Core-mantle Boundary Heat Flow Research Articles

Indicators of mantle control on the geodynamo from observations and simulations

There has been longstanding controversy about whether the influence of lateral variations in core-mantle boundary heat flow can be detected in paleomagnetic records of geomagnetic field behavior. Their signature is commonly sought in globally distributed records of virtual geomagnetic pole (VGP) paths that have been claimed to exhibit specific longitudinal preferences during polarity transitions and excursions. These preferences have often been linked to thermal effects from large low seismic velocity areas (LLVPs) in the lowermost mantle, but the results have been contested because of potential sensitivity to sparse temporal and spatial sampling. Recently developed time varying global paleofield models spanning various time intervals in 1–100 ka, three of which include excursions, allow us to complement assessments of spatial distributions of transitional VGP paths with distributions of minimum field intensity. Robustness of the results is evaluated using similar products from four distinct numerical dynamo simulations with and without variable thermal boundary conditions and including stable geomagnetic polarity, excursions and reversals. We determine that VGP distributions are less useful than minimum field intensity in linking the influences of thermal CMB structure to geographical variations in actual paleofield observables, because VGP correlations depend strongly on good spatial sampling of a sufficient number of relatively rare events. These results provide a basis for evaluating comparable observations from four paleofield models. The distribution of VGP locations provide unreliable results given the restricted time span and available data locations. Rough correlations of global distributions of minimum intensity with areas outside the LLVPs give some indications of mantle control during excursions, although the results for the eastern hemisphere are complex, perhaps highlighting uncertainties about the hemispheric balance between thermal and compositional variations in the lowermost mantle. However, access to other geomagnetic properties (such as intensity and radial field at the CMB) provides a strong argument for using extended and improved global paleofield models to resolve the question of mantle influence on the geodynamo from the observational side.

Dynamo constraints on the long-term evolution of Earth’s magnetic field strength

SUMMARY Elucidating the processes in the liquid core that have produced observed palaeointensity changes over the last 3.5 Gyr is crucial for understanding the dynamics and long-term evolution of Earth’s deep interior. We combine numerical geodynamo simulations with theoretical scaling laws to investigate the variation of Earth’s magnetic field strength over geological time. Our approach follows the study of Aubert et al., adapted to include recent advances in numerical simulations, mineral physics and palaeomagnetism. We first compare the field strength within the dynamo region and on the core–mantle boundary (CMB) between a suite of 314 dynamo simulations and two power-based theoretical scaling laws. The scaling laws are both based on a Quasi-Geostropic (QG) force balance at leading order and a Magnetic, Archimedian, and Coriolis (MAC) balance at first order and differ in treating the characteristic length scale of the convection as fixed (QG-MAC-fixed) or determined as part of the solution (QG-MAC-free). When the data set is filtered to retain only simulations with magnetic to kinetic energy ratios greater than at least two we find that the internal field together with the root-mean-square and dipole CMB fields exhibit power-law behaviour that is compatible with both scalings within uncertainties arising from different heating modes and boundary conditions. However, while the extrapolated intensity based on the QG-MAC-free scaling matches Earth’s modern CMB field, the QG-MAC-fixed prediction shoots too high and also significantly overestimates palaeointensities over the last 3.5 Gyr. We combine the QG-MAC-free scaling with outputs from 275 realizations of core–mantle thermal evolution to construct synthetic true dipole moment (TDM) curves spanning the last 3.5 Gyr. Best-fitting TDMs reproduce binned PINT data during the Bruhnes and before inner core nucleation (ICN) within observational uncertainties, but PINT does not contain the predicted strong increase and subsequent high TDMs during the early stages of inner core growth. The best-fitting models are obtained for a present-day CMB heat flow of 11–16 TW, increasing to 17–22 TW at 4 Ga, and predict a minimum TDM at ICN.

Did the cessation of convection in Mercury's mantle allow for a dynamo supporting increase in heat loss from its core?

