The fusion crust of the Winchcombe meteorite: A preserved record of atmospheric entry processes

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity.     

Regional ash fall hazard II: Asia-Pacific modelling results and implications

In a companion paper (this volume), the authors propose a methodology for assessing ash fall hazard on a regional scale. In this study, the methodology is applied to the Asia-Pacific region, determining the hazard from 190 volcanoes to over one million square kilometre of urban area. Ash fall hazard is quantified for each square kilometre grid cell of urban area in terms of the annual exceedance probability (AEP), and its inverse, the average recurrence interval (ARI), for ash falls exceeding 1, 10 and 100?mm. A surrogate risk variable, the Population-Weighted Hazard Score: the product of AEP and population density, approximates the relative risk for each grid cell. Within the Asia-Pacific region, urban areas in Indonesia are found to have the highest levels of hazard and risk, while Australia has the lowest. A clear demarcation emerges between the hazard in countries close to and farther from major subduction plate boundaries, with the latter having ARIs at least 2 orders of magnitude longer for the same thickness thresholds. Countries with no volcanoes, such as North Korea and Malaysia, also face ash falls from volcanoes in neighbouring countries. Ash falls exceeding 1?mm are expected to affect more than one million people living in urban areas within the study region; in Indonesia, Japan and the Philippines, this situation could occur with ARIs less than 40?years.     

Computing cross-isotherm volume transports from ocean temperature observations and surface heat fluxes,with application to the Barents Sea inflow

A method for determining the cross-isotherm ocean transport from surface heat flux and ocean temperature data is derived. By computing the volume flux through the isotherm that extend from 19°E, 74°N to the eastern part of the Kola Peninsula, the flow through the western entrance of the Barents Sea south of 74°N is estimated. Using three different surface heat flux datasets, the inflow is found to range from 2.9 to 4.5 Sv in winter (October–March) and from 0.4 to 1.4 Sv in summer (April–September; 1 Sv=10⁶ m³ s⁻¹). The seasonal variations are stronger than indicated by results from direct current measurements, probably because the seasonal cycle of the surface heat fluxes is overestimated along the considered isotherm. The annual mean inflow ranges from 1.9 to 2.2 Sv during a cold period (1986–1988), and from 2.4 to 3.0 Sv during a warm period (1990–1992), close to reported observations.     

Landscape Controls on Structural Variation in Eucalypt Vegetation Communities: Woronora Plateau,Australia

Quantification of landscape-based vegetation structural variation and pattern is a significant goal for a variety of ecological, monitoring and biodiversity studies. Vegetation structural metrics, derived from airborne laser scanning (ALS or aerial light detection and ranging—LiDAR) and QuickBird satellite imagery, were used to establish the degree of plot-based vegetation variation at a hillslope scale. Topographic position is an indicator of energy and water availability, and was quantified using DEM-based insolation and topographic wetness, respectively, stratifying areas into hot-warm-cold and wet-moist-dry topographic classes. A range of vegetation metrics—maximum and modal canopy height, crown cover, foliage cover, NDVI and semivariance—were compared among randomly selected plots from each topographic class. NDVI increases with increasing landscape wetness, whereas ALS-derived foliage cover decreases with increasing insolation. Foliage cover is well correlated with crown cover (R ² =0.65), and since foliage cover is readily calculable for whole-of-landscape application, it will provide valuable and complementary information to NDVI. Between-plot heterogeneity increases with increasing wetness and decreasing insolation, indicating that more sampling is required in these locations to capture the full range of landscape-based variability. Pattern analysis in landscape ecology is one of the fundamental requirements of landscape ecology, and the methods described here offer statistically significant, quantifiable and repeatable means to realise that goal at a fine spatial grain.     

