Deep Earth DIALOG
|Number 13||Fall, 2004|
- 9th SEDI Symposium
- Session 1: Physical properties of Earth materials
- Session 2: Earth's magnetic field: constraints on deep processes
- Session 3: Core dynamics and the geodynamo
- Session 4: Structure and interactions in the system inner core, outer core and mantle
- Session 5: Interaction of plate tectonics and deep mantle processes
- Session 6: High resolution images of the Earth's interior
- Session 7: Physical and chemical discontinuities and reservoirs in the Earth's mantle
- Session 8: Deep interior of other planets
This is the thirteenth 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 email@example.com. Items for the
next issue or notifications of change of address should be sent to firstname.lastname@example.org.
9th SEDI Symposium, Garmisch-Partenkirchen, Germany, 4th - 9th July 2004.
The ninth SEDI symposium was held at Garmisch-Partenkirchen, Germany, in the beautiful Bavarian Alps, from 4th - 9th, July 2004, with approximately 150 participants. The local organizing committee comprised Uli Christensen, Julien Aubert, Peter Bunge, Reini Boehler, Rosemarie Gross, Ulli Hansen, Al Hoffman, Heiner Igel, Rainer Kind, Karin Peschke, and Johannes Wicht. Richard Holme gave the first SEDI Zatman lecture, in honor of Stephen Zatman. A special issue of Physics of the Earth and Planetary Interiors will be based on talks and posters presented at the meeting. Brief summaries of the sessions are found below.
At the business meeting, Uri Shamir, President of IUGG, commended SEDI for the quality and vigor of the scientific discussion at the meeting, and also for the inclusion of so many young scientists at the meeting. He also would like to see us sponsor more sessions at the IUGG and Association meetings, and think of other activities that might help link to the Associations. Also, the Doornboos prizes for outstanding contributions from young scientists were awarded, commemorating the life and work of Durk Doornboos. The recipients were Arwen Deuss and Cinzia Farnetani.
At the business meeting, there was also discussion of the next two meeting
sites, with possible locations for SEDI 2006 being Merida, Mexico (on the
Yucatan peninsula), Banff, Canada, Prague, The Czech Republic, and somewhere
in the eastern USA, in conjunction with the COMPRES meeting. Prague appears
to be the favorite, and Pavel Hejda is looking into the possibilities.
Sidao Ni made a presentation for having SEDI 2008 in China, with several
Session 1: Physical properties of Earth materials
Session chair: Reini Boehler
Invited speakers: Guillaume Fiquet, Gerd Steinle-Neumann, and
Fiquet discussed several issues of lower mantle mineralogy. Because of the tradeoff between the effect of Fe content and temperature on the bulk moduli of perovskite, the chemical composition of the lower mantle is unclear. A better knowledge of the shear properties of the the lower mantle will help discriminate between different compositional models. There is recent experimental evidence for a post-perovskite change in phase that might help explain some of the anomalous elastic properties of the lowermost mantle. Fiquet also discussed recent work on low spin states of Fe, which could have implications for the Fe content of these lower mantle minerals, which is also influenced by the presence of Al. Geophysical consequences include understanding the low velocity zone at the base of the mantle, a postive temperature derivative of the shear velocity, the large elastic anisotropy in D", and convection in the lowermost mantle. He also discussed his results concerning S in outer core, which shows that S decreases the incompressibility, so tha the outer core is not likely to have more than 1.5% S. The melting temperature, stable phase, elastic anisotropy, and miscilibitly gaps all remain subjects of controversy.
Steinle-Neumann reviewed briefly the current state of DFT calculations. Large systems are now standard, so that studying chemical heterogeneities is now possible. Construction of phase diagrams is also now reliable. He emphasized the importance of collaboration between computational and experimental mineral physicists, and gave two examples. Synchrotron studies have shown that K can alloy with Fe above 26 GPa, and DFT calculations show that substitution is favored over interstitial incorporation. Substitution causes the hcp structure to expand. The presence of K in the core might help explain the crustal depletion, and the age of the inner core. The second example dealt with a phase transition in a clay mineral. The interest is in dehydration of layered silicate minerals during subduction.
Frost gave a petrologist's view of lower mantle structure, with an interest in phase transitions, Fe partitioning, and primodial fractionation. In particular, he showed experimentally that ferric iron concentration is strongly affected by the Al concentration, though there can be issues of atmospheric O affecting the experiments. He argued that the O in perovskite may come from ferrous Fe producing ferric and metallic Fe in a redox reaction. On upwelling this metallic Fe recombines with the ferric Fe to produce ferrous Fe.