Mercury's large core generates a magnetic field and harbors a solidified inner component. These features constrain models of the planet's thermal history. The mantle provides the boundary condition on Mercury's core that determines heat loss. Recent studies suggest Mercury's mantle may have a higher thermal diffusivity than the silicate shells of other rocky bodies due to iron depletion. Considering the role of diffusivity, we model Mercury's mantle starting from a post magma-ocean state by calculating core-mantle boundary heat flow and the ratio of inner-core boundary radius to core-mantle boundary radius, fc, for periods comparable to the age of the solar system. Core-mantle boundary (CMB) heat flow is calculated using 3D mantle convection simulations for thermal diffusivities, κ, ranging from 1.0 - 3.0 ×10−6 m2/s and initial radiogenic uniform mantle heating rates, χ, of 0 - 40 pW/kg (that decay with a 3 Gyr half-life). Several scenarios can unfold for the range of parameters considered: these include cases featuring both the cessation of mantle convection and its continuation at 4.5 Gyr. We map a trend in κ - χ space and find that for some parameters present-day core heat flux and inner-core size estimates are satisfied for a Mercurian mantle that is cooling by conduction. The influence of initial sulfur fraction in the core was examined for a subset of cases. For a sulfur fraction of 0.10, fc falls between 0.1 and 0.55 which corresponds to planetary radius contractions below 7 km (since the Late Heavy Bombardment). However, fc exceeds 0.55 for a lower initial sulfur fraction and results in planetary contraction in excess of 7 km. In general, we find that the mean core heat flux reaches a temporal local minimum when the mantle transitions from a convective to a conductive regime and subsequently climbs before decreasing. The transition to conduction is delayed with increased mantle internal heating rate but the maximum mean heat flux from the core into the conducting and cooling mantle is always greater than the heat flux observed at the cessation of the stagnant-lid convection. We find that for an initially more vigorously convecting mantle the onset of inner-core growth is earlier while the basal heat flux is marginally reduced at present day. In general, we find that current-day conductive cooling of the Mercurian mantle can satisfy estimates on Mercury's core heat loss inferred from the strength of its magnetic field while also satisfying the limits on the present-day size of its inner core.

Geodynamo models with a stable layer and heterogeneous heat flow at the top of the core

The mean heat flow at the core–mantle boundary (CMB) could be less than the heat that can be transported by conduction along a core adiabat. If the difference is significant, it may render the top few hundred kilometres of the core to be stably stratified. The geodynamo then operates in deeper parts of the core, driven from the inner-core boundary by thermochemical convection. The CMB heat flow is probably strongly heterogeneous. Locally the thermal gradient below the CMB could be unstable while on average it is stable. The way in which lateral heat flux differences are accommodated and the consequences for the dynamics of a stable layer are not clear a priori. This is studied in numerical dynamo simulations with a strongly stabilizing average density gradient near the outer boundary and large lateral differences in the heat flux. The results suggest that a circulation in a thin layer immediately below the CMB balances the differences in heat flow by lateral advection. Deeper parts of the stable layer and the convecting region remain unaffected by the thermal heterogeneity at the CMB. A simple scaling theory for the shallow circulation layer is developed and is found to be in rough agreement with the numerical results. Applied to the Earth’s core, it predicts that the thickness of the layer is a few hundred metres, the horizontal velocity reaches values around 1 mm s−1, and lateral temperature differences at the CMB are slightly less than 1 K. The morphology of the magnetic field is found to be only marginally compatible with characteristic properties of the geomagnetic field when the stable layer thickness is around 400 km and is incompatible for a thicker layer. Therefore, the total CMB heat flow should exceed 10 TW unless unaccounted compositional buoyancy effects close to the CMB play a major role. If a sufficiently stable layer exists at the top of the core, thermal heterogeneity at the CMB would not influence the flow pattern in deeper parts of the core or affect heterogeneous growth of the inner core. Likewise, proposed mechanisms for the dipole offset from the centre of Mercury based on heterogenous CMB heat flow are unlikely to work if the upper part of Mercury’s core is thermally stable.

