Earth's Outer Core Research Articles

Nitrogen Content in the Earth's Outer Core

AbstractUsing first principles molecular dynamic simulations, we explore the effects of nitrogen (N) on the density and sound velocity of liquid iron and evaluate its potential as a light element in the Earth's outer core. Our results suggest that Fe‐N melt cannot simultaneously explain the density and seismic velocity of the Earth's outer core. Although ~2.0 wt.% N can explain the bulk sound velocity of the outer core, such N content only lowers the density of liquid Fe by ~3%. Matching both the velocity and density by the other light elements limits the N in the core to ≪2.0 wt.%. Our finding suggests that nitrogen is a minor to trace element in the Earth's core and is consistent with the geochemical mass balance with terrestrial abundance of N and alloy‐silicate partitioning data, which suggest that there cannot be significant N in the core.

Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth

AbstractThe Earth's outer core is believed to be laterally homogeneous because of its low viscosity. However, a hemispherical difference in the inner core likely exists due to its uneven growth, which may be accompanied by localized light‐element releases to the outer core. A few seismological studies proposed heterogeneous lowermost outer core (called F layer) but using methods that are not very sensitive to F layer structures. In a previous study we developed a new method sensitive to the F layer structure and insensitive to others. The method analyzes differences in P wave traveltimes reflected on the inner core boundary and those that turn above the boundary as well as dispersion in waves bottoming or diffracting in the F layer, and was applied to obtain an F layer model of P wave velocity for the northeastern Pacific Ocean. In this paper, we examine the F layer structure beneath Australia using the same method. The observed dispersion requires a lower‐velocity gradient beneath Australia than beneath the northeastern Pacific, whereas the observed traveltime differences require higher average velocities beneath Australia. The results obtained for the two regions indicate that the F layer is laterally heterogeneous and that the layer beneath Australia has a higher velocity and velocity gradient in its upper part and a much smaller gradient in its lower part than beneath northeastern Pacific. The maximum velocity difference between the two regions is 0.04 km/s, which corresponds to 0.8 wt% excess oxygen according to ab initio calculations of elastic properties. These results suggest regional light‐element concentration beneath Australia.

On Destruction of a Thermally Stable Layer by Compositional Convection in the Earth's Outer Core

We discuss destruction of a thermally stable layer in the upper part of the Earth's outer core by compositional convection excited at the inner core boundary. We propose to use the radial distribution of power induced by thermal and compositional buoyancy (rate of kinetic energy production) as a measure of occurrence of thermal and compositional convection. The power consists of the terms proportional to convective entropy flux and convective compositional flux. In the region with positive power, convection is active because kinetic energy can be produced by buoyancy force, and a stably stratified layer could not be formed there. On the other hand, in the region with negative power, convection is suppressed and a stably stratified layer may be produced. Considering penetration effect of convection, we discuss possible maximum and minimum thicknesses of the stable layer based on the radial distribution of power and its radial integral, respectively. We construct a 1-dimensional thermal and compositional balance model of the Earth's core with a larger value of thermal conductivity recently suggested by high-pressure experiments and first principle calculations, and estimate radial distributions of power for various values of core mantle boundary (CMB) heat flux Q_CMB. When Q_CMB > Q_sCMB no thermally stable layer can exist, where Q_sCMB is conductive heat flux along the adiabat at CMB. On the other hand, when Q_CMB < Q_sCMB, formation of an upper thermally stable layer becomes possible, depending on the extent of penetration of compositional convection excited below. When Q_CMB is sufficiently lower than Q_sCMB, a thermally stable layer survives the maximum penetration of compositional convection. The results show that a thermally stable layer becomes effectively thinner when the effect of compositional convection is considered compared with the results of previous studies where the existence of a stable layer is evaluated based on the convective flux only.

