Post-depositional migration of elements during diagenesis in brown clay and turbidite sequences in the North East Atlantic     

S Colley  J Thomson  T.R.S Wilson  N.C Higgs 《Geochimica et cosmochimica acta》1984,48(6):1223-1235

Two cores from a NE Atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age, thickness and composition. Diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes. The diagenetic process is thought to be driven by bacterially-mediated organic oxidation, and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process.In one core, a long turbidite section emplaced 330,000 years ago is overlain by clay. Organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a U concentration peak below the oxidation front marked by a colour change. The U source is believed to be U initially associated with the organic matter in the oxidised section of the turbidite. Vanadium and Cu behave similarly, but upward migration of the redox-sensitive metals Mn and Ni is also seen. In the second core an 8 cm turbidite section was emplaced about 170,000 years ago in a clay column. In this case organic consumption now appears complete, but evidence for diagenetic effects is found in a 16 cm compositional alteration ‘halo’ of the underlying pelagic clay. Fe(II) has been enriched and the hydrogenous component of Mn, Co, Ni and Cu removed from the pelagic clay to form the halo.  相似文献   

Further studies of the nature,composition and ages of contemporary phosphorite from the Namibian Shelf     

J. Thomson  S.E. Calvert  S. Mukherjee  W.C. Burnett  J.M. Bremner 《Earth and Planetary Science Letters》1984,69(2):341-353

Major and minor element and radiochemical data are examined for a set of phosphorite concretions and other phosphatic material from the Namibian Shelf. The concretions were separated on a morphological basis from grab samples taken in a transect of shallowing depth over the diatomaceous ooze belt. Compositional data indicate that the samples are fairly pure carbonate-fluorapatite, and trend towards a maximum 32% P₂O₅ content. A range of compositional variability similar to other samples from the Namibian Shelf is observed.Radiochemical systematics confirm that pelletal phosphorite and glauconitized pelletal phosphorite from depths > 170 m are at radioactive equilibrium, and are therefore old and reworked. The concretions, collected from depths shallower than 78 m, are entirely Recent, with maximum ages in the range 500–3800 years. They may be much younger. Despite distinct textural and small compositional differences, similar ages are found for separates from the same grab. Samples of fish scales and an unidentified high-magnesian phosphate mineral are also shown to have incorporated uranium recently.  相似文献   

PL whistlers     

R.J. Thomson 《Planetary and Space Science》1977,25(11):1037-1043

Simultaneous ground and satellite VLF observations together with raytracing studies clearly establishes the existence of ground observed PL whistlers. The dynamic spectrum $\text{(?-ν-t}\text{shape}\text{)}$ of observed PL whistlers may be reproduced exactly by raytracing in TLG magnetospheric models consistent with lower ionosphere, topside ionosphere and equatorial density measurements. The Transition Level Gradient (TLG) model is based on the observation that the transition level altitude increases towards the plasmapause (Titheridge, 1976). PL ground whistlers (i) are observed downgoing over large latitudinal ranges, for up to 2000 km of satellite travel, by ISIS II at 1400 km altitude, (ii) have almost the same dynamic spectrum over the entire latitudinal range observed by ISIS II, (iii) are indistinguishable from ducted whistlers over the observed frequency range (i.e. linear Q for ? < 10 kHz), (iv) have nose frequencies > 16 kHz, (v) at 1400 km altitude have a lower latitudinal cutoff at L ～ 2 and a higher latitudinal cutoff between L ～ 3 and L ～ 4 and (vi) probably only occur at night-time during or immediately following disturbed magnetic activity.  相似文献   

Long-term changes in salt marsh extent affected by channel deepening in a modified estuary   总被引：2，自引：0，他引：2  

Ruth Cox  R. A. Wadsworth  A. G. Thomson 《Continental Shelf Research》2003,23(17-19):1833

The aim of this study is to quantify the long-term (54 years) rates of marsh extension and retreat at two sites in the Westerschelde Estuary, SW Netherlands. Nine sets of aerial photographs were obtained for each of the two salt marsh sites, Zuidgors and Waarde, taken at various times between 1944 and 1998. The seaward edges of the marshes were digitised from rectified images after the photographs had been scanned and georegistered to the Dutch National Grid. Comparison of the extents of the marshes at these nine time points revealed that the areas of both marshes had fluctuated during these 54 years with periods of both extension and retreat of the seaward marsh edges. These periods of extension and retreat appeared to be approximately synchronised until the 1990s, with mean changes in position of marsh front ranging from −7.92 to 6.04 m a⁻¹.The rate of landward retreat and seaward extension of the marsh front is shown to be related to an increase in the tidal prism brought about by dredging operations to maintain or increase the depth of the main navigable channel of the estuary. The consequent greater frequency with which the high tides reach the edge of the fringing marshes increases the risk of erosion. Strong westerly winds may also be a factor in increasing erosion. No relationship between the volume of shipping traffic using the estuary and marsh erosion was found.  相似文献   