Venus Resurfacing Constrained by Geoid and Topography

AbstractThe primary explanations for the young crater age of the surface of Venus are progressive volcanic resurfacing and a period of mobile‐lid tectonics. The role of initial conditions, mantle potential temperature, and core potential temperature on Venus surface mobility are explored using 3‐D spherical mantle convection calculations with a lithospheric yield stress that allows the fluid to adopt either a stagnant or mobile lid. The small offset between the center of mass and center of figure (CM‐CF) of Venus is reproduced in calculations that never undergo a period of mobile‐lid convection. The CM‐CF offset has never been used in the evaluation of geodynamic models for Venus. The mobile‐lid calculations have a CM‐CF offset that is four times the observed offset, and the offset increases significantly at the onset of a mobile lid/resurfacing event. Furthermore, the rapid increase in core mantle boundary heat flow that occurs as the cold lithospheric material interacts with the core mantle boundary during a mobile‐lid event is sufficient to power a core dynamo, even in the absence of inner core solidification. The core mantle boundary heat flow decays over 1 Gyr after the cessation of mobile‐lid tectonics. The heat flow never reaches a level that would power a thermally driven dynamo in calculations that do not undergo mobile‐lid convection. The calculations indicate that Venus young surface age is not the result of a mobile‐lid resurfacing event while progressive resurfacing is consistent with the observations.

On the thermo-chemical origin of the stratified region at the top of the Earth’s core

I developed a combined model of the thermal and chemical evolution of the Earth’s core and investigated its influence on a thermochemically stable region beneath the core-mantle boundary (CMB). The chemical effects of the growing stable region are caused by the equilibrium chemical reaction between silicate and the metallic core. The thermal effects can be characterized by the growth of the sub-isentropic shell, which may have a rapid growth rate compared to that of the chemically stable region. When the present-day CMB heat flow was varied, the origin of the stable region changed from chemical to thermochemical to purely thermal because the rapid growth of the sub-isentropic shell can replace the chemically stable region. Physically reasonable values of the present-day CMB heat flow that can maintain the geodynamo action over 4 billion years should be between 8 and 11TW. To constrain the thickness of the thermochemically stable region beneath the CMB, the chemical diffusivity is important and should be ∼O(10−8) m2/s to obtain a thickness of the thermochemically stable region beneath the CMB consistent with that inferred from geomagnetic secular variations (140km). However, the strength of the stable region found in this study is too high to be consistent with the constraint on the stability of the stable region inferred from geomagnetic secular variations.

The deep Earth may not be cooling down

Abstract The Earth is a thermal engine generating the fundamental processes of geomagnetic field, plate tectonics and volcanism. Large amounts of heat are permanently lost at the surface yielding the classic view of the deep Earth continuously cooling down. Contrary to this conventional depiction, we propose that the temperature profile in the deep Earth has remained almost constant for the last ∼4.3 billion years. The core–mantle boundary (CMB) has reached a temperature of ∼4400 K in probably less than 1 million years after the Moon-forming impact, regardless the initial core temperature. This temperature corresponds to an abrupt increase in mantle viscosity atop the CMB, when ∼60% of partial crystallization was achieved, accompanied with a major decrease in heat flow at the CMB. Then, the deep Earth underwent a very slow cooling until it reached ∼4100 K today. This temperature at, or just below, the mantle solidus is suggested by seismological evidence of ultra-low velocity zones in the D”-layer. Such a steady thermal state of the CMB temperature excludes thermal buoyancy from being the predominant mechanism to power the geodynamo over geological time. An alternative mechanism to sustain the geodynamo is mechanical forcing by tidal distortion and planetary precession. Motions in the outer core are generated by the conversion of gravitational and rotational energies of the Earth–Moon–Sun system. Mechanical forcing remains efficient to drive the geodynamo even for a sub-adiabatic temperature gradient in the outer core. Our thermal model of the deep Earth is compatible with an average CMB heat flow of 3.0 to 4.7 TW. Furthermore, the regime of core instabilities and/or secular changes in the astronomical forces could have supplied the lowermost mantle with a heat source of variable intensity through geological time. Episodic release of large amounts of heat could have remelted the lowermost mantle, thereby inducing the dramatic volcanic events that occurred during the Earth's history. In this scenario, because the Moon is a necessary ingredient to sustain the magnetic field, the habitability on Earth appears to require the existence of a large satellite.

