Earth's Core Research Articles

Elasticity and viscosity of hcp iron at Earth's inner core conditions from machine learning-based large-scale atomistic simulations

Elasticity and viscosity of hcp iron at Earth's inner core conditions from machine learning-based large-scale atomistic simulations

The inherent instability of axisymmetric magnetostrophic dynamo models

Recent studies have demonstrated the possibility of constructing magnetostrophic dynamo models, which describe the slowly evolving background state of Earth's magnetic field when inertia and viscosity are negligible. Here we explore the properties of steady, stable magnetostrophic states as a leading order approximation to the slow dynamics within Earth's core. For the case of an axisymmetric magnetostrophic system driven by a prescribed α-effect, we confirmed the existence of four known steady states: , , where is purely dipolar and is purely quadrupolar. Importantly, here we show that in all but the most weakly driven cases, an initial magnetic field that is not purely dipolar or quadrapolar never converges to these states. Despite this instability, we also show that there are a plethora of instantaneous solutions that are quasi-steady, but nevertheless unstable. If the dynamics in Earth's core are reasonably modelled by a strongly driven α-effect, this work suggests that the background state can never be steady. We discuss the difficulties in comparing our magnetostrophic models with geomagnetic timeseries.

CCMOC: A new view of the Earth's outer core through the global coda correlation wavefield

Increasing seismic evidence has accumulated, suggesting that the Earth's outer core consists of distinct zones of low P-wave velocities in the top and bottom regions relative to the Preliminary Reference Earth Model (PREM). Seismically detected low velocities in the outer core could be linked with the stratification, essential for understanding the geodynamo and thermochemical evolution of the liquid core. However, a consistent globally-averaged radial structure of the outer core has not been obtained due to the incomplete coverage of sampling body waves. To remedy this problem, we explore the seismic structure of Earth's outer core by employing a new theoretical and observational concept termed coda correlation wavefield. We construct the global correlogram in the 15–50 s period range by stacking cross-correlations of the long-duration coda waves from the selected ten large earthquakes. We then assemble a dataset of prominent correlation features from the global correlogram that are sensitive to the outer core. The waveforms of these features are fit by computing synthetic correlograms through various outer core models. The obtained optimal model displays P-wave velocities in both the outer core's top and bottom, consistent with Coda Correlation Reference Earth Model (CCREM) and reduced relative to PREM. The P-wave velocity is ∼1% lower in the core's top than that in PREM, and the slow anomaly gradually approaches zero at about 800 km below the core-mantle boundary. The low seismic velocities in the top of the outer core could likely imply the formation of a thermal and/or compositional stratification.

The root to the Galápagos mantle plume on the core-mantle boundary


 Ultra-low velocity zones (ULVZs) are thin anomalous patches on the boundary between the Earth's core and mantle, revealed by their effects on the seismic waves that propagate through them. Here we map a broad ULVZ near the Galápagos hotspot using shear-diffracted waves. Forward modelling assuming a cylindrical shape shows the patch is ~600 km wide, ~20 km high, and its shear velocities are ~25% reduced. The ULVZ is comparable to other broad ULVZs mapped on the core-mantle boundary near Hawaii, Iceland, and Samoa. Strikingly, all four hotspots where the mantle plume appears rooted by these ‘mega-ULVZs’, show similar anomalous isotopic signatures in He, Ne, and W in their ocean island basalts. This correlation suggests mega-ULVZs might be primordial or caused by interaction with the core, and some material from ULVZs is entrained within the plume. For the Galápagos, the connection implies the plume is offset to the west towards the base of the mantle. 

