Crystal-poor, multiply saturated rhyolites (obsidians) from the Cascade and Mexican arcs: evidence of degassing-induced crystallization of phenocrysts     

Laura E. Waters  Rebecca A. Lange 《Contributions to Mineralogy and Petrology》2013,166(3):731-754

A detailed petrological study is presented for six phenocryst-poor obsidian samples (73–75 wt% SiO₂) erupted as small volume, monogenetic domes in the Mexican and Cascade arcs. Despite low phenocryst (+microphenocryst) abundances (2–6 %), these rhyolites are each multiply saturated with five to eight mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± zircon ± hornblende ± clinopyroxene ± sanidine ± pyrrhotite). Plagioclase and orthopyroxene phenocrysts (identified using phase-equilibrium constraints) span ≤30 mol % An and ≤15 % Mg#, respectively. Eruptive temperatures (±1σ), on the basis of Fe–Ti two oxide thermometry, range from 779 (±25) to 940 (±18) °C. Oxygen fugacities (±1σ) range from ?0.4 to 1.4 (±0.1) log units relative to those along the Ni–NiO buffer. With temperature known, the plagioclase-liquid hygrometer was applied; maximum water concentrations calculated for the most calcic plagioclase phenocryst in each sample range from 2.6 to 6.5 wt%. This requires that the rhyolites were fluid-saturated at depths ≥2–7 km. It is proposed that the wide compositional range in plagioclase and orthopyroxene phenocrysts, despite their low abundance, can be attributed to changing melt water concentrations owing to degassing during magma ascent. Phase-equilibrium experiments from the literature show that higher dissolved water concentrations lead to more Fe-rich orthopyroxene, as well as more calcic plagioclase. Loss of dissolved water leads to a progressive increase in melt viscosity, and phenocrysts often display diffusion-limited growth textures (e.g., dendritic and vermiform), consistent with large undercoolings caused by degassing. A kinetic barrier to microlite crystallization occurred at viscosities from 4.5 to 5.0 log₁₀ Pa s for these rhyolites, presumably because the rate at which melt viscosity changed was high owing to rapid loss of dissolved water during magma ascent.  相似文献   

Spatial variations in surface sediment structure in riffle–pool sequences: a preliminary test of the Differential Sediment Entrainment Hypothesis (DSEH)     

Rebecca A. Hodge  David A. Sear  Julian Leyland 《地球表面变化过程与地形》2013,38(5):449-465

Riffle–pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles. This preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows; however the Differential Sediment Entrainment Hypothesis (DSEH) postulates that differential entrainment can instead result from spatial sedimentological contrasts. Here we use a novel suite of in situ grain‐scale field measurements from a riffle–pool sequence to parameterize a physically‐based model of grain entrainment. Field measurements include pivoting angles, lift forces and high resolution digital elevation models (DEMs) acquired using terrestrial laser scanning, from which particle exposure, protrusion and surface roughness were derived. The entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles. This is because pool grains have looser packing, hence greater exposure and lower pivoting angles. Conversely, riffle and pool exit grains have denser packing, lower exposure and higher pivoting angles. A cohesive matrix further stabilizes pool exit grains. The resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a two‐dimensional computational fluid dynamics (CFD) model of the reach. The CFD model predicts that, under bankfull conditions, pools experience lower shear stresses than riffles and pool exits. However, the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles, resulting in differential entrainment under bankfull flows. Significantly, this differential entrainment does not require a reversal in flow velocities or shear stress, suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle–pool sequences. This finding has implications for the prediction of sediment transport and the morphological evolution of gravel‐bed rivers. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Modeling and simulation of two-phase two-component flow with disappearing nonwetting phase   总被引：1，自引：0，他引：1  

Rebecca Neumann  Peter Bastian  Olaf Ippisch 《Computational Geosciences》2013,17(1):139-149