Weakly nonlinear magnetic equatorial Kelvin waves in rotating spherical coordinates

ABSTRACTIn a recent paper, Marquez-Artavia, Jones and Tobias (referred to as MJT) studied wave propagation on a rotating sphere using the shallow water ideal magnetohydrodynamic (MHD) equations. These equations were linearised about a toroidal magnetic field (the Malkus field) which depends only on the latitude and is proportional to where θ is the co-latitude. These waves may play an important role in the Earth's outer core and the solar tachocline. The purpose of this paper is to extend MJT's solution for magnetic equatorial Kelvin waves to the case of weakly nonlinear waves. To do this, we use a formal asymptotic approximation when two main parameters are separately assumed to be large. The approximation recovers MJT's results in the linear case and extends those results to the weakly nonlinear case. The main result of this paper is that the zonal wave function is obtained from a first-order linear wave equation so that, unlike in the hydrodynamic case, these waves do not break or form shocks. We demonstrate a mechanism by which the presence of a magnetic field regularises the governing equations in the sense that a sufficiently large magnetic field can suppress the nonlinear terms in the Burgers equation of the hydrodynamic problem. We also find weak north–south currents, not present in the linear problem. The nonlinearities do not appear to significantly modify the linear results.

Solving Controversies on the Iron Phase Diagram Under High Pressure

AbstractAs the main constituent of planetary cores, pure iron phase diagram under high pressure and temperature is of fundamental importance in geophysics and planetary science. However, previously reported iron‐melting curves show large discrepancies (up to 1000 K at the Earth's core–mantle boundary, 136 GPa), resulting in persisting high uncertainties on the solid‐liquid phase boundary. Here we unambiguously show that the observed differences commonly attributed to the nature of the used melting diagnostic are due to a carbon contamination of the sample as well as pressure overestimation at high temperature. The high melting temperature of pure iron under core‐mantle boundary (4250 ± 250 K), here determined by X‐ray absorption experiments at the Fe K‐edge, indicates that volatile light elements such as sulfur, carbon, or hydrogen are required to lower the crystallization temperature of the Earth's liquid outer core in order to prevent extended melting of the surrounding silicate mantle.

Geomagnetic Dipole Changes and Upwelling/Downwelling at the Top of the Earth's Core

The convective state of the top of Earth's outer core is still under debate. Conflicting evidence from seismology and geomagnetism provides arguments for and against a thick stably stratified layer below the core-mantle boundary. Mineral physics and cooling scenarios of the core favor a stratified layer. However, a non-zero secular variation of the total geomagnetic energy on the core-mantle boundary is evidence for the presence of radial motions extending to the top of the core. We compare the secular variation of the total geomagnetic energy with the secular variation of the geomagnetic dipole intensity and tilt. We demonstrate that both the level of cancellations of the sources and sinks of the dipole intensity secular variation, as well as the level of cancellations of the sources and sinks of the dipole tilt secular variation, are either larger than or comparable to the level of cancellations of the sources and sinks of the total geomagnetic energy secular variation on the core-mantle boundary, indicating that the latter is numerically significant hence upwelling/downwelling reach the top of the core. Radial motions below the core-mantle boundary are either evidence for no stratified layer or to its penetration by various dynamical mechanisms, most notably lateral heterogeneity of core-mantle boundary heat flux.

Geomagnetic polar minima do not arise from steady meridional circulation

Observations of the Earth's magnetic field have revealed locally pronounced field minima near each pole at the core-mantle boundary (CMB). The existence of the polar magnetic minima has long been attributed to the supposed large-scale overturning circulation of molten metal in the outer core: Fluid upwells within the inner core tangent cylinder toward the poles and then diverges toward lower latitudes when it reaches the CMB, where Coriolis effects sweep the fluid into anticyclonic vortical flows. The diverging near-surface meridional circulation is believed to advectively draw magnetic flux away from the poles, resulting in the low intensity or even reversed polar magnetic fields. However, the interconnections between polar magnetic minima and meridional circulations have not to date been ascertained quantitatively. Here, we quantify the magnetic effects of steady, axisymmetric meridional circulation via numerically solving the axisymmetric magnetohydrodynamic equations for Earth's outer core under the magnetostrophic approximation. Extrapolated to core conditions, our results show that the change in polar magnetic field resulting from steady, large-scale meridional circulations in Earth's outer core is less than [Formula: see text] of the background field, significantly smaller than the [Formula: see text] polar magnetic minima observed at the CMB. This suggests that the geomagnetic polar minima cannot be produced solely by axisymmetric, steady meridional circulations and must depend upon additional tangent cylinder dynamics, likely including nonaxisymmetric, time-varying processes.