Composition of the Earth’s core: A review

This paper provides the state-of-the-art discussion of major aspects of the composition and evolution of the Earth’s core. A comparison of experimentally-derived density of Fe with seismological data shows that the outer liquid core has a homogeneous structure and a ~10% density deficit, whereas the solid inner core has a complex heterogeneous anisotropic structure and a ~5% density deficit. Recent estimations of the core-mantle boundary (CMB) and inner core boundary temperatures are equal to 3800–4200 K and 5200–5700 K, respectively. Si and O (up to 5–7 wt.%) are considered to be the most likely light element candidates in the liquid core. Cosmochemical estimates show that the core must contain about 2 wt.% S and new experimental data indicate that the inner core structure gives the best match to the properties of Fe carbides. Our best estimate of the Earth’s core calls for 5–6 wt.% Si, 0.5-1.0 wt.% O, 1.8-1.9 wt.% S, and 2.0 wt.% C, with the Fe7C3 carbide being the dominant phase in the inner core. The study of short-lived isotope systems shows that the core could have formed early in the Earth’s history within about 30–50 Myr after the formation of the Solar System, t0 = 4567.2 ± 0.5 Ma. Studies on the partitioning of siderophile elements between liquid iron and silicate melt suggest that the core material would be formed in a magma ocean at ~1000-1500 km depths and 3000–4000 K. The oxygen fugacity for the magma ocean is estimated to vary from 4–5 to 1–2 log units below the Iron-Wustite oxygen buffer. However, the data for Mo, W, and S suggest addition of a late veneer of 10-15% of oxidized chondritic material as a result of the Moon-forming giant impact. Thermal and energetics core models agree with the estimate of a mean CMB heat flow of 7–17 TW. The excess heat is transported out of the core via two large low shear velocity zones at the base of superplumes. These zones may not be stable in their positions over geologic time and could move according to cycles of mantle plume and plate tectonics. The CMB heat fluxes are controlled either by high heat production from the core or subduction of cold slabs, but in both cases are closely linked with surface geodynamic processes and plate tectonic motions. Considerable amounts of exchange may have occurred between the core and mantle early in the Earth’s history even up to the formation of a basal magma ocean. However, the extent of material exchange across the CMB upon cooling of the mantle was no greater than 1-2% of the core’s mass, which, however, was sufficient to supply thermochemical plumes with volatiles H, C, and S.

The 13 million year Cenozoic pulse of the Earth

The geomagnetic polarity reversal rate changes radically from very low to extremely high. Such process indicates fundamental changes in the Earth's core reorganization and core–mantle boundary heat flow fluctuations. However, we still do not know how critical such changes are to surface geology and climate processes. Our analysis of the geomagnetic reversal frequency, oxygen isotope record, and tectonic plate subduction rate, which are indicators of the changes in the heat flux at the core mantle boundary, climate and plate tectonic activity, shows that all these changes indicate similar rhythms on million years' timescale in the Cenozoic Era occurring with the common fundamental periodicity of ∼13 Myr during most of the time. The periodicity is disrupted only during the last 20 Myr. Such periodic behavior suggests that large scale climate and tectonic changes at the Earth's surface are closely connected with the million year timescale cyclical reorganization of the Earth's interior.

Constraints from material properties on the dynamics and evolution of Earth’s core

The Earth’s magnetic field is powered by energy supplied by the slow cooling and freezing of the liquid iron core. Efforts to determine the thermal and chemical history of the core have been hindered by poor knowledge of the properties of liquid iron alloys at the extreme pressures and temperatures that exist in the core. This obstacle is now being overcome by high-pressure experiments and advanced mineral physics computations. Using these approaches, updated transport properties for Fe–Si–O mixtures have been determined at core conditions, including electrical and thermal conductivities that are higher than previous estimates by a factor of two to three. Models of core evolution with these high conductivities suggest that the core is cooling much faster than previously thought. This implies that the solid inner core formed relatively recently (around half a billion years ago), and that early core temperatures were high enough to cause partial melting of the lowermost mantle. Estimates of core–mantle boundary heat flow suggest that the uppermost core is thermally stratified at the present day.

