The Experts below are selected from a list of 261 Experts worldwide ranked by ideXlab platform
Sebastian Rost - One of the best experts on this subject based on the ideXlab platform.
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Regional stratification at the top of Earth's core due to Core-Mantle Boundary heat flux variations
Nature Geoscience, 2019Co-Authors: J. E. Mound, Sebastian Rost, Christopher J. Davies, Jon AurnouAbstract:Earth’s magnetic field is generated by turbulent motion in its fluid outer core. Although the bulk of the outer core is vigorously convecting and well mixed, some seismic, geomagnetic and geodynamic evidence suggests that a global stably stratified layer exists at the top of Earth’s core. Such a layer would strongly influence thermal, chemical and momentum exchange across the core–mantle Boundary and thus have important implications for the dynamics and evolution of the core. Here we argue that the relevant scenario is not global stratification, but rather regional stratification arising solely from the lateral variations in heat flux at the core–mantle Boundary. Using our extensive suite of numerical simulations of the dynamics of the fluid core with heterogeneous core–mantle Boundary heat flux, we predict that thermal regional inversion layers extend hundreds of kilometres into the core under anomalously hot regions of the lowermost mantle. Although the majority of the outermost core remains actively convecting, sufficiently large and strong regional inversion layers produce a one-dimensional temperature profile that mimics a globally stratified layer below the core–mantle Boundary—an apparent thermal stratification despite the average heat flux across the core–mantle Boundary being strongly superadiabatic.
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Deep Earth: Core-Mantle Boundary landscapes
Nature Geoscience, 2013Co-Authors: Sebastian RostAbstract:The molten-iron alloy of the core meets the mantle's silicate rock at Earth's core–mantle Boundary. Seismological images reveal hummocks of iron-enriched material above the Boundary, highlighting the heterogeneous nature of the mantle.
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Identifying regions of strong scattering at the Core-Mantle Boundary from analysis of PKKP precursor energy
Earth and Planetary Science Letters, 2010Co-Authors: Sebastian Rost, Paul S. EarleAbstract:Abstract We detect seismic scattering from the core–mantle Boundary related to the phase PKKP (PK • KP) in data from small aperture seismic arrays in India and Canada. The detection of these scattered waves in data from small aperture arrays is new and allows a better characterization of the fine-scale structure of the deep Earth especially in the southern hemisphere. Their slowness vector is determined from array processing allowing location of the heterogeneities at the core–mantle Boundary using back-projection techniques through 1D Earth models. We identify strong scattering at the core–mantle Boundary (CMB) beneath the Caribbean, Patagonia and the Antarctic Peninsula as well as beneath southern Africa. An analysis of the scattering regions relative to sources and receivers indicates that these regions represent areas of increased scattering likely due to increased heterogeneities close to the CMB. The 1 Hz array data used in this study is most sensitive to heterogeneity with scale lengths of about 10 km. Given the small size of the scatterers, a chemical origin of the heterogeneities is likely. By comparing the location of the fine-scale heterogeneity to geodynamical models and tomographic images, we identify different scattering mechanisms in regions related to subduction (Caribbean and Patagonia) and dense thermo chemical piles (Southern Africa).
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Detection of an ultralow velocity zone at the core‐mantle Boundary using diffracted PKKPab waves
Journal of Geophysical Research, 2006Co-Authors: Sebastian Rost, Edward J. GarneroAbstract:[1] Seismic phases diffracted around Earth's core contain information about lowermost mantle wave speeds. By measuring the slowness of incident diffracted energy from array recordings, seismic velocity along the diffracted path can be estimated. Here we apply this principle to diffraction of the major arc seismic phase PKKPab recorded at the Canadian Yellowknife array to estimate P wave velocity variations along the Core-Mantle Boundary. We observe PKKPabdiff about 7.5° past the ray theoretical cutoff distance for PKKPab. We utilize 330 western Pacific rim earthquakes that allow us to probe the Core-Mantle Boundary beneath the North Atlantic and the South Pacific oceans using PKKPabdiff. Slowness and back azimuth are measured by frequency–wave number analysis. Mapping PKKPabdiff slowness variations suggest 4–19% P wave velocity reductions relative to PREM, in good agreement with the magnitude of velocity reductions previously mapped in ultralow velocity zones. The PKKPabdiff slowness and back-azimuth variations combined with results from previous ULVZ studies using SPdiffKS imply that the lowered velocities occur at the base of the mantle beneath the North Atlantic Ocean, along the receiver side of raypaths. PKKPabdiff array measurements thus hold important potential for mapping ultralow velocity zone structure in so far unprobed regions of the lower mantle as well as for providing additional and independent information about lower mantle structure.