High‐Latitude Westward Jets in the Earth's Outer Core Due to Small‐Scale Convection

AbstractThe general circulation of the Earth's outer core is driven by small‐scale convection. A model based on the dynamics of individual buoyant parcels is put forward to explain how the main features of the circulation are generated by the convection. The model predicts a distribution of buoyancy in the core such that the fluid is lighter/warmer at the poles and heavier/colder at the equator. Zonal velocity as a function of latitude is predicted by the model and compared with the previous observational data inferred from the variations of the Earth's magnetic field. The model demonstrates that the westward jets observed at the co‐latitude of approximately 210 are in a geostrophic balance with the dynamic pressure created by convecting parcels and with the excess hydrostatic pressure due to the redistribution of buoyancy in the meridional direction.

Internally driven inertial waves in geodynamo simulations

© The Authors 2017. Inertial waves are oscillations in a rotating fluid, such as the Earth's outer core, which result from the restoring action of the Coriolis force. In an earlier work, it was argued by Davidson that inertial waves launched near the equatorial regions could be important for the α2 dynamo mechanism, as they can maintain a helicity distribution which is negative (positive) in the north (south). Here,we identify such internally driven inertial waves, triggered by buoyant anomalies in the equatorial regions in a strongly forced geodynamo simulation. Using the time derivative of vertical velocity, ∂uz/∂t, as a diagnostic for traveling wave fronts, we find that the horizontal movement in the buoyancy field near the equator is well correlated with a corresponding movement of the fluid far from the equator. Moreover, the azimuthally averaged spectrum of ∂uz/∂t lies in the inertial wave frequency range. We also test the dispersion properties of the waves by computing the spectral energy as a function of frequency, ω, and the dispersion angle, θ. Our results suggest that the columnar flow in the rotation-dominated core, which is an important ingredient for the maintenance of a dipolar magnetic field, is maintained despite the chaotic evolution of the buoyancy field on a fast timescale by internally driven inertial waves.

High‐Pressure Geophysical Properties of Fcc Phase FeHX

AbstractFace centered cubic (fcc) FeHX was synthesized at pressures of 18–68 GPa and temperatures exceeding 1,500 K. Thermally quenched samples were evaluated using synchrotron X‐ray diffraction (XRD) and nuclear resonant inelastic X‐ray scattering (NRIXS) to determine sample composition and sound velocities to 82 GPa. To aid in the interpretation of nonideal (X ≠ 1) stoichiometries, two equations of state for fcc FeHX were developed, combining an empirical equation of state for iron with two distinct synthetic compression curves for interstitial hydrogen. Matching the density deficit of the Earth's core using these equations of state requires 0.8–1.1 wt % hydrogen at the core‐mantle boundary and 0.2–0.3 wt % hydrogen at the interface of the inner and outer cores. Furthermore, a comparison of Preliminary Reference Earth Model (PREM) to a Birch's law extrapolation of our experimental results suggests that an iron alloy containing ∼0.8–1.3 wt % hydrogen could reproduce both the density and compressional velocity (VP) of the Earth's outer core.

Composition of the low seismic velocity E′ layer at the top of Earth's core

AbstractUsing ab initio simulations on Fe‐Ni‐S‐C‐O‐Si liquids, we constrain the origin and composition of the low‐velocity layer E′ at the top of Earth's outer core. We find that increasing the concentration of any light element always increases velocity and so a low‐velocity and low‐density layer (for stability) cannot be made by simply increasing light element concentration. This rules out barodiffusion or simple sedimentation of a light phase for its origin. However, exchanging elements can—depending on the elements exchanged—produce such a layer. We evaluate three possibilities. First, crystalization of a phase from a core may make such a layer, but only if the core contains more than one light element and only if crystalizing phase is very Fe rich. Second, the E′ layer may result from incomplete mixing of an early Earth core with a late impactor, depending on the light element compositions of the impactor and Earth's core. Third, using thermodynamic models for metal‐silicate partitioning, we show that a reaction between the core and an FeO‐rich basal magma ocean can result in a light and slow layer.

