Physics of the Earth and Planetary Interiors最新文献

We examine magnesium and potassium solubility in liquid Fe mixtures, representative of Earth's core composition, in equilibrium with liquid silicate mixtures representative of an early magma ocean. Our study is based on the calculation of the chemical potentials of MgO and K₂O in both phases, using density functional theory. For MgO, we also study stability against precipitation of the solid phase. We use thermal evolution models of the core and mantle to assess whether either radiogenic heating from ⁴⁰K decay or Mg precipitation from the liquid core can resolve the new core paradox by powering the geodynamo prior to inner core formation. Our results for K show that concentrations in the core are likely to be small and the effect of ⁴⁰K decay on the thermal evolution of the core is minimal, making it incapable of sustaining the early geodynamo alone. Our results also predict small concentrations of Mg in the core which might be sufficient to power the geodynamo prior to inner core formation, depending on the process by which it is transported across the core mantle boundary.

The Earth's interior, especially the mantle (MT), is believed to deviate from the state of hydrostatic equilibrium. This is mainly because the customary approaches in computing the period of the free-core nutation (FCN) for Earth models in hydrostatic equilibrium yield predicted values larger than the observed value by an average of about 30 sidereal days (sd). However, results from alternative computational approaches yield predicted periods of this mode that are close to the observed value. This suggests that the Earth's interior maybe closer to hydrostatic equilibrium than previously envisioned. In this work I study the dynamics of a compressible liquid core (LC) model, bounded by the rigid MT, and vary the square of the Brunt Väsälä frequency, within its known limits, such that the LC density profile is stably, neutrally, and unstably stratified, and compute the periods of the CW and FCN. I show that the period of the CW is unaffected, as expected, by the variations of the Brunt Väsälä frequency. Unless the LC is moderately unstably stratified, for which the computed period of this mode does not converge to a single value, the period of the FCN is also not significantly affected by these variations except for a small range of Brunt Väsälä frequency for which the core is unstably stratified. I conclude that either the LC is moderately-unstably stratified, or the discrepancy between the observed and computed period of the FCN, once the effects of the viscous and electromagnetic torques on the inner-core boundary (ICB) and core-mantle boundary (CMB) are considered, may be in the theoretical/computational treatment of the dynamics of the elastic MT.

Comprehensive paleomagnetic study from the Paraná-Etendeka Igneous Province (PEIP), Serra Geral Formation, southern Brazil (∼ 135 Ma) adds to the scarce southern hemisphere paleomagnetic database. Twelve paleomagnetic sites from the exposed stratigraphy of the PEIP were recovered from basaltic lava flows in northern Rio Grande do Sul state. These volcanic rocks erupted between 131.5 and 135.6 Ma. The reliability of the rock magnetism data was confirmed before being accepted for paleo-intensity analysis. The Thellier-Coe paleointensity protocol was applied to 109 collected specimens, and 26 individual samples from 9 sites were selected to test for consistency of the paleointensity determinations. The new high-quality mean paleointensity value of 30.6 ± 7.2μT corresponds to a virtual dipole moment (VDM) of 5.75 ± 0.49 × 10²² Am². These values correspond to approximately 72% of the present-day Earth's magnetic field and suggest that the Earth's magnetic field strength was slightly elevated compared to the long-term average geomagnetic field that prevailed during the Cretaceous Normal Superchron. The calculated paleomagnetic pole (Plong = −27.5° and Plat = −78.6°, A95 = 3.5°) presents an angular difference of 22° with respect to the expected pole declination with an inclination of 5°. This difference may correspond to secular variations considering the confidence cones. Our results, when compared with previous paleomagnetic data, approach the accuracy of recent models for the Cretaceous interval.

The Zagros fold-and-thrust belt is the principal response to the Arabia–Eurasia continental collision, which generates significant seismic activity in Iran. Surface wave tomography can be a proper tool for crustal structure imaging in this region. We used observed dispersion curves of Rayleigh wave derived from high-quality local-earthquake waveforms to obtain 2D tomographic maps in the period range 1–28 s beneath the Zagros collision zone. Subsequently, local pseudo-dispersion curves for individual grid points are inverted for shear-wave velocity structure, in order to obtain a 3D velocity model of the crust. Our results indicate shallow low-velocity structures near the suture zone. Moreover, a low-velocity flexure shape in the south of the Zagros Mountain Front Fault could be due to the thick sedimentary basin beneath the Dezful embayment. Relatively an uplifting of the deeper structures beneath Sanandaj-Sirjan and the high Zagros might could be led to high-velocity structures at mid-crustal depths. The variation of shear-wave velocity along the Zagros indicates the inhomogeneous structures along this folding and thrust belt.

