Deep Earth DIALOG
|Number 14||Summer, 2006|
- 10th SEDI Symposium, Prague, The Czech Republic, 9th - 14th July 2006.
- Session 1: Structure and chemistry of the core and lower mantle
- Session 2: Seismic observations of the Earth's deep interior
- Session 3: Mantle convection
- Session 4: Inner core-outer core-mantle coupling
- Session 5: Laboratory modeling of deep Earth processes
- Session 6: Core dynamics and thermodynamics
- Session 7: Geodynamo, theory and observations
- Session 8: Deep interiors and magnetic fields of other planets
This is the fourteenth issue of the newsletter of SEDI, an IUGG Union
Committee to Study the Earth's Deep Interior. Requests for copies of the
earlier issues should be addressed to David Loper, Geophysical Fluid Dynamics
Institute, Florida State University, Tallahassee, Florida 32306-3017, U.S.A,
faxed to (904) 644-8972 or emailed to firstname.lastname@example.org. Items for the
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10th SEDI Symposium, Prague, The Czech Republic, 9th - 14th July 2006.
The tenth SEDI symposium was held in Prague, The Czech Republic, from 9th - 14th, July 2006, with approximately 150 participants. The local organizing committee comprised Pavel Hejda, Eduard Petrovsky, Jan Simkanin, and Marta Tuckova. Jonathan Aurnou gave the Zatman lecture, and Julien Aubert, Keith Koper, and Jonathan Mound won the Doornbos prizes. Special issues of Geophysical and Astrophysical Fluid Dynamics and Studia Geophysica et Geodetica will be based on talks and posters presented at the meeting. Brief summaries of the sessions are found below.
At the business meeting, Bruce Buffett awarded the Doornboos prizes for outstanding contributions from young scientists, commemorating the life and work of Durk Doornboos. He also announced that a nominating committee consisting of Bruce Buffett, Satoru Tanaka, and Kathy Whaler will help draw up a list of possible SEDI Vice-Chairs, as Gauthier Hulot will become Chair, and Bruce steps down. Mike Bergman will continue as Secretary. The nominating committee will also seek to rotate members of the SEDI advisory board.
There was also discussion of the next meeting site. China (perhaps in
Kunming) remains the first choice for 2008, with Sidao Ni's student Daoyuan
Sun making a presentation. There remain questions about the details of
the venue, costs, and the makeup of the local organizing committee. Other
possible meeting sites include Norway (suggested by Bernhard Steinberger)
and Maryland, USA (suggested by Dan Lathrop). Finally, Philippe Cardin,
and Johannes Wicht are convening sessions at IUGG that SEDI is co-sponsoring.
Session 1: Structure and chemistry of the core and lower mantle
Session chair: Lidunka Vocadlo
Invited speakers: Tim Eliott, Dario Alfe, and Christine Reif
Eliott examined W isotopic composition differences between S. African kimberlites and Hawaiian basalts. Since W isotopic differences are generated only within the first 50 My of solar system history, and W is siderophile, they provide a good diagnostic of core/mantle interactions. Neither the kimberlites nor the basalts show isotopic differences relative to other terrestrial reservoirs, suggesting little (less than .1%) exchange of material from core to mantle. Eliott further argued that the core cannot contribute to elevated Os186 isotope levels because the core is likely to be undersaturated with respect to oxygen, so that siderophile oxides are likely to be extracted from the mantle. In this view Os isotopic data cannot be used to make inferences concerning the age of the inner core.
Alfe covered two areas that first principles calculations have addressed: the melting temperature of pure iron under core conditions, and the nature of core impurities and their partitioning between inner and outer cores. Discrepancies remain between various experimental results and the quantum mechanical calculations. The seismologically inferred density jump at the ICB is 5%, but less than 2% of that is ascribed to the phase change. Alfe looked at the chemical potentials of the liquid and solid for various concentrations of O, S, and Si. He finds little partitioning for S or Si, so that O is required for the density jump, but O (in the amount required to match the OC density) by itself creates too large a jump, so that S and or Si are also required.