As always, there was discussion about the nature of mantle convection: whole, layered up to mid-mantle, or layered at the bottom. Part of the problem is that many mineral physics models can fit the seismic data, and geochemistry is not yet able to contrain the mineral physics. To resolve this, we need better knowledge of expansivity and thermal conductivity at high pressure and temperature. We also need a better understanding of the rheological properties. There was also discussion about the melting temperature of Fe. Experiments favor a normal but low mleting temperature of pure Fe, but ab initio calculations show more variation. Other issues that were discussed include the role of core formation in determining impurities in the core, and the ICB density contrast, in particular, how much partitioning of light elements between the OC and IC is required. Whereas low frequency seismology clearly sees a density jump that cannot be explained by the phase change, the high frequency data is perhaps not as clear.
Two posters (Aizawa & Yoneda; Singh) dealt with the properties of
perovskite. The former carried out an experimental study to determine the
temperature derviatives of the elastic moduli, and the latter looking at
the equations of state. Lee & Steinle-Neumann used a first principles
approach to find that up to 3 mol% Xe might be incorporated into the Earth's
core. Bunge used the mantle dynamics code TERRA with high resolution to
explore the effects of non-adiabatic temperatures on plume heat fluxes.
Kennett and Jackson advocate use of 1-D Earth models that are based on
physical requirments rather than mathematical convienence, i.e., simple
Session 2: Earth's magnetic field: constraints on deep processes
Session chair: Gauthier Hulot
Invited speakers: Richard Holme, Cor Langereis, and Monika Korte
Holme reviewed progress in determining the secular variation and its use in determining core flow. He discussed how length of day data on the decadal timescale can be used, assuming torsional oscillations are responsible for exchange of angular momentum between core and mantle, to find the cylindrically radial component of the core magnetic field, a topic that Stephen Zatman worked on. Holme also discussed the connection between torsional oscillations and geomagnetic jerks, and how jerks may provide information on mantle conductivity. Problems with studying torsional oscillations include bias due to non-uniform data, crustal filtering, and minimum norm techiques underfitting extrema. He discussed recent advances involving maximum entropy methods, and better data with the Oersted and CHAMP satellites. He then discussed the frozen-flux approximation needed for finding core flow, suggesting that even for l > 13 diffusion may still not be dominant. While we think we can see some predicted flows such as polar vortices, there are other simple flows we cannot see.
Langereis spoke about the Earth's paleofield. A primary emphasis of current research is to extend the timescale for detailed knowledge of the field, and the reversal timescale. He emphasized the need for high resolution and high quality data during superchrons. Good age and time control are crucial.
Korte reviewed our knowledge of the geomagnetic field over a thousand year timescale, longer than the historcal record, using the archeological record, lava flows, and lake sediments. The record is deficient due to the lack of data from the southern hemisphere, and due to uncertainty about the acquisition of remanence. Dating can also be an issue, though correlation of samples can help reduce uncertainty. One result of recent work is that the dipole moment has been overestimated.
The discussion centered on the posters. Hulot also promoted the SWARM satellite.
Ten posters dealt with the secular variation. Two (Amit, et al. and Horncastle & Holme) attempted to recover core flow from dynamo models. They have some, though not total, success. Three (Amit & Olson, Whaler, et al., and Ballani, et al.) invert secular variation data for core flow. Amit & Olson use a new contraint, termed helical flow, in which the tangential divergence is correlated with the radial vorticity, to remove non-uniqueness. Two posters by Simonyan & Shahparonyan looked at the structure and mechanism of geomagnetic jerks. Wardinski and Holme also studied jerks while inverting for core flow. Finlay & Jackson studied whether secular variation might be due to MAC waves rather than fluid advection. Chulliat, et al. performed tests of the frozen flux assumption, finding that at the large scale diffusion is low, but that it increases with spherical harmonic degree.
Bargery, et al. and Wicht dealt with the structure of the field during
reversals. The former examined the role of the non-dipole components during
a reversal, and the latter used numerical dynamos to examine the site dependence
of reversal duration. Four papers looked at the statistical issues of paleomagnetism.
Hulot & Bouligand examined axial and equatorial symmetry of the paleomagnetic
field, while Bouligand, et al. tested the Giant Gaussian Process
modelling approach by using data from numerical dynamo simulations, finding
that there can be some issues when there are non-homogneous CMB boundary
conditions. Constable & Johnson looked at the paleomagnetic power spectrum,
while Lawrence, et al. carried out a detailed study of the paelosecular
variation of the Pacific. Dumberry & Bloxham extended the work of Zatman
on MAC waves and changes in the length of day to millennial timescales.