The lowermost mantle has been studied with a variety of seismic waves which show that different structures on many length-scales exist there but it is possible that several seismic phases could be generated by one structure. Specifically, we use a randomly distributed scattering layer that is able to generate precursors to the core phase PKPdf as well as waves that resemble reflections off the D″ layer (PdP wave) and here we present a parameter study to understand how scattering parameters in this layer affect the two tested seismic waves and whether it is possible to discriminate between a reflector or a scattering layer as cause for apparent D″-reflected waves. We compute synthetic data for several source-receiver combinations and vary the parameters of the heterogeneity model above the core-mantle boundary: correlation length, velocity (and density) perturbation, taper to soften the transition, thickness, location of the scattering layer and the randomness of the models. We measure the effects on the amplitudes and arrival times of the seismic waves, and in addition for the apparent D″ reflection, we also record the number of reflected waves, their polarity and slowness. We find that the precursor and reflection amplitudes are influenced by all scattering parameters and that trade-offs exist. One conclusion therefore is that it is difficult to match local scattering parameters to fit observations with a unique model since several combinations of scattering parameters result in similar amplitudes and travel times. Interestingly, amplitudes for different correlation lengths are affected differently for larger and smaller epicentral distances, therefore testing the amplitudes of precursors at different distance ranges could potentially help to distinguish between correlation length and perturbation effects. We were not able to further narrow down the scattering parameters of the layer when using a combination of PKP precursors and PdP-like reflections due to the non-uniqueness. Analyzing the effects on PdP arrivals that arrive between P and PcP, we find influences on all seismic observables albeit without clear trends. But based on our modeling it seems prudent to assume that the waves are caused by a scattering layer if 1) the observations show strong scatter in arrival time, slowness and amplitude and 2) if more than one arrival is present between P and PcP.

Opening of the South China Sea (SCS) was triggered by the breakup of south-eastern Eurasia and the southward drifting of the Palawan-Reed Bank microcontinent, as various prior studies have suggested. The young, moderately magmatic, rifted northern margin of the SCS is vital for investigating the relationship among magmatism, rheology, and structural evolution. This paper integrates satellite gravity anomaly, elevation/bathymetry, geoid, and seismic velocities to investigate continental breakup, magmatism, and rifting beneath the South China Sea. Source depths of 146, 31.5, and 8 km derived from the spectrum of Bouguer gravity anomalies suggest the average depths of the lithospheric base, continental, and oceanic crustal bases, respectively. Correspondingly, gravity Moho ranges from 8 km beneath the rifting center of SCS to 42 km in the Indo-China block. It is worth mentioning that the isostatic Moho from the Airy and flexural models were highly correlated with gravity Moho with correlation coefficients of ∼1. From the ratio between geoid and topography and our estimate of the vertical tectonic stress, the seamounts and reefs (Shuangfeng Basin: SB, Reed Bank: RB, Macclesfield Bank: MB) have a deep compensation depth. In contrast, the other parts (Manila Trench: MT, Phu Khan Basin: PK, East Sub Basin: ESB) have a smaller depth of compensation. 2D gravity modelling suggests the crustal thinning of the oceanic basin, and the thickening of the continental boundary implies the opening of the SCS, which is connected and happened at the same time as the northern subduction of the proto-SCS. The Gravity modelling also suggests a large rifting event within the lithosphere that favors mantle upwelling.

Temperature-dependent viscosity convection is investigated for various horizontal wavelengths of the convective cells. Finite-amplitude steady solutions are obtained by the Newton method in a two-dimensional layer for various values of the Rayleigh number and strength of temperature-dependence of viscosity, and their stability is examined through numerical time integrations. The viscosity $\eta$ of the model varies with temperature $T$ as $\eta \propto exp\left(-\mathrm{\gamma T}\right)$, where the parameter $\gamma$ denotes the strength of the temperature-dependency of $\eta$. Although approximately square convection cells are stable when $\gamma$ is small, the stable convective structure elongates horizontally as $\gamma$ increases in the middle range of $\gamma$ less than about 10. When $\gamma$ exceeds that range, the stable convection approaches a square cell.

Scaling relations for the Nusselt number that include the effect of the horizontal wavelength are developed. The results obtained by the numerical steady solutions are well explained by the proposed novel scaling relations. When the solutions with the maximum Nusselt number are traced using the scaling relations for various $\gamma$, we find that the convective cells elongate gradually as $\gamma$ increases until $\gamma <8.6$, and then the convection becomes narrower. The most elongated convection is expected to appear at the threshold with a horizontal length $\lambda$ of $6.6$, which may not depend on the Rayleigh number. Our results suggest that rocky exoplanets (such as super-Earths), which will be studied in detail in the future, may have surface plates with various horizontal scales.