Testing the dynamic coupling of the core‐mantle and inner core boundaries

AbstractThe proposal that the seismically observed hemispherical asymmetry of Earth's inner core is controlled by the heat flux structure imposed on the outer core by the lower mantle is tested with numerical dynamo models driven by mixed thermochemical convection. We find that models driven by a single core‐mantle boundary (CMB) spherical harmonic of degree and mode 2, the dominant mode in lower mantle seismic shear velocity tomography, produce a similar structure at the inner core boundary (ICB) shifted 30∘ westward. The sensitivity of the ICB to the CMB is further tested by increasing the CMB heterogeneity amplitude. In addition, two seismic tomographic models are tested: first with CMB resolution up to degree and order 4, and second with resolution up to degree and order 8. We find time‐averaged ICB heat flux in these cases to be similar at large scale, with small‐scale differences due to higher CMB harmonics (above degree 4). The tomographic models produce “Earth‐like” magnetic fields, while similar models with twice the CMB heat flow amplitudes produce less Earth‐like fields, implying that increasing CMB heterogeneity forces the model out of an Earth‐like regime. The dynamic ICB heat fluxes are compared to the proposed translation mode of the inner core to test whether the CMB controls inner core growth and structure. This test indicates that, although CMB tomography is unlikely to be driving inner core translation, the ICB heat flux response is weak enough to not interfere with the most unstable translation mode, if it is occurring.

Cooling history of Earth’s core with high thermal conductivity

Thermal evolution models of Earth’s core constrain the power available to the geodynamo process that generates the geomagnetic field, the evolution of the solid inner core and the thermal history of the overlying mantle. Recent upward revision of the thermal conductivity of liquid iron mixtures by a factor of 2–3 has drastically reduced the estimated power available to generate the present-day geomagnetic field. Moreover, this high conductivity increases the amount of heat that is conducted out of the core down the adiabatic gradient, bringing it into line with the highest estimates of present-day core–mantle boundary heat flow. These issues raise problems with the standard scenario of core cooling in which the core has remained completely well-mixed and relatively cool for the past 3.5Ga. This paper presents cooling histories for Earth’s core spanning the last 3.5Ga to constrain the thermodynamic conditions corresponding to marginal dynamo evolution, i.e. where the ohmic dissipation remains just positive over time. The radial variation of core properties is represented by polynomials, which gives good agreement with radial profiles derived from seismological and mineralogical data and allows the governing energy and entropy equations to be solved analytically. Time-dependent evolution of liquid and solid light element concentrations, the melting curve, and gravitational energy are calculated for an Fe–O–S–Si model of core chemistry. A suite of cooling histories are presented by varying the inner core boundary density jump, thermal conductivity and amount of radiogenic heat production in the core. All models where the core remains superadiabatic predict an inner core age of ≲600Myr, about two times younger than estimates based on old (lower) thermal conductivity estimates, and core temperatures that exceed present estimates of the lower mantle solidus prior to the last 0.5–1.5Ga. Allowing the top of the core to become strongly subadiabatic in recent times pushes the onset of inner core nucleation back to ∼1.5Gyr, but the ancient core temperature still implies a partially molten mantle prior to ∼2Ga. Based on these results, the scenario of a long-lived basal magma ocean and subadiabatic present-day core seems hard to avoid.

Thermal evolution of the core with a high thermal conductivity

The rate at which heat is extracted across the core mantle boundary (CMB) is constrained by the requirement of dynamo action in the core. This constraint can be computed explicitly using the entropy balance of the core and depends on the thermal conductivity, whose value has been revised upwardly. A high order model (fourth degree polynomial of the radial position) for the core structure is derived and the implications for the core cooling rate and thermal evolution obtained, using the recent values of the thermal conductivity. For a thermal conductivity increasing with depth as proposed by some of these recent studies, a CMB heat flow equal to the isentropic value (13.25TW at present) leads to a 700km thick layer at the top of the core where a downward convective heat flow is necessary to maintain an isentropic and well mixed average state. Considering a CMB heat flow larger than the well mixed isentropic value leads to an inner core less than 700Myr old and the thermal evolution of the core is largely constrained by the conditions for dynamo action without an inner core. Analytical calculations for that period show that a CMB temperature larger than 7000K must have prevailed 4.5Gyr ago if the geodynamo has been driven by thermal convection for that whole time. This raises questions regarding the onset of the geodynamo and its continuous operation for the last 3.5Gyr. Implications regarding the evolution of a basal magma ocean are also considered.