Shimizu & Utada looked into the possibility of using decadal changes
in the geoelectric field to observe the Earth's core. A major difficulty
is the need to remove motionally induced emf's in the oceans via global
Session 3: Core dynamics and the geodynamo
Session chair: Johannes Wicht
Invited speakers: Chris Jones, Henri-Claude Nataf, and Rainer
Jones's talk concentrated on the issues confronting those trying to numerically simulate the geodynamo. In some sense it has been surprising that modellers have found 'Earth-like' dynamos because the parameter regime that has so far been realized is not always close to that of the Earth. It is unclear why this is so. On the other hand, when modellers try to reduce the ratio of magnetic to thermal Prandtl numbers to more realistic values, numerical dynamos have failed. Jones also discussed issues concerning the Boussinesq approximation, in particular, concerns about its thermodynamic implications. He also talked about the power requirements of the dynamo. Finally, there has been a lot of recent work on sub-grid methods to understand the influence of the small scale on the large, and Jones reviewed the early work on this promising method to understand parameter regimes that are not likely to be achieveable by brute force.
Nataf reviewed the progress of experimental aspects of dynamos, and questions that such work can bear on. For instance, the recent work of Christensen and Tilgner attempts to use the Joule dissipation time from the Karlsruhe experiment to find scaling that can be applied to find the power requirements of the dynamo. This value, .2 - .5 TW, is low enough that the IC could be older than a billion years without appealing to radioactivity in the core. However, there are questions about whether the Riga dynamo follows the same scaling. If it does not, it may have to do with differences in the way the two dynamos saturate: the Riga dynamo is 'flexible', i.e., there is excess power and saturation is due to flow deformation, whereas the Karlsruhe dynamo is 'stiff', i.e., saturation is due to an increase in pressure. Since we don't know what controls magnetic field saturation in the core, we must be careful about what we can infer about the core. Nataf then showed how laboratory experiments can help us understand turbulence in a way that neither observations nor numerical simulations can. In particular, he examined the scaling of the turbulence for various parameter regimes.
Hollerbach discussed some of the advantages and disadvantages of traditional spherical harmonic expansions for geodynamo codes. Advantages include that divB and divu equalling zero are automatically satisfied, the pressure is elimated by taking the curl, and the boundary conditions are also easily incorporated. The disadvantages are that there is no fast Legendre transformation (though see Risbo & Jackson's poster), and that the codes do not parallelize. Hence, Hollerbach is exploring methods that invoke only local coupling, so that less communication is needed, making parallelization easier. By using the magnetic vector potential, divB = 0 is guaranteed, but this is harder to do for u, since the pressure is a physical quantity. Hollerbach then spent time showing ways of dealing with the velocity when using local methods.
During the discussion, Wicht pondered whether we should be optimistic or pessimistic about numerical dynamo work. On the one hand, even though the parameters are not Earth-like, but the results are, perhaps the results are fairly universal. On the other hand, perhaps we are not looking at the right things. There is also widespread concern about the issue of the ratio of Prandtl numbers. Moreover, mere comparison of models with data doesn't "explain" the dynamo. There is also consensus that we need to understand turbulence and what is going on at the small scale, so that sub-grid scale methods look to be helpful.
Matsui & Buffett, Matsushima, Soltis, et al., and Ivers looked at various methods of understanding what is going on at scales smaller than can be modelled globally, so tha the small scale can be correctly parameterized for large scale models. Matsui & Buffett employ the non-linear gradient method, and compare their results with that of a higher resolution dynamo calculation. Matsushima employs a scale similarity approach. Soltis, et al. and Ivers incorprate turbulence via anisotrpic thermal diffusion, finding that the stability is similar to that for isotropic thermal diffusion.
Risbo & Jackson presented a new functional representation on a sphere that is complete and that has a fast transform between physical and spectral space.
Six posters in this session involved various aspects of numerical simulation of spherical dynamos. Sreenivasan & Jones performed a parametric study to better understand the influence of the ratio of Prandtl numbers. As both thermal and magnetic Prandtl numbers are increased together, inertial forces become weaker, and the magnetic field is confined to smaller lengthscales. Aubert examined the interaction between zonal flows and magnetic field generation. When dynamo action is present, the flow tends to be ageostrophic, as opposed to pure rotating convection. Maclean and Fearn investigated how the direction of the buoyancy force influenced the parity of the solutons for non-linear alpha-omega dynamos. Wicht & Olson studied Taylor-Couette instability in a spherical shell, finding that the instabilities have a helicity similar to that of rotating convection. They can thus generate a magnetic field, and might be interesting for experimental dynamos. Willis & Gubbins looked at kinematic dynamo action resulting from periodic time-dependent flows. The time dependence often enhances dynamo action. Oishi, et al. presented a numerical method that uses spectral expansion in longitude and finite differences in meridional planes with the goal of increasing speed so that more Earth-like parameters can be used.
