnetic field behavior [ABSTRACT FROM AUTHOR]

After accretion and formation, terrestrial planets go through at least one magma ocean episode. As the magma ocean crystallises, it creates the first layer of solid rocky mantle. Two different scenarios of magma ocean crystallisation involve that the solid mantle either (1) first appears at the core–mantle boundary and grows upwards or (2) appears at mid-mantle depth and grows in both directions. Regardless of the magma ocean freezing scenario, the composition of the solid mantle and liquid reservoirs continuously change due to fractional crystallisation. This chemical fractionation has important implications for the long-term thermo-chemical evolution of the mantle as well as its present-day dynamics and composition. In this work, we use numerical models to study convection in a solid mantle bounded at one or both boundaries by magma ocean(s) and, in particular, the related consequences for large-scale chemical fractionation. We use a parameterisation of fractional crystallisation of the magma ocean(s) and (re)melting of solid material at the interface between these reservoirs. When these crystallisation and remelting processes are taken into account, convection in the solid mantle occurs readily and is dominated by large wavelengths. Related material transfer across the mantle–magma ocean boundaries promotes chemical equilibrium and prevents extreme enrichment of the last-stage magma ocean (as would otherwise occur due to pure fractional crystallisation). The timescale of equilibration depends on the convective vigour of mantle convection and on the efficiency of material transfer between the solid mantle and magma ocean(s). For Earth, this timescale is comparable to that of magma ocean crystallisation suggested in previous studies , which may explain why the Earth's mantle is rather homogeneous in composition, as supported by geophysical constraints. [ABSTRACT FROM AUTHOR]

The pyrite-type FeO 2 and FeO 2 H were synthesized at the pressure-temperature conditions relevant to Earth's deep lower mantle. Through the water-iron reaction, the pyrite-phase is a good candidate to explain the chemical heterogeneities and seismological anomalies at the bottom of the mantle. The solid solution of pyrite-type FeO 2 and FeO 2 H, namely the FeO 2 H x (0 ​≤ ​ x ​≤ ​1), is particularly interesting and introduces puzzling chemical states for both the O and H atoms in the deep mantle. While the role of H in the FeO 2 –FeO 2 H system has been primarily investigated, discrepancies remain. In this work, we summarize recent progress on the pyrite-phase, including FeO 2 , FeO 2 H, and FeO 2 H x , which is critical for understanding the water cycling, redox equilibria, and compositional heterogenicities in the deep lower mantle. Image 1 • Previous findings on the pyrite-type FeO 2 –FeO 2 H system are summarized and reviewed. • The mobility of H in the FeO 2 –FeO 2 H system was characterized by x-ray diffraction in diamond anvil cell. • The co-existence of pyrite-type phase with high-low concentrations of H was observed in a quickly decompressed sample. [ABSTRACT FROM AUTHOR]

Detection of Hadean isotopic signatures within modern ocean island basalts (OIB) has greatly influenced understanding of Earth's earliest history and long‐term dynamics. However, a relationship between two isotopic tools for studying early Earth processes, the short‐lived 146Sm‐142Nd and 182Hf‐182W systems, has not been established in this context. The differing chemical behavior of these two isotopic systems means that they are complementary tracers of a range of proposed early Earth events, including core formation, magma ocean processes, and late accretion. There is a negative trend between 142Nd/144Nd and 182W/184W ratios among Réunion OIB that is extended by Deccan continental flood basalts. This finding is contrary to expectations if both systems were affected by silicate differentiation during the lifetime of 182Hf. The observed isotopic compositions are attributed to interaction between magma ocean remnants and Earth's core, coupled with later assimilation of recycled Hadean mafic crust. The effects of this scenario on the long‐lived 143Nd‐176Hf isotopic systematics mirror classical models invoking mixing of recycled trace‐element enriched (sedimentary) and depleted (igneous) domains in OIB mantle sources. If the core provides a detectible contribution to the tungsten element budget of the silicate Earth, this represents a critical component to planetary‐scale tungsten mass balance. A basic model is explored that reconciles the W abundance and isotopic composition of the bulk silicate Earth resulting from both late accretion and core‐mantle interaction. The veracity of core‐mantle interaction as proposed here would have many implications for long‐term thermochemical cycling. Plain Language Summary: Radioactive elements with relatively short half‐lives can be used as tools to study the geological processes that took place in the earliest part of Earth's history. Two of these short‐lived radioactive tools, the samarium‐neodymium and hafnium‐tungsten systems, are correlated in the Réunion hotspot source and it is suggested that this results from influences from Earth's metallic core and the preservation of four‐billion‐year old crust in the deep Earth. The idea that a geochemical fingerprint of Earth's core may make it to the surface has important consequences for broader understanding of Earth's thermal and chemical evolution and possibly changes previous assumptions about the role of late addition of meteorites in establishing Earth's modern tungsten isotopic composition. Key Points: There is a negative trend between the μ142Nd‐μ182W compositions of Réunion ocean island basalts (OIB) and primitive Deccan continental flood basaltsRéunion hotspot Nd‐W isotopic compositions may reflect early magma ocean crystallization and core‐mantle interactionStripping of W during subduction can explain the difference in μ142Nd‐μ182W compositions between Réunion OIB and Samoan or Hawaiian OIB [ABSTRACT FROM AUTHOR]