Reif discussed recent advances in seismic tomography that allow distinguishing between thermal and chemical contributions to variations in mantle seismic velocities. By using both P and S wave velocity anomalies Reif finds that most anomalies are thermal in origin, but that in the central Pacific, compositional contributions are important. An experimentally determined equation of state suggests this is due to an increase in iron and perovskite.
Posters covered experimental studies of the phase diagram of perovskite, first principles calculations concerning the elastic properties of post-perovskite, as well as its role in the global geotherm. Several other posters involved first principles calculations of the properties of iron under core conditions, including the melting temperature of iron alloys, the stability of the tetragonal phase and the body centered cubic phase in the presence of impurities, and the elasticity of iron under inner core conditions. One poster suggested a significantly higher shear wave velocity than observed seismically, which could be resolved by 8 % melt in the inner core. Other posters examined the nature of the lower mantle using electrical conductivity profiles, and shear wave seismic studies.
The discussion centered on impurities in the core, the structure of
iron under inner core conditions, and the influence of impurities on that
structure. It was pointed out that the geochemical signature puts an upper
bound on the amount of Si in the core, and that 8 % melt in the inner core
seems high. There was also discussion on phase transitions in D", and the
role of post-perovskite.
Session 2: Seismic observations of the Earth's deep interior
Session chair: Christine Thomas
Invited speakers: Keith Koper, James Wookey, and Lapo Boschi
Koper spoke about IC variations on scales from the hemispheric down to 10-100 km. He used a study of PKiKP codas to show that heterogeneity on the small scale must be present within the inner core, not just at the ICB, CMB, or lower mantle. He also found that scattering attenuation is likely to be at least as important as intrinsic attenuation. There may also be lateral variations in the scattering that anticorrelate with the IC elastic anisotropy.
Wookey discussed the seismic anisotropy of the lowermost mantle, using a shear wave splitting technique to isolate the anisotrpic contribution from the lower mantle. In order to interpret the results he suggested one needs to understand the anisotropy of perovskite and post-perovskite, their slip systems, and the likely deformation direction.
Boschi reviewed progress in seismological techniques, in particular, the use of finite frequency methods rather than the ray theory approximation. By using analytical formulations and efficient numerical techniques, these methods are likely to become increasingly important for tomography. The methods also allow more precise quantification of accuracy and resolution.
The discussion centered on the posters. Several posters concerned the
660 km transition, and there was discussion why it has not been seen in
PP. There may be issues because of the shorter wavelength, or it could
be due to the effects of a combination of a phase transition in olivine
as well as changes in chemical composition. Other posters concerned mantle
convection and slab penetration, and the question arose as to slabs' thermal
signature, which depends on how fast they go down. Other posters concerned
D", and yet others the inner core, where no global discontinuity was seen,
but Q does seem to be laterally variable..
Session 3: Mantle convection
Session chair: Ondrej Cadek
Invited speakers: Paul Tackley, Ctirad Matyska, and Anne Davaille
Tackley emphasized the role of compositional effects. For instance, compositional variations can affect the CMB heat flow, and hence the thermal evolution of the core and mantle. He also emphasized the need for a stress dependent viscosity to yield plates, and that it might also be necessary to allow subducted slabs to remain above the CMB. He also pointed out that post-perovskite is more likely near cold slabs, but that we still do not understand subduction.
Matyska also spoke about the effects of lateral variations in material properties, and how they lead to multiple scales in mantle plumes in numerical calculations, with a change in spatial scales between the upper and lower mantles.
In the third talk Davaille examined episodic phenomena such as volcanism, plate tectonics, and continental growth, which can be produced by thermal convection, especially in the presence of heterogeneities. Spatial and temporal variations in the heat flux can be as large as the mean value of the heat flux, and the time scale of variations can be a large fraction of the Earth's age. Hence, the Earth's cooling may not be monotonous.
During the discussion it was agreed that lab studies and numerical studies
are often complementary: lab studies are better for studying entrainment,
but have a difficult time examining plate tectonics. Tim Elliott pointed
out that although much focus has been on post-perovskite, there are certain
isotope ratios that are not yet understood. Seismologists also asked how
they might best help geodynamicists, with the answer being a distinction
between temperature and compositional variations, information on the smaller
scale, and more knowledge of anisotropy and scattering.