Six other posters dealt primarily with numerical simulation of convection in spherical geometry. Rotvig implemented a pseudospectral model for compressible thermal convection. Tortorella, et al. were also interested in anelasticity, making a comparison of anelastic and Boussinesq solutions. Hejda & Reshetnyak presented a finite volume formulation of thermal convection, with particular interest in the anisotropy of turbulence. Harder & Hansen also carried out finite volume simulations, reaching Ekman numbers as low as 10^-5 and Rayleigh numbers 30 times critical. Sakuraba studied magnetoconvection with an imposed axial magnetic field. He was particularly interested in the non-linear evolution of flow parallel to the spin axis and crossing the equatorial plane because of its possible role in reversals. Phillips & Ivers have developed a code to examine the stability of MHD models.
Three posters examined numerically dynamo action in non-spherical geometry. McMillan & Sarson take advantage of a Cartesian code that is efficiently parallelised, and flexible, and might be adaptable to studying the terrestrial dynamo. Stellmach & Hansen explored the Childress-Soward model numerically to better understand sub-critical dynamo action. Simkanin & Tilgner studied dynamo action within a conducting region surrounded by an insulator, where the field does not penetrate into the insulator. They studied the structure of such fields in cylindrical geometry, as a start.
Breuer, et al., looked at the effects of inertia on Rayleigh-Benard convection and rotating convection in a spherical shell, concentrating on the role of the Prandtl number. Fearn also studied the role of inertia, for magnetoconvection. Here the magnetic Prandtl number plays a key role. As it decreases (adding inertia), dynamo action is facilitated.
Schmitt, Tilgner, and Fournier, et al. continued to consider precessionally driven flow. Schmitt studied the effect of the container ellipticity. Tilgner showed that dynamo action is possible from precession driven flow, though there is no clear separation of timescale between rotation and precession. Fournier are interested in the interaction between convectionally and precessionally driven flows, finding that precession can inhibit convection.
Legaut & Jault considered the possibility that torsional oscillations might be excited by auroral currents. Mound also was interested in torsional oscillations, finding that they cannot simultaneoulsy explain both LOD and polar motion data. Aldridge & Baker looked for evidence of rotating parametric instability in paleomagnetic intensity records.
Three posters had a primarily analytic approach. Kotelnikova, et al. studied the shear layers between two differentially rotating spheres. Loper & Chulliat presented a Green's function approach for solving buoyancy driven flow in a rapidly rotating fluid. Anufriev showed that Cowling's theorem need not hold when the fluid is compressible.
Six posters described experimental work. Gillet, et al. carried out
an experimental and accompanying numerical study of magnetoconvection in
a rapidly rotating spherical shell of gallium, with good agreement between
the two. By measuring fluid velocity in spin-up experiments on gallium
and on water, Deleplace, et al. were able to measure the effects of turbulent
viscosity. The results appear to depend not only on the Renolds number,
but also the Prandtl number. Yanagisawa, et al. reported on experiments
on Rayleigh-Benard convection in gallium, with the goal of improving
the spatial and temporal resolution of ultrasonic velocimetry. Christensen
& Tilgner used the results of numerical dynamos and the Karlsruhe dynamo
to find that the scaling of the magnetic dissipation time depends only
on the magnetic Reynolds number. They then use this scaling to estimate
Ohmic dissipation in the core at about .2 - .5 TW, on the low side, allowing
for an old inner core without radioactivity in the core. Lathop & Sisan
studied magnetic instabilities in liquid sodium between differentially
rotating spheres. Alboussiere & Cardin studied the formation of Taylor
columns by suction of water on a rotating table. The experiments imply
that for the Earth, columns with diameters less than 5 km are not geostrophic.
Session 4: Structure and interactions in the system inner core, outer core and mantle
Session chair: Dave Loper
Invited speakers: Bruce Buffett, Jeannot Trampert, and Ed Garnero
Buffett spoke about the different coupling mechanisms between the fluid outer core and the solid mantle and inner core. These can be divided intor four types: thermal, chemical, electromagnetic, and mechanical. The key question as regards thermal interaction between the mantle and core is whether the heat flux across the CMB exceeds the heat conducted up the core adiabat. If it does, the core is thermally convecting. If not, the core is either mixing compositionally, or there is a stable layer at the top of the outer core. Lateral variations in heat flow can produce non-dipolar features that affect the primary dynamo process and affect reversals. Chemical interactions result in mass transfer between core and mantle, with Fe going into the mantle, and light elements into the core. Supersaturation of light elements, a result of inner core solidification, may result in sediments at the top of the outer core. There has been research on whether there is a geochemical signal (osmium ratios) in the mantle coming from the core. Electromagnetic coupling between the outer core and a finitely conducting mantle has been invoked to understand exchange of angular momentum that results in changes in the LOD and nutations, and em coupling between inner and outer core may play a role in IC rotation and anisotropy. Lateral variations in mantle conductivity can result in dynamo action and affect the observed surface field. Mechanical interactions include viscous, topographic, and gravitational coupling. Topography may lock flow. Gravitational coupling between the mantle and inner core may contrain IC rotation, depending on the timescale of the IC to deform.