Turbulent Rotating Rayleigh–Bénard Convection

Rotation with thermally induced buoyancy governs many astrophysical and geophysical processes in the atmosphere, ocean, sun, and Earth's liquid-metal outer core. Rotating Rayleigh–Bénard convection (RRBC) is an experimental system that has features of rotation and buoyancy, where a container of height H and temperature difference Δ between its bottom and top is rotated about its vertical axis with angular velocity Ω. The strength of buoyancy is reflected in the Rayleigh number (∼ H3Δ) and that of the Coriolis force in the Ekman and Rossby numbers (∼Ω−1). Rotation suppresses the convective onset, introduces instabilities, changes the velocity boundary layers, modifies the shape of thermal structures from plumes to vortical columns, affects the large-scale circulation, and can decrease or enhance global heat transport depending on buoyant and Coriolis forcing. RRBC is an extremely rich system, with features directly comparable to geophysical and astrophysical phenomena. Here we review RRBC studies, suggest a unifying heat transport scaling approach for the transition between rotation-dominated and buoyancy-dominated regimes in RRBC, and discuss non-Oberbeck–Boussinesq and centrifugal effects.

Zur Gründung des 140 m hohen Staudamms Kühtai im Längental

AbstractKühtai Dam is an embankment dam with a central earth core that is founded on rock. The rock surface is treated by shaping, adding dental concrete and, finally, grouting. A large excavation was required to expose the rock in the earth core foundation. Before starting the excavation, the Längentalbach stream had to be diverted. The upstream and downstream shells of Kühtai Dam are founded on the remaining overburden. Under the earth core, an inspection tunnel is located along the dam axis. The tunnel serves to accommodate measuring equipment and decouple fill and grout curtain works in terms of sequence of construction. In deviation from the tender project, the inspection gallery on each side of the valley is constructed as a combination of two conventional adits and two raise‐boring shafts. To connect the adits and shafts, reinforced concrete structures are situated at half dam height, on which the earth core is founded directly similar to an inspection gallery in trench type. On the one hand, this adjustment is the result of a geological fault detected during the preliminary excavation works in 2020. The fault crosses the core footprint in the orographic left flank of the valley. On the other hand, the subdivision of the two long conventional shafts will eliminate the construction time constraint resulting from the requirement to not start grouting in the dam foundation until the conventional tunnelling below the dam has been completed.

A new incorporative element to modify plastic concrete mechanical characteristics for cut-off wall construction in very soft soil media: Identification of tensile galvanized open-mesh distributer (TGOD) element

• Tensile galvanized open-mesh distributor (TGOD) element boost deformability. • Incorporation of TGOD element does not result in any strength loss. • TGOD is multi-functional element involved in tensile and frictional mechanisms. Establishing cut-off walls in high deformable foundations or constructing a water-tight diaphragm wall within the clayey-damaged core of earth and rockfill dams requires plastic concrete with a typical higher deformability characteristic to ensure a decent wall’s mechanical performance compatible with the surrounding media. The present study investigates an appropriate method to achieve a proper mix design for the plastic concrete by high deformability and maintaining the required strength. In this regard, different methods in the mix designs were employed to evaluate the plastic concrete cylindrical 100 × 200 mm specimens under unconfined and confined conditions. A series of uniaxial compression tests accompanied by conventional triaxial compression tests were performed. The obtained results led to distinguishing a new multi-functional element named “Tensile Galvanized Open-mesh Distributor (TGOD)” as the most effective element to boost deformability and establish the preferred mix design. The results showed that incorporating the introduced elements fulfilled the needed mechanical specifications for plastic concrete construction within the very soft media and decreased the elastic modulus by more than 50 % compared to the control mix, without any loss in the load-bearing capacity.