Thermal and compositional stratification of the inner core

The improvements of the knowledge of the seismic structure of the inner core and the complexities thereby revealed ask for a dynamical origin. Sub-solidus convection was one of the early suggestions to explain the seismic anisotropy, but it requires an unstable density gradient either from thermal or compositional origin, or from both. Temperature and composition profiles in the inner core are computed using a unidimensional model of core evolution including diffusion in the inner core and fractional crystallisation at the inner core boundary (ICB). The thermal conductivity of the core has been recently revised upwardly and, moreover, found to increase with depth. Values of the heat flow across the core mantle boundary (CMB) sufficient to maintain convection in the whole outer core are not sufficient to make the temperature in the inner core super-isentropic and therefore prone to thermal instability. An unreasonably high CMB heat flow is necessary to this end. The compositional stratification results from a competition of the increase of the concentration of light elements in the outer core with inner core growth, which makes the inner core concentration also increase, and of the decrease of the liquidus, which makes the partition coefficient decrease as well as the concentration of light elements in the solid. While the latter (destabilizing) effect dominates at small inner core sizes, the former takes over for a large inner core. The turnover point is encountered for an inner core about half its current size in the case of S, but much larger for the case of O. The combined thermal and compositional buoyancy is stabilizing and solid-state convection in the inner core appears unlikely, unless an early double-diffusive instability can set in.

Magnetodynamo lifetimes for rocky, Earth‐mass exoplanets with contrasting mantle convection regimes

We used a thermal model of an iron core to calculate magnetodynamo evolution in Earth‐mass rocky planets to determine the sensitivity of dynamo lifetime and intensity to planets with different mantle tectonic regimes, surface temperatures, and core properties. The heat flow at the core‐mantle boundary (CMB) is derived from numerical models of mantle convection with a viscous/pseudoplastic rheology that captures the phenomenology of plate‐like tectonics. Our thermal evolution models predict a long‐lived (~8 Gyr) field for Earth and similar dynamo evolution for Earth‐mass exoplanets with plate tectonics. Both elevated surface temperature and pressure‐dependent mantle viscosity reduce the CMB heat flow but produce only slightly longer‐lived dynamos (~8–9.5 Gyr). Single‐plate (“stagnant lid”) planets with relatively low CMB heat flow produce long‐lived (~10.5 Gyr) dynamos. These weaker dynamos can cease for several billions of years and subsequently reactivate due to the additional entropy production associated with inner core growth, a possible explanation for the absence of a magnetic field on present‐day Venus. We also show that dynamo operation is sensitive to the initial temperature, size, and solidus of a planet's core. These dependencies would severely challenge any attempt to distinguish exoplanets with plate tectonics and stagnant lids based on the presence or absence of a magnetic field.

Controls on geomagnetic reversals and core evolution by mantle convection in the Phanerozoic

Numerical dynamos driven by non-uniform heat flux at the core-mantle boundary are used to investigate the connections between geomagnetic field structure, geomagnetic reversal frequency, core evolution, and mantle convection through Phanerozoic time. Polarity reversal sequences and time average magnetic field structures are calculated using dynamos driven by two representations of lower mantle history: a reconstruction of mantle convection with plate motions by Zhang and Zhong (2011) that produces time variable core-mantle boundary (CMB) heat flux and an irregular evolution of the core, and a second model based on hotspot locations with a time independent pattern of CMB heat flux derived from the present-day seismic shear wave heterogeneity of the lower mantle that produces a monotonic evolution of the core. For both mantle histories, present-day values of the dynamo control parameters are tuned to match Geomagnetic Polarity Time Scale reversal statistics for 0–5Ma, and the time dependences of the dynamo control parameters are determined from the thermal evolution of the core, including time variability of CMB heat flow, inner core size, inner core chemical buoyancy flux, and rotation rate. The dynamo with time independent CMB heat flux shows minor fluctuations in reversal frequency with age, whereas the dynamo with time variable CMB heat flux shows reversal rate fluctuations including stable polarity at 275 and 475Ma and frequent reversals at other times. This dynamo also produces departures from geocentric axial dipole symmetry during the time of supercontinent Pangaea and a heterogeneous growth history of the inner core.