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Seismological constraints on a possible plume root at the core–mantle Boundary
Nature, 2005Co-Authors: Sebastian Rost, Edward J. Garnero, Quentin Williams, Michael MangaAbstract:Recent seismological discoveries have indicated that the Earth's core–mantle Boundary is far more complex than a simple Boundary between the molten outer core and the silicate mantle. Instead, its structural complexities probably rival those of the Earth's crust1. Some regions of the lowermost mantle have been observed to have seismic wave speed reductions of at least 10 per cent2,3,4,5,6,7, which appear not to be global in extent7,8,9. Here we present robust evidence for an 8.5-km-thick and ∼50-km-wide pocket of dense, partially molten material at the core–mantle Boundary east of Australia. Array analyses of an anomalous precursor to the reflected seismic wave ScP reveal compressional and shear-wave velocity reductions of 8 and 25 per cent, respectively, and a 10 per cent increase in density of the partially molten aggregate. Seismological data are incompatible with a basal layer composed of pure melt, and thus require a mechanism to prevent downward percolation of dense melt within the layer. This may be possible by trapping of melt by cumulus crystal growth following melt drainage from an anomalously hot overlying region of the lowermost mantle. This magmatic evolution and the resulting cumulate structure seem to be associated with overlying thermal instabilities, and thus may mark a root zone of an upwelling plume.
John P. Brodholt - One of the best experts on this subject based on the ideXlab platform.
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Subducted banded iron formations as a source of ultralow-velocity zones at the core–mantle Boundary
Nature, 2005Co-Authors: David P. Dobson, John P. BrodholtAbstract:Ultralow-velocity zones (ULVZs) are regions of the Earth's core–mantle Boundary about 1–10 kilometres thick exhibiting seismic velocities that are lower than radial-Earth reference models by about 10–20 per cent for compressional waves and 10–30 per cent for shear waves. It is also thought that such regions have an increased density of about 0–20 per cent (ref. 1). A number of origins for ULVZs have been proposed, such as ponding of dense silicate melt2, core–mantle reaction zones3 or underside sedimentation from the core4. Here we suggest that ULVZs might instead be relics of banded iron formations subducted to the core–mantle Boundary between 2.8 and 1.8 billion years ago. Consisting mainly of interbedded iron oxides and silica, such banded iron formations were deposited in the world's oceans during the late Archaean and early Proterozoic eras. We argue that these layers, as part of the ocean floor, would be recycled into the Earth's interior by subduction5, sink to the bottom of the mantle and may explain all of the observed features of ULVZs.
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Subducted banded iron formations as a source of ultralow-velocity zones at the Core-Mantle Boundary
NATURE, 2005Co-Authors: John P. BrodholtAbstract:Ultralow-velocity zones (ULVZs) are regions of the Earth's core mantle Boundary about 1 - 10 kilometres thick exhibiting seismic velocities that are lower than radial-Earth reference models by about 10 - 20 per cent for compressional waves and 10 - 30 per cent for shear waves. It is also thought that such regions have an increased density of about 0 - 20 per cent (ref. 1). A number of origins for ULVZs have been proposed, such as ponding of dense silicate melt(2), core - mantle reaction zones(3) or underside sedimentation from the core(4). Here we suggest that ULVZs might instead be relics of banded iron formations subducted to the core - mantle Boundary between 2.8 and 1.8 billion years ago. Consisting mainly of interbedded iron oxides and silica, such banded iron formations were deposited in the world's oceans during the late Archaean and early Proterozoic eras. We argue that these layers, as part of the ocean floor, would be recycled into the Earth's interior by subduction(5), sink to the bottom of the mantle and may explain all of the observed features of ULVZs.
