The importance of grain size and shape in controlling the dispersion of the Vedde cryptotephra     

Jennifer Saxby  Alison Rust  Katharine Cashman  Frances Beckett 《第四纪科学杂志》2020,35(1-2):175-185

Volcanic ash is dispersed in the atmosphere according to meteorology and particle properties, including size and shape. However, the multiple definitions of size and shape for non-spherical particles affect our ability to use physical particle properties to understand tephra transport. Moreover, although particles are often excluded from operational ash dispersion model setups, ash in tephra deposits 1000 km from source can exceed . Here we measure the shape and size of samples of Vedde ash from Iceland, an exceptionally widespread tephra layer in Europe, collected in Iceland and Norway. Using X-ray computed tomography and optical microscopy, we show that distal ash is more anisotropic than proximate ash, suggesting that shape exerts an important control on tephra dispersion. Shape also impacts particle size measurements. Particle long axis, a parameter often reported by tephrochronologists, is on average greater than geometric size, used by dispersion modellers. By using geometric size and quantifying shape, we can explain the transport of Vedde ash particles more than 1200 km from source. We define a set of best practices for measuring the size and shape of cryptotephra shards and discuss the benefits and limitations of using physical particle properties to understand cryptotephra transport.  相似文献   

Alkali ion migration in albite and K-feldspar     

A.?JonesEmail author  D.?Palmer  M. S.?Islam  M.?Mortimer 《Physics and Chemistry of Minerals》2004,31(5):313-320

As part of a study of ion migration mechanisms in feldspars, the dynamical behaviour of the alkali metal cations ions in albite and K-feldspar has been investigated using a combination of dielectric spectroscopy and atomistic computer simulation techniques. The low-frequency dielectric properties of these minerals have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectra was observed for both albite and K-feldspar; the activation energy for these processes was determined to be 1.33 eV in both albite and K–feldspar. Atomistic simulation techniques were used to elucidate the mechanism and energetics of the cation migration processes. Mechanisms involving the conventional hopping of Na⁺ and K⁺ ions between cation sites in the (010) plane were found to give calculated energy barriers in good agreement with the experimentally determined activation energies. These results assist in understanding the nature of the processes responsible for the observed dielectric behaviour.  相似文献   

Out there and in here: studying Eastern Europe in the West     

Alison Stenning 《Area》2005,37(4):378-383

This paper seeks to explore the challenge of studying geographies of Eastern Europe in Western Europe. It reflects on the growing relationships – imagined and material – between Eastern and Western Europe and engages in contemporary debates over 'relational geographies' to make a case for geographies which imagine our plural, European spatial connections, which connect different parts of the continent and which encourage the re-examination and renegotiation of ideas across East and West in the post-Cold War world. In these ways, the paper borrows from postcolonial frameworks and the ongoing reassessment of area studies to call for studies of Eastern and Western Europe which reinvigorate our "sense of Europe".  相似文献   

A Hybrid Multi-Agent/Spatial Interaction Model System for Petrol Price Setting   总被引：2，自引：0，他引：2  

Alison J Heppenstall  rew J Evans  Mark H Birkin 《Transactions in GIS》2005,9(1):35-51

This paper examines the use of multi‐agent simulations (MAS) to model the petrol market. The development of a purely agent based model for petrol prices is presented. Failings within this model are discussed and an alternative strategy for controlling the price of each petrol station based on population of customers is considered. Individual level modelling of customers is too computationally intensive; therefore a spatial interaction model was used to estimate the sales and linked to the agent system to create a hybrid model. To evaluate how effective this hybrid model was, a comparison was made with an existing data set of real petrol prices collected over a two month period. This was achieved both statistically and visually with the use of a Geographical Information System (GIS). Experimentation revealed that the hybrid model outperformed the agent model. Investigation into the behaviour and sensitivity of the system (for example, how prices diffuse spatially) was undertaken by means of idealised simulations.  相似文献   

In situ mesocosms: Denitrification in the Elk Valley aquifer     

by Scott F. Korom  Allen J. Schlag  William M. Schuh  Alison Kammer Schlag 《Ground Water Monitoring & Remediation》2005,25(1):79-89

  相似文献   

The relative importance of chlorophyll and colored dissolved organic matter (CDOM) to the prediction of the diffuse attenuation coefficient in shallow estuaries   总被引：2，自引：0，他引：2  

Alison B. Branco  James N. Kremer 《Estuaries and Coasts》2005,28(5):643-652

The availability of underwater light is a critical factor in the growth and abundance of primary producers in shallow embayments. The goal of this study was to examine the relative importance of factors influencing light availability in this type of water body. Many simulation models of aquatic ecosystems predict light attenuation from chlorophyll or phytoplankton stock. In the three southern New England sites studied here, no useful relationship was found to exist between chlorophyll and K_PAR (the diffuse attenuation coefficient of photosynthetically active radiation; Kirk 1994; Mobley 1994). In 40 of 53 cases, a regression of chlorophyll versus K_PAR was not statistically significant. Variation in K_PAR did demonstrate a correlation to salinity, implicating a freshwater source of light attenuating material. This was true even in a system with little freshwater inflow. Colored dissolved organic matter (CDOM) is one such terrestrial input that enters estuaries from their watersheds and can strongly influence the availability of light to aquatic primary producers. This study demonstrated that over 70% of the variability in the K_PAR coefficient can be attributed to CDOM in the shallow estuaries studied. This illustrates the need for improved model formulations that include CDOM in the prediction of light attenuation in shallow coastal systems. A new equation has been developed to predict K_PAR with CDOM.  相似文献   

Undressing the researcher: feminism, embodiment and sexuality at a queer bathhouse event   总被引：2，自引：0，他引：2  

