Applying a new waveform imaging methodology that takes advantage of accurate numerical seismic wavefield computations, Barbara Romanowicz's group has constructed a global shear velocity model in the upper mantle that reveals the presence of low velocity channels at the base of the oceanic asthenosphere. In a paper recently published in Science (http://www.sciencemag.org/content/342/6155/227), graduate student Scott French, former graduate student Vedran Lekic (now assistant professor at the University of Maryland) and Barbara Romanowicz show that these quasi-periodic finger-like structures of wavelength ~2000 km, stretch parallel to the direction of absolute plate motion for thousands of kilometers. Below 400 km depth, velocity structure is organized into fewer, undulating but vertically coherent, low-velocity plume-like features, which appear rooted in the lower mantle. This suggests the presence of a dynamic interplay between plate-driven flow in the low-velocity zone, and active influx of low-rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer. Hotspots are not the direct consequence of plumes impinging on the lithosphere
Over the last three years, Burkhard Militzer's group has been working on understanding water at megabar pressures with ab initio computer simulations. In the interiors of Uranus and Neptune (dashed lines in figure) where such pressures exist, water is predicted to occur in a superionic state where the oxgyen atoms remain stationary like in a solid while the hydrogen atoms diffuse throughout the crystal like a fluid. In the most recent article that appeared in the journal Physical Review Letters, postdoc Hugh Wilson, summer student Michael Wong, and Burkhard Militzer, show that, at 1.0±0.5 megabars, the oxygen sub-lattice in superionic water changes from a body-centered cubic lattice to an face-cented cubic lattice (inset). This transformation lead to a more efficient packing but also reduces the diffusion rate of the hydrogen atoms, which may have further implications for electronic conductivity and magnetic dynamo in Uranus and Neptune. This theorectical prediction is expected to be verified with laboratory experiments using shock wave and x-ray diffration techniques.
Over the last years Rudy Wenk’s group in the department has developed methods to investigate deformation of materials at ultrahigh pressures with diamond anvil cells. It was observed that brittle minerals such as olivine, perovskite and postperovskite become ductile above 20 GPa. This approach has been applied to study deformation mechanisms of rocks at deep Earth conditions and was applied to explain seismic anisotropy. A recent collaboration of Rudy and graduate student Jane Kanitpanyacharoen with scientists at the high pressure beamline 12.2.2. of the Advanced Light Source applied the technique to nanocrystalline metals which were generally thought not to be subject to dislocation glide. Yet at 37 GPa synchrotron X-ray diffraction images reveal that preferred orientation developed in nanocrystalline nickel, suggesting that also here pressure promotes dislocation activity. It highlights the university as a forum for interdisciplinary interaction, where methods developed by earth scientists to investigate the deep earth are used by engineers to shed light on puzzles of nanomaterials, one of the big issue in materials science. The novel results are reported in the recent issue of Science.
In a report published in the April 1, 2012 issue of Nature Geoscience, EPS faculty member Kristie Boering (also Dept. of Chemistry), former EPS graduate student Sunyoung Park, and their co-authors measured the nitrogen and oxygen isotopes in nitrous oxide in air samples collected since 1978 at the Cape Grim Air Pollution Baseline Station (pictured) and in air trapped in snow in Antarctica dating back to 1940. The trends in the isotopes represent a "smoking gun" showing unequivocably that increasing fertilizer use is responsible for the dramatic rise in atmospheric nitrous oxide, which is a major greenhouse gas contributing to global climate change. Read Press Release from UC Berkeley at http://newscenter.berkeley.edu/2012/04/02/fertilizer-use-responsible-for-increase-in-nitrous-oxide-in-atmosphere/ Read the article at http://www.nature.com/ngeo/journal/v5/n4/abs/ngeo1421.html
