Superplumes, superpiles or superpuddings? Understanding the thermochemical dynamics of the mantle with waveform seismology

We understand plate tectonics. What we still do not understand is how the mantle and plates work together to generate this unique behaviour. This is an outstanding fundamental problem. In particular we do not understand the dynamics of mantle convection including what role composition plays. Seismological studies have shown us that the mantle has two giant mysterious structures at its base, one beneath Africa the other beneath the Pacific. We do not understand their role in mantle convection.

There are three hypotheses for these structures, each with different dynamical implications. First that they are the result of thermal convection e.g. result from a cluster of plumes (super-plumes); second that they represent dense detrital pile (super-piles); and third that they are thermo-compositional, resulting from recycling of oceanic crust, leading to distributed heterogeneity (equated by some to plums in a pudding) through the mantle, which maybe more concentrated in these regions (super-puddings).

We will produce computer simulations of mantle circulation to investigate each class of hypothesis. We will apply plate motion history to these to produce models that can be compared with the real Earth. Earlier work of our team has shown that both preliminary superplume and superpile models produce structures similar to the two large structures imaged with seismic tomography. The different hypotheses though will have different internal and top structures, which cannot be resolved with current seismic tomography. They can be differentiated seismically, but it requires more advanced methods.

This project will bring these more advanced methods to bear. Models of the predicted seismic structure will be produced from the present-day stage of the resulting mantle circulation models. This will be done using a world-class thermodynamic database of the mineralogy and its elastic properties, derived from hundreds of laboratory experiments.

The models will be tested using seismic probes that can look inside the structures, and another set that focus at their upper edge. The predictions of the probes for the different hypotheses will be produced by accurately directly simulating the propagating seismic waves. This will be done using the spectral finite-element code SPECFEM3D_GLOBE on the National Supercomputer, HECToR (and soon ARCHER). One set of probes is the so called 'ScS' seismic wave. This is a wave that reflects off the core mantle boundary - this provides a tool to look inside the structure with high lateral resolution. The second set of probes will be body-waves that bottom around the top of the structures. If the structures extend high above the core mantle boundary then waves that only sample them will be affected.

The distinctive predicted seismic signatures of the different models will then be compared to the large datasets now available allowing the hypotheses to be exactingly tested. The weak signatures in the data will be amplified by using the advanced techniques of observational seismology of stacking waveforms (adding multiple seismograms), which are best done using arrays of seismometers. They will also provide a good test for current approximate methods used to image and model the mantle structure.

The research team has the resources (access to high performance computing), tools (code to model mantle circulation (TERRA, Fluidity) and seismic wave propagation (SPECFEM3D_GLOBE)), data, track record and expertise (including partners for plate motion histories, mineral physics, modelling, and seismic data analysis) in place to undertake this ambitious project.

Despite the fundamental science focus of the project, there is a broad range of potential beneficiaries and end-users of the knowledge generated beyond academia.

Industry knowledge exchange

The hydrocarbon industry are interested in a better understanding of mantle dynamics and how it has affected surface motions - vertical and horizontal - and near-surface temperatures - through Earth history. These are of value in understanding basin evolution and prospectivity. We note that the geological history of oil and gas reservoirs is also important in considering sequestration of CO2. Multiple companies are funding Mantle Geodynamics studies at Cardiff, including Shell International Exploration and Production (SEIP) who are funding a geodynamic project testing plate motion histories. The Global Frontiers group of SEIP, and other sponsors will be directly informed of the new knowledge at our ongoing regular meetings.

Such industries will also be interested in technical aspects of the project, such as developing improved seismic imaging. This can be applied to, for example, using non-traditional data sources such as passively recorded natural seismicity and linking dynamic models to seismic observables. Bristol has a long pedigree of applying global scale imaging to reservoir scale problems, through BUMPS, a Bristol consortium project (involving PI Wookey). This is funded by not only multiple industrial partners but also government stakeholders such as UKERC and NERC, for applications such as the monitoring of CO2 sequestration.

Public engagement

While this project clearly focuses on part of the Earth beyond people's direct experience, its fundamental nature as the very force which shapes the planet we live on makes it a compelling subject to engage the public. Given the potential for its visual and dynamic nature, with the correct presentation this work can help us forge an appreciation in the public of the significance of the deep interior, and the geosciences in general. The National Centre for Public Engagement based at Bristol, and 'Beacon for Wales' hosted at Cardiff University will facilitate this. These both organise and provide access to events at which our science can be showcased.

The fundamental nature of this kind of research and its capacity to fire the imagination of the public is reflected in the media attention it draws. In recent years, project investigators have been contacted by journalists (e.g., New Scientist, S4C, BBC and online media outlets) in relation to their research. For example PI Davies' work has recently been covered by major popular publications New Scientist, Earth (American Geological Institute's news stand publication); and he has taken part in TV news and documentaries for S4C and BBC. PI Wookey has written articles for general audience publications such as NERC's PlanetEarth, and consulted on a major upcoming BBC series, 'The Story of the Continents'. We will engage politicians / policy makers by attending the Annual 'Science and the Assembly' meeting organized by the Royal Society of Chemistry which occurs in May every year at the Wales Millennium Centre and Senedd, Cardiff Bay. PI Davies has already presented his work to Wales's First Minister and we will continue to pursue such opportunities.