Session 4: Inner core-outer core-mantle coupling
Session chair: Mathieu Dumberry
Invited speakers: Jonathan Mound, Marianne Greff-Lefftz, and
Mound reviewed variations in the length-of-day (LOD) at time scales from years to decades, and the various coupling mechanisms between core and mantle. Although correlations between the LOD and the magnetic field secular variation on the decade scale can be explained by torsional oscillations, they do not constrain the coupling mechanism. Electromagnetic coupling can produce large angular momentum transfer, but damps out oscillations. Gravitational coupling can lock the inner core to the mantle and may explain the newly observed six year oscillation. This requires an inner core with a viscosity greater than 10^17 Pa-s (and is also incompatible with the suggested IC super-rotation). Topographic coupling could also be important, but viscous coupling is not likely to be.
Greff-Lefftz examined the constraints that geodesy puts on core-mantle coupling, in particular, the effects of the nearly-diurnal luni-solar tidal potential on surface gravity and on the spatial nutations. VLBI observations suggest an effective kinematic viscosity as large as 10^-6 m^2/s, much larger than most molecular estimates, around 10^-15 m^2/s.
Jellinek pondered the question of what thermal boundary condition the core sees and over what timescale. Significant differences in mantle convection occur with stagnant-lid versus plate tectonic (active-lid) boundary conditions, and these affect the core thermal boundary conditions and hence the generation of planetary magnetic fields.
In the discussion a question arose about whether the individual core-mantle
coupling processes could be studied on their own, or whether they needed
to be coupled. There was also discussion about the OC viscosity. Smylie
claimed that he could constrain the viscosity, and that it was rather high
(number?), and that this was a molecular value. The posters centered around
three themes: thermal core-mantle coupling, lunar/solar torques and nutations,
and mechanical core-mantle coupling.
Session 5: Laboratory modeling of deep Earth processes
Session chair: Dan Latrhop
Invited speakers: Philippe Cardin, Romain Volk, and Daniel Lathrop
Cardin presented the current status of the Grenoble work, emphasizing the magnetostrophic regime of the experiment, which consists of differentially rotating spheres, with the inner sphere containing a permanent magnet to provide a quasi-dipolar magnetic field. Instabilties in the form of an m = 3 retrograde wave are present, as are two regions in the fluid: one controlled by the imposed magnetic field and one where inertial effects dominate. There are several unexplained observations, such as an apparent oscillation between two regimes as measured by the mean velocity of the flow.
Volk described the progress of the work in Lyon. They have examined amplification in a von Karman sodium setup with a magnetic Reynolds number of about 10, Reynolds number of order 10^6, and magnetic Prandtl number of order 10^5, finding a generated field in the same direction and about half as large as the applied field.
Lathrop talked about scaling of laboratory dynamos, and then about specific observations from laboratory experiments, including magnetorotational instabilities and inertial waves. He also discussed the possibility of dynamo action in spherical Couette flow, which is possible in theory, but has not been observed in the laboratory.
There was discussion that lab experiments are producing good quality
data, but that it remains difficult to connect the experiments with theory,
nevermind the earth. There was also considerable discussion about the role
Session 6: Core dynamics and thermodynamics
Session chair: Dave Loper
Invited speakers: Jonathan Aurnou, Mike Bergman, and Dave Gubbins
Continuing on the theme from Lathrop's talk, Aurnou gave the Zatman lecture, on scaling behavior in rotating fluids, after first reviewing rotating fluids in general. In particular, he focused on the Nusselt number dependence on the Rayleigh number. The Nusselt number may also depend on the Prandtl number. In any case, for Rayleigh numbers as large as ten times critical, the Nusselt number remains only slightly greater than one.