Assessment of crack resistance of ultra-high earth core rockfill dam by pore pressure

Assessment of crack resistance of ultra-high earth core rockfill dam by pore pressure

Non‐Isotropic Contraction and Expansion of Samples in Diamond Anvil Cells: Implications for Thermal Conductivity at the Core‐Mantle Boundary

AbstractThe thermal conductivities of mantle and core materials have a major impact on planetary evolution, but their experimental determination requires precise knowledge of sample thickness at high pressure. Despite its importance, thickness in most diamond anvil cell (DAC) experiments is not measured but inferred from equations of state, assuming isotropic contraction upon compression or assuming isotropic expansion upon decompression. Here we provide evidence that in DAC experiments both assumptions are invalid for a range of mechanically diverse materials (KCl, NaCl, Ar, MgO, silica glass, Al2O3). Upon compression, these samples are ∼30–50% thinner than expected from isotropic contraction. Most surprisingly, all the studied samples continue to thin upon decompression to 10–20 GPa. Our results partially explain some discrepancies among the highly controversial thermal conductivity values of iron at Earth's core conditions. More generally, we suggest that in situ characterization of sample geometry is essential for conductivity measurements at high pressure.

The role of slow magnetostrophic waves in the formation of the axial dipole in planetary dynamos

The preference for the axial dipole in planetary dynamos is investigated through the analysis of wave motions in spherical dynamo models. Our study focuses on the role of slow magnetostrophic waves, which are generated from localized balances between the Lorentz, Coriolis and buoyancy (MAC) forces. Since the slow waves are known to intensify with increasing field strength, simulations in which the field grows from a small seed towards saturation are useful in understanding the role of these waves in dynamo action. Axial group velocity measurements in the energy-containing scales show that fast inertial waves slightly modified by the magnetic field and buoyancy are dominant under weak fields. However, the dominance of the slow waves is evident for strong fields satisfying ∣ωM/ωC∣∼ 0.1, where ωM and ωC are the frequencies of the Alfvén and inertial waves respectively. A MAC wave window of azimuthal wavenumbers is identified in which helicity generation by the slow waves strongly correlates with dipole generation. Analysis of the magnetic induction equation suggests a poloidal–poloidal field conversion in the formation of the dipole. Finally, the attenuation of slow waves may result in polarity reversals in a strongly driven Earth's core.

A Journey Through Nonlinear Dynamics: The Case of Temperature Gradients

The overall effect of temperature gradients is stressed for the Earth's core and surface, but also for the Sun's surface. Using Rayleigh–Bénard convection in helium and mercury, we measured all of the scaling properties of the period-doubling cascade and quasiperiodicity. Hard turbulence scaling properties are presented in an experiment using helium gas at low temperature. A [Formula: see text] scaling law is measured and also an exponential distribution for temperature fluctuations is observed. We present a study of a Rayleigh–Bénard convection cell with an open top and a floater. One of the simplest limit cycles is observed for the floater position. It follows a model proposed by Wilson for continent motion. Using the Soret effect, we study how temperature differences lead to strong accumulation of DNA suspensions. Also using polyethylene glycol concentration gradients, we measured local DNA and RNA accumulation. Finally, using thermal convection, we build one of the smallest PCR machines.

Length of day at c . 1.1 Ga based on cyclostratigraphic analyses of the Nanfen Formation in the North China craton, and its geodynamic implications

Length of day (LOD) varies throughout Earth history, dictated in the long term by the retreat of the Moon and growth of Earth's core. Reconstruction of LOD in deep time has greatly benefited from analyses of the orbital cycle from geological records. The Precambrian database, however, is very limited. Here we present a cyclostratigraphic study on c. 1.1 Ga laminated argillaceous limestones of the Nanfen Formation from the North China craton. Milankovitch cycles are successfully recognized from the consistent cyclical variations of the high-resolution magnetic susceptibility data measured on drillcores from the Nanfen Formation. Sedimentary cycles with wavelengths of 12.45, 10.72 and 9.96 m in three drillcores are interpreted as 405 kyr eccentricity cycles. An obliquity period of 20.9–21.4 kyr and a precession period of 12.7–16.0 kyr are also identified. The Earth–Moon distance and LOD are thus estimated at 3.43 ± 0.04 × 10 8 m and 18.94 ± 0.39 h, respectively. Our new results together with the data previously published reveal an important discontinuity between 1.9 and 1.4 Ga in the evolutionary trend of the LOD. We suggest that Earth's core grew rapidly during the interval, which significantly changed the moment of inertia of the Earth–Moon system, and tidal dissipation again ruled the LOD variations after that stage. Supplementary material: A description of the calculation method, magnetic susceptibility data, anisotropy of magnetic susceptibility data and supplementary figures are available at https://doi.org/10.6084/m9.figshare.c.6086544

