Examination of core samples from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope: Effects of retrieval and preservation

Collecting and preserving undamaged core samples containing gas hydrates from depth is difficult because of the pressure and temperature changes encountered upon retrieval. Hydrate-bearing core samples were collected at the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well in February 2007. Coring was performed while using a custom oil-based drilling mud, and the cores were retrieved by a wireline. The samples were characterized and subsampled at the surface under ambient winter arctic conditions. Samples thought to be hydrate bearing were preserved either by immersion in liquid nitrogen (LN), or by storage under methane pressure at ambient arctic conditions, and later depressurized and immersed in LN. Eleven core samples from hydrate-bearing zones were scanned using x-ray computed tomography to examine core structure and homogeneity. Features observed include radial fractures, spalling-type fractures, and reduced density near the periphery. These features were induced during sample collection, handling, and preservation. Isotopic analysis of the methane from hydrate in an initially LN-preserved core and a pressure-preserved core indicate that secondary hydrate formation occurred throughout the pressurized core, whereas none occurred in the LN-preserved core, however no hydrate was found near the periphery of the LN-preserved core. To replicate some aspects of the preservation methods, natural and laboratory-made saturated porous media samples were frozen in a variety of ways, with radial fractures observed in some LN-frozen sands, and needle-like ice crystals forming in slowly frozen clay-rich sediments. Suggestions for hydrate-bearing core preservation are presented.     

Zero graffiti for a beautiful city: the cultural politics of urban space in San Francisco

San Francisco, like many cities in the United States and across the world, has an official zero-tolerance policy on graffiti. In this article, we examine the academic literature concerned with graffiti and then present a case study of zero-tolerance abatement policies in San Francisco. Our analysis yields three main findings. First, zero-tolerance policy stimulates an anti-graffiti industry with vested interest in perpetuating an endless war for control of public space. Second, zero tolerance may generate an unintended result—the proliferation of tags and other forms of graffiti that people tend to dislike the most. Third, we find little evidence that the general public shares the same desire for zero tolerance as the San Francisco Government. Ultimately, we believe that more nuanced readings of graffiti allow greater numbers of people to make sensible, local, place-specific policies regarding graffiti.     

Estimating sea-level extremes under conditions of uncertain sea-level rise

Estimation of expected extremes, using combinations of observations and model simulations, is common practice. Many techniques assume that the background statistics are stationary and that the resulting estimates may be used satisfactorily for any time in the future. We are now however in a period of climate change, during which both average values and statistical distributions may change in time. The situation is further complicated by the considerable uncertainty which accompanies the projections of such future change. Any useful technique for the assessment of future risk should combine our knowledge of the present, our best estimate of how the world will change, and the uncertainty in both. A method of combining observations of present sea-level extremes with the (uncertain) projections of sea-level rise during the 21st century is described, using Australian data as an example. The technique makes the assumption that the change of flooding extremes during the 21st century will be dominated by the rise in mean sea level and that the effect of changes in the variability about the mean will be relatively small. The results give engineers, planners and policymakers a way of estimating the probability that a given sea level will be exceeded during any prescribed period during the present century.     

Localized versus regional adaptation in limpet shell morphology across the Iberian Peninsula

The climate‐envelope approach to predicting climate‐induced species range shift is limited. There are many possible reasons for this, but one novel explanation is that species adapt to changes in temperature at the expense of adaptation to other stressors. Here we test this hypothesis using the limpet Patella depressa (Mollusca, Patellidae), over a large geographical area covering most of the Atlantic coast of the Iberian Peninsula, known to consist of a genetically inter‐connected population. We examine limpet shell morphology on four shores in each of three regions, from Northern Spain to Southern Portugal. Within each region, shell morphology (measured as maximum shell profile to length ratio) varied between shore types differing in their insolation, wave action, microhabitat availability and biological factors. However, this ratio, which is known to be an adaptive response to heat stress, was found to be consistently higher in more southern latitudes despite differences between shore types being found in all regions. This implies that localized adaptation to shore type (most likely through phenotypic plasticity) is compromised by factors that change over latitudinal or regional scales, or which could occur in response to climate change. Although such climate‐induced changes may initially be localized, compromised adaptation (through phenotypic or genetic plasticity) may result in altered community interactions and potentially large shifts in community structure.     