Trampert reviewed our current understanding of the inner core. Both normal mode and body wave data indicate a rigid inner core, with a shear wave speed of 3.6 km/s. Estimates in the density jump at the ICB vary from as high as .9 g/cc from normal mode data to as low as .2 g/cc from some PKiKP data. Such low values imply little difference between IC and OC composition, and impact the energetics of the geodynamo. Prograde inner core rotation of 0 - .2 deg/yr can be inferred from both types of data, though the rotation rate appears to slow with time of research! Both types of data indicate IC anisotropy, but there remain questions about the depth dependence and lateral variations of the anisotropy. Many models show less anisotropy near the top of the IC, and at least one model (Beghein & Trampert) shows a reversal of the anisotropy deep in the IC. The level of heterogeneity in the IC appears small, and there is also little evidence for OC structure. Simultaneous inversion for bulk and shear modulus is an ill-posed and non-unique problem, but normal mode data suggests a higher Q in compression than in shear.
The D" is a seismically rich area whose dynamics are not well understood. It exhibits elastic heterogeneity at all scales, a high velocity reflector of variable depth, a low velcoty zone beneath this reflector, and thin ultra-low velocity zones right above the CMB. There is also seismic anisotropy. The correlation of these features with mantle features such as subduction zones and plumes is still not clear. Garnero looked at the presence of these features in four regions: Caribean/South America, central Pacific, southwest Pacific, and central Atlantic. Progress can be made by more array analysis, using shorter wavelengths, studying the frequency dependence, using new phases, improving wave propagation tools, and carrying out finite frequency analysis.
Discussion centered about several topics. Concerning the density jump across the ICB, Koper hypothesized that it might be possible to reconcile the low value found in his study with a layer at the top of the ICB. There was also discussion about the ability of seismologists to see a stable layer at the top of the OC, or any other observable associated with a stable layer. There was also a question about the effect of impurities on conduction in liquid Fe. Stevenson also questioned the interpretation of the osmium results, pointing out that the IC is small. Someone posed the question, "what is the core?". Reminiscent of the crust vs. mantle and lithosphere vs. aesthenosphere, is the CMB a chemical or a rheologic boundary? The answer, of course, may depend on the process. It was also pointed out that melt in D" is not necessarily due to crossing the liquidus, but could be due to chemical reactions with the core.
Six posters dealt with IC seismology. Tanaka, searching for another tool to probe under the ICB, considers PKP-C postcursors. It is not clear if their origin is the IC or the upper mantle. Tanaka's study cannot rule out that their source might still be the IC. Koper had two posters concerning the IC. One, discussed above, concerned the density jump at the ICB. The other found no evidence for radial discontinuities in the IC, as has been suggested. Ishii & Dziewonski performed a joint absolute and differential body wave and normal mode inversion, finding that a simple model of anisotropy, with the symmetry axis parallel to the spin axis, fits the data well, except for the innermost 300 km, where a model with the slow direction oriented 45 degrees from the equator gives a better fit. Sun & Song examined whether systematic earthquake mislocation might be responsible for the time shift in differential time residuals, but they do not find this to be the case. Black & Thomas examined the source of differential travel time anomalies, finding that sometimes it appears to be due to the IC, but sometimes due to mantle structure.
Two posters dealt with OC seismology. Tanaka uses SmKS phases to explore structure in the outer core. His data suggests there could be a low velocity zone in the outermost 50 km of the OC, but the small number of data and nonuniqueness makes the conclusion uncertain. Pino, et al. reach a similar conclusion, with the possibility that the anomaly could be due to the D".
Seven posters dealt with CMB, D", and lower mantle seismology. Rost & Garnero use arrays to probe the fine scale structure of the CMB. Avants, et al. used stacking of ScS data to map ULVZ in D". Hutko & Revenaugh also imaged D", using PcP precursors. Three posters looked at anisotropy. Usui, et al. investigated SV and SH waves beneath the Antarctic Ocean, finding that their observations can be attributed to D" anisotropy attributed to paleoslabs. Ford & Garnero carried out a similar study beneath the southern Pacific Ocean. Wookey, et al. explored using differential splitting for studying anisotropy in the lower mantle, in an effort to eliminate near source and upper mantle contamination. The method appears promising, but larger scale studies are needed. Using Sdiff, SKS, SKKS, S, and ScS records, along with synthetics, Helmberger & Ni image the South African superplume. They do not find an ULVZ beneath the plume, but they do around its edges. Compressional velocities do not appear anomalous.