Carbon capture and storage is a recently discussed new technology, aimed at allowing an ongoing use of fossil fuels while preventing the produced CO₂ to be released to the atmosphere. CCS can be modeled with two components (water and CO₂) in two phases (liquid and CO₂). To simulate the process, a multiphase flow equation with equilibrium phase exchange is used. One of the big problems arising in two-phase two-component flow simulations is the disappearance of the nonwetting phase, which leads to a degeneration of the equations satisfied by the saturation. A standard choice of primary variables, which is the pressure of one phase and the saturation of the other phase, cannot be applied here. We developed a new approach using the pressure of the nonwetting phase and the capillary pressure as primary variables. One important advantage of this approach is the fact that we have only one set of primary variables that can be used for the biphasic as well as the monophasic case. We implemented this new choice of primary variables in the DUNE simulation framework and present numerical results for some test cases.  相似文献   

Climate-induced vegetation dynamics and the Bantu Expansion: Evidence from Bantu names for pioneer trees (Elaeis guineensis,Canarium schweinfurthii,and Musanga cecropioides)     

Koen Bostoen  Rebecca Grollemund  Joseph Koni Muluwa 《Comptes Rendus Geoscience》2013,345(7-8):336-349

The present article examines whether Late Holocene climate-induced vegetation changes in the Central African forest block may have facilitated the Bantu Expansion. This is done through a body of evidence that is not commonly used for the reconstruction of vegetation dynamics, i.e. language data. The article focuses on common Bantu vocabulary for three pioneer species abundantly present in the Central African pollen record between ca. 2500 and 2000 BP: Musanga cecropioides, Elaeis guineensis, and Canarium schweinfurthii. The geographical distribution patterns of the vernacular names for these pioneer trees add weight to the hypothesis according to which the rainforest contraction that emerged in the first millennium BC had an impact on the way Bantu languages dispersed.  相似文献   

How Reliable are Citizen‐Derived Scientific Data? Assessing the Quality of Contrail Observations Made by the General Public     

Amy Fowler  J. Duncan Whyatt  Gemma Davies  Rebecca Ellis 《Transactions in GIS》2013,17(4):488-506

Citizen science projects encourage the general public to participate in scientific research. Participants can contribute large volumes of data over broad spatial and temporal frames; however, the challenge is to generate data of sufficient quality to be useable in scientific research. Most observations made by citizen‐scientists can be independently verified by “experts”. However, verification is more problematic when the phenomena being recorded are short‐lived. This article uses a GIS methodology to verify the quality of contrail observations made by the general public as part of the OPAL Climate Survey. We verify observations using datasets derived from a variety of different sources (experts, models and amateur enthusiasts) with different spatial and temporal properties which reflect the complex 3D nature of the atmosphere. Our results suggest that ～70% of citizen observations are plausible, based on favorable atmospheric conditions and the presence or absence of aircraft; a finding which is in keeping with other, more conventional citizen science projects. However, questions remain as to why the quality of the citizen‐based observations was so high. Given the lack of supporting data on observers, it is impossible to determine whether the dataset was generated by the activities of many participants or a small but dedicated number of individual observers.  相似文献   

An analysis of river bank slope and unsaturated flow effects on bank storage   总被引：1，自引：0，他引：1  

Doble R  Brunner P  McCallum J  Cook PG 《Ground water》2012,50(1):77-86

Recognizing the underlying mechanisms of bank storage and return flow is important for understanding streamflow hydrographs. Analytical models have been widely used to estimate the impacts of bank storage, but are often based on assumptions of conditions that are rarely found in the field, such as vertical river banks and saturated flow. Numerical simulations of bank storage and return flow in river-aquifer cross sections with vertical and sloping banks were undertaken using a fully-coupled, surface-subsurface flow model. Sloping river banks were found to increase the bank infiltration rates by 98% and storage volume by 40% for a bank slope of 3.4° from horizontal, and for a slope of 8.5°, delay bank return flow by more than four times compared with vertical river banks and saturated flow. The results suggested that conventional analytical approximations cannot adequately be used to quantify bank storage when bank slope is less than 60° from horizontal. Additionally, in the unconfined aquifers modeled, the analytical solutions did not accurately model bank storage and return flow even in rivers with vertical banks due to a violation of the dupuit assumption. Bank storage and return flow were also modeled for more realistic cross sections and river hydrograph from the Fitzroy River, Western Australia, to indicate the importance of accurately modeling sloping river banks at a field scale. Following a single wet season flood event of 12 m, results showed that it may take over 3.5 years for 50% of the bank storage volume to return to the river.  相似文献   