Lattice thermal conductivity of MgSiO3 perovskite and post-perovskite at the core–mantle boundary

Thermal conductivity is essential for controlling the rate of core heat loss and long-term thermal evolution of the Earth, but it has been poorly constrained at the high pressures of Earth's lowermost mantle. We measured the lattice component of thermal diffusivity, heat transport by scattering of phonons, of both MgSiO3 perovskite (Pv) and post-perovskite (PPv) at high pressures of up to 144GPa and at room temperature. Lattice thermal conductivity of Pv-dominant lowermost mantle assemblage obtained in this study is about 11W/m/K, while PPv-bearing rocks exhibit ∼60% higher conductivity. Since such Pv value is comparable to the conventionally assumed lowermost mantle conductivity, our findings do not significantly alter but support the recent notion of high core–mantle boundary heat flow along with a young inner core and high temperatures in the early deep Earth.

Possible links between long-term geomagnetic variations and whole-mantle convection processes

The Earth's internal magnetic field varies on timescales of months to billions of years. The field is generated by convection in the liquid outer core, which in turn is influenced by the heat flowing from the core into the base of the overlying mantle. Much of the magnetic field's variation is thought to be stochastic, but over very long timescales, this variability may be related to changes in heat flow associated with mantle convection processes. Over the past 500 Myr, correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late Mesozoic era, beginning about 180 Myr ago. Simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability — such as occurred between the Middle Jurassic and Middle Cretaceous periods — may have been triggered by a decrease in core-mantle boundary heat flow either globally or in equatorial regions. This decrease in heat flow could have been linked to reduced mantle-plume-head production at the core-mantle boundary, an episode of true polar wander, or a combination of the two.

Thermal and electrical conductivity of iron at Earth’s core conditions

The Earth acts as a gigantic heat engine driven by the decay of radiogenic isotopes and slow cooling, which gives rise to plate tectonics, volcanoes and mountain building. Another key product is the geomagnetic field, generated in the liquid iron core by a dynamo running on heat released by cooling and freezing (as the solid inner core grows), and on chemical convection (due to light elements expelled from the liquid on freezing). The power supplied to the geodynamo, measured by the heat flux across the core-mantle boundary (CMB), places constraints on Earth's evolution. Estimates of CMB heat flux depend on properties of iron mixtures under the extreme pressure and temperature conditions in the core, most critically on the thermal and electrical conductivities. These quantities remain poorly known because of inherent experimental and theoretical difficulties. Here we use density functional theory to compute these conductivities in liquid iron mixtures at core conditions from first principles--unlike previous estimates, which relied on extrapolations. The mixtures of iron, oxygen, sulphur and silicon are taken from earlier work and fit the seismologically determined core density and inner-core boundary density jump. We find both conductivities to be two to three times higher than estimates in current use. The changes are so large that core thermal histories and power requirements need to be reassessed. New estimates indicate that the adiabatic heat flux is 15 to 16 terawatts at the CMB, higher than present estimates of CMB heat flux based on mantle convection; the top of the core must be thermally stratified and any convection in the upper core must be driven by chemical convection against the adverse thermal buoyancy or lateral variations in CMB heat flow. Power for the geodynamo is greatly restricted, and future models of mantle evolution will need to incorporate a high CMB heat flux and explain the recent formation of the inner core.

A statistical boundary layer model for the mantleD″ region

[1] The D″ region above the core-mantle boundary (CMB) plays critical roles in the dynamics of both the mantle and the core; however, the complexity of this region observed over a broad range of spatial scales defies simple interpretations as either purely thermal or purely chemical heterogeneity. Here, we formulate a 1-D, time-dependent boundary layer model for the D″ region, which provides statistical properties of the dynamics and seismic heterogeneity, by coupling thermal, chemical, and phase (TCP) variability in the layer. We assume a Gaussian-like time-dependent mantle flow, compositional stratification due to variations in iron content, heat flow variations because of changes in the local temperature gradient, and a postperovskite (PPV) phase transformation. We compute a range of TCP boundary layer model cases that are consistent with the observed seismic shear wave velocity heterogeneity in the region 50–300 km above the CMB. These results imply an average CMB heat flow near 13 TW with ±3 TW variations, CMB temperature of 4000 K, a large positive PPV Clapeyron slope, and an average heat transport of about 3 TW associated with deep mantle plumes.