Cold dark matter variant cosmological models — I. Simulations and preliminary comparisons     

Michael A. K. Gross  Rachel S. Somerville  Joel R. Primack  Jon Holtzman  & Anatoly Klypin 《Monthly notices of the Royal Astronomical Society》1998,301(1):81-94

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Fluoride-Selective Electrode as a Tool to Evaluate the Degradation of PFAS in Groundwater: A Bench-Scale Investigation     

Janice Stonebridge  Rachel Baldwin  Neil R. Thomson  Carol Ptacek 《Ground Water Monitoring & Remediation》2020,40(2):73-80

Fluoride (F⁻) concentration in groundwater can be used as an independent measure of the degradation of per- and polyfluoroalkyl substances (PFAS) at contaminated sites. This study assessed the impact of the groundwater matrix, oxidizing reagents, quenching agent, and sample handling procedures (filtration) on determinations of F⁻. F⁻ was quantified using a fluoride-selective electrode (FSE) using matrix spike recovery (acceptable range 75 to 125%) and electrode slope performance (ideal range −54 to −60 mV). For the unaltered aqueous matrices considered (ultrapure water, ultrapure water with sodium chloride, and simulated groundwater), matrix spike recoveries were greater than 93% and acceptable electrode slopes (−59.5 ± 0.2 mV) were obtained. Matrix spike recoveries and electrode slopes when persulfate or permanganate was added were 87 and 93%, and −58.4 ± 1.3 and −47.3 ± 2.2 mV, respectively. Therefore, quantifying F⁻ in matrices containing permanganate should involve use of either a matrix-matched calibration curve or the method of standard additions due to the low electrode slope. The addition of ascorbic acid as a quenching agent resulted in a decrease in matrix spike recoveries to <74% and suggests alternate quenching procedures should be used if an FSE is employed to measure F⁻. Removal of sediments by filtration prior to the addition of the fluoride spike led to improved matrix spike recoveries (>96%), which were otherwise underestimated using the FSE in the presence of sediments. While the FSE may be a quick and portable tool, its significant limitations need to be fully understood before it can be used to quantify the production of F⁻ resulting from the potential degradation of PFAS at field sites.  相似文献   

Order of diagenetic events controls evolution of porosity and permeability in carbonates     

Aleksandra Hosa  Rachel Wood 《Sedimentology》2020,67(6):3042-3054

The properties of carbonate rocks are often the result of multiple, diagenetic events that involve phases of cementation (porosity occlusion) and dissolution (porosity enhancement). This study tests the hypothesis that the order of these events is a major control on final porosity and permeability. A three-dimensional synthetic model of grainstone is used to quantify trends that show the effect of early cementation, non-fabric selective dissolution, and then a second-generation of (post-dissolution) cement. Models are 3 mm³ with a resolution of 10 μm. Six simple paragenetic sequences are modelled from an identical starting sediment (without accounting for compaction) where the same diagenetic events are placed in different sequences, allowing for quantification of relative changes in the resultant porosity and permeability for each diagenetic event, the trajectory through time, as well as for each final rock. All modelled paragenetic sequences result in reductions in porosity and permeability, but the order of diagenetic events controls the trajectory and final rock properties. Differences in the order of early cement precipitation alone produce variable final values, but all follow the porosity–permeability relationship as expressed by the Kozeny-Carman equation. However, final values for the sequences which include a phase of dissolution fall on a new curve, which departs from that predicted by the Kozeny-Carman relationship. This allows an alternative form of porosity–permeability relationship to be proposed: κ = 2280ϕ–30,400, where ϕ is porosity (%) and κ is permeability (mD). Hence while the Kozeny-Carman relationship predicts porosity–permeability changes that occur with cementation, it is unable to capture accurately changes within the pore network as a result of dissolution. Although the results may be dependent on the properties of the initial carbonate sediment and simplified diagenetic scenarios, it is suggested that this new porosity–permeability relationship may capture some generalized behaviour, which can be tested by modelling further sediment types and diagenetic scenarios.  相似文献   