Correlation of the upper devonian rocks of Australia

A heavy mineral concentrate from the undeformed Mundi Mundi Granite N of Broken Hill yielded very few zircons. U‐Th‐Pb measurements on microgram fractions of those extracted showed no indication of the stock's true 1500–1600 Ma intrusive event but revealed something inherited and of an age probably greater than 2 Ga. These zircons, either survivors of those inherited from the magma source or accidental inclusions from the wall rocks, may either represent sedimentary accumulations in the lower Willyama Supergroup with an older craton source i.e. provenance, or indicate the presence of a pre‐Willyama Supergroup basement. Considerable loss of Pb from the zircons is deduced to have occurred at (1) the time of granite intrusion, (2) in the lower Palaeozoic, and, (3) in the case of ²⁰⁸Pb, probably right up to recent time.     

Clustering of galaxy clusters in cold dark matter universes

We use very large cosmological N -body simulations to obtain accurate predictions for the two-point correlations and power spectra of mass-limited samples of galaxy clusters. We consider two currently popular cold dark matter (CDM) cosmogonies, a critical density model ( τ CDM) and a flat low density model with a cosmological constant (ΛCDM). Our simulations each use 10⁹ particles to follow the mass distribution within cubes of side 2  h ⁻¹ Gpc ( τ CDM) and 3  h ⁻¹ Gpc (ΛCDM) with a force resolution better than 10⁻⁴ of the cube side. We investigate how the predicted cluster correlations increase for samples of increasing mass and decreasing abundance. Very similar behaviour is found in the two cases. The correlation length increases from     for samples with mean separation     to     for samples with     The lower value here corresponds to τ CDM and the upper to ΛCDM. The power spectra of these cluster samples are accurately parallel to those of the mass over more than a decade in scale. Both correlation lengths and power spectrum biases can be predicted to better than 10 per cent using the simple model of Sheth, Mo & Tormen. This prediction requires only the linear mass power spectrum and has no adjustable parameters. We compare our predictions with published results for the automated plate measurement (APM) cluster sample. The observed variation of correlation length with richness agrees well with the models, particularly for ΛCDM. The observed power spectrum (for a cluster sample of mean separation     ) lies significantly above the predictions of both models.     

The effects of reduced land fraction and solar forcing on the general circulation: results from the NCAR CCM

Land fraction and the solar energy at the top of the atmosphere (solar constant) may have been significantly lower early in Earth's history. It is likely that both of these factors played some important role in the climate of the early earth. The climate changes associated with a global ocean(i.e. no continents) and reduced solar constant are examined with a general circulation model and compared with the present-day climate simulation. The general circulation model used in the study is the NCAR CCM with a swamp ocean surface. First, all land points are removed in the model and then the solar constant is reduced by 10% for this global ocean case.Results indicate that a 4 K increase in air temperature occurs with global ocean simulation compared to the control. When solar constant is reduced by 10% under global ocean conditions a 23 K decrease in air temperature is noted. The global ocean warms much of the troposphere and stratosphere, while a reduction in the solar constant cools the troposphere and stratosphere. The largest cooling occurs near the surface with the lower solar constant.Global mean values of evaporation, water vapor amounts, absorbed solar radiation and the downward longwave radiation are increased under global ocean conditions, while all are reduced when the solar constant is lowered. The global ocean simulation produces sea ice only in the highest latitudes. A frozen planet does not occur when the solar constant is reduced—rather, the ice line settles near 30° of latitude. It is near this latitude that transient eddies transport large amounts of sensible heat across the ice line acting as a negative feedback under lower solar constant conditions keeping sea ice from migrating to even lower latitudes.Clouds, under lower solar forcing, also act as a negative feedback because they are reduced in higher latitudes with colder atmospheric temperatures allowing additional solar radiation to reach the surface. The overall effect of clouds in the global ocean is to act as a positive feedback because they are slightly reduced thereby allowing additional solar radiation to reach the surface and increase the warming caused by the removal of land. The relevance of the results to the “Faint-Young Sun Paradox” indicates that reduced land fraction and solar forcing affect dynamics, heat transport, and clouds. Therefore the associated feedbacks should be taken into account in order to understand their roles in resolving the “Faint-Young Sun Paradox”.