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Forschungsarbeit

Earth's deep interior: How hot is the mantle?

The Thermal, Elastic and Seismic Signature of High-Resolution Mantle Circulation Models

Von Bernhard Schuberth (7.12.2009)

The evolution of Earth's mantle over geologic time is one of the major controlling factors for a variety of phenomena that directly affect the daily life of many people world wide. The continuous motions within the mantle on the order of millimeters to centimeters per year result in the motion of tectonic plates and the corresponding faulting of the crust. This steady transport of material at the Earth's surface regularly produces earthquakes and volcanism all over the world. Zones of such active tectonic processes are, for example, located all around the Pacific Ocean, where the Nazca, Cocos, Juan de Fuca, Philippine and Pacific Plate are subducted beneath the Americas, and the Eurasian and Australian continents. Another famous region of long-lived subduction is from the Mediterranean to India, where the closure of the ancient Tethys Ocean has led to the formation of large mountain ranges from the Alps in the West to the Himalaya in the East.

The idea of plate tectonics has its origin in the theory of continental drift put forward in the pioneering work of Alfred Wegener in the early 20th century. Several decades later, only in the 1960s, unequivocal evidence for continuous plate motions has been found and since then, our understanding of planet Earth has undergone dramatic changes. Early ideas about mechanisms that lead to the observed plate motions included the fundamental concept of convection in the mantle, and the hypothesis that the upper and lower mantle convect as two separate layers. But despite the awareness of the potential threat from natural hazards, rather few is still known on the forces and processes inside the Earth that drive mantle flow.

Three-dimensional representation of temperature variations in a mantle convection model with strong core heat flux.[Bildunterschrift / Subline]: Three-dimensional representation of temperature variations in a mantle convection model with strong core heat flux. The four adjacent cross sections are centered on (top left) 35, (bottom right) 125, (bottom left) 215, and (top right) 305 degrees longitude. The color scale indicating temperature variations with respect to the mean temperature in each depth is saturated at -400 K and +400 K. Continents with color-coded topography and plate boundaries (cyan lines) are overlain for geographic reference. Isosurfaces of temperature variations are displayed for -600 K and +400 K.

In the last two decades, seismic tomography has made great progress in mapping the elastic mantle structure at great depths, which is not accessible directly. Seismic waves that are generated during large earthquakes travel through the whole Earth and can be recorded all over the surface. Similar to the concept of medical tomography, the Earth is thus illuminated from all directions due to thousands of earthquakes happening all over the world, which allows the imaging of variations in the velocity of seismic waves. To date, the origin of this seismic heterogeneity and the evolution of Earth's mantle are still a matter of debate. High expectations to gain more insight currently lie within the application of scientific high-performance computing to geophysical problems.  Modern Tera-flop supercomputers allow, for example, the simulation of global mantle flow at Earth-like convective vigor or seismic wave propagation through complex three-dimensional structures. The sophisticated computational tools incorporate a variety of physical phenomena and result in synthetic datasets that show a complexity comparable to real observations.  However, it is so far not clear how to combine the results from various disciplines in a consistent manner to obtain a better understanding of deep Earth structure from the expensive large-scale numerical simulations.

One of the key questions still unanswered is how to build theoretical models of material properties in Earth's mantle from geodynamic considerations. This relates to the fact that still rather little is known about the lateral variations in temperature and composition. The challenge is to consistently couple theoretical tools and methods from geodynamics, mineral physics and seismology so that specific geophysical hypotheses can be tested quantitatively against observations.  One specific goal is to generate seismic heterogeneity from dynamic mantle flow calculations that can be used in global wave propagation simulations so that synthetic seismograms can be directly compared to seismic data.

In the multi-disciplinary study presented here, a new method is developed to theoretically predict and assess seismic mantle heterogeneity. Forward modeling of global mantle flow is combined with information from mineral physics and seismology. Temperatures inside the mantle are obtained by generating a new class of mantle circulation models at very high numerical resolution with a global average grid spacing of ~25 km (around 80 million finite elements). To assess the predicted present day temperature fields, the geodynamic models are post-processed with thermodynamic models of mantle mineralogy to convert thermal structure into elastic parameters. The resulting structures are compared to tomographic models based on a variety of statistical measures.

One specific question that can be addressed with this approach is the origin of two large regions of strongly reduced seismic velocities in the lowermost mantle (roughly between 2000 and 2890 km depth). Several seismological observations point to the possibility that the structure in these regions is caused by variations in chemical composition. However, a large number of recent geodynamical, mineralogical and also seismological studies argue for a strong thermal gradient across the core-mantle boundary that might provide an alternative explanation for the reduced seismic velocities through the resulting large temperature variations. Here, the forward modeling approach is used to test the assumption whether the presence of a strong thermal gradient in isochemical whole mantle flow is compatible with a variety of geophysical observations. The results show that the temperature variations deduced from the new high-resolution models are capable of explaining gross statistical features of mantle structure mapped by tomography. The main finding is that models with strong core heating, which also give a surface heat flux consistent with observations, yield realistic variations of shear wave velocity.  Most importantly, only models with a large core contribution to the mantle energy budget are compatible with the strong negative seismic anomalies in the large low velocity provinces of the lower mantle. This illustrates that seismic heterogeneity is likely dominated by thermal variations and thus limits the possible role of chemical heterogeneity in the lower mantle.

Figure reference:

Schuberth, B. S. A., H.-P. Bunge, G. Steinle-Neumann, C. Moder, and J. Oeser, (2009), Thermal versus elastic heterogeneity in high-resolution mantle circulation models with pyrolite composition: High plume excess temperatures in the lowermost mantle, Geochem. Geophys. Geosyst., 10(1), Q01W01, doi:10.1029/2008GC002235.


Dr. Bernhard Schuberth
Dr. Bernhard Schuberth
* 1977, Regensburg

Stationen
  • Nov. 1997 - Nov. 2003
  • Studies of Geophysics, Ludwig-Maximilians-Universität Munich, Graduation with degree "Diplom" in Geophysics
  • 2004-2009
  • PhD student in computational seismology and geodynamics within the Graduate College THESIS of the Elite Network Bavaria, Geophysics Section, Dept. of Earth and Environmental Sciences, Ludwig-Maximilians-Universität Munich
  • July 2009 - present
  • Post-Doc in computational seismology and geodynamics, Geophysics Section, Dept. of Earth and Environmental Sciences, Ludwig-Maximilians-Universität Munich

Praktika
  • October 1999
  • Internship at the Institute of Atmospheric Physics, German Aerospace Center (DLR), Oberpfaffenhofen
  • Nov. 1999 - Oct. 2000, March - July 2001
  • Student Assistant at the German Geodetic Research Institute Munich
  • Nov. 2002 - Feb. 2004
  • Assistant, Geophysics Section, Dept. of Earth and Environmental Sciences, LMU Munich

Preise und Auszeichnungen
  • June 2004
  • Edison Award - Silver Prize of the General Electrics Foundation and the Institute of International Education
  • March 2008
  • Best Oral Presentation Award of the German Geophysical Society (DGG) for a talk given at the 67th Annual Meeting 2007 in Aachen, Germany

Stipendien
  • From 2010
  • Marie-Curie Post-Doc Scholarship for a two-year-long scientific research stay in Nice, France