Convective mesoscale turbulence at very low Prandtl numbers

Horizontally extended turbulent convection, termed mesoscale convection in natural systems, remains a challenge to investigate in both experiments and simulations. This is particularly so for very low molecular Prandtl numbers, such as occur in stellar convection and in the Earth's outer core. The present study reports three-dimensional direct numerical simulations of turbulent Rayleigh–Bénard convection in square boxes of side length $L$ and height $H$ with the aspect ratio $\varGamma =L/H$ of 25, for Prandtl numbers that span almost 4 orders of magnitude, $10^{-3}\le Pr \le 7$ , and Rayleigh numbers $10^5 \le Ra \le 10^7$ , obtained by massively parallel computations on grids of up to $5.36\times 10^{11}$ points. The low end of this $Pr$ -range cannot be accessed in controlled laboratory measurements. We report the essential properties of the flow and their trends with the Rayleigh and Prandtl numbers, in particular, the global transport of momentum and heat – the latter decomposed into convective and diffusive contributions – across the convection layer, mean vertical profiles of the temperature and temperature fluctuations and the kinetic energy and thermal dissipation rates. We also explore the degree to which the turbulence in the bulk of the convection layer resembles classical homogeneous and isotropic turbulence in terms of spectra, increment moments and dissipative anomaly, and find close similarities. Finally, we show that a characteristic scale of the order of the mesoscale seems to saturate to a wavelength of $\lambda \gtrsim 3H$ for $Pr\lesssim 0.005$ . We briefly discuss possible implications of these results for the development of subgrid-scale parameterization of turbulent convection.

The electronic and mechanical properties of Fe16N2 up to 150 GPa: First-principles calculation

The electronic and mechanical properties of a tetragonal Fe–N binary compound have been investigated under extreme pressure using density functional theory with the generalized gradient approximation. The results of formation enthalpy, phonon dispersion, and elastic stability criterion show that an Fe–N binary compound is stable up to 150 GPa. On the one hand, the electronic properties of Fe16N2 have been demonstrated by the band structures and density of states. On the other hand, the elastic constants, shear, and compressional modulus derived from elastic constants impact on compressional velocity (Vp) greatly. The shear modulus of a binary compound with N is about 14.7% lower than the data of pure Fe. The density and Vp of an Fe–N binary compound, which are closer with the value of a preliminary reference earth model (PREM), are lower than the pure Fe's, especially in the part of higher pressure. In addition, the density of an Fe–N binary compound is about 5.3% lower than the Fe's. The effect of the temperature is not considered in this work, but the varying tendency indicates that the light element N incorporating in pure Fe is helpful for building an earth's core model.

Offline-Online Submission and Assessment System (OSAS) chatbot as an E-tool on retrieval of learners’ outputs in Earth and Life Science during the Distance Learning

The COVID-19 had markedly impacted education that moved to distance learning. The Basic Education Learning Continuity Plan (BE-LCP) emphasized the fairness of learners' access to technology, gadgets, and household assistance. Besides, low-tech and no-tech approaches should consider. In response to the challenge, the researcher created Offline-Online Submission and Assessment System (OSAS) chatbot employed in the Messenger app with features on submission and assessment strategy used in Earth and Life Science core subject where both offline and online can access. The 170 participants were among the selected grade 11 senior high school learners exposed to OSAS chatbot for 6-Week. The 15-item OSAS Usability Scale was assessed, while the 3-item Open-ended Questions to analyze the learners' personal experiences made the study a mixed-method. The result of OSAS's 15-item Usability Scale in terms of (1) Convenience, (2) Applicability, (3) Accessibility, (4) Organization, and (5) Satisfaction were with all high-level interpretations. Besides, the 3-item Open-ended Questions focused on learners' experience revealed the OSAS advantages, such as Accessibility even with a low internet connection, Ease of assessment, Convenience of submission, Ability to track and monitor progress, User-friendly, No stress to use, and Lower cost. On-the-other-hand two drawbacks reported poor network signals and the pressing buttons. Overall, all learners recommended OSAS due to its convenience, ability to access even offline, less hassle on internet cost, effective aid in the teaching-learning process, and ability to track their progress personally.
 