Alison L Bain  Catherine J Nash 《Area》2006,38(1):99-106

In this paper we examine how the researcher's body can be used as a tool for data collection in the process of ethnographic fieldwork. We focus in particular on the tensions inherent in undertaking embodied ethnographic research in the sexualized setting of a queer women's bathhouse event in Toronto, Canada. Our discussion addresses three moments within the research process: preparing our bodies to attend the bathhouse; positioning our bodies within the spaces of the bathhouse; and interacting with our bodies during the event. Through this discussion we argue that the body of the researcher is a contested site of knowledge production.  相似文献   

Lacustrine ostracodes as hydrochemical indicators in lakes of the north-central United States     

Alison J. Smith 《Journal of Paleolimnology》1993,8(2):121-134

The modern ostracode distribution in lakes of the north-central United States shows how ostracode abundances are related to concentrations of major ions such as calcium, sulfate, and bicarbonate. These relationships are quantified for species living in lakes that range from fresh water (200 S cm^-1) to saline water (17 000 S cm^-1). Lakes located in the Minnesota forests have different ostracode assemblages and different water chemistry than lakes located on the prairie of North Dakota and South Dakota. These differences are attributed to differences in precipitation and rock-water interactions. Multivariate analyses of the ostracode and water chemistry data set indicate that different ostracode species are associated with different water types. For example, Limnocythere sappaensis and Heterocypris glaucus are found in bicarbonate-enriched sulfate-dominated waters, whereas Limnocythere staplini is found in bicarbonate-depleted sulfate-dominated waters. Candona ohioensis and Limnocythere itasca are found in fresh water, and Candona rawsoni is eurytopic and found in both bicarbonate-enriched and bicarbonate-depleted sulfate-dominated waters. Ostracodes can be used to identify changes in both ionic composition (solutes) and ionic concentration.This publication is the second of a series of papers presented at the Conference on Sedimentary and Paleolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr Evans is serving as Guest Editor for this series.  相似文献   

The DEEP2 galaxy redshift survey: evolution of the colour–density relation at  0.4 < z < 1.35      

Michael C. Cooper  Jeffrey A. Newman †  Alison L. Coil †  Darren J. Croton  Brian F. Gerke  Renbin Yan  Marc Davis  S. M. Faber  Puragra Guhathakurta  David C. Koo  Benjamin J. Weiner  Christopher N. A. Willmer ‡ 《Monthly notices of the Royal Astronomical Society》2007,376(4):1445-1459

Using a sample of 19 464 galaxies drawn from the DEEP2 Galaxy Redshift Survey, we study the relationship between galaxy colour and environment at  0.4 < z < 1.35  . We find that the fraction of galaxies on the red sequence depends strongly on local environment out to   z > 1  , being larger in regions of greater galaxy density. At all epochs probed, we also find a small population of red, morphologically early-type galaxies residing in regions of low measured overdensity. The observed correlations between the red fraction and local overdensity are highly significant, with the trend at   z > 1  detected at a greater than 5σ level. Over the entire redshift regime studied, we find that the colour–density relation evolves continuously, with red galaxies more strongly favouring overdense regions at low z relative to their red-sequence counterparts at high redshift. At   z ≳ 1.3  , the red fraction only weakly correlates with overdensity, implying that any colour dependence to the clustering of  ∼ L *  galaxies at that epoch must be small. Our findings add weight to existing evidence that the build-up of galaxies on the red sequence has occurred preferentially in overdense environments (i.e. galaxy groups) at   z ≲ 1.5  . Furthermore, we identify the epoch  ( z ∼ 2)  at which typical  ∼ L *  galaxies began quenching and moved on to the red sequence in significant number. The strength of the observed evolutionary trends at  0 < z < 1.35  suggests that the correlations observed locally, such as the morphology–density and colour–density relations, are the result of environment-driven mechanisms (i.e. 'nurture') and do not appear to have been imprinted (by 'nature') upon the galaxy population during their epoch of formation.  相似文献   

How well do ecosystem indicators communicate the effects of anthropogenic eutrophication?     

Abigail McQuatters-Gollop  Alison J. Gilbert  Laurence D. Mee  Jan E. Vermaat  Yuri Artioli  Christoph Humborg  Fred Wulff   《Estuarine, Coastal and Shelf Science》2009,82(4):583-596

Anthropogenic eutrophication affects the Mediterranean, Black, North and Baltic Seas to various extents. Responses to nutrient loading and methods of monitoring relevant indicators vary regionally, hindering interpretation of ecosystem state changes and preventing a straightforward pan-European assessment of eutrophication symptoms. Here we summarize responses to nutrient enrichment in Europe's seas, comparing existing time-series of selected pelagic (phytoplankton biomass and community composition, turbidity, N:P ratio) and benthic (macro flora and faunal communities, bottom oxygen condition) indicators based on their effectiveness in assessing eutrophication effects. Our results suggest that the Black Sea and Northern Adriatic appear to be recovering from eutrophication due to economic reorganization in the Black Sea catchment and nutrient abatement measures in the case of the Northern Adriatic. The Baltic is most strongly impacted by eutrophication due to its limited exchange and the prevalence of nutrient recycling. Eutrophication in the North Sea is primarily a coastal problem, but may be exacerbated by climatic changes. Indicator interpretation is strongly dependent on sea-specific knowledge of ecosystem characteristics, and no single indicator can be employed to adequately compare eutrophication state between European seas. Communicating eutrophication-related information to policy-makers could be facilitated through the use of consistent indicator selection and monitoring methodologies across European seas. This work is discussed in the context of the European Commission's recently published Marine Strategy Directive.  相似文献