Stokes‐Einstein relation in liquid iron‐nickel alloy up to 300 GPa

AbstractMolecular dynamic simulations were applied to investigate the Stokes‐Einstein relation (SER) and the Rosenfeld entropy scaling law (ESL) in liquid Fe0.9Ni0.1 over a sufficiently broad range of temperatures (0.70 &lt; T/Tm &lt; 1.85; Tm is melting temperature) and pressures (from 50 GPa to 300 GPa). Our results suggest that the SER and ESL hold well in the normal liquid region and break down in the supercooled region under high‐pressure conditions, and the deviation becomes larger with decreasing temperature. In other words, the SER can be used to calculate the viscosity of liquid Earth's outer core from the self‐diffusion coefficients of iron/nickel and the ESL can be used to predict the viscosity and diffusion coefficients of liquid Earth's outer core form its structural properties. In addition, the pressure dependence of effective diameters cannot be ignored in the course of using the SER. Moreover, ESL provides a useful, structure‐based probe for the validity of SER, while the ratio of the self‐diffusion coefficients of the components cannot be used as a probe for the validity of SER.

Sound velocity of liquid Fe‐Ni‐S at high pressure

AbstractThe sound velocity of liquid Fe47Ni28S25 and Fe63Ni12S25 was measured up to 52 GPa/2480 K in externally resistance‐heated and laser‐heated diamond‐anvil cells using high‐resolution inelastic X‐ray scattering. From these experimental data, we obtained the elastic parameters of liquid Fe47Ni28S25, KS0 = 96.1 ± 2.7 GPa and KS0′ = 4.00 ± 0.13, where KS0 and KS0′ are the adiabatic bulk modulus and its pressure derivative at 1 bar, when the density is fixed at ρ0 = 5.62 ± 0.09 g/cm3 for 1 bar and 2000 K. With these parameters, the sound velocity and density of liquid Fe47Ni28S25 were calculated to be 8.41 ± 0.17 km/s and 8.93 ± 0.19 to 9.10 ± 0.18 g/cm3, respectively, at the core mantle boundary conditions of 135 GPa and 3600–4300 K. These values are 9.4% higher and 17–18% lower than those of pure Fe, respectively. Extrapolation of measurements and comparison with seismological models suggest the presence of 5.8–7.5 wt % sulfur in the Earth's outer core if it is the only light element.

Experimental determination of oxygen diffusion in liquid iron at high pressure

Oxygen diffusion experiments in liquid iron have been performed at 3–18 GPa and 1975–2643 K using a multi-anvil apparatus. Diffusion couples consisted of a pure iron rod and a sintered disk of Fe0.85O0.15 placed end-to-end in a vertical orientation. Images and chemical spot analyses were acquired along the full length of the quenched sample on lines perpendicular to the diffusion interface. Exsolution features that formed during quenching consist mostly of spherical oxide blobs of at least two size populations, as well as feathery dendritic textures in more oxygen-rich regions near the top of the samples. Diffusion during heating (i.e. prior to reaching the peak annealing temperature, Tf) is treated numerically to refine Arrhenian parameters from simultaneous least-squares fits to several concentration profiles obtained from experiments at constant pressure and variable Tf. Diffusion coefficients range from ∼6×10−9 to ∼2×10−8 m2s−1 over the P–T range of the study, with activation enthalpies of less than 100 kJ mol−1. We find a very weak effect of pressure on oxygen diffusion with an activation volume of 0.1±0.1 cm3mol−1, in agreement with computational studies performed above 100 GPa. Arrhenian extrapolation of diffusion coefficients for oxygen to P–T conditions of the Earth's outer core yields faster average diffusion rates (∼3×10−8 m2s−1) than for Si or Fe in silicon-rich liquid iron alloys or pure liquid iron (∼5×10−9 m2s−1) reported previously. Oxygen diffusion data are used to constrain the maximum size of descending liquid metal droplets in a magma ocean that is required for chemical equilibration to be achieved. Our results indicate that if the Earth's core composition is representative of equilibrium chemical exchange with a silicate magma ocean, then it could only have been accomplished by large-scale break-up of impactor cores to liquid iron droplet sizes no larger than a few tens of centimeters.

