The effect of thermal core–mantle interactions on the palaeomagnetic secular variation

Bloxham, Jeremy

doi:10.1098/rsta.2000.0579

Cited by 47 publications

(35 citation statements)

References 31 publications

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“…Of particular interest is the regime in which the fluid flow and magnetic field become locked to the boundary anomalies because this can explain the observation of the four relatively stationary main concentrations of flux seen on the surface of the Earth's core. Both papers used the pattern of seismic shear wave velocity at the base of the Earth's mantle for the heat flow boundary condition, in common with several previous studies , Bloxham 2000, Olson and Christensen 2002, Christensen and Olson 2003. Paper I found a near-steady solution with surface magnetic flux concentrated into four main lobes located very close to those in the Earth.…”

Section: Introductionmentioning

confidence: 53%

Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction

Sreenivasan

Gubbins

2008

Geophysical & Astrophysical Fluid Dynamics

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Section: Introductionmentioning

confidence: 53%

Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction

Sreenivasan

Gubbins

2008

Geophysical & Astrophysical Fluid Dynamics

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“…Depending on the amount of radiative conduction, the conductive component to the heat loss across the CMB is thus uncertain by (at least) a factor of two (Q cmb ≈ 7.5 to 15 TW). It is likely, however, that the flux into the base of the mantle exceeds the heat conducted down the core adiabat, which has implications for models of thermal coremantle coupling, core evolution, and the distribution of heat-producing elements (36,37).…”

Section: D′′ Structure and Temperaturementioning

confidence: 99%

Seismostratigraphy and Thermal Structure of Earth's Core-Mantle Boundary Region

Hilst

Hoop

Wang

et al. 2007

Science

242

172

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We used three-dimensional inverse scattering of core-reflected shear waves for large-scale, high-resolution exploration of Earth's deep interior (D′′) and detected multiple, piecewise continuous interfaces in the lowermost layer (D′′) beneath Central and North America. With thermodynamic properties of phase transitions in mantle silicates, we interpret the images and estimate in situ temperatures. A widespread wave-speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite. Internal D′′ stratification may be due to multiple phase-boundary crossings, and a deep wave-speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick. The core-mantle boundary temperature is estimated at 3950 ± 200 kelvin. Beneath Central America, a site of deep subduction, the D′′ is relatively cold (DT = 700 ± 100 kelvin). Accounting for a factor-of-two uncertainty in thermal conductivity, core heat flux is 80 to 160 milliwatts per square meter (mW m A t a depth of~2890 km, the core-mantle boundary (CMB) separates turbulent flow of liquid metals in the outer core from slowly convecting, highly viscous mantle silicates. The 200-to 300-km-thick thermochemical boundary layer on the mantle sidethe so-called D′′ layer-is enigmatic (1, 2), but a recently discovered phase transition from perovskite (pv) to postperovskite (ppv) in (Mg,Fe)SiO 3 (3-5) begins to explain seismologically observed complexity [e.g., (6)]. If the ppv transition occurs, one can, in principle, estimate in situ variations in temperature from the pressure-temperature dependence (that is, the Clapeyron slope) and the seismologically inferred location of the associated interface (7). Steep (conductive) thermal gradients in D′′ can produce multiple crossings of the phase boundary, and identification of associated seismic signals offers new opportunities for constraining (local) core heat flux (8, 9).Seismic (transmission) tomography delineates smooth changes in wave speed associated with mantle convection (Fig. 1A), but one must focus on the scattered wave field to image interfaces associated with transitions in mineralogy or composition. Scattering of PKP (the main P wave propagating through the core) in D′′, first recognized in the early 1970s (10), has been used to constrain stochastic models of deep mantle structure [e.g., (11)], but the most detailed and accurate constraints on D′′ structure to date have come from forward modeling of shear waves reflected at or near the CMB (12, 13). This approach has its drawbacks, however. First, it requires prior knowledge about the target structure and often assumes relatively simple geometries, the uniqueness of which is not easily established. Second, it relies on signal associated with near-and postcritical incidence, which limits radial resolution and the CMB regions that can be studied (14). The small distance window can also reduce the available source-receiver azimuths, which can degrade imaging in dir...

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“…The dynamically self-consistent models of Bloxham (2002) supported this. Bloxham (2000), also using the tomographic model of Masters et al (1996), was able to match numerical and observed magnitudes of secular variation (SV). He found a rapid increase in SV near the equator but no average difference in SV between the Pacific and Atlantic hemispheres.…”

Section: Introductionmentioning

confidence: 99%

Time-averaged paleomagnetic field and secular variation: Predictions from dynamo solutions based on lower mantle seismic tomography

Davies

Gubbins

Willis

et al. 2008

Physics of the Earth and Planetary Interiors

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We compare 3 dynamo solutions incorporating laterally varying boundary heat flux with paleomagnetic models and data. The boundary condition is defined by the D seismic shear wave velocity and the 3 solutions have boundary anomalies with different amplitudes. The generated fields appear to divide into a stationary, boundary-locked part and a time-varying part with persistent centres of activity. Both parts contribute to the time average. A very Page 2 of 32 A c c e p t e d M a n u s c r i p t long averaging time can be needed for nearly-locked solutions, but a rough time average that remains within the threshold set by the accuracy of paleomagnetic data is achieved in a few diffusion times. The locked part dominates for larger amplitude boundary anomalies.In previous work the locked field was shown to have strong similarities with the modern geomagnetic field. Previous dynamo solutions that were not locked to the boundary show similarities with our solutions with weak boundary forcing. The axisymmetric time average has small g 0 2 and larger g 0 3 components and peaks in inclination anomaly in high latitudes (associated with the locked field) and low latitudes (associated with the time average of the time-varying fields). The non-axisymmetric time average displays a striking longitudinal variation in inclination anomaly, with a large negative anomaly in the Pacific region in agreement with observations. None of the dominant geomagnetic coefficients are axisymmetric and g 0 2 negligible in all 3 models. Secular variation is concentrated in equatorial latitudes, as in some recent paleomagnetic models. The locked field agrees with the inclination difference found between Hawai'i and Réunion, in agreement with paleomagnetic averages. The locked field agrees with the paleomagnetic time average rather better than the fields with less boundary variations. We conclude that, because the locked field agrees with the modern field as well as some aspects of the long-term time average, the geomagnetic field spends a considerable time in the present 4-lobe configuration: it is not a coincidence that the present field resembles the time average. Longitudinal variations are likely to be at least as important as latitudinal variations in the paleomagnetic time average. This presents a challenge for dynamo theory, since a move to more geophysically realistic parameters would appear to destroy the locked solutions.

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The effect of thermal core–mantle interactions on the palaeomagnetic secular variation

Cited by 47 publications

References 31 publications

Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction

Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction

Seismostratigraphy and Thermal Structure of Earth's Core-Mantle Boundary Region

Time-averaged paleomagnetic field and secular variation: Predictions from dynamo solutions based on lower mantle seismic tomography

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