Greiner-Mai, et al. examined whether IC motions might be detectable in Earth rotation and gravity data. They determine that forced IC wobble might be detectable, but that the free wobble will be tougher. Bergman, et al. constructed a lab model for the solidification of the inner core, showing how rotation influences convection, and in turn, solidification. They also showed how the flow in the melt affects the texture of the solid. In other experimental work, Andrault, et al. have found that at 25 GPa, the solubility of S in solid Fe is limited to 1.2%, and that upon extrapolating to the ICB, the width of the phase loop remains wide enough that S could contribute to the density jump at the ICB.
Deleplace & Cardin studied viscous and em coupling at the CMB. They
suggest that to retrieve VLBI data the magnetic field at the CMB must be
smaller than found by others. Jacoby considered whether difficulties reconciling
CMB undulations, global gravity, and seismic data might be due to isostatic
D" variations in thickness and density. Barkin & Schatina performed
an analytical study of the deformation of the mantle by small displacements
of the core, and Barkin, et al. looked at tidal displacements due to external
Session 5: Interaction of plate tectonics and deep mantle processes
Session chair: Ulli Hansen
Invited speakers: Dave Bercovici, Carolina Lithgow-Bertelloni,
and Julian Lowman
Bercovici's talk concerned trying to understand plate generation from mantle convection. Questions include why strike-slip motion, why narrow ridges, why subduction in cold, strong lithosphere, why is subduction one-sided, why have there been changes in plate geometry, and why Earth? Possible and partial answers include a variable viscosity, the presence of water (lubrication, low viscosity, grain szie reduction), and time dependence for healing. He also investigated the role of symmetry breakers.
Lithgow-Bertelloni reviewed the progress of reconstructing plate motions over the past 200 million years by examining slabs via seismic wavespeed and anisotropy, surface topography, continental uplift, and polar wander. The difficulty of converting seismic velocity to density remains an issue.
Lowman concentrated on advances in calculations on the feedback between plate motion and mantle convection. Using the force-balance method it is possible to incorporate plumes, though there are limitations such as the plate geometry not evolving. In the future we hope to be able to carry out fully dynamically self-consistent calculations.
The discussion brought out that to fully understand the details and history of plate tectonics and mantle convection we must understand the fundamentals, and not just rely on an empirical approach. For instance, do we know the equation of state? Other issues that were discussed included the more complex nature of thermochemical plumes than thermal plumes, and the global distribution of torioidal power.
Six posters examined numerically aspects of mantle convection. Nettlefield and Lowman discussed in depth plumes and heat transport, suggesting that previous studies have underestimated core heat loss. Farnetani and Samuel concentrated on thermochemical plumes, finding that there can be a shift in location between surface volcanism and plume source. Stein and Hansen and Stemmer and Hansen examined self-consistent generation of low viscosity zones. Yoshida and Kageyama presented a Yin-Yang grid for a finite difference code to study 3-D thermal convection. Ichikawa, et al. studied the influence of internal heating on cell size.
Pouilloux, et al. carried out a linear stability analysis with respect
to various crystallographic symmetries. Fu, et al. used seismic tomography
models to calculate mantle flow models for a range of Rayleigh numbers.
Regional and multiple scale convection is common. Ates and Bilim examined
block rotations of the Anatolian plate.
Session 6: High resolution images of the Earth's interior
Session chair: Guy Masters
Invited speakers: Gustav Nolet, Jeroen Tromp, and Mike Kendall
Nolet, et al. examined the resolution of mantle plumes, and where good resolution is possible, translated velcoity anomalies to temperature anomalies. Assuming a wide range of physical parameters, they then translated these to heat and mass fluxes. They found that these fluxes are likley too high unless the plumes are compensated by a heavy element, i.e., iron, though there is also a tradeoff between iron content and viscosity. They favor a plume enrichment of a few tenths of a percent.
Tromp, et al. described their development of a technique, the spectral-element method, for calculating synthetic seismograms for 3-D earth models. The combined choice of numerical integration and basis functions yields a diagnol mass matrix, which reduces the computational costs. The technique can be efficiently implemented on parallel computers. Using the Earth Simulator Tromp demonstrated that the method can synthesize anisotropy and attenuation, but that global 3-D synthetics at 1-2 kHz will require petaflop machines, that will perhaps be available within 5-10 years.
