Occurrence of gorgosterol in diatoms of the genus Delphineis

The presence of the C₃₀ sterol gorgosterol (22,23-methylene-23,24-dimethylcholest-5-en-3ß-ol) and its analogues in some marine and freshwater environments is generally associated with invertebrate animals or dinoflagellates since there have been no reports of them in other microalgal classes. Here we show that two unialgal cultures of different species of the marine diatom Delphineis contain gorgosterol in addition to sterols more commonly found in diatoms. Our findings suggest that some reports of gorgosterol in seawater and marine sediments may well have an origin, at least in part, from diatoms.     

Permeability changes in coal: Analytical modeling

Permeability changes can be very large during depletion of coalbed methane wells: up to 100 times in the San Juan basin. Analytic models of permeability increase during depletion are accessible, easy to use, and practical. They are more transparent, and less complicated, than coupled numerical models. However, there are minor differences between the analytic models, and these are reviewed here. There are also discrepancies in attempting to match field data, which is the final test of a model, and these are discussed. Two different sets of San Juan data have been modeled, after suppressing the pressure-dependent permeability, with reasonably consistent reservoir parameters. The matching parameters appear to be justifiable, the small porosities do not seem to be ruled out by reservoir experts, and the changes of elastic parameters (including pore–volume compressibility) with depletion do not seem unreasonable. The analytic models are used in reservoir simulators to model and predict injection of greenhouse gases in coal seams. Only when we can match the permeability increase of coalbed methane wells during depletion, will we have the confidence to use the analytic models in reservoir simulators to match or predict sequestration of greenhouse gases such as CO₂.     

Finite element investigation of steel built-up shear links subjected to inelastic deformations

Non-linear finite element models accounting for large displacements have been used to investigate the behavior of steel built-up shear links that had previously been tested using large-scale experiments. The links were designed using steel grades with yield points ranging from high to low strengths. The objectives of the numerical analyses were to further investigate the non-linear behavior and to correlate the numerical results with experimental observations. Elasto-plastic as well as cyclic stress-strain material properties were incorporated to study the influence of material behavior on the overall shear link response. Non-linear monotonic analyses of the shear links incorporating the cyclic stress-strain steel properties resulted in similar trends in the response as the backbone curves recorded from the physical experiments. The numerical models of built-up shear links utilizing structural grade steels closely correlated to the experimentally recorded shear strength.Models utilizing low yield point steels overestimated the shear strength, which was caused by the characteristics of cyclic behavior of those steels. The detailed numerical models also allowed for investigation of the plastic strain demands on the different components of the link. It was shown that finite element models combined with appropriate stress-strain relationship may be used with confidence to check the design of shear links of different steel grades and sectional geometries.     

On the significance of form-induced stress in rough wall turbulent boundary layers

This paper presents a review of recent experimental and numerical studies which deal with the analysis of form-induced stress in rough wall turbulent boundary layers. The aim of the paper is to assess the importance of this stress for various rough wall geometries and flow conditions. Analysis of the significance of form-induced stress is first performed by comparing its magnitude with the magnitude of Reynolds stress for each data set available in literature. Then, by selecting a special set of data, we analyze the comparison between the gradients of both stresses. We point out that the comparison of stress gradients gives a different perspective on the role of form-induced stress in rough wall boundary layers.     

On the response of yielding seismically isolated structures

This paper investigates the seismic response of yielding isolated structures. To establish a general understanding of the nonlinear response of seismically isolated structures, this study first investigates the nonlinear response of isolated structures subjected to steady‐state harmonic motion and nonlinear transient ground excitation. The response of both viscously damped and hysteretically damped isolation systems is investigated in three phases. Initially, basic insights are gained through simple nonlinear two degrees of freedom (2‐DOF) models subjected to harmonic motion of varying frequencies. Next, the transient response analysis of the nonlinear 2‐DOF model is investigated for a wide range of isolation system and superstructure properties. The results obtained from both approaches indicate that the yielding behavior of a structure on an isolation system is significantly different from that of the comparable fixed‐base structure. Finally, the response of the nonlinear 2‐DOF system model is compared with that of a 15‐story, three‐dimensional model. Based on the results of these analytical investigations, some important considerations for the design of seismically isolated structures are presented. Copyright © 2007 John Wiley & Sons, Ltd.     

The effects of surface moisture on aeolian sediment transport threshold and mass flux on a beach

This paper presents results from a study designed to explore the effects of beach surface moisture and fetch effects on the threshold of movement, intensity of sand transport by wind and mass flux. The experiment was carried out over a period of five weeks at Greenwich Dunes, Prince Edward Island, Canada in May and June 2002. Moisture content was measured with a Delta‐T moisture probe over a 50 m by 25 m grid established on the beach. Measurements of wind speed and direction were made with arrays of cup anemometers and a two‐dimensional sonic anemometer. Transport intensity was measured at a height of 2–4 cm above the bed using omnidirectional saltation probes which count the impact of saltating grains on a piezoelectric crystal. Anemometers and saltation probes were sampled at 1 Hz. Sand transport was measured with vertical integrating sand traps over periods of 10–20 minutes. Results show that where there is a considerable supply of dry sand the saltation system responds very rapidly (1–2 s) to fluctuations in wind speed, i.e. to wind gusts. Where sand supply from the surface is limited by moisture, mean transport rates are much lower and this reflects in both a reduction in the instantaneous transport rate and in a transport system that becomes increasingly intermittent. Threshold wind speed is significantly correlated with an increase in surface moisture content near the upwind end of the beach fetch, but the relationship is not significant at the downwind end where sediment transport is initiated primarily by saltation impact from upwind. Mass flux increases with increasing fetch length and the relationship is described best by a power function. Further work is necessary to develop a theoretical function to predict the increase in transport with fetch distance as well as the critical fetch distance. Copyright © 2007 John Wiley & Sons, Ltd.     

