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.
In their recently published article in Nature, EPS graduate student Alexander (Zan) Stine, Harvard fauculty member Peter Huybers, and EPS faculty member Inez Fung have found a shift towards earlier seasonal transitions in the temperature record over extratropical land in the last 57 years. This shift is anomolous when compared to the variability seen in the preceeding 100 years, and is not predicted by any of the model-based simulations of 20th century climate reported by the IPCC. (reprint)
Zan Stine discusses this work on Nature's podcast.
In the Press
EPS graduate students Michael Lamb and Sarah Aciego and EPS faculty members Bill Dietrich, Michael Manga and Don DePaolo have found new evidence that amphitheater-headed canyons on Earth and Mars might be carved by catastrophic floods rather than slow erosion by seepage erosion. Studying Box Canyon Idaho, which has morphologic attributes long inferred to result from gradual erosion by spring water, the team reported in the journal Science (reprint) that the canyon was instead carved during a megaflood about 45 thousand years ago. Press coverage.
EPS faculty member Paul Renne and colleagues at the Berkeley Geochronology Center, the Free University of Amsterdam, and Utrecht University, are fine-tuning geochronology to unprecedented levels of accuracy. By calibrating the uniquely versatile 40Ar/39Ar radioisotopic dating method with climate proxy signals tracking Earths orbital cycles, the team reported a ten-fold increase in accuracy. As an illustration of the consequences of their study, reported in Science (reprint), the age of the Cretaceous/Tertiary boundary, and the extinction of the dinosaurs, has been adjusted by almost 500,000 years to 65.95 Ma. UC Berkeley news release.
EPS faculty member Jim Bishop and Phoebe Lam, former graduate student and now an assistant scientist at Woods Hole Oceanographic Institution, are challenging the theory that almost all iron for fertilizing oceanic plankton blooms comes from wind-blow dust. In a recent issue of Geophysical Research Letters (reprint) they show that the key source of iron in the Western North Pacific is not dust, but the volcanic shelf sediments of the Kuril - Kamchatka island arc system. Understanding the origins, transport mechanisms and fate of naturally occurring iron in high-nutrient, low-chlorophyll surface waters is important in climate change calculations.
In a recent issue of Science (reprint) researchers from Berkeley (Miller fellows Sebastien Merkel and Sergio Speziale, graduate student Lowell Miyagi and EPS faculty Rudy Wenk), Arizona State University, Princeton and the Advanced Photon Source at Argonne report on experimental deformation of the mineral phase postperovskite MgSiO3 with diamond anvil cells at pressures of 150 GPa. This phase is supposed to constitute the D" layer in the earth, just above the liquid core boundary. Using information about deformation mechanisms derived from the experiments the team then model the evolution of anisotropy in the deep earth that seismologists have observed.