Pacific secular variation a result of hot lower mantle
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Pacific Secular Variation A result of hot lower mantle. David Gubbins School of Earth Sciences University of Leeds. Thermal Core-Mantle Interaction. (hot). (cold). Lateral variations in heat flux boundary condition on spherical rotating convection can:. Drive thermal winds

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Pacific secular variation a result of hot lower mantle

Pacific Secular VariationA result of hot lower mantle

David Gubbins

School of Earth Sciences

University of Leeds



Lateral variations in heat flux boundary condition on spherical rotating convection can
Lateral variations in heat flux boundary condition on spherical rotating convection can:

  • Drive thermal winds

  • “lock” core convection…

  • …and delay drift of convection rolls

  • Produce resonance of length scales…

  • …and secondary resonances

  • Force a lateral scale on the convection

  • Indirectly produce similar scales on the magnetic field


The effect of lateral variations is weakened by
The effect of lateral variations is weakened by: spherical rotating convection can:

  • Low Prandtl number (inertia)

  • Disparity of length scales between convection and boundary conditions

  • High Rayleigh number (time dependence)


Geophysical input for core heat flux
Geophysical Input for Core Heat Flux spherical rotating convection can:

  • Mantle convection studies suggest large variations in lateral heat flow (100%)

  • …and thermal boundary layer at the base of the mantle (D”)

  • Seismology suggests a boundary layer 250 km thick

  • …with temperature variations of 500 K


Observational evidence of lateral variations
Observational Evidence of Lateral Variations spherical rotating convection can:

  • Modern geomagnetic field

  • Time-average of paleomagnetic field

  • Persistent reversal paths

  • Non-axisymmetric variations in secular variation

  • Low secular variation in Pacific


Overview
OVERVIEW spherical rotating convection can:

  • Evidence for low secular variation in the Pacific -historical and paleomagnetic

  • Lateral heat variations on the core-mantle boundary

  • Simple thermal convection influenced by the boundary

  • Relationship with numerical dynamo simulations and application to the Earth’s core

  • Implications for the thermal state of the core


Declination ad 1650
Declination AD 1650 spherical rotating convection can:


Declination ad 1990
Declination AD 1990 spherical rotating convection can:


Pacific secular variation a result of hot lower mantle

Declination at Hawaii and Greenwich Meridian spherical rotating convection can:


Inclination hawaii and greenwich meridian
Inclination Hawaii and Greenwich meridian spherical rotating convection can:


Looking for weak secular variation
Looking for weak Secular Variation spherical rotating convection can:

  • Historical record shows little SV in Pacific

  • 400 years is not long enough to be definitive

  • We need 5-50 kyr

  • Big Island, Hawaii, offers 35 kyr with dating


Volcanoes of big island hawaii
Volcanoes of Big Island, Hawaii spherical rotating convection can:


Pacific secular variation a result of hot lower mantle

Mean residual -2.8 spherical rotating convection can:o +/- 0.3o


D from flows dated by c 14 big island hawaii
D spherical rotating convection can: from flows dated by C14, Big Island, Hawaii


I from flows dated by c 14 big island hawaii
I spherical rotating convection can: from flows dated by C14, Big Island, Hawaii


Kilauea east rift zone drilling
Kilauea East Rift Zone Drilling spherical rotating convection can:



The tangent cylinder
The flowsTangent Cylinder


Convection with laterally varying heat flux depends on 3 important parameters
Convection with laterally varying heat flux depends on 3 important parameters

1. Ekman number

2. Vertical Rayleigh number

where h is the mean surface heat flux

3. Horizontal Rayleigh number

where q is the lateral variation of

heat flux, average zero


3 limiting cases
3 LIMITING CASES important parameters

  • Rv=0: thermal wind

  • Rh=0: convection with uniform boundaries

  • Rh=0.3Rv: convection heated from below and influenced by the boundary variations


Thermal wind r v 0 e 2x10 4
“Thermal Wind”, important parametersRv=0, E=2x10-4


Uniform boundaries e 2x10 4 r h 0 r v 1 1 r v c
Uniform boundaries important parametersE=2x10-4, Rh=0, Rv=1.1 Rvc



Pacific secular variation a result of hot lower mantle
Inhomogeneous boundary conditions (periodic solution) important parameterssurface flow and temperature Rh=0.3 Rv, E=2x10-4, Rv=1.1 Rvc


Inhomogeneous boundary conditions r h 0 3 r v e 2x10 4 r v 1 1 r v c
Inhomogeneous boundary conditions important parametersRh=0.3 Rv, E=2x10-4, Rv=1.1 Rvc


Summary
SUMMARY important parameters

  • Boundary heat flux based on shear wave anomalies can inhibit convection at the top of the core below the hot region corresponding to the Pacific…

  • …because the anomaly there is longitudinally broader than in the Atlantic/Africa

  • This convective flow does not generate a magnetic field


Comparison with a geodynamo simulation
COMPARISON WITH A GEODYNAMO SIMULATION important parameters

  • This convective flow does not generate a magnetic field

  • Bloxham’s geodynamo simulation exhibits a time average that reflects the boundary conditions…

  • …but does not give low Pacific SV or a field that resembles the time average at any instant of time

  • The principle difference is not the magnetic field…

  • It is probably the higher Rv in the dynamo simulation


Application to the earth
APPLICATION TO THE EARTH important parameters

  • Resonance with the boundary arises because of similarity in length scales of convection and boundary anomalies

  • Small E (10-9) in the core implies a small scale but magnetic forces increase it

  • A higher supercritical Rv is needed for dynamo action, but this produces magnetic fields that are too complex, both spatially and temporally

  • Again, the in the low E regime dynamo action may occur at lower supercritical Rv because of its organising effect on the flow


Implications for core heat flux
IMPLICATIONS FOR CORE HEAT FLUX important parameters

high heat flux

low heat flux

D’’

slow

fast

Difference in Vs implies temperature difference 500 K in 250 km


Horizontal vs vertical heat flux
HORIZONTAL VS VERTICAL HEAT FLUX important parameters

  • Lateral temperature difference 500 K

  • Within D’’ thickness 200 km

  • Thermal conductivity 10 W/m/K

  • Gives heat flux variation 1 TW =…

  • 20% of conventional estimate of vertical heat flux

  • May be larger locally


Conclusions
CONCLUSIONS important parameters

  • The evidence for weak secular variation in the Pacific is quite strong

  • Simple thermal convection calculations show this can come about from lateral variations in heat flux through the boundary

  • These flows are too simple to generate a magnetic field, and numerical dynamo simulations give magnetic fields that appear more complex than is observed

  • Lateral heat flux variations in D’’ appear to be large enough to cause this effect, provided large scale flow is maintained in the core