The evolution of mantle composition can be viewed as a process of destruction whereby the initial chemical state is overprinted and reworked with time. Analyses of ocean island basalts reveals that some portion of the mantle has survived this process, retaining a chemically "primitive" signature. A question that remains is how this primitive signature has survived four and a half billion years of vigorous convection. We hypothesize that some of Earth's primitive mantle is buried within a slab graveyard at the core‐mantle boundary. We explore this possibility using high‐resolution finite element models of mantle convection, in which oceanic lithosphere is produced at zones of plate spreading and subducted at zones of plate convergence. Upon subduction, dense oceanic crust sinks to the base of the mantle and gradually accumulates to form broad, robust thermochemical piles. Sinking oceanic crust entrains the surrounding mantle whose composition is predominantly primitive early in the model's evolution. As a result, thermochemical piles are initially supplied with relatively high concentrations of primitive material—summing up to ∼30% their total mass. The dense oceanic crust dominating the piles resists efficient mixing and preserves the primitive material that it is intermingled with. The significance of this process is shown to be proportional the rate of mantle processing through time and the excess density of oceanic crust at mantle pressures and temperatures. Unlike other theories for the survival of Earth's primitive mantle, this one does not require the early Earth to have large‐scale domains of anomalously high density and/or viscosity. Plain Language Summary: When oceanic plates pull apart the mantle melts to form slabs of lithosphere, which are later recycled back into the mantle at subduction zones. This process of melting and subduction destroys the initial chemical signature of the mantle. Geochemical analyses reveal that some portion of the mantle has avoided this process and retained a chemically "primitive" signature. How this material has survived vigorous convection for ∼4.5 Gyr is an open question. Here we propose that it may be preserved at the base of the mantle in large accumulations of subducted lithosphere. These accumulations are dominated by dense oceanic crust but can comprise up to 30% primitive material. The intermingling of oceanic crust and primitive material may explain why the chemical signatures of both coexist in volcanic eruptions at Earth's surface. Key Points: Subducting oceanic lithosphere entrains primitive mantle as it sinks to the core‐mantle boundaryDense oceanic crust forms robust thermochemical piles that can trap and preserve primitive material over the age of the EarthThe mixture of primitive and recycled material may explain the co‐existence of these signatures observed in ocean island basalts [ABSTRACT FROM AUTHOR]

We present the GFZ candidate field models for the 13 th Generation International Geomagnetic Reference Field (IGRF-13). These candidates were derived from the Mag.num.IGRF13 geomagnetic core field model, which is constrained by Swarm satellite and ground observatory data from November 2013 to August 2019. Data were selected from magnetically quiet periods, and the model parameters have been obtained using an iteratively reweighted inversion scheme approximating a robust modified Huber norm as a measure of misfit. The root mean square misfit of the Mag.num.IGRF13 model to Swarm and observatory data is in the order of 3–5 nT for mid and low latitudes, with a maximum of 44 nT for the satellite east component data at high latitudes. The time-varying core field is described by order 6 splines and spherical harmonic coefficients up to degree and order 20. We note that the temporal variation of the core field component of the Mag.num.IGRF13 model is strongly damped and shows a smooth secular variation that suits well for the IGRF, where secular variation is represented as constant over 5-year intervals. Further, the external field is parameterised by a slowly varying part and a more rapidly varying part controlled by magnetic activity and interplanetary magnetic field proxies. Additionally, the Euler angles of the magnetic field sensor orientation are co-estimated. A widely discussed feature of the geomagnetic field is the South Atlantic Anomaly, a zone of weak and decreasing field strength stretching from southern Africa over to South America. The IGRF and Mag.num.IGRF13 indicate that the anomaly has developed a second, less pronounced eastern minimum at Earth's surface since 2007. We observe that while the strong western minimum continues to drift westwards, the less pronounced eastern minimum currently drifts eastward at Earth's surface. This does not seem to be linked to any eastward motion at the core–mantle boundary, but rather to intensity changes of westward drifting flux patches contributing to the observed surface field. Also, we report a sudden change in the secular variation measured at two South Atlantic observatories around 2015.0, which occurred shortly after the well-known jerk of 2014.0. [ABSTRACT FROM AUTHOR]

