Widespread uplift and 'trapdoor' faulting on Galápagos volcanoes observed with radar interferometry

Volcanic uplift, caused by the accumulation of magma in subsurface reservoirs, is a common precursor to eruptions. But, for some volcanoes, uplift of metres or more has not yet led to an eruption. Here we present displacement maps of volcanoes in the Galápagos Islands, constructed using satellite radar interferometry, that might help explain this dichotomy. We show that all but one of the seven volcanoes on the islands of Isabela and Fernandina deformed during 1992-99. Cerro Azul and Fernandina erupted during the observation period and show evidence of inflation, co-eruptive deflation and shallow dyke intrusion. In contrast, the largest volcano, Sierra Negra, has not erupted, yet exhibits spatially and temporally variable deformation, with a maximum uplift of 2.7 m between 1992 and 1999, which can be modelled by a shallow inflating sill. Inflation during 1997-98, however, was accompanied by 'trapdoor' faulting on a steeply dipping fracture system within the caldera. Repeated trapdoor faulting over geological time has formed an arcuate intra-caldera ridge within Sierra Negra and may have acted to relax stresses above the magma chamber, inhibiting summit eruptions. Similar processes may help explain large uplift unaccompanied by eruptive activity at other volcanoes.     

Geochemical evidence for terrestrial ecosystems 2.6 billion years ago

Microorganisms have flourished in the oceans since at least 3.8 billion years (3.8 Gyr) ago, but it is not at present clear when they first colonized the land. Organic matter in some Au/U-rich conglomerates and ancient soils of 2.3-2.7 Gyr age has been suggested as remnants of terrestrial organisms. Some 2.7-Gyr-old stromatolites have also been suggested as structures created by terrestrial organisms. However, it has been disputed whether this organic matter is indigenous or exogenic, and whether these stromatolites formed in marine or fresh water. Consequently, the oldest undisputed remnants of terrestrial organisms are currently the 1.2-Gyr-old microfossils from Arizona, USA. Unusually carbonaceous ancient soils--palaeosols--have been found in the Mpumalanga Province (Eastern Transvaal) of South Africa. Here we report the occurrences, elemental ratios (C, H, N, P) and isotopic compositions of this organic matter and its host rocks. These data show that the organic matter very probably represents remnants of microbial mats that developed on the soil surface between 2.6 and 2.7 Gyr ago. This places the development of terrestrial biomass more than 1.4 billion years earlier than previously reported.     

Strain effects and phase transitions in photonic resonator crystals

Optical structures with periodic variations of the dielectric constant in one or more directions (photonic crystals) have been employed extensively for studying optical diffraction phenomena. Practical interest in such structures arises from the possibilities they offer for tailoring photon modes, and thereby the characteristics of light propagation and light-matter interactions. Photonic resonator crystals comprising two-dimensional arrays of coupled optical microcavities have been fabricated using vertical-cavity surface-emitting laser wafers. In such structures, the light propagates mostly normal to the periodic plane. Therefore, the corresponding lateral Bragg-periodicities are larger, a feature that is advantageous for device manufacture as it allows for larger lattice constants in the lateral direction. Here we investigate strain effects in a photonic resonator crystal by shifting neighbouring lattice rows of microcavities in opposite directions, thereby introducing an alternating square or quasi-hexagonal pattern of shear strain. We find that, for strain values below a critical threshold, the lasing photon mode is virtually locked to the corresponding mode supported by the unstrained photonic crystal. At the critical strain value, we observe a phase-transition-like switching between the square and quasi-hexagonal lattice modes. The tolerance of subcritical strains suggests that the resonator crystal may be useful for applications requiring high spatial coherence across the lattice, while the mode switching could potentially be exploited in free-space optical communications.     

Structural basis of glutamate recognition by a dimeric metabotropic glutamate receptor

The metabotropic glutamate receptors (mGluRs) are key receptors in the modulation of excitatory synaptic transmission in the central nervous system. Here we have determined three different crystal structures of the extracellular ligand-binding region of mGluR1--in a complex with glutamate and in two unliganded forms. They all showed disulphide-linked homodimers, whose 'active' and 'resting' conformations are modulated through the dimeric interface by a packed alpha-helical structure. The bi-lobed protomer architectures flexibly change their domain arrangements to form an 'open' or 'closed' conformation. The structures imply that glutamate binding stabilizes both the 'active' dimer and the 'closed' protomer in dynamic equilibrium. Movements of the four domains in the dimer are likely to affect the separation of the transmembrane and intracellular regions, and thereby activate the receptor. This scheme in the initial receptor activation could be applied generally to G-protein-coupled neurotransmitter receptors that possess extracellular ligand-binding sites.     

Somatic support cells restrict germline stem cell self-renewal and promote differentiation

Stem cells maintain populations of highly differentiated, short-lived cell-types, including blood, skin and sperm, throughout adult life. Understanding the mechanisms that regulate stem cell behaviour is crucial for realizing their potential in regenerative medicine. A fundamental characteristic of stem cells is their capacity for asymmetric division: daughter cells either retain stem cell identity or initiate differentiation. However, stem cells are also capable of symmetric division where both daughters remain stem cells, indicating that mechanisms must exist to balance self-renewal capacity with differentiation. Here we present evidence that support cells surrounding the stem cells restrict self-renewal and control stem cell number by ensuring asymmetric division. Loss of function of the Drosophila Epidermal growth factor receptor in somatic cells disrupted the balance of self-renewal versus differentiation in the male germline, increasing the number of germline stem cells. We propose that activation of this receptor specifies normal behaviour of somatic support cells; in turn, the somatic cells play a guardian role, providing information that prevents self-renewal of stem cell identity by the germ cell they enclose.     

Enhanced supercurrent density in polycrystalline YBa2Cu3O(7-delta) at 77 K from calcium doping of grain boundaries

With the discovery of high-temperature superconductivity, it seemed that the vision of superconducting power cables operating at the boiling point of liquid nitrogen (77 K) was close to realization. But it was soon found that the critical current density Jc of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small. In many materials, Jc is suppressed at grain boundaries, by phenomena such as interface charging and bending of the electronic band structure. Partial replacement ('doping') of the yttrium in YBa2Cu3O(7-delta) with calcium has been used to increase grain-boundary Jc values substantially, but only at temperatures much lower than 77 K (ref. 9). Here we show that preferentially overdoping the grain boundaries, relative to the grains themselves, yields values of Jc at 77 K that far exceed previously published values. Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective superconducting power cable operating at liquid-nitrogen temperatures.     

Controlled surface charging as a depth-profiling probe for mesoscopic layers

Probing the structure of material layers just a few nanometres thick requires analytical techniques with high depth sensitivity. X-ray photoelectron spectroscopy (XPS) provides one such method, but obtaining vertically resolved structural information from the raw data is not straightforward. There are several XPS depth-profiling methods, including ion etching, angle-resolved XPS (ref. 2) and Tougaard's approach, but all suffer various limitations. Here we report a simple, non-destructive XPS depth-profiling method that yields accurate depth information with nanometre resolution. We demonstrate the technique using self-assembled multilayers on gold surfaces; the former contain 'marker' monolayers that have been inserted at predetermined depths. A controllable potential gradient is established vertically through the sample by charging the surface of the dielectric overlayer with an electron flood gun. The local potential is probed by measuring XPS line shifts, which correlate directly with the vertical position of atoms. We term the method 'controlled surface charging' and expect it to be generally applicable to a large variety of mesoscopic heterostructures.