Jeanlouis Le Mouel - One of the best experts on this subject based on the ideXlab platform.
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tangentially geostrophic flow at the core mantle Boundary compatible with the observed geomagnetic secular variation the large scale component of the flow
Physics of the Earth and Planetary Interiors, 1990Co-Authors: C Gire, Jeanlouis Le MouelAbstract:Abstract We present a method for determining the large-scale component of a tangentially geostrophic flow beneath the Core-Mantle Boundary compatible with magnetic secular variation observations. We use a tangentially geostrophic basis to ensure the geostrophy of the motion. The fit of the secular variation (SV) generated by the motion to the observed SV (in fact SV models) is adequate, taking into account the existing error level. As in any horizontal geostrophic motion, the flow is expressed as a sum of two independent tangentially geostrophic flows: a zonal component, which is toroidal, and a non-zonal component, which is directly linked with the motions deeper in core. The flow derived for the recent epoch (1970–1985) presents interesting symmetry properties: the non-zonal velocities are the same at two antipodal points, while the zonal velocities are the same at two points symmetrical about the Equator. The non-zonal component of the flow is more vigorous than the zonal one; the consoidal ingredient, though weaker than the toroidal ingredient, is essential and indicates strong vertical motion at depth in low-latitude areas. The SV is actually compatible with a geostrophic motion at the Core-Mantle Boundary and appears to be mainly due to the action of the non-zonal component of the flow.
Edward J. Garnero - One of the best experts on this subject based on the ideXlab platform.
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Compositionally-distinct ultra-low velocity zones on Earth's Core-Mantle Boundary
Nature Communications, 2017Co-Authors: Mingming Li, Edward J. Garnero, Allen K. Mcnamara, Shule YuAbstract:The Earth’s lowermost mantle large low velocity provinces are accompanied by small-scale ultralow velocity zones in localized regions on the Core-Mantle Boundary. Large low velocity provinces are hypothesized to be caused by large-scale compositional heterogeneity (i.e., thermochemical piles). The origin of ultralow velocity zones, however, remains elusive. Here we perform three-dimensional geodynamical calculations to show that the current locations and shapes of ultralow velocity zones are related to their cause. We find that the hottest lowermost mantle regions are commonly located well within the interiors of thermochemical piles. In contrast, accumulations of ultradense compositionally distinct material occur as discontinuous patches along the margins of thermochemical piles and have asymmetrical cross-sectional shape. Furthermore, the lateral morphology of these patches provides insight into mantle flow directions and long-term stability. The global distribution and large variations of morphology of ultralow velocity zones validate a compositionally distinct origin for most ultralow velocity zones. Ultralow velocity zones are detected on the Core-Mantle Boundary, but their origin is enigmatic. Here, the authors find that the global distribution and large variations of morphology of ultralow velocity zones are consistent with most having a compositionally-distinct origin.
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Detection of an ultralow velocity zone at the core‐mantle Boundary using diffracted PKKPab waves
Journal of Geophysical Research, 2006Co-Authors: Sebastian Rost, Edward J. GarneroAbstract:[1] Seismic phases diffracted around Earth's core contain information about lowermost mantle wave speeds. By measuring the slowness of incident diffracted energy from array recordings, seismic velocity along the diffracted path can be estimated. Here we apply this principle to diffraction of the major arc seismic phase PKKPab recorded at the Canadian Yellowknife array to estimate P wave velocity variations along the Core-Mantle Boundary. We observe PKKPabdiff about 7.5° past the ray theoretical cutoff distance for PKKPab. We utilize 330 western Pacific rim earthquakes that allow us to probe the Core-Mantle Boundary beneath the North Atlantic and the South Pacific oceans using PKKPabdiff. Slowness and back azimuth are measured by frequency–wave number analysis. Mapping PKKPabdiff slowness variations suggest 4–19% P wave velocity reductions relative to PREM, in good agreement with the magnitude of velocity reductions previously mapped in ultralow velocity zones. The PKKPabdiff slowness and back-azimuth variations combined with results from previous ULVZ studies using SPdiffKS imply that the lowered velocities occur at the base of the mantle beneath the North Atlantic Ocean, along the receiver side of raypaths. PKKPabdiff array measurements thus hold important potential for mapping ultralow velocity zone structure in so far unprobed regions of the lower mantle as well as for providing additional and independent information about lower mantle structure.