Kendall reviewed anisotropy in the mantle. A basic question is whether the ansiotropy is parallel or perpendicular to flow, with the answer depending on the mechanism-lattice or shape preferred orientation, or layering; and the mode of deformation, which depends on variables such as grain size, temperature, and strain rate. Some conclusions include that anisotropy is weak (<1%) in shallow upper mantle, anisotropy is trench parallel, water can have major effects, there is significant ansiotropy above and below slabs, anisotropy is weak in the transition zone primarily because the minerals there are weakly anisotropic, and that below 660 km the cause of the ansiotropy is unclear and may be sensitive to viscosity.
During the discussion several themes for near future of global seismology were laid out. These include the introduction of new seismic techniques, the effects of finite frequency kernals on imaging (and the importance of parameterization), plume structures (thermal halos?, activation energy effects on viscosity?), and discerning structure in the transition zone and D".
Several posters involved aspects of theoretical seismology. Dahlen had a poster on the resolution limits of 3-D traveltime tomography, while McGee and Dahlen looked at the effects of near-field and surface contributions waveform and traveltime sensitivity kernals. Simon, et al., presented a method to extract localized spatial data from spherical harmonic basis functions. The method may have applications for a range of geophysical data. Sieminski, et al. examined the validity of the "great-circle ray theory" that is used in surface wave tomography. They find that the method is generally sound provided dense coverage and not too strong velocity anomalies in regions not covered by great circle paths. Browaeys proposed an analytical model for decomposing the elastic tensor into a orthogonal tensors belonging to different symmetry classes.
Three posters examined the lithosphere. Beucler and Woodhouse addressed the issue of the global distribution of the low velocity zone at the base of the lithosphere by looking at the Poisson ratio. Maggi, et al. and Laske and Hogg looked at surface waves to study the cooling of the lithosphere under the Pacific. The former find some evidence for a velocity anomaly associated with the south Pacific Superswell. Sigloch and Nolet worked on a technique for measuring body wave amplitudes of shallow earthquakes, which are complicated by crustal echoes.
Several posters probed deeper into the Earth. Debayle, et al., used
a large number of Rayleigh waveforms to examine the correlation of seismic
heterogeneity and tectonics with depth, finding that the correlation is
generally lost beneath 250 km. Lebedev and van der Hilst looked for plumes
in the transition zone using S waveforms, finding some evidence for them,
suggesting a deep origin. Rokosky, et al., used a large number of records
to assess shear wave splitting beneath Central America and the central
Pacific. Deuss and Woodhouse examined issues concerning the use of free
oscillations to determine long-period mantle structure. In particular,
they examined whether modes can assumed to be isolated when using splitting
functions. Finally, Calvet and Chevrot looked at the sensitivity kernels
for PKP waves sampling the D" and inner core.
Session 7: Physical and chemical discontinuities and reservoirs in the Earth's mantle
Session chair: Francis Albarede
Invited speakers: Peter Shearer, Richard Carlson, and Anne Davaille
Shearer discussed mantle interfaces from a seismologist's view. Some interfaces are easy to observe, global, and with physical explanations. Others are more intermittently observed, either because they are not global or because they cause a small perturbation. Stacking algorithms can miss these if the depths are variable. Other discontinuities have been proposed on geochemical arguments, but not observed. Chief among these is the 'stealth' layer.
Carlson gave insight from geochemistry on mantle structure. Geochemistry suggests that the likely 2% compositional buoyancy added to the lithospheric mantle by partial melt removal may have kept it from participating in mantle convection for 3 Gyr. A surprising conclusion is that chemical heterogeneities in the mantle may be dominated by imperfect mixing of subducted oceanic lithosphere.
Following up on this theme Davaille asked the questions, 'why do we need reservoirs? how big? how long do they persist in the presence of convection?'. She approached these last two questions from an experimental view. It is clear that through stretching and folding large unmixed islands could not survive. Moreover, long plumes connecting the bottom thermal boundary layer to the lithosphere are not likely to be stable for more than 40 Myr. A stratified fluid can help the plumes persist, and topography can also anchor the plumes, if the height of the topography exceeds the thickness of the thermal boundary layer. If the plumes have a density contrast of <3%, 'marble cakes' are likely to rapidly form.
Again, several themes were identified, inlcuding mapping discontinuities, mapping surface expression of reservoirs, the nature of discontinuities, the size distribution and creation of discontinuities and reservoirs, and the distribution of heterogeneities. Questions that arose include, 'does the He^4/heat flow ratio need to be the same? could that be a red herring? how do the initial conditions affect convection ? (perhaps differentiation early enough that the initial conditions do not matter?), how thin could a stealth layer be? (but why would a sharp layer remain distinct?), the role of water?'.