 Keywords: submission, assessment, chatbot education, earth and life science

A Search for Large‐Scale Variations in the Fine‐Scale Structure of Earth's Inner Core

AbstractA hemispherical pattern in inner core attenuation (Q) has been observed from the apparent Q of inner core transmitted waves (PKPDF). How the elastic scattering (QSC) originating from kilometer‐scale inner core heterogeneities relates to large‐scale apparent Q variations remains elusive. Such inner core scattered (ICS) energy is characterized by emergent, long‐lasting, high‐frequency (1–4 Hz) coda immediately following pre‐critical reflections (PKiKP) from the inner core boundary (ICB). In this study, we develop a framework to systematically investigate ICS and examine its hemispherical pattern using data from two arrays that sample opposing sides of the Pacific quasi‐hemispherical boundary. We use all the viable data from earthquakes (Mw ≥ 5.8) within the past 2–3 decades recorded at distances of 50°–75° by YKA and ILAR—small‐aperture (∼10–20 km) seismic arrays situated in northwestern North America. Two independent waveform stripping methods are applied to extract ICS from the background energy, and various stacks are performed to identify ICS and its spatial pattern. We found that individual beams lacking clear evidence of PKiKP coda waves reveal ICS characteristics when stacked together, implying that ICS is ubiquitous. We used a modified phonon‐based simulation to reinforce the idea that ICS is primarily created by volumetric heterogeneity within the inner core as opposed to ICB topography. With simplified two‐layer ICS models, our results suggest that ICS within the eastern quasi‐hemisphere is slightly stronger than in the western quasi‐hemisphere, although intra‐hemispherical variations are as significant, and our sampling is limited to patches of the northern hemisphere.

Reconstruction of the Virtual Geomagnetic Pole (VGP) path at high latitude for the last 22 kyr: The role of radial field flux patches as VGP attractor

Reconstruction of geomagnetic field changes has a strong potential to complement geodynamo modeling and improve the understanding of Earth's core dynamics. Recent works based on geomagnetic measurements pointed out that over the last two decades the position of the north magnetic pole has been largely determined by the influence of two competing flux lobes under Canada and Siberia.In order to understand if the waxing and waning of magnetic flux lobes have driven the path of geomagnetic paleopoles in the past, we present an augmented and updated record of the chronology and paleosecular variation of geomagnetic field for the last 22 kyr derived from sedimentary cores collected along the north-western margin of Barents Sea and western margin of Spitsbergen (Arctic). The path of the virtual geomagnetic pole (VGP) has been reconstructed over this time period and compared with the maps of the radial component of the geomagnetic field at the core-mantle boundary, obtained from the most recent models. The VGP path includes centuries during which the VGP position is stable and centuries during which its motion accelerates. We recognize both clockwise and counterclockwise VGP paths, mostly developing inside the surface projection of the inner core tangent cylinder in the Arctic region. The VGP path seems to follow the appearance of Br patches of normal magnetic flux, especially those located under Siberia and Canada areas, but also those that may cause peculiar paleomagnetic features such as the Levantine Iron Age Anomaly.

Extremely weak early Cambrian dipole moment similar to Ediacaran: Evidence for long-term trends in geomagnetic field behaviour?