After some 350 years – zero declination again in Paris

Abstract. The main part of the geomagnetic field – produced by a dynamo process in the Earth's outer core – changes its direction and strength in time, over timescales from months to centuries, even millennia. Its temporal variations, known as secular variation and secular acceleration, are crucial ingredients for understanding the physics of the deep Earth. Very long series of measurements therefore play an important role. Here, we provide an updated series of geomagnetic declination in Paris, shortly after a very special occasion: its value has reached zero after some 350 years of westerly values. Indeed, during October and November 2013, the declination at the Chambon la Forêt geomagnetic observatory changed from westerly to easterly values, the agonic line then passing through this place. We take this occasion to emphasize the importance of long series of continuous measurements.

Frequency spectrum of the geomagnetic field harmonic coefficients from dynamo simulations

The construction of geomagnetic, archeomagnetic or paleomagnetic field models requires some prior knowledge about the actual field, which can be gathered from the statistical properties of the field over a variety of length-scales and time-scales. However, available geomagnetic data on centennial to millennial periods are too sparse to infer directly these statistical properties. We thus use high-resolution numerical simulations of the geodynamo to test a method for estimating the temporal power spectra (or equivalently the auto-covariance functions) of the individual Gauss coefficients that describe the geomagnetic field outside the Earth's fluid outer core. Based on the spectral analysis of our simulations, we argue that a prior for the observational geomagnetic field over decennial to millennial periods can be constructed from the statistics of the field during the short satellite era. The method rests on the assumption that time series of spherical harmonic coefficients can be considered as realisations of stationary and differentiable stochastic processes, namely order 2 autoregressive (AR2) processes. In the framework of these processes, the statistics of Gauss coefficients are well constrained by their variance and one or two time-scales. We find that the time spectra in the dynamo simulations of all Gauss coefficients but the axial dipole are well approximated by the spectra of AR2 processes characterized by only one timescale. The process parameters can simply be deduced from instantaneous estimates of the spatial power spectra of the magnetic field and of its first time derivative. Some deviations of the Gauss coefficients statistics from this minimal model are also discussed. Characterizing the axial dipole clearly requires a more sophisticated AR2 process, with a second distinct time-scale.

A computationally efficient spectral method for modeling core dynamics

AbstractAn efficient, spectral numerical method is presented for solving problems in a spherical shell geometry that employs spherical harmonics in the angular dimensions and Chebyshev polynomials in the radial direction. We exploit the three‐term recurrence relation for Chebyshev polynomials that renders all matrices sparse in spectral space. This approach is significantly more efficient than the collocation approach and is generalizable to both the Galerkin and tau methodologies for enforcing boundary conditions. The sparsity of the matrices reduces the computational complexity of the linear solution of implicit‐explicit time stepping schemes to O(N) operations, compared to operations for a collocation method. The method is illustrated by considering several example problems of important dynamical processes in the Earth's liquid outer core. Results are presented from both fully nonlinear, time‐dependent numerical simulations and eigenvalue problems arising from the investigation of the onset of convection and the inertial wave spectrum. We compare the explicit and implicit temporal discretization of the Coriolis force; the latter becomes computationally feasible given the sparsity of the differential operators. We find that implicit treatment of the Coriolis force allows for significantly larger time step sizes compared to explicit algorithms; for hydrodynamic and dynamo problems at an Ekman number of , time step sizes can be increased by a factor of 3 to 16 times that of the explicit algorithm, depending on the order of the time stepping scheme. The implementation with explicit Coriolis force scales well to at least 2048 cores, while the implicit implementation scales to 512 cores.

Geophysics: Earth's core problem.