Four posters looked at upper mantle discontinuities from a seismic viewpoint. T.-R. A. Song, et al. found evidence for a low velocity zone above the 410 km discontinuity beneath the northwestern US, perhaps evidence of a high water content. Chambers, et al. and Schmerr, et al., also looked at the 410 km discontinuity. By using precursors they found lateral variations in impedance contrast. Ni and Helmberger examined waves turning beneath Africa, and find evidence for large scale vertical structure, perhaps chemical in origin.
Six posters used numerical simulations to examine various aspects of mantle convection. Yamagishi, et al., were interested in the timescale for heat flow variations as a function of the negative Clapeyron slope at the 660 km discontinuity. Leahy and Bercovici performed a test of the 'water-filter' model-does such a model yield geophysically plausible convection? Kellogg studied the stability of a hot, abyssal layer at the base of the mantle. Hansen, et al. examined whether an initially unlayered system could become layered, and hence a potential geochemical reservoir, as a result of double diffusive convection. Steinberger studied the shape of plume conduits, comparing them with seismic observations. Hoeink, et al., looked at cystal settling in a magma ocean such as might have occurred after accretion.
Four posters took experimental approaches. Kurita, et al., and Kumagai and Kurita quantified entrainment and studied entrainment by a thermal plume through discontinuities. Smyth and Jacobsen carried out ultrasonic measurements on hydrated silicates in a diamond anvil cell, showing that the effects of water can be greater than that of temperature for geophysically and geochemically realistic values. Basu and Murty studied the isotopic composition of nitrogen in carbonatites.
Mattern, et al., attempt to invert seismic data for lower mantle composition
and temperature. They find a temperature tradeoff between pyrolite and
perovskite, and that iron partitioning makes little difference.
Session 8: Deep interior of other planets
Session chair: Dave Stevenson
Invited speakers: Tilman Spohn, Philippe Lognonne, and Herbert
In his talk, Spohn highlighted the structure and dynamics of the terrestrial and icy planets and moons. For Mars and the Moon, density and moment of inertia (MOI) constrain models of interior structure, but for the other bodies, much is speculative. Nevertheless the hypothesized range of structure and dynamics is quite large. Important issues include, of course, the size of the cores, the presence and style of convection, the presence of magnetic fields, surface and atmospheric composition, and the presence of liquid.
Logonne discussed the history (70% deployment failure!) and future of seismic exploration of other planets. So far only the Moon has been explored seismically, with Apollo data being recently reprocessed. Future lunar missions are hoped for soon, and a Mars mission by 2011. The latter should help constrain geochemical models. Because of the high soundspeed in Venus' atmosphere it may be possible to carry out remote sensing seismology.
Palme summarized our geochemical understanding of planetary composition and evolution. In spite of the large variety of structure within the solar system, the terrestrial planets are all differentiated, with surface indications of silicate differentiation. They are also all depleted in volatiles. Systematic compositional trends with heliocentric distance are not evident, though the Earth and Mercury are enriched in Fe, perhaps due to impact removal.
At the time of the meeting, Titan was seen from Cassini, and would soon enter the atmosphere. There was a question about the rotation of Saturn-now six minutes longer than 25 years ago-presumably an error! There was discussion of the thermal evolution of the terrestrial planets-is it monotonic, episodic, or have regime change as a function of time. The role of S and cores/dynamos was pointed out to likely be important. There was also discussion that the moon has non-radial structure, and about Tharsis' formation and importance.
Three posters examined planetary convection from a numerical approach. Glatzmaier, et al. carried out simulations for a range of density, entropy, and compositional stratifications, finding that the resulting flows vary dramatically. Buske and Christensen examined ways in which plume activity can be concentrated, such as for Tharsis on Mars. Loddoch and Hansen were interested in the interplay between surface tectonics and internal dynamics on Mars. A fourth numerical paper by Rotvig looked at the zonal jets on Jupiter, discussing the forming of zonal winds with multiple zeros.
Two posters took experimental approaches. Wenzel, et al. studied the formation of Tharsis, finding that compositional stratification and a variable upper boundary heat flux can play a major role. Le Bars and Davaille looked at episodes of enhance convection, perhaps due to the surface arrival of domes.
Johnson, et al., attempt to form a mineralogical model of the mantle consistent with seismic data from Apollo. They use seismic data directly, rather than seismic models. Kimura, et al., looked at the stresses due to volume changes resulting from solidification on the icy satellites. Pauer was interested in using gravity localization for inferring sub-surface structure. Finally, Khristoforova looked at heat flow from within the Earth and planets, and hypothesized that there could be a relation to the interior neutrino flux.
[Report by Michael Bergman]