Observations of the geomagnetic field by surface observatories and dedicated satellite missions such as the Swarm constellation provide constraints on the dynamics in Earth's outer core. In particular, global core flow models estimated by inversion of the radial magnetic induction equation provide an image of the circulation of the electrically conductive fluid at the top of the core. However, in these models the poloidal flow is much less robust than the toroidal core flow. Here, we infer regional outer core kinematics from the temporal variability of high-latitude intense geomagnetic flux patches. We develop an algorithm to fit anisotropic 2D-Gaussians to the shape of those flux patches in order to infer their area, amplitude and level of anisotropy. The temporal variabilities of these properties are used to quantify contraction, expansion, amplification, weakening and horizontal shear. Comparisons with idealized kinematic scenarios based on synthetic field and flow models allow to infer regional outer core kinematics. We found that some geomagnetic flux patches exhibit expansion and weakening corresponding to fluid upwellings, whereas other patches exhibit contraction and intensification corresponding to downwellings. In both cases the patches' area and amplitude relations follow hyperbolic curves. Our results show that the geomagnetic flux patches are affected by upwelling more often than by downwelling during the historical period. Equatorially symmetric poloidal flow prior to $\approx 1910$ is inferred for the western intense patches. Kinematic scenarios where the field and flow structures centers coincide failed to reproduce the geomagnetic flux patches behavior. We recover the flux concentration efficiency of intense geomagnetic flux patches with an upwelling that resides two times its radius size away from the center of the flux patch. We also found a significant level of anisotropy over long periods for the historical geomagnetic flux patches. Anisotropic magnetic flux patches that are elongated in the direction of the shear flow may explain the east-west oriented present-day field at high latitudes of the southern Hemisphere. Overall, stretching effects at the top of the core can be deduced from our analysis of regional SV and allow further inferences on the poloidal part of the core flow.

Here, we present a synthetic validation for the inversion of transient fluid motions at the surface of Earth's core. It is based on a numerical simulation of the geodynamo in which the main time-scales (based on rotation, magnetic field and velocity) are sufficiently separated to give rise to a variety of hydro-magnetic waves. We focus the study on wave-like motions with periods commensurate to the Alfvén time, which is based on the strength of the magnetic field in the core interior. Synthetic magnetic data are generated over 90 Alfvén times, representative of the era covered by observatory and satellite measurements. These synthetic data are inverted to estimate a magnetic field model. Thereafter, we apply the pygeodyn data assimilation tool to recover core surface flows. We investigate the quality of their reconstruction as a function of their time scale. The success of the reconstruction depends on the data accuracy and coverage and on the magnitude of the flow. We also retrieve axi-symmetric torsional Alfvén waves, despite their relatively weak magnitude.

We use the synthetic data to investigate the exchanges of angular momentum between core and mantle that induce length-of-day (LOD) changes. These exchanges result from the electromagnetic torque between the fluid core and the mantle and the gravitational torque between the inner core and the mantle. The inverted flows convincingly predict LOD variations in the dynamo calculation. We find that core surface zonal motions match well with the geostrophic (axially invariant) motions at the origin of the LOD changes, on all considered time-scales. We also investigate the different contributions to the electro-magnetic torque. In the dynamo simulation, only a small part can be attributed to the leakage torque caused by electrical currents flowing from the core to the mantle. The relative contribution from the poloidal field induced in the mantle, which amounts to about 1/3 of the total torque, is significantly larger than estimated in previous studies, based on geomagnetic observations. The remaining torque, which is associated with the toroidal induced field, mostly stems from the solid body rotation interacting with the radial magnetic field up to spherical harmonic degree 30.

The secular variation of the geomagnetic field suggests that there are anticyclonic polar vortices in the Earth's core. Under the influence of a magnetic field, the polar azimuthal flow is thought to be produced by one or more coherent upwellings within the tangent cylinder, offset from the rotation axis. In this study, convection within the tangent cylinder in rapidly rotating dynamos is investigated through the analysis of forced magnetic waves. The first part of the study investigates the evolution of an isolated buoyancy disturbance in an unstably stratified rotating fluid subjected to an axial magnetic field. It is shown that the axial flow intensity of the slow Magnetic-Archimedean-Coriolis (MAC) waves becomes comparable to that of the fast MAC waves when $|{\omega }_M/{\omega }_C|\sim 0.1$, where ${\omega }_M$ and ${\omega }_C$ are the Alfvén wave and inertial wave frequencies respectively. In spherical shell dynamo simulations, the isolated upwellings within the tangent cylinder are shown to originate from the localized excitation of slow MAC waves in the dipole-dominated regime. Axial flow measurements in turn reveal the approximate parity between the slow and fast wave intensities in this regime, which corresponds to the existence of strong polar vortices in the Earth's core. To obtain the observed peak azimuthal motions of $0.6$–${0.9}^{\circ }$ ${\mathrm{yr}}^{{}^{-1}}$, the Rayleigh number in the low-inertia geodynamo must be $\sim {10}^3$ times the Rayleigh number for the onset of nonmagnetic convection. However, if the forcing is so strong as to cause polarity reversals, the field within the tangent cylinder decays away, and the convection takes the form of an ensemble of plumes supported entirely by the fast waves of frequency $\omega \sim {\omega }_C$. The resulting weak polar circulation is comparable to that obtained in nonmagnetic convection.