Measurements of the electrical resistance and thermal conductivity of iron at extreme pressures and temperatures cast fresh light on controversial numerical simulations of the properties of Earth's outer core. See Letters p.95 & 99 The thermal conductivity of iron and its alloys at high pressure and temperature is a critical factor in the evolution and dynamics of Earth-like planets. Recently, increasing uncertainty in these values has produced dramatically variable predictions for Earth's history that challenge traditional geophysical theories. Two groups reporting in this issue of Nature use laser-heated diamond-anvil cells to study the properties of iron at the extreme temperatures and pressures relevant to Earth's core, but using different methodologies, and they arrive at contrasting results. Kenji Ohta and co-authors measured the electrical resistivity of iron at up to 4,500 kelvin and obtained an estimate that is even lower than the low values predicted from recent ab initio studies. They conclude that this suggests a high thermal conductivity for Earth's core, which would imply rapid core cooling by conduction and a relatively young inner core. Zuzana Konopkova and co-authors measured heat pulses propagating through solid iron after heating with a laser pulse at pressures and temperatures relevant to the cores of planets ranging in size from Mercury to Earth. Their measurements place the thermal conductivity of Earth's core near the low end of previous estimates, implying that thermal convection in Earth's core could have driven the geodynamo for billions of years, and allowing for an ancient inner core. In a linked News & Views, David Dobson discusses the interpretation of these two tours de force of experimental geophysics.

Magnetic and spin transitions in wüstite: A synchrotron Mössbauer spectroscopic study

This is a M\ossbauer study of w\ustite at pressures above 200 GPa using synchrotron M\ossbauer spectroscopy. Synthetic $\mathrm{F}{\mathrm{e}}_{0.96}\mathrm{O}$-w\ustite was investigated at 91(2), 95(4), 109(2), 114.5(3), 131.1(7), 133.2(2), 155(2), 167(2), 193(2) and 203(1) GPa at 300 K at the SPring-8 BL11XU beamline. The M\ossbauer spectrum at 91 GPa consists of both magnetic and nonmagnetic components. The magnetic high-spin component decreases gradually with increasing pressure from 91 to 203 GPa, while the nonmagnetic low-spin component increases with pressure in the same pressure range. The result suggests that the spin state of Fe in the outer core at pressures above 203 GPa is the low-spin state. If oxygen exists in the core, the low-spin Fe-O bonding is shorter than high-spin Fe-O bonding, suggesting dense Fe-O liquid in the Earth's outer core. The gradual increase of the density of the metallic liquid with depth by the spin transition of Fe-O bonding in the shallow outer core region will stabilize the outer core against thermal convection.

On the water delivery to terrestrial embryos by ice pebble accretion

Standard accretion disk models suggest that the snow line in the solar nebula migrated interior to the Earth's orbit in a late stage of nebula evolution. In this late stage, a significant amount of ice could have been delivered to 1 AU from outer regions in the form of mm to dm-sized pebbles. This raises the question why the present Earth is so depleted of water (with the ocean mass being as small as 0.023% of the Earth mass). Here we quantify the amount of icy pebbles accreted by terrestrial embryos after the migration of the snow line assuming that no mechanism halts the pebble flow in outer disk regions. We use a simplified version of the coagulation equation to calculate the formation and radial inward drift of icy pebbles in a protoplanetary disk. The pebble accretion cross section of an embryo is calculated using analytic expressions presented by recent studies. We find that the final mass and water content of terrestrial embryos strongly depends on the radial extent of the gas disk, the strength of disk turbulence, and the time at which the snow lines arrives at 1 AU. The disk's radial extent sets the lifetime of the pebble flow, while turbulence determines the density of pebbles at the midplane where the embryos reside. We find that the final water content of the embryos falls below 0.023 wt% only if the disk is compact (< 100 AU), turbulence is strong at 1 AU, and the snow line arrives at 1 AU later than 2-4 Myr after disk formation. If the solar nebula extended to 300 AU, initially rocky embryos would have evolved into icy planets of 1-10 Earth masses unless the snow-line migration was slow. If the proto-Earth contained water of ~ 1 wt% as might be suggested by the density deficit of the Earth's outer core, the formation of the proto-Earth was possible with weaker turbulence and with earlier (> 0.5-2 Myr) snow-line migration.