The South Atlantic Anomaly (SAA) is a region at Earth's surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Here, we propose new measures to better characterize the SAA area and center, accounting for surface intensity changes outside the SAA region and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the SAA area and center are more time dependent, including episodes of steady area, eastward drift and rapid southward drift. We interpret these special events in terms of the secular variation of relevant large-scale geomagnetic flux patches on the core–mantle boundary. Our characterization may be used as a constraint on Earth-like numerical dynamo models. [ABSTRACT FROM AUTHOR]

The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth's lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β -factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its β -factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β -factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰. [ABSTRACT FROM AUTHOR]

Geomagnetic jerks are sudden changes in the geomagnetic field secular variation related to changes in outer core flow patterns. Finding geophysical phenomena related to geomagnetic jerks provides a vital contribution to better understand the geomagnetic field behavior. Here, we link the geomagnetic jerks occurrence with one of the most relevant features of the geomagnetic field nowadays, the South Atlantic Anomaly (SAA), which is due to the presence of reversed flux patches (RFPs) at the Core-Mantle Boundary (CMB). Our results show that minima of acceleration of the areal extent of SAA calculated using the CHAOS-7 model (CHAOS-7.2 release) coincide with the occurrence of geomagnetic jerks for the last 2 decades. In addition, a new pulse in the secular acceleration of the radial component of the geomagnetic field has been observed at the CMB, with a maximum in 2016.2 and a minimum in 2017.5. This fact, along with the minimum observed in 2017.8 in the acceleration of the areal extent of SAA, could point to a new geomagnetic jerk. We have also analyzed the acceleration of the areal extent of South American and African RFPs at the CMB related to the presence of the SAA at surface and have registered minima in the same periods when they are observed in the SAA at surface. This reinforces the link found and would indicate that physical processes that produce the RFPs, and in turn the SAA evolution, contribute to the core dynamics at the origin of jerks.

Abstract The locations of ultralow‐velocity zones (ULVZs) at the core‐mantle boundary (CMB) have been linked to a variety of features including hot spot volcanoes and large low‐velocity province (LLVP) boundaries, yet only a small portion of the CMB region has been probed for ULVZ existence. Here we present a new map of lower mantle heterogeneity locations using a global collection of highly anomalous SPdKS recordings based on a dataset of more than 58,000 radial component seismograms, which sample 56.9% of the CMB by surface area. The inference of heterogeneity location using the SPdKS seismic phase is challenging due to source‐versus receiver‐side ambiguity. Due to this ambiguity, we conducted an inversion using the principle of parsimony. The inversion is conducted using a genetic algorithm which is repeated several thousand times in order to construct heterogeneity probability maps. This analysis reveals that at probabilities ≥0.5, 0.25, and 0.125 up to 1.3%, 8.2%, or 19.7% of the CMB may contain ULVZ‐like heterogeneities. These heterogeneities exist in all lower mantle settings, including both high‐ and low‐velocity regions. Additionally, we present evidence that the Samoan ULVZ may be twice as large as previously estimated, and also present evidence for the existence of additional mega‐sized ULVZs, such as a newly discovered ULVZ located to the east of the Philippines. We provide new evidence for the ULVZ east of the Philippines through an analysis of ScP records.

We present observations on a new precursory phase of seismic waves scattered in the deep Earth. This phase arrives prior to the PKPab wave at epicentral distances larger than 155°, and we call it PKPab precursor. We show that the presence of the PKPab precursor is a necessary consequence of scattering in D″, which is the commonly accepted cause of the PKPdf precursor at distances smaller than 145°. PKPdf waves that propagate through the inner core should arrive before the PKPab precursor but those, are strongly attenuated in the inner core at frequencies between 4 Hz and 8 Hz used here, making the PKPab precursor the earliest teleseismic signal at distances larger than 155°. Calculated PKPab precursor sensitivity kernel shows that this phase is mostly sensitive to scattering along the closest PKPbc path between source and receiver. It can thus help to constrain the lateral distribution of heterogeneity along D″. Plain Language Summary: A new discovered seismic signal recorded far away from earthquakes, by stations on the other side of Earth, will help to study the properties of the core–mantle boundary. We use high frequencies at which seismic waves do not propagate through the Earth's inner core but are instead propagated around it by deflection at heterogeneity located along the core–mantle boundary. Key Points: PKP precursor observed at distance beyond 155°D″ scattering of teleseismic waves at 6 HzRadiative transfer simulation used to locate regions of heterogeneity [ABSTRACT FROM AUTHOR]