Dynamic history of the inner core constrained by seismic anisotropy

Abstract

Progressive crystallisation of Earth's inner core over geological times drives convection in the outer core and the generation of the Earth's magnetic field. Resolving the rate and pattern of inner core growth is thus crucial to understanding the evolution of the geodynamo. The growth history of Earth's inner core is likely recorded in the distribution and strength of seismic anisotropy arising from 19 deformation texturing constrained by boundary conditions at the inner-core solid-20 fluid boundary. Travel times of seismic body waves indicate that seismic anisotropy 21 increases with depth. Here we find that the strongest anisotropy is offset from Earth's rotation axis. Using geodynamic growth models and mineral physics calculations, we simulate the development of inner core anisotropy in a self-consistent manner. We show for the first time that an inner core model composed of hexagonally closepacked iron-nickel alloy, deformed by a combination of preferential equatorial growth and slow translation can match the seismic observations without requiring the introduction of hemispheres with sharp boundaries. We find a model of the inner core 28 growth history compatible with external constraints from outer core dynamics, supporting arguments for a relatively young inner core (~0.5-1.5 Ga) and a viscosity >10 18 Pa-s.The presence of seismic anisotropy -the dependence of seismic wavespeed on direction of propagation -in the inner core (IC) was proposed over 30 years ago to explain the early arrival times of IC sensitive seismic body waves (PKPdf) travelling on paths parallel to the Earth's rotation axis 1,2 and anomalous splitting of core-sensitive free oscillations 3 . This anisotropy is thought to result from alignment of iron crystals caused by deformation in a flow field induced by the evolution of the core, i.e. deformation texturing. In previous work, different geodynamic 4 and plastic deformation mechanisms 5 were explored to explain the variation of PKPdf travel times with angle of the ray path with respect to the rotation axis.Here, for the first time, we combine geodynamic modelling of the evolution of flow in the IC, allowing for slow lateral translation, with presently available knowledge on the mineralogy and deformation mechanisms proposed for the IC to explain spatially varying patterns of observed seismic travel times in an updated global dataset.

Early Cambrian renewal of the geodynamo and the origin of inner core structure

“…Although model details differ [2][3][4]47,48 , recent analyses show that the slow direction of seismic anisotropy changes from an equatorial orientation to 54 o relative to the rotation axis, at a radius of ~650 km 1 . A thermal evolution model 49 using a ~550 Ma onset age suggests the inner core would have grown to this size by 450 þ10 À15 Ma (Fig.…”

Neutrino Oscillations through the Earth's Core

“…Moreover, there are connections between understanding the core of our planet and the cores of exoplanets [42]. In addition, it is speculated that the core may have modest eccentricity, although this is not yet fully understood [43][44][45][46][47][48][49][50]. Finally, the outer core's radius is composed of the so-called D" layer between the core and the mantle, which is ∼ 200 km thick and whose properties and topology are fairly uncertain [51] and may be the location of the formation of hotspots within the mantle [52].…”

Shear‐Wave Anisotropy in the Earth's Inner Core

“…The anisotropy asymmetry may imply a slow translation in addition to the preferential equatorial growth of the IC (Frost et al., 2021; Romanowicz et al., 1996). Currently, the coda‐correlation observations cannot provide constraints on some proposed complex models, but they do not rule out models of asymmetric anisotropy and large‐scale low‐order convection in the IC (Frost et al., 2021; Romanowicz et al., 1996). A delicate grouping of I2‐J with respect to longitude may shed light on the asymmetric anisotropy; however, care should be taken due to the unequal distribution of great‐circle planes as a function of longitude (Figure ).…”

“…A more complex asymmetric IC model departing from the cylindrical anisotropy (Romanowicz et al., 1996) can increase travel time variability and further decrease the amplitude of I2‐J. The anisotropy asymmetry may imply a slow translation in addition to the preferential equatorial growth of the IC (Frost et al., 2021; Romanowicz et al., 1996). Currently, the coda‐correlation observations cannot provide constraints on some proposed complex models, but they do not rule out models of asymmetric anisotropy and large‐scale low‐order convection in the IC (Frost et al., 2021; Romanowicz et al., 1996).…”