ALBERT

Description: The presence of potential microbial trace fossils (endolithic microborings) has been well documented in oceanic basaltic pillow lavas, hyaloclastites, tuffs, and transitional subglacial marine lavas in the past 30 yr. Despite their evident abundance in oceanic to subglacial environments, they have not been observed in continental basalts that were not erupted in marine or subglacial settings. To expand the record of putative endolithic microborings in volcanic rocks to nonmarine, continental lacustrine environments, we examined hydrovolcanic pyroclastic deposits in the Fort Rock volcanic field, central Oregon. This study presents the textures, mineralogy, and geochemistry of basaltic tuffs containing possible endolithic microborings comparable in morphology, size, and distribution to those described in earlier oceanic and subglacial basalt studies. We observed a variety of tubular and granular textures that show evidence of biogenic morphologies and behavior, and a primary geological context that expresses their age and syngenicity. Petrographic relationships with secondary phases (chabazite, nontronite, calcite) indicate that the construction of microtunnels occurred in saline, alkaline fluids at temperatures of 25–80 °C. In addition, positive correlations were observed between the extent of aqueous (abiotic) alteration and both the abundance of microtunnels and morphological type. These correlations suggest that microtunnels were more readily formed where there was greater abiotic alteration-fluid flux and that the resulting change in chemical composition of those fluids may have had a direct influence on the formation process or possibly the type of constructing microbe. This work adds to understanding of factors controlling microtunnel formation and is the first account of putative endolithic microborings in a continental lacustrine setting. This new information may also have implications in the search for habitable extraterrestrial environments, such as on Mars.

Description: To evaluate compaction and interstitial melt expulsion during cumulate formation, a 20 m cumulate section including the UG2 and UG3 chromitites from a 264 m drill core through the Upper Critical Zone of the Bushveld Complex (South Africa) has been studied. The cumulates in the studied section are as follows: 3 m plagioclase pyroxenite to pyroxenite, pegmatoid footwall pyroxenite at the lower contact to UG2, 0·7 m UG2 chromitite, 6·8 m pyroxenite, 0·24 m UG3 chromitite, 2·0 m plagioclase-rich pyroxenite changing locally to norite, the two 5 cm leader stringers UG3a and UG3b, and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites. All pyroxenites are dominated by orthopyroxene (opx) and the cumulate sequence is topped by mottled anorthosite grading into norite. Stratigraphic concentrations of major and trace elements of 52 bulk-rock samples were determined. Bulk-rock Mg-numbers are 0·79–0·81 throughout the silicate cumulate units, and 0·40–0·46 in the chromitite layers. The stratigraphic distribution of six incompatible trace elements (K, Rb, Ba, Cs, Zr and Th) has been used to determine the amount of trapped liquid ( F TL ) or paleo-porosity in the cumulate rocks. Final porosities (volume fractions), based on averages from the six trace elements, are 0·06–0·33 in the pyroxenites. In chromitite layers, trapped melt fractions of 0·12–0·36 are calculated from incompatible trace element concentrations, but bulk SiO 2 concentrations and X-ray tomography yield 0·04–0·17 higher porosities. Hence, the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction, as poikilitic opx was crystallizing while the silicate melt still equilibrated. Using a previously derived experiment-based model for compaction time scales, gravitationally driven chemical compaction in the UG2–UG3–pyroxenite section is calculated to occur within 1–10 years. This time frame corresponds to the times necessary to cool a 20 m layer by 10–50°C, the temperature interval argued to encompass the liquidus and almost complete solidification. Compaction within a decade can in fact easily develop the paleo-porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid. Contrary to previous assertions, melt expulsion from the cumulate pile does not hinder compaction; calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate. The pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites. Their entrapment below the chromitite layers suggests that these act as permeability barriers. This is in part due to their finer grain size compared with the pyroxenites, but is mainly due to the crystallization of large poikilitic opx during compaction.

Description: Coinductive definitions, such as that of an infinite stream, may often be described by elegant logic programs, but ones for which SLD-refutation is of no value as SLD-derivations fall into infinite loops. Such definitions give rise to questions of lazy corecursive derivations and parallelism, as execution of such logic programs can have both recursive and corecursive features at once. Observational and coalgebraic semantics have been used to study them abstractly. The programming developments have often occurred separately and have usually been implementation-led. Here, we give a coherent semantics-led account of the issues, starting with abstract category theoretic semantics, developing coalgebra to characterize naturally arising trees and proceeding towards implementation of a new dialect, CoALP, of logic programming, characterised by guarded lazy corecursion and parallelism.

Description: Fermi arcs are the surface manifestation of the topological nature of Weyl semimetals, enforced by the bulk-boundary correspondence with the bulk Weyl nodes. The surface of tantalum arsenide, similar to that of other members of the Weyl semimetal class, hosts nontopological bands that obscure the exploration of this correspondence. We use the spatial structure of the Fermi arc wave function, probed by scanning tunneling microscopy, as a spectroscopic tool to distinguish and characterize the surface Fermi arc bands. We find that, as opposed to nontopological states, the Fermi arc wave function is weakly affected by the surface potential: it spreads rather uniformly within the unit cell and penetrates deeper into the bulk. Fermi arcs reside predominantly on tantalum sites, from which the topological bulk bands are derived. Furthermore, we identify a correspondence between the Fermi arc dispersion and the energy and momentum of the bulk Weyl nodes that classify this material as topological. We obtain these results by introducing an analysis based on the role the Bloch wave function has in shaping quantum electronic interference patterns. It thus carries broader applicability to the study of other electronic systems and other physical processes.

Description: During opening of a new ocean, magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock, creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g., during the Paleocene-Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as sediment volcanoes, and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black smoker–type chimneys rather than sediment volcanoes. We have evidence for two buried and one active hydrothermal vent systems outside the rift axis. The active vent releases fluids of several hundred degrees Celsius containing abundant methane, mid-ocean ridge basalt–type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

Description: : Intrinsically disordered proteins (IDPs) play central roles in many biological processes. Consequently, an accurate description of the disordered state is an important step towards a comprehensive understanding of a number of important biological functions. In this work we describe a new web server, Mollack, for the automated construction of unfolded ensembles that uses both experimental and molecular simulation data to construct models for the unfolded state. An important aspect of the method is that it calculates a quantitative estimate of the uncertainty in the constructed ensemble, thereby providing an objective measure of the quality of the final model. Overall, Mollack facilitates structure-function studies of disordered proteins. Availability and Implementation: http://cmstultz-mollack.mit.edu Contact: cmstultz@mit.edu Supplementary information: Supplementary data are available at Bioinformatics online.

Description: Understanding the role of electron correlations in strong spin-orbit transition-metal oxides is key to the realization of numerous exotic phases including spin-orbit–assisted Mott insulators, correlated topological solids, and prospective new high-temperature superconductors. To date, most attention has been focused on the 5 d iridium-based oxides. We instead consider the Pt-based delafossite oxide PtCoO 2 . Our transport measurements, performed on single-crystal samples etched to well-defined geometries using focused ion beam techniques, yield a room temperature resistivity of only 2.1 microhm·cm (μ-cm), establishing PtCoO 2 as the most conductive oxide known. From angle-resolved photoemission and density functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along k z . Despite being predominantly composed of d -orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14 m e . Moreover, the sharp spectral features observed in photoemission remain well defined with little additional broadening for more than 500 meV below E F , pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO 2 as a model nearly-free–electron system in a 5 d delafossite transition-metal oxide.