A simple technique for estimating an allowance for uncertain sea-level rise

Projections of climate change are inherently uncertain, leading to considerable debate over suitable allowances for future changes such as sea-level rise (an ??allowance?? is, in this context, the amount by which something, such as the height of coastal infrastructure, needs to be altered to cope with climate change). Words such as ??plausible?? and ??high-end?? abound, with little objective or statistically valid support. It is firstly shown that, in cases in which extreme events are modified by an uncertain change in the average (e.g. flooding caused by a rise in mean sea level), it is preferable to base future allowances on estimates of the expected frequency of exceedances rather than on the probability of at least one exceedance. A simple method of determining a future sea-level rise allowance is then derived, based on the projected rise in mean sea level and its uncertainty, and on the variability of present tides and storm surges (??storm tides??). The method preserves the expected frequency of flooding events under a given projection of sea-level rise. It is assumed that the statistics of storm tides relative to mean sea level are unchanged. The method is demonstrated using the GESLA (Global Extreme Sea-Level Analysis) data set of roughly hourly sea levels, covering 198 sites over much of the globe. Two possible projections of sea-level rise are assumed for the 21st century: one based on the Third and Fourth Assessment Reports of the Intergovernmental Panel on Climate Change and a larger one based on research since the Fourth Assessment Report.     

Garnet peridotite from Colorado Plateau ultramafic diatremes: Hydrates,carbonates, and comparative geothermometry

Crystal fragments of pyrope from diatremes of ultramafic microbreccia in the Navajo Province of the Colorado Plateau contain inclusions of olivine, pyroxene, spinel, chlorite, amphibole, chlorapatite, and dolomite. The included suite supports earlier hypotheses that hydrous phases and carbonates were primary parts of some garnet peridotite assemblages in the Plateau lithosphere. Garnets with spinel and orthopyroxene inclusions likely all were sampled at pressures less than 36 kb and perhaps as low as 15–20 kb; no evidence was found for inclusions from greater depths. Temperature estimates are 800°–900° C for garnet-clinopyroxene equilibration, but only 500°–700° C for garnetolivine equilibration. Inherent differences between geothermometry methods account for only part of the discrepancy. Pronounced Fe-Mg zoning in garnet at olivine contacts and the lack of such zoning at clinopyroxene contacts are evidence that the difference in part relates to relative reequilibration rates with cooling. The garnet-olivine temperature estimates may be the best approximations to mantle temperatures before eruption. Our data are compatible both with the hypothesis that the garnet peridotite was emplaced in the mantle by large-scale, horizontal transport in the lithosphere and with the hypothesis that rocks were sampled from Precambrian lithosphere cooled to temperatures like those along a low heat flow geotherm. Discordances between the geothermometers here and in other lherzolite localities may be keys to evaluating tectonic histories of lherzolite masses.     

Surface and downhole shear wave seismic methods for thick soil site investigations

Shear wave velocity–depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites.     

Sediment mobility due to currents and waves in the Torres Strait–Gulf of Papua region

The MECO hydrodynamic model (MECO Technical Report No. OMR-118/120, CSIRO Marine Research, 1998) was adapted for the Torres Strait–Gulf of Papua region at 0.05° resolution. Validation of the hydrodynamic model was carried out against observed current meter data and calculated tidal sea levels. Dispersal pathways of sediments derived from the Fly River, and from a resuspension event along the northern Great Barrier Reef were investigated using an Eulerian approach. Sediment input into Torres Strait is found to be greater during the Trade season by approximately 10%. Wave data were also obtained, and together with hydrodynamic model output, sediment mobility due to currents, waves and wave–current interactions was considered for both the Trade and Monsoon seasons. Sediment mobility in the Gulf of Papua is dominated by wave motion, whereas Torres Strait is a mixed environment of waves and tidal currents.