Fluxes to sediments underlying the Rainbow hydrothermal plume at 36°14′N on the Mid-Atlantic Ridge     

R.R. Cave  C.R. GermanJ. Thomson  R.W. Nesbitt 《Geochimica et cosmochimica acta》2002,66(11):1905-1923

A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO₃) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of “hydrothermal” elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr⁻¹, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores’ surface mixed layers are presented which, typically, are an order of magnitude greater than “background” authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 ¹⁴C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time—perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.  相似文献   

Elevation Dependence of Winter Wheat Production in Eastern Washington State with Climate Change: A Methodological Study     

Allison M. Thomson  Robert A. Brown  Steven J. Ghan  R. Cesar Izaurralde  Norman J. Rosenberg  L. Ruby Leung 《Climatic change》2002,54(1-2):141-164

Crop growth models, used in climate change impact assessments to project production on a local scale, can obtain the daily weather information to drive them from models of the Earth's climate. General Circulation Models (GCMs), often used for this purpose, provide weather information for the entire globe but often cannot depict details of regional climates especially where complex topography plays an important role in weather patterns. The U.S. Pacific Northwest is an important wheat growing region where climate patterns are difficult to resolve with a coarse scale GCM. Here, we use the PNNL Regional Climate Model (RCM) which uses a sub-grid parameterization to resolve the complex topography and simulate meteorology to drive the Erosion Productivity Impact Calculator (EPIC) crop model. The climate scenarios were extracted from the PNNL-RCM baseline and 2 × CO₂ simulationsfor each of sixteen 90 km² grid cells of the RCM, with differentiation byelevation and without correction for climate biases. The dominant agricultural soil type and farm management practices were established for each grid cell. Using these climate and management data in EPIC, we simulated winter wheat production in eastern Washington for current climate conditions (baseline) and a 2 × CO₂ `greenhouse' scenario of climate change.Dryland wheat yields for the baseline climate averaged 4.52 Mg ha^–1 across the study region. Yields were zero at high elevations where temperatures were too low to allow the crops to mature. The highest yields (7.32 Mgha^–1) occurred at intermediate elevations with sufficientprecipitation and mild temperatures. Mean yield of dryland winter wheat increased to 5.45 Mg ha^–1 for the 2 × CO₂ climate, which wasmarkedly warmer and wetter. Simulated yields of irrigated wheat were generally higher than dryland yields and followed the same pattern but were, of course, less sensitive to increases in precipitation. Increases in dryland and irrigated wheat yields were due, principally, to decreases in the frequency of temperature and water stress. This study shows that the elevation of a farm is a more important determinant of yield than farm location in eastern Washington and that climate changes would affect wheat yields at all farms in the study.  相似文献   

Design of Dry Barriers for Containment of Contaminants in Unsaturated Soils     

Carl E. Morris  Bruce M. Thomson  John C. Stormont 《Ground Water Monitoring & Remediation》1999,19(3):145-156

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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   

An improved geomagnetic data selection algorithm for global geomagnetic field modelling     

Alan W. P. Thomson  Vincent Lesur 《Geophysical Journal International》2007,169(3):951-963

A spherical harmonic degrees 60, global internal field model is described (called BGS/G/L/0706). This model includes a degree 15 core and piecewise-linear secular variation model and is derived from quiet-time Ørsted and Champ satellite data sampled between 2001.0 and 2005.0. For the satellite data selection, a wide range of geomagnetic index and other data selection filters have been used to best isolate suitably quiet magnetospheric and ionospheric conditions. Only a relatively simple, degree one spherical harmonic, external field model is then required. It is found that a new 'Vector Magnetic Disturbance' index ( VMD ), the existing longitude sector A indices, the auroral zone index IE , and the polar cap index PC are better than Kp and Dst at rejecting rapidly varying external field signals at low, middle, auroral and polar latitudes. The model quality is further enhanced by filling spatial and temporal gaps in the quiet data selection with a second selection containing slightly more disturbed data. It is shown that VMD provides a better parametrization than Dst of the large-scale, rapidly changing, external field. The lithospheric field model between degrees 16 and 50 is robust and displays good coherence with other recently published models for this epoch. BGS/G/L/0706 also shows crustal anomalies consistent with other studies, although agreement is poorer in the southern polar cap. Intermodel coherency reduces above about degree 40, most likely due to incompletely filtered signals from polar ionospheric currents and auroral field aligned currents. The absence of the PC index for the southern hemisphere for 2003 onwards is a particular concern.  相似文献   

The SuperCOSMOS Sky Survey – I. Introduction and description     

N.C. Hambly  H.T. MacGillivray  M.A. Read  S.B. Tritton  E.B. Thomson  B.D. Kelly  D.H. Morgan  R.E. Smith  S.P. Driver  J. Williamson  Q.A. Parker  M.R.S. Hawkins  P.M. Williams  A. Lawrence 《Monthly notices of the Royal Astronomical Society》2001,326(4):1279-1294

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