The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin     

David H. Roberts  Colm Ó Cofaigh  Colin K. Ballantyne  Matthew Burke  Richard C. Chiverrell  David J. A. Evans  Chris D. Clark  Geoff A. T. Duller  Jeremy Ely  Derek Fabel  David Small  Rachel K. Smedley  S. Louise Callard 《Boreas: An International Journal of Quaternary Research》2020,49(3):438-460

This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.  相似文献   

The trophic ecology of key megafaunal species at the Pakistan Margin: Evidence from stable isotopes and lipid biomarkers     

Rachel M. Jeffreys  George A. Wolff  Sarah J. Murty 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(10):1816-1833

The Arabian Sea is subject to intense seasonality resulting from biannual monsoons, which lead to associated large particulate fluxes and an abundance of organic carbon, a potential food source at the seafloor for benthic detritivores. We used the stable isotopes of carbon and nitrogen alongside lipid analyses to examine potential food sources (particulate and sedimentary organic matter, POM and SOM respectively) in order to determine trophic linkages for the twelve most abundant megafaunal species (Pontocaris sp., Solenocera sp., Munidopsis aff. scobina, Actinoscyphia sp., Actinauge sp., Echinoptilum sp., Pennatula aff. grandis, Astropecten sp. Amphiura sp. Ophiura euryplax, Phormosoma placenta and Hyalinoecia sp.) at the Pakistan Margin between 140 and 1400 m water depth. This transect spans a steep gradient in oxygen concentrations and POM flux. Ranges of δ¹³C and δ¹⁵N values were narrow in POM and SOM (4‰ and 2‰ for δ¹³C and δ¹⁵N, respectively) with little evidence of temporal variability. Labile lipid compounds in SOM originating from phytoplankton did exhibit seasonal change in their concentrations at the shallowest sites, 140 and 300 m. Benthic megafauna had broad ranges in δ¹³C and δ¹⁵N (>10‰ and >8‰ for δ¹³C and δ¹⁵N, respectively) suggesting they occupy several trophic levels and utilize a variety of food sources. There is evidence for feeding niche separation between and within trophic groups. Lipid biomarkers in animal tissues indicate a mixture of food sources originating from both phytoplankton (C_20:5(n-3) and C_22:6(n-3)) and invertebrate prey (C_20:1 and C_22:1). Biomarkers originating from phytodetritus are conserved through trophic transfer to the predator/scavengers. Six species (Pontocaris sp., Solenocera sp., Actinoscyphia sp., Echinoptilum sp., Amphiura sp. and Hyalinoecia sp.) showed a significant biochemical response to the seasonal supply of food and probably adapt their trophic strategy to low food availability. Biotransformation of assimilated lipids by megafauna is evident from polyunsaturated fatty acid distributions, for example, Echinoptilum sp. converts C_20:5(n-3) to C_24:6(n-3).  相似文献   

The Norwegian system and the distribution of claims to redfeed     

Rachel Gjelsvik Tiller   《Marine Policy》2008,32(6):928-940

Identifying competing interests and potential claims to a new marine resource, the plankton redfeed, which may be central as a solution to diminishing supplies of marine oils world wide, is critical prior to a commercial harvest thereof. The competing interests are found to be between those Norwegian fisheries organization taking an encompassing view of fisheries interests and those wanting more focus on the well-being of the coastal fishermen. Working within a corporatist setting, it is found, that interest organizations in Norway are included comprehensively by the national government in the decision-making process, but that the main group having the most power is Norges Fiskarlag, which will draw the quota distribution of a future redfeed harvest off shore and away from the coastal fishermen.  相似文献   

Results from the Wide Angle Search for Planets Prototype (WASP0) – III. Planet hunting in the Draco field     

Stephen R. Kane †  rew Collier Cameron  Keith Horne  David James  T. A. Lister  Don L. Pollacco  Rachel A. Street  Yiannis Tsapras 《Monthly notices of the Royal Astronomical Society》2005,364(3):1091-1103

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Spatial patterns of ice storm disturbance on a forested landscape in the Appalachian Mountains, Virginia   总被引：3，自引：0，他引：3  