Paleointensity data can yield insight on the state of the geodynamo, providing constraints on deep Earth events and enabling analysis of long-term trends in the paleomagnetic field. The Ediacaran (635 Ma–539 Ma) is a period of discrepant paleomagnetic behaviour that was recently characterised by sustained, extremely weak, paleointensity. The interval also coincides with some of the most recent estimates for Earth's inner core nucleation (ICN) age, determined from numerical geodynamo models and analysis of long-term paleointensity data. However, the field strength during the subsequent Cambrian period (540 Ma–485 Ma) is largely unknown with almost no data.Here, we provide high-quality paleointensity results for the Cambrian. A Grenville dyke (∼590 Ma) that was baked by the Chatham-Grenville stock (532 Ma), slowly cooled at a rate controlled by the stock and recorded the paleointensity averaged over this interval (up to several tens of thousands of years). The characteristic paleomagnetic directions of the dyke are well-defined and consistent with those previously obtained from the Chatham-Grenville and Mont Riguad stocks.Paleointensity data were obtained using multiple methods and indicate an extremely weak field during a period coincidental with evidence for hyper-reversing activity extending into the late Cambrian. The dipole strength is similar to that of the ‘ultra-weak’ Ediacaran and may suggest that this paleomagnetic behaviour persisted into the Cambrian. The cause of this weak-field interval remains enigmatic but an approximate 200-million-year quasi-periodicity in dipole strength extending across the entire Phanerozoic is not ruled out.

Combined dynamical and morphological characterisation of geodynamo simulations

Numerical dynamo simulations cannot operate at the physical conditions of Earth's core, yet they often produce fields that appear morphologically similar to the present geomagnetic field. A key issue is therefore to decipher under what conditions “Earth-like” simulations can be achieved. Recent work has shown that a set of simulations undertaken along a specific path in parameter space smoothly approach the QG-MAC dynamics that are expected in Earth's core, whereby the leading order force balance is Quasi-Geostrophic with Magnetic, Archimedean and Coriolis forces equilibrating at first order. However, a systematic link between QG-MAC balance and morphological features of the simulated fields has yet to be established. Here we assess a suite of 67 simulations using established compliance criteria for the field morphology and scale-dependent force balances to quantify the internal dynamics. Morphological compliance with the modern geomagnetic field does not imply a single underlying force balance or vice versa; however, the majority of compliant simulations, including all those approaching a realistic value of the magnetic Reynolds number Rm, are in QG-MAC balance. Simulations that simultaneously achieve excellent morphological compliance with Earth's modern field, QG-MAC balance, and high Rm, are confined to an intermediate range of dipolarity (the ratio of energy in the dipole field to the energy truncated at degree 12 at the outer boundary). Reversing simulations in this dipolarity range maintain dominant QG-MAC balance during polarity transition, though inertia makes a non-negligible contribution to the force balance.

Sound Velocity Measurements of B2‐Fe‐Ni‐Si Alloy Under High Pressure by Inelastic X‐Ray Scattering: Implications for the Composition of Earth's Core

AbstractElastic properties of B2‐Fe0.67Ni0.06Si0.27 (15 wt.% Si) alloy have been investigated by combined high‐resolution inelastic X‐ray scattering and powder X‐ray diffraction in diamond anvil cells up to 100 GPa at room temperature. Densities (ρ), compressional (VP) and shear (VS) wave velocities were extrapolated to inner core conditions to enable comparison with the preliminary reference Earth model. The modeled aggregate compressional and shear wave velocities and densities of the two‐phase mixture of B2‐Fe0.67Ni0.06Si0.27 and hcp‐Fe‐Ni are consistent with inner core PREM values of VP, VS, and ρ based on a linear mixing model with 30(5) vol % B2‐Fe0.67Ni0.06Si0.27 and 70(5) vol % hcp Fe‐Ni, which corresponds to ∼3–5 wt.% Si and ∼5–12 wt.% Ni.