Conference summary for the 18th Symposium of SEDI
The 18th Symposium of SEDI, a Committee of IUGG, was held at Simon’s Rock College in Great Barrington, western MA, from 23rd to 28th of June, 2024 and we wish to provide a short conference summary of IAGA relevant highlights. We especially wish to thank the conference organisers, especially Mike Bergman, and to Jon Mound (Leeds) and Peter Driscoll (Carnegie) for their inputs in this summary. The full conference details and abstracts are available at: https://sedi-conference-2024-2675c.ingress-baronn.ewp.live/.
Session 4 – Geomagnetism, and Outer Core Structure and Chemistry
John Brodholt (UCL) gave a nice overview of the current state of understanding of Earth's core composition and material properties. One takeaway from that talk was that there has not been much progress in understanding the exact light element composition of the outer and inner core, and there remains a rather wide range of light element abundances (although there are trade-offs amongst the candidate elements). There has been some debate in recent years whether there is geochemical evidence for core-mantle chemical interaction and Brodholt made the convincing point that if 3He and 22Ne were coming out of the core they would exsolve out with a ratio 3He/22Ne of about 103, but the highest this ratio that has been found is around 10 in OIB's. This seems pretty clear that if there is anything coming out of the core it is not much, or at least not detectable.
Roger Fu (Harvard) gave an interesting research talk on Archean 3.5 Ga paleomagnetism preserved in the Pilbara. The magnetic minerals seem to be carrying a primary magnetization that can provide a time-averaged magnetic field direction. It's not clear over what time scales the minerals lock in the ambient field, or the time span over which the magnetic field is being averaged. This makes it tricky to test if there was a geocentric axial dipole (GAD) field at the time, but Fu did see what appear to be reversal-like behavior in the samples. The inferred paleogeography at that time implies that the Pilbara and South African cratons were moving at speeds around 12 cm/yr, which is about twice modern spreading rates. This classic paleomagnetic study demonstrated how valuable such efforts are in providing novel constraints on the dynamics of the Earth's deep interior and surface in deep time.
Finally, Andreas Nilsson (Lund) discussed observational constraints on the dynamics of Earth’s core on multi-centennial to millennial timescales. He has been using the pygeodyn core flow inversion code, a data assimilation tool, to investigate core surface flows and requires input from Earth-like geodynamo simulations. This ongoing work could provide insights into excursion mechanisms, the change of the dipole field strength and long-lived flow dynamics. The choice of dynamo prior does influence the output from the data assimilation routine and longitudinal preference of flow features continue to be examined.
Session 5 – Outer Core Dynamics
The session on outer core dynamics started with an overview talk by Julien Aubert (IPGP) covering the insight into the balance of forces within the core that can be obtained from the interrogation of numerical simulations and comparison with observed geomagnetic variations. Numerical models should be in the correct dynamical regime when the force balances reflect those of the actual core (namely a magnetic, buoyancy, and Coriolis force balance), and that inertia and viscous diffusion should be very weak. Improvements in computational power and numerical approaches mean that simulations can now closely approximate the conditions expected in Earth’s outer core. Nevertheless, challenges remain in understanding variations on the very longest timescales, such as the mechanism by which reversals occur and why the frequency of reversals varies through geological time. Ongoing research is currently focused on how imposing a stratified layer on a dynamo model can cause it to reverse.
Stratified layers may exist at both the top and bottom of the core, and there have been a variety of events and processes that could cause them to form. This was the focus of the research talk by Mathieu Bouffard (Nantes). Layers in the core have been associated with inner core growth, the magnitude of both thermal and chemical fluxes across the CMB, and the consequences of the moon-forming impact. Each mechanism makes specific predictions for the thickness, stratification strength, and long-term evolution of the layer; determining which (if any) of these possible layering mechanisms apply would provide insight into both the present-day state of the core and the thermal and chemical evolution of the whole planet.
Celine Guervilly (Newcastle) described how the so-called “fingering convection” instability can develop if stratified layers arise due to a combination of chemical and thermal effects. When “fingering convection” occurs, the compositional field convects in narrow upwellings through a region of thermal stratification. In the conditions expected for planetary cores, these narrow upwelling and downwelling fingers are predicted to be on the order of 1 metre wide. However, large-scale structures can emerge from these small-scale figures, the nature of which depends on the relative orientations of gravity and rotation, and the strength of the stratification. Possibilities include banded structures in the polar regions and clustering of fingers near the equator, the influence of such dynamic structures on the geomagnetic field remain to be explored. She showed that the compositional convection, or "fingers" tend to become smaller, more numerous, and more radial (as opposed to cyclindrical) as the thermal stratification is increased.
Other highlights
During one of the discussions, the topic of using tectonic plate reconstructions to drive mantle convection models to then infer outer boundary conditions and dynamo behavior is still being pursued by several groups despite the fact that earlier attempts had been unable to match the magnetic observations with the models. The big concern with this effort is how different mantle convection models produce different time evolving CMB conditions given the same (or roughly the same) surface plate motions as the driving force. This indicates that we don't understand the dynamics of the lower mantle well enough to produce a unique solution, but maybe the resulting magnetic field behavior could possibly be used to infer what the lower mantle conditions might have to be.
The idea of a basal magma ocean (BMO) dynamo was raised given that new measurements of the electrical conductivity of Fe-rich silicate liquids implies they have conductivities about a factor of 10 lower than Fe metal. This means that a convicting liquid silicate BMO that is enriched in Fe could potentially generate a large enough electrical current to produce a magnetic field. This is an interesting new idea, but questions remain about the thermal conductivity of these materials and whether it will behave like a classic Wiedmann-Franz style metal (where the thermal and electrical conductivity are correlated), or not. A BMO dynamo could potentially be efficient if it has a higher electrical conductivity and lower thermal conductivity than a typical metal, but such behavior is not known and would be surprising.
Most of the efforts to measure a (either thermally or chemically) stratified layer at the top of Earth's core seem to be finding no significant result. In other words, if there is global stratified layer it must be smaller or weaker than the observational capabilities. This implies that the core is not likely strongly thermally stratified, if it is at all. This in turn would imply the CMB heat flow is near the adiabatically conductive limit (~12-15 TW) and that there may be no issue driving a thermal dynamo prior to inner core formation (i.e. the New Core Paradox).
Seismic measurements aimed at constraining inner core structure and possible super-rotation seem to be somewhat agnostic as to whether the inner core is super-rotating at all. It remains unclear whether neighboring seismic ray paths can discriminate between super-rotation and the presence of fine scale structure influencing the seismic waves or mantle-based anomalies.
