The Spatial Structure of Land Use from 1970–2000 in the Phoenix,Arizona, Metropolitan Area

This article explores the dimensions of rapid urbanization in the Phoenix Standard Metropolitan Statistical Area (SMSA) over the past thirty years with respect to land use change. We devote primary attention to developing an understanding of what land transformations took place, the extent to which they occurred, and where they occurred. Our findings indicate that 32 percent of the Phoenix SMSA changed between 1970 and 2000. More than half of the overall change was from agriculture to some form of urban land use, and although a large percentage of the region remains open desert the parcels of desert are increasingly fragmented. This has significant implications for urban ecology and biodiversity. The growth indicates that rather than a pattern that reflects the agglomeration effects of a polycentric metropolis, the central business district of the City of Phoenix dominates the region. This has implications with respect to employment patterns, traffic congestion, and urban air quality and climate.     

Assessing disaster needs in megacities: Perspectives from developing countries

Megacities are extreme manifestations of urban complexity that present aid agencies with major disaster assessment challenges. Geographers can contribute to improving megacity disaster assessments by: 1) defining megacity disasters; 2) improving socioeconomic measures of disaster to allow for clearer differentiation of effects among different population sub-groups; 3) specifying criteria for identifying thresholds of disaster and trends in the status of disaster-affected populations; and 4) developing credible data collection and analysis methods appropriate for complex urban environments. In addition, long term research is needed to encourage the formulation of models of megacity life that provide better understanding of human survival under normal conditions of severe adversity.The views expressed herein are entirely those of the author and are not attributable to organizations which have sponsored the author's research.     

Oscillaxanthin and myxoxanthophyll in two cores from Lake Wabamun,Alberta, Canada

A sedimentary blue-green algal record has been investigated through measurement of myxoxanthophyll and oscillaxanthin in two cores taken from deep and shallow sites in Lake Wabamun, Alberta, Canada (Longitudes 114° 26 and 114° 44 W; Latitudes 50° 30 and 50° 35 N). Blue-green algae have been a component of the algal flora of this lake throughout the Holocene period. Myxoxanthophyll and oscillaxanthin maxima occur in early Holocene sediments (ca. 9000 years BP), whereas oscillaxanthin concentrations are high between 7000 and 3800 years BP. High oscillaxanthin levels suggest that a phytoplankton assemblage, which included Oscillatoria spp., existed during this latter period and the lake was more eutrophic than at present. Decreases in the number of planktonic diatoms in the core from the deep site (Seba core) appear to be related to increased eutrophy, increased salinity, and sediment redistribution as well as possible competition with Oscillatoria. That the lake has been less productive during the last 2500 years in supported by the diatom record, the diatom: chrysophyte statospore (stomatocyst) ratio and concentrations of the blue-green algal pigments. In the core from the shallow site (Moonlight Bay) concentrations of blue-green algal pigments are initially high, which along with the diatom assemblage, indicates a younger basal age of the sediments. It is possible that benthic blue-green algae contributed significantly to sedimentary pigment concentrations in the Moonlight Bay core. Major fluctuations in the Osc: Myx ratio, particularly in the Seba core, casts some doubt upon the usefulness of this ratio, and suggests that it is not degradation-independent.     

Precaution and proportionality in the management of global environmental change

The precautionary principle is a mandate to tread cautiously when managing novel threats to the environment or human health. A major obstacle when applying the principle at the international level is disagreement about how precautionary efforts should be constrained to ensure that policy costs are proportional to the attained level of protection. Proportionality is an unresolved question when preliminary evidence precludes decision-makers from assigning probabilities over future events. The paper suggests practical analytical tools for communicating ex ante trade-offs when probabilities are unavailable. The tools could be used to facilitate discussion and compromise when implementing precautionary decisions in international settings where cooperation is important. The approach is demonstrated in an application to climate policy that uses the integrated assessment model DICE (Nordhaus, 2008). The paper also situates the task of precautionary decision-making within the broader context of implementing a precautionary response at the international level.     

The Cerro los Calvos and La Banderia chondrites

Abstract— The Cerro los Calvos meteorite is a single stone of 68.5 g found in the Nuevo Mercurio strewn field of Zacatecas, Mexico (24°20′N, 102°8′W). It is an unusual H4 chondrite. Its olivine (Fa_12.5) and orthopyroxene (Fs 11.7, Wo 0.8) are reduced relative to typical H chondrites. The La Banderia meteorite of 54.3 g from the same vicinity is an LL5 chondrite of shock classification e.     

Field evidence for the influence of weathering on rock erodibility and channel form in bedrock rivers

Erosion processes in bedrock‐floored rivers shape channel cross‐sectional geometry and the broader landscape. However, the influence of weathering on channel slope and geometry is not well understood. Weathering can produce variation in rock erodibility within channel cross‐sections. Recent numerical modeling results suggest that weathering may preferentially weaken rock on channel banks relative to the thalweg, strongly influencing channel form. Here, we present the first quantitative field study of differential weathering across channel cross‐sections. We hypothesize that average cross‐section erosion rate controls the magnitude of this contrast in weathering between the banks and the thalweg. Erosion rate, in turn, is moderated by the extent to which weathering processes increase bedrock erodibility. We test these hypotheses on tributaries to the Potomac River, Virginia, with inferred erosion rates from ~0.1 m/kyr to >0.8 m/kyr, with higher rates in knickpoints spawned by the migratory Great Falls knickzone. We selected nine channel cross‐sections on three tributaries spanning the full range of erosion rates, and at multiple flow heights we measured (1) rock compressive strength using a Schmidt hammer, (2) rock surface roughness using a contour gage combined with automated photograph analysis, and (3) crack density (crack length/area) at three cross‐sections on one channel. All cross‐sections showed significant (p < 0.01 for strength, p < 0.05 for roughness) increases in weathering by at least one metric with height above the thalweg. These results, assuming that the weathered state of rock is a proxy for erodibility, indicate that rock erodibility varies inversely with bedrock inundation frequency. Differences in weathering between the thalweg and the channel margins tend to decrease as inferred erosion rates increase, leading to variations in channel form related to the interplay of weathering and erosion rate. This observation is consistent with numerical modeling that predicts a strong influence of weathering‐related erodibility on channel morphology. Copyright © 2017 John Wiley & Sons, Ltd.     

Viscosity profile of the lower mantle

Summary. We determine the variation of effective viscosity η across the lower mantle from models of the Gibb's free energy of activation G * and the adiabatic temperature profile. The variation of G * with depth is calculated using both an elastic strain energy model, in which G * is related to the seismic velocities, and a model which assumes G * is proportional to the melting temperature. The melting temperature is assumed to follow Lindemann's equation. The adiabatic temperature profile is calculated from a model for the density dependence of the Grüneisen parameter. Estimates of η depend on whether the lower mantle is a Newtonian or power law fluid. In the latter case separate estimates of η are obtained for flow with constant stress, constant strain rate, and constant strain energy dissipation rate. For G * based on the melting temperature, increases in η with depth range from a factor of about 100 for Newtonian deformation or power-law flow with constant stress to about 5 for non-Newtonian deformation with constant strain rate. For G * based on elastic defect energy, increases in η with depth range from a factor of about 1500 for Newtonian deformation or power-law flow with constant stress to about 10 for non-Newtonian deformation with constant strain rate. Among these models, only a non-Newtonian lower mantle convecting with constant strain rate or constant strain energy dissipation rate is consistent with recent estimates of mantle viscosity from post-glacial rebound and true polar wander data.