Bergman reviewed deformation and solidification in the inner core, and their connection with inner core elasticity and attenuation anisotropy. The details of the anisotropy and heterogeneity in the IC remain unclear. Key issues include the deformation mechanism and viscosity of the inner core, the source of stress, and the effects of post-solidification deformation on the solidification texture. He also looked at fluid flow effects during solidification.
Gubbins pointed out a central paradox concerning the IC: the three billion year old geomagnetic field requires the presence of radial motion in the OC, and hence an old IC to provide a source of buoyancy, but heat flow calculations suggest an IC younger than a billion years. It is not clear that K40 can resolve this paradox. Another possibility is a density stratified layer above the ICB, which may be consistent with seismic inferences, and could arise if the core is on the light element side of the eutectic. This would steepen the liquidus, allow more heat to be conducted away from the ICB, and resolve the paradox.
During the discussion it was pointed out that it is difficult to get
a large Nusselt number in liquid metals in the lab. It was also pointed
out that an IC viscosity as small as 10^11 Pa s, as a Harper-Dorn deformation
mechanism suggests, would make IC normal modes essentially unobservable,
as well as having other consequences. It was also pointed out that a light
element-rich core would preclude dendritic growth in the IC, and yield
a eutectic IC composition, which may not agree with seismic data. Many
parameters are perhaps insufficiently known that we may not yet need to
revise completely our views.
Session 7: Geodynamo, theory and observations
Session chair: Andrew Soward
Invited speakers: Uli Christensen, Hiroaki Matsui, and Gauthier
The pessimistic view of numerical geodynamos is that they give the right answers for the wrong reasons. The optimistic view is that the Earth is already in the diffusive regime. Christensen spoke about the emerging importance of the Rossby number in determining whether the field is dipole dominated, by examining some 70 models. He also finds that dynamo properties depend mainly on the driving buoyancy flux and are weakly dependent on the viscosity and diffusivities.
Matsui gave a talk on progress in finite element and sub-grid scale modeling. The main advantage is its suitability for parallel computation. The main disadvantage is the difficulty of handling the divergenceless nature of and CMB boundary conditions on the magnetic field. The sub-grid scale modeling results show that it can approcimate the influence of small scale fields.
Hulot compared the modern, historical, and archeo magnetic fields with dynamo simulations, concentrating on high latitude flux concentrations, preferred latitude paths during reversals, polar lows, and hemispherical asymmetries. In general the dynamo simulations do well, though they do not reproduce such features as geomagnetic jerks or torsional oscillations.
The discussion covered topics such as whether dynamo models can give
insight into the age of the IC, the Rayleigh number dependence of the Nusselt
number, the need to add the heat conducted down the adiabat if that has
first been subtracted out, the sufficiency of paleomagnetic data, and why
a laterally variable heat flux can kill the dynamo.
Session 8: Deep interiors and magnetic fields of other planets
Session chair: Bruce Buffett
Invited speakers: Dave Stevenson and Johannes Wicht
Stevenson gave a tour of the solar system, relating the presence and style of dynamo action to planetary structure and other factors, since magnetic fields can serve as a diagnostic of planetary interiors. Bodies in the solar system can be classified as terrestrial, icy satellites, gas giants, and ice giants. For the terrestrial planets, the mantle is master (and the earth is special because of the presence of plate tectonics), the presence of S guarantees a liquid core, and the heat conducted is almost always very close to the total heat transported, making convection difficult. For the fluid planets, the atmosphere is master, there are no sharp boundaries, and convection is easy.
Wicht spoke about thin shell dynamos, and their generation of non-dipolar fields. One can also obtain quadruple dynamos numerically at Rayleigh numbers only slightly supercritical. He spoke at length about Mercury's magnetic field, which is likely to generated by compositionally driven convection, with a strong solar wind contribution.
During the discussion, the philosophical question arose, "When is a planetary magnetic field considered dynamo-generated, and when is it merely induced from an externally applied field?" The relevance is to Mercury and some of the giant planets' satellites. Another question arose concerning the importance of S, as opposed to other light elements. O may be more important in the earth because of it siderophility at high pressure. The importance of plate tectonics in governing the presence and style of a magnetic field was emphasized.
[Report by Michael Bergman]