Kirk M Stueve  Charles W Lafon  Rachel E Isaacs 《Area》2007,39(1):20-30

Ice storms (major freezing rain events) periodically disturb forests in eastern North America. The damage may vary spatially, especially in complex terrain. This study uses satellite imagery to investigate spatial heterogeneity of forest damage caused by ice storms that affected the Appalachian Mountains, Virginia during 1994. The results display a region-scale (southwest-to-northeast) gradient in damage that apparently corresponds to a gradient in the depth of ice that accumulated during the storms. Damage also varied topographically, particularly by aspect. Damage was most extensive on east-, southeast- and south-facing slopes; at middle elevations; and on slopes of moderate steepness.  相似文献   

Particle suspensions and their regions of effect in the Neuse River Estuary: Implications for water quality monitoring     

J. Stephen Fries  Rachel T. Noble  Hans W. Paerl  Gregory W. Characklis 《Estuaries and Coasts》2007,30(2):359-364

Monitoring of estuarine condition depends on water quality parameters that have significant and interpretable ecological effects and can be assessed either in situ or via rapid laboratory techniques. Two commonly measured parameters are water column turbidity (NTU) and total suspended solids (TSS). Under certain conditions, either of these measures could represent a proxy for runoff and provide rapid, in situ measures to improve protection of the public by decreasing the time required to detect and monitor associated effects (e.g., reduced water clarity and eutrophication). The Neuse River Estuary (NRE) has experienced a decline in water quality due to anthropogenic inputs, including stormwater containing nutrient and particle loads. Water samples were collected from the NRE during both dry weather and storm events over 16 mo across the entire estuarine gradient. Particle size distributions, ratio of particulate organic carbon to nitrogen, chlorophylla (chla), TSS, and NTU were measured in each of these samples, with the data separated into regions based on salinity and depth of sample collection. Particle analyses were directed at identifying suspensions dominated by phytoplankton, runoff particles, or resuspended sediments. Particle size distributions for suspensions in Region I (all samples with salinity less than 2) varied little during sampling, resulting in a robust NTU-TSS relationship. This relationship confirmed the usefulness of turbidity as a measure of runoff and resuspension of recently deposited runoff in the upper NRE. Phytoplankton cells represented a majority of the particles in Region II (surface samples with salinity greater than 2), based on the close relationship between chla and total particle volume in these samples. Suspensions of large, nearly uniform diameter particles, which are likely aggregates and resuspended sediment, were observed in Region III (bottom samples with salinity greater than 2). Using these techniques as part of routine monitoring, particle suspension measures in different hydrographic regions of an estuary provide evidence useful for identifying the source and water quality consequences of particle suspensions (e.g., microbial contamination and algal blooms).  相似文献   

Evaluating approximations for halo merging histories     

Rachel S. Somerville  Gerard Lemson  Tsafrir S. Kolatt  Avishai Dekel 《Monthly notices of the Royal Astronomical Society》2000,316(3):479-490

We study the merging history of dark matter haloes in N -body simulations and semi-analytical 'merger trees' based on the extended Press–Schechter (EPS) formalism. The main focus of our study is the joint distribution of progenitor number and mass as a function of redshift and parent halo mass. We begin by investigating the mean quantities predicted directly by the Press–Schechter (PS) and EPS formalism, such as the halo mass and conditional mass functions, and compare these predictions with the results of the simulations. The higher moments of this distribution are not predicted by the EPS formalism alone and must be obtained from the merger trees. We find that the Press–Schechter model deviates from the simulations at the level of 30–50 per cent on certain mass scales, and that the sense of the discrepancy changes as a function of redshift. We show that this discrepancy is reflected in the higher moments of the distribution of progenitor mass and number. We investigate some related statistics such as the accretion rate and the mass ratio of the largest two progenitors. For galaxy sized haloes ( M ∼10¹² M_⊙), we find that the merging history of haloes, as represented by these statistics, is well reproduced in the merger trees compared with the simulations. The agreement deteriorates for larger mass haloes. We conclude that merger trees based on the extended Press–Schechter formalism provide a reasonably reliable framework for semi-analytical models of galaxy formation.  相似文献