A review of sediment and nutrient concentration data from Australia for use in catchment water quality models   总被引：1，自引：0，他引：1  

Bartley R  Speirs WJ  Ellis TW  Waters DK 《Marine pollution bulletin》2012,65(4-9):101-116

Land use (and land management) change is seen as the primary factor responsible for changes in sediment and nutrient delivery to water bodies. Understanding how sediment and nutrient (or constituent) concentrations vary with land use is critical to understanding the current and future impact of land use change on aquatic ecosystems. Access to appropriate land-use based water quality data is also important for calculating reliable load estimates using water quality models. This study collated published and unpublished runoff, constituent concentration and load data for Australian catchments. Water quality data for total suspended sediments (TSS), total nitrogen (TN) and total phosphorus (TP) were collated from runoff events with a focus on catchment areas that have a single or majority of the contributing area under one land use. Where possible, information on the dissolved forms of nutrients were also collated. For each data point, information was included on the site location, land use type and condition, contributing catchment area, runoff, laboratory analyses, the number of samples collected over the hydrograph and the mean constituent concentration calculation method. A total of ～750 entries were recorded from 514 different geographical sites covering 13 different land uses. We found that the nutrient concentrations collected using "grab" sampling (without a well defined hydrograph) were lower than for sites with gauged auto-samplers although this data set was small and no statistical analysis could be undertaken. There was no statistically significant difference (p<0.05) between data collected at plot and catchment scales for the same land use. This is most likely due to differences in land condition over-shadowing the effects of spatial scale. There was, however, a significant difference in the concentration value for constituent samples collected from sites where >90% of the catchment was represented by a single land use, compared to sites with <90% of the upstream area represented by a single land use. This highlights the need for more single land use water quality data, preferably over a range of spatial scales. Overall, the land uses with the highest median TSS concentrations were mining (～50,000mg/l), horticulture (～3000mg/l), dryland cropping (～2000mg/l), cotton (～600mg/l) and grazing on native pastures (～300mg/l). The highest median TN concentrations are from horticulture (～32,000μg/l), cotton (～6500μg/l), bananas (～2700μg/l), grazing on modified pastures (～2200μg/l) and sugar (～1700μg/l). For TP it is forestry (～5800μg/l), horticulture (～1500μg/l), bananas (～1400μg/l), dryland cropping (～900mg/l) and grazing on modified pastures (～400μg/l). For the dissolved nutrient fractions, the sugarcane land use had the highest concentrations of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP). Urban land use had the highest concentrations of dissolved inorganic phosphorus (DIP). This study provides modellers and catchment managers with an increased understanding of the processes involved in estimating constituent concentrations, the data available for use in modelling projects, and the conditions under which they should be applied. Areas requiring more data are also discussed.  相似文献   

Alignment time-scale of the microquasar GRO J1655−40     

Rebecca G. Martin  Christopher A. Tout  J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2008,387(1):188-196

The microquasar GRO J1655−40 has a black hole with spin angular momentum apparently misaligned to the orbital plane of its companion star. We analytically model the system with a steady-state disc warped by Lense–Thirring precession and find the time-scale for the alignment of the black hole with the binary orbit. We make detailed stellar evolution models so as to estimate the accretion rate and the lifetime of the system in this state. The secondary can be evolving at the end of the main sequence or across the Hertzsprung gap. The mass-transfer rate is typically 50 times higher in the latter case but we find that, in both the cases, the lifetime of the mass-transfer state is at most a few times the alignment time-scale. The fact that the black hole has not yet aligned with the orbital plane is therefore consistent with either model. We conclude that the system may or may not have been counter aligned after its supernova kick but that it is most likely to be close to alignment rather than counter alignment now.  相似文献   

