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931.
Antony G. Brown Phillip S. Toms Chris J. Carey Andy J. Howard Keith Challis 《地球表面变化过程与地形》2013,38(3):237-249
Although river confluences have received geomorphic attention in recent years it is difficult to upscale these studies, so confluence‐dominated reaches are commonly presumed to be either: (1) braided; or (2) meandering and characterized by laterally migrating channels. If the geomorphology of a confluence zone is to be considered over longer timescales, changes in river style need to be taken into account. This paper uses a combination of remote sensing techniques (LiDAR, GPR, ER), borehole survey and chronometric dating to test this differentiation in the confluence‐zone of a medium‐sized, mixed‐load, temperate river system (Trent, UK), which on the basis of planform evidence appears to conform to the meandering model. However, the analysis of ‘confluence sediment body stratigraphy’ demonstrates that the confluence does not correspond with a simple meander migration model and chronostratigraphic data suggests it has undergone two major transformations. Firstly, from a high‐energy braid‐plain confluence in the Lateglacial (25–13 K yrs cal BP), to a lower‐energy braided confluence in the early to middle Holocene (early Holocene‐2.4 kyr BP), which created a compound terrace. Second, incision into this terrace, creating a single‐channel confluence (2.4–0.5 kyr cal BP) with a high sinuosity south bank tributary (the River Soar). The confluence sediment‐body stratigraphy is characterized by a basal suite of Late Pleistocene gravels bisected by younger channel fills, which grade into the intervening levee and overbank sediments. The best explanation for the confluence sediment body stratigraphy encountered is that frequent switching (soft‐avulsions sensu Edmonds et al., 2011) of the tributary are responsible for the downstream movement of the channel confluence (at an average rate of approximately 0.5 m per year) dissecting and reworking older braid‐plain sediments. The late Holocene evolution of the confluence can be seen as a variant of the incisional‐frequent channel reorganization (avulsion) model with sequential downstream migration of the reattachment point. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
932.
Eric C. J. Oliver Jinyu Sheng Keith R. Thompson Jorge R. Urrego Blanco 《Natural Hazards》2012,64(2):1425-1446
This study presents a methodology for estimating extreme current speeds from numerical model results using extremal analysis techniques. This method is used to estimate the extreme near-surface and near-bottom current speeds of the northwest Atlantic Ocean with 50-year return periods from 17?years of model output. The non-tidal currents produced by a three-dimensional ocean circulation model for the 1988?C2004 period were first used to estimate and map the 17-year return period extreme current speeds at the surface and near the bottom. Extremal analysis techniques (i.e., fitting the annual maxima to the Type I probability distribution) are used to estimate and map the 50-year extreme current speeds. Tidal currents are dominant in some parts of the northwest Atlantic, and a Monte Carlo-based methodology is developed to take into account the fact that large non-tidal extrema may occur at different tidal phases. The inclusion of tidal currents in this way modifies the estimated 50-year extreme current speeds, and this is illustrated along several representative transects and depth profiles. Seasonal variations are examined by calculating the extreme current speeds for fall-winter and spring?Csummer. Finally, the distribution of extreme currents is interpreted taking into account (1) variability about the time-mean current speeds, (2) wind-driven Ekman currents, and (3) flow along isobaths. 相似文献
933.
934.
Shaun Cole Will J. Percival John A. Peacock Peder Norberg Carlton M. Baugh Carlos S. Frenk Ivan Baldry Joss Bland-Hawthorn Terry Bridges Russell Cannon Matthew Colless Chris Collins Warrick Couch Nicholas J. G. Cross Gavin Dalton Vincent R. Eke Roberto De Propris Simon P. Driver George Efstathiou Richard S. Ellis Karl Glazebrook Carole Jackson Adrian Jenkins Ofer Lahav Ian Lewis Stuart Lumsden Steve Maddox Darren Madgwick Bruce A. Peterson Will Sutherland Keith Taylor 《Monthly notices of the Royal Astronomical Society》2005,362(2):505-534
935.
Abstract The surface distribution of salinity, temperature, nitrate‐nitrogen (N03‐N), and chlorophyll a in the southern New Zealand, Foveaux Strait region in February 1977, 1978, 1979, and 1980 was highly variable. The source of new nitrogen appears to be incursions of high‐salinity water west and east of Stewart Island. Although it seems likely that the source of this high‐nutrient, high‐salinity water is vertical, a horizontal advective source cannot be ruled out The chlorophyll a content of surface waters was not related directly to the NO3‐N concentrations. This lower food chain variability may be linked to variability in economically important species. Oysters grew twice as fast in the summer of 1978/79 as they did in 1979/80. But the mean chlorophyll a values were very similar for February of both years (2.5 and 2.2 μg 1?1, respectively). The elevated NO3‐N levels in 1979 may have resulted in much higher phytoplankton levels later that summer and resulted in the higher oyster growth rate that year. The mechanisms driving this variability have yet to be determined. 相似文献
936.
Meinte Blaas Changming Dong Patrick Marchesiello James C. McWilliams Keith D. Stolzenbach 《Continental Shelf Research》2007
Suspended sediment-transport processes in Santa Monica and San Pedro Bay are analyzed using the sediment-transport capabilities of the Regional Oceanic Modeling System (roms). A one-month simulation for December 2001 has been carried out with a set of nested domains. The model inputs include tides, winds, surface waves, and idealized initial sediment conditions for sand and non-cohesive silt. Apart from the control run, the sensitivity of the results to surface waves, ripple roughness and bed armoring has been analyzed. From the control experiment, the horizontal transport of sand turns out to be limited to