40Ar/39Ar thermochronology using alkali feldspars: real thermal history or mathematical mirage of microtexture?

The Multiple Diffusion Domain (MDD) theory of Ar loss, based on the ⁴⁰Ar/³⁹Ar method, has been used to obtain thermal histories, over tens of millions of years, in the range 500 °C to near-surface temperatures, of alkali feldspars from slowly cooled geological environments and recovered in situ from sedimentary basins and geothermal systems. It assumes that Ar diffuses in Nature, over geological time, from the same domains, and by the same processes, that control its diffusion during laboratory step-heating experiments. Mathematically the model has many stringent requirements: the domains must form at T above those being modelled and must remain unchanged during geological time and during step heating, must not interact, must occur in discrete size ranges sharing one simple geometrical shape, contain no compositional gradients in ⁴⁰K, and release Ar into an infinite reservoir. We describe the nature of possible diffusion domains, and the fast pathways for diffusion between them, known to exist in alkali feldspars from direct electron microscopic observations, and explore their time–temperature evolution, based on established phase behaviour, both over geological time and during step heating. Strain-controlled microtextures, whose morphology is controlled by the minimization of coherency strain energy, do not provide fast pathways for Ar loss, and behave as crystallographically complex but single domains for Ar. Subregular dislocations may form over a range of T in some strain-controlled intergrowths, but in Nature the majority form closed loops and do not provide pathways to the crystal surface. Deuteric microtextures cut across strain-controlled microtextures, and are composed of microporous subgrain and grain mosaics with many pathways and a wide spectrum of possible domain sizes and shapes. The deuterically altered zones provide pathways for Ar diffusion out of the crystal, although some Ar is retained in fluid inclusions. All microtextures likely to define diffusion domains form below 500 °C, over a range of T and (in slowly cooled rocks) of times. The most important involve fluid–feldspar reactions, which can reset Ar, and continue to surface T. Feldspars in geothermal systems and young sedimentary basins are subject to ongoing chemical and microtextural changes. Because the microtextures are metastable during step-heating experiments in the T range over which most Ar is released, domains and pathways for rapid diffusion of Ar between domains will be significantly modified during step heating, particularly above 750 °C, when departures from a simple Arrhenius diffusion law are most marked. Pre-melting is likely to be an important cause of complexity in Ar loss behaviour. Few, if any, of the requirements of the MDD model are met, and we conclude that thermal histories cannot be obtained from alkali feldspars using this method. Received: 21 September 1998 / Accepted: 1 March 1999     

The discovery of ERO counterparts to faint submillimetre galaxies

We have used deep ground-based imaging in the near-infrared (near-IR) to search for counterparts to the luminous submillimetre (submm) sources in the catalogue of Smail et al. For the majority of the submm sources the near-IR imaging supports the counterparts originally selected from deep optical images. However, in two cases (10 per cent of the sample) we find a relatively bright near-IR source close to the submm position, sources that were unidentified in the deep Hubble Space Telescope ( HST ) and ground-based R -band images used by Smail et al. We place limits on colours of these sources from deep high-resolution Keck II imaging and find they have 2 σ limits of ( I − K )≳6.8 and ( I − K )≳6.0, respectively. Both sources thus class as extremely red objects (EROs). Using the spectral properties of the submm source in the radio and submm we argue that these EROs are probably the source of the submm emission, rather than the bright spiral galaxies previously identified by Smail et al. This connection provides important insights into the nature of the enigmatic ERO population and faint submm galaxies in general. From the estimated surface density of these submm-bright EROs we suggest that this class accounts for the majority of the reddest members of the ERO population, in good agreement with the preliminary conclusions of pointed submm observations of individual EROs. We conclude that the most extreme EROs represent a population of dusty, ultraluminous galaxies at high redshifts; further study of these will provide useful insights into the nature of star formation in obscured galaxies in the early Universe. The identification of similar counterparts in blank-field submm surveys will be extremely difficult owing to their faintness ( K ∼20.5, I ≳26.5). Finally, we discuss the radio and submm properties of the two submm-bright EROs discovered here and suggest that both galaxies lie at z ≳2.     

Applicability of time-to-failure analysis to accelerated strain before earthquakes and volcanic eruptions

We examine quantitatively the ranges of applicability of the equation Ω= A+B [1− t/t _f ] ^m for predicting 'system-sized' failure times t _f in the Earth. In applications Ω is a proxy measure for strain or crack length, and A , B and the index m are model parameters determined by curve fitting. We consider constitutive rules derived from (a) Charles' law for subcritical crack growth; (b) Voight's equation; and (c) a simple percolation model, and show in each case that this equation holds only when m < 0. When m > 0, the general solution takes the form Ω = A + B [1 + t / T  ] ^m , where T   is a positive time constant, and no failure time can be defined. Reported values for volcanic precursors based on rate data are found to be within the range of applicability of time-to-failure analysis ( m < 0). The same applies to seismic moment release before earthquakes, at the expense of poor retrospective predictability of the time of the a posteriori -defined main shock. In contrast, reported values based on increasing cumulative Benioff strain occur in the region where a system-sized failure time cannot be defined ( m > 0; commonly m ≈ 0.3). We conclude on physical grounds that cumulative seismic moment is preferred as the most direct measure of seismic strain. If cumulative Benioff strain is to be retained on empirical grounds, then it is important that these data either be re-examined with the independent constraint m < 0, or that for the case 0 < m + 1 < 1, a specific correction for the time-integration of cumulative data be applied, of the form ΣΩ = At + B '{1 − [1 − t/t _f ] ^m+1 }.