In situ characterization of grain‐scale fluvial morphology using Terrestrial Laser Scanning     

Rebecca Hodge  James Brasington  Keith Richards 《地球表面变化过程与地形》2009,34(7):954-968

The grain‐scale morphology of fluvial sediments is an important control on the character and dynamics of river systems; however current understanding of its role is limited by the difficulties of robustly quantifying field surface morphology. Terrestrial Laser Scanning (TLS) offers a new methodology for the rapid acquisition of high‐resolution and high‐precision surface elevation data from in situ sediments. To date, most environmental and fluvial applications of TLS have focused on large‐scale systems, capturing macroscale morphologies. Application of this new technology at scales necessary to characterize the complexity of grain‐scale fluvial sediments therefore requires a robust assessment of the quality and sources of errors in close‐range TLS data. This paper describes both laboratory and field experiments designed to evaluate close‐range TLS for sedimentological applications and to develop protocols for data acquisition. In the former, controlled experiments comprising high‐resolution scans of white, grey and black planes and a sphere were used to quantify the magnitude and source of three‐dimensional (3D) point errors resulting from a combination of surface geometry, reflectivity effects and inherent instrument precision. Subsequently, a methodology for the collection and processing of grain‐scale TLS data is described through an application to a coarse grained gravel system, the River Feshie (D₅₀ 32 to 63 mm). This stepwise strategy incorporates averaging repeat scans and filtering scan artefact and non‐surface points using local 3D search algorithms. The sensitivity of the results to the filter parameter values are assessed by careful internal validation of Digital Terrain Models (DTMs) created from the resulting point cloud data. The transferability of this methodology is assessed through application to a second river, Bury Green Brook, dominated by finer gravel (D₅₀ 18 to 33 mm). The factor limiting the resolution of DTMs created from this second dataset was found to be the relative sizes of the laser footprint and smallest grains. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Changes in sediment and organic carbon accumulation in a highly-disturbed ecosystem: The Sacramento-San Joaquin River Delta (California, USA)     

Elizabeth A. Canuel  Elizabeth J. Lerberg  Rebecca M. Dickhut  Steven A. Kuehl  Thomas S. Bianchi  Stuart G. Wakeham 《Marine pollution bulletin》2009,59(4-7):154

We used the Sacramento-San Joaquin River Delta CA (Delta, hereafter) as a model system for understanding how human activities influence the delivery of sediment and total organic carbon (TOC) over the past 50–60 years. Sediment cores were collected from sites within the Delta representing the Sacramento River (SAC), the San Joaquin River (SJR), and Franks Tract (FT), a flooded agricultural tract. A variety of anthropogenic tracers including ¹³⁷Cs, total DDE (∑DDE) and brominated diphenyl ether (BDE) congeners were used to quantify sediment accumulation rates. This information was combined with total organic carbon (TOC) profiles to quantify rates of TOC accumulation. Across the three sites, sediment and TOC accumulation rates were four to eight-fold higher prior to 1972. Changes in sediment and TOC accumulation were coincident with completion of several large reservoirs and increased agriculture and urbanization in the Delta watershed. Radiocarbon content of TOC indicated that much of the carbon delivered to the Delta is “pre-aged” reflecting processing in the Delta watershed or during transport to the sites rather than an input of predominantly contemporary carbon (e.g., 900–1400 years BP in surface sediments and 2200 yrs BP and 3610 yrs BP at the base of the SJR and FT cores, respectively). Together, these data suggest that human activities have altered the amount and age of TOC accumulating in the Delta since the 1940s.  相似文献