In a report published in the April 1, 2011 issue of Science, EPS faculty members David Shuster, Kurt Cuffey (also Dept. of Geography), former EPS graduate student Johnny Sanders, and Greg Balco of the Berkeley Geochronology Center, used apatite (U-Th)/He and 4He/3He thermochronometry to investigate topographic evolution in the archetypal glacial landscape of Fiordland, New Zealand. They found that the topography near Milford Sound was clearly not in steady state over the last 2 million years, while erosion removed the entire pre-Pleistocene landscape. Their data are best explained by up-valley propagation of erosion through the glacier-carved landscape during this time. This scenario is consistent with a subglacial erosion rate dependent on ice sliding velocity, but not ice discharge. Read Press Release from UC Berkeley at http://newscenter.berkeley.edu/2011/03/31/novel-technique-reveals-how-glaciers-sculpted-their-valleys/ Read the full report at http://www.sciencemag.org/content/332/6025/84.full
Water ice is one of the most prevalent substances in the solar system, with the majority of it existing at high pressures in the interiors of giant planets. The known phase diagram of water is extremely rich, with at least fifteen crystal phases observed experimentally. In our article in Physical Review Letters (see also cond-mat), Hugh Wilson and I (Burkhard Militzer) explore the phase diagram of water ice by means of ab initio computer simulations and predict two new phases to occur at megabar pressures. In the figure from top to bottom, you see
- ice X the highest pressure phase seen in experiments,
- the Pbcm phase that was predicted with computer simulations in 1996,
- our new Pbca phase that transforms out of the Pbcmphase via a phonon instability at 7.6 Mbar, and finally
- our new Cmcm structure that is metallic and predicted to occur at 15.5 Mbar.
The known high pressure ice phases VII, VIII, X and Pbcmas well as our Pbca phase are all insulating and composed of two interpenetrating hydrogen bonded networks, but the Cmcm structure is metallic and consists of corrugated sheets of H and O atoms. The H atoms are squeezed into octahedral positions between next-nearest O atoms while they occupy tetrahedral positions between nearest O atoms in the ice X, Pbcm, and Pbca phases.
In a report published in the Sept. 24 issue of Science, current and former graduate students Lowell Miyagi, Waruntorn (Jane) Kanitpanyacharoen, Pamela Kearcher and Kanani Lee (Lowell and Kanani are now at Yale), working with faculty member Rudy Wenk, describe diamond anvil high pressure deformation experiments performed at ALS on the enigmatic mineral phase postperovskite MgSiO3. They observe strong mineral alignment due to intracrystalline dislocation movements that can be captured in inverse pole figures. This alignment, when applied to lowest mantle rheology, predicts fast S-waves to be polarized parallel to the core mantle boundary which is just what seismologists observe. Linking microscopic processes to macroscopic geodynamics provides new insight about the deep earth. Read Press Release from UC Berkeley.
In a recent article published in the Aug. 26 issue of the journal Nature, BSL posdoc Huaiyu Yuan and faculty member Barbara Romanowicz report that the North American cratonic upper mantle is anisotropically stratified. The strong layering, inferred from rapid changes in the direction of azimuthal anisotropy with depth, reveals two distinct lithospheric layers (Chemical and Thermal layer in figure) throughout the stable part of the continent, and a relatively flat lithosphere-asthenosphere boundary (LAB) further separates the underlying asthenosphere. The findings tie together seismological, geochemical and geodynamical studies of the cratonic lithosphere in North America. Read press release from UC Berkeley and Science on msnbc.com.
In a recent paper published in Physical Review Letters, EPS postdoc Hugh Wilson and faculty member Burkhard Militzer report calculations showing that the large deficiency of neon in the atmosphere of Jupiter observed by the Galileo probe can be explained by the existence of a hydrogen-helium immiscibility layer deep within the planet. The new calculations show that neon atoms are absorbed into helium-rich droplets which then rain deeper into the planet's interior, leading to an atmosphere that is depleted of both helium and neon. Read commentary by J. Fortney, press release from UC Berkeley, helium rain forcast on the Discovery Channel, and LA Times report about helium rain washing away neon.
EPS graduate student Amanda Thomas, BSL Researcher Bob Nadeau and EPS faculty member Roland Bürgmann identify a robust correlation between extremely small, tidally induced shear stress parallel to the San Andreas fault and non-volcanic tremor activity near Parkfield, California. In their recently published article in Nature (Reprint), they suggest that this tremor represents shear failure on a critically stressed fault in the presence of near-lithostatic pore pressure. UC Berkeley News Release.