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Seismological constraints on a possible plume root at the core–mantle Boundary
Nature, 2005Co-Authors: Sebastian Rost, Edward J. Garnero, Quentin Williams, Michael MangaAbstract:Recent seismological discoveries have indicated that the Earth's core–mantle Boundary is far more complex than a simple Boundary between the molten outer core and the silicate mantle. Instead, its structural complexities probably rival those of the Earth's crust1. Some regions of the lowermost mantle have been observed to have seismic wave speed reductions of at least 10 per cent2,3,4,5,6,7, which appear not to be global in extent7,8,9. Here we present robust evidence for an 8.5-km-thick and ∼50-km-wide pocket of dense, partially molten material at the core–mantle Boundary east of Australia. Array analyses of an anomalous precursor to the reflected seismic wave ScP reveal compressional and shear-wave velocity reductions of 8 and 25 per cent, respectively, and a 10 per cent increase in density of the partially molten aggregate. Seismological data are incompatible with a basal layer composed of pure melt, and thus require a mechanism to prevent downward percolation of dense melt within the layer. This may be possible by trapping of melt by cumulus crystal growth following melt drainage from an anomalously hot overlying region of the lowermost mantle. This magmatic evolution and the resulting cumulate structure seem to be associated with overlying thermal instabilities, and thus may mark a root zone of an upwelling plume.
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A New Paradigm for Earth's Core-Mantle Boundary
Science, 2004Co-Authors: Edward J. GarneroAbstract:Understanding the Boundary between Earth9s rock mantle and the molten core is one of the key challenges in geoscience. In his Perspective, Garnero discusses recent geophysical discoveries that are forcing the adoption of a new paradigm for studying the Core-Mantle Boundary. Long thought to be a simple division between solid silicate rock and liquid iron alloy, the Core-Mantle Boundary is now emerging as an active, complex, and heterogeneous region of Earth9s interior.
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Fuzzy patches on the Earth's Core-Mantle Boundary?
Geophysical Research Letters, 2000Co-Authors: Edward J. Garnero, Raymond JeanlozAbstract:Recent seismological investigations reveal the presence of highly anomalous structures at the base of the mantle, modeled as patches ≤5–50 km thick having ultralow-velocities (−δVP∼10–20%, −δVS∼10–50%). Waveform modeling shows seismological data are compatible with the patches exhibiting a wide range of density increases, up to δρ∼60%, which can be ascribed to chemical contamination of the deep mantle by the core. Not all anomalies require lowermost mantle partial melting, and may be located just below or right at the Core-Mantle Boundary (CMB): a ∼1–3 km thick zone of finite rigidity (crystallization?) at the top of the outer core or, more generally, of gradational properties across the CMB can also explain observations. Fuzzy patches at the Boundary may be zones of intense chemical and physical interactions between the mantle and core.
V. V. Bykova - One of the best experts on this subject based on the ideXlab platform.
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Can the Core-Mantle Boundary Topography Influence the Earth’s Nutation?
Inertial Coordinate System on the Sky, 1990Co-Authors: V. V. BykovaAbstract:The nutation of the Earth with slightly nonelliptical liquid core is investigated by the perturbation theory method. It is shown that first-order terms affect the core ellipticity and its triaxiality. The most sensitive nutation terms in the second approximation were found to be retrograde 18.6-year term and retrograde annual term. The observed nutation amplitude values can be satisfied by special Core-Mantle Boundary form.
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Can the Core-Mantle Boundary Topography Influence the Earth's Nutation?
Symposium - International Astronomical Union, 1990Co-Authors: V. V. BykovaAbstract:The nutation of the Earth with slightly nonelliptical liquid core is investigated by the perturbation theory method. It is shown that first-order terms affect the core ellipticity and its triaxiality. The most sensitive nutation terms in the second approximation were found to be retrograde 18.6-year term and retrograde annual term. The observed nutation amplitude values can be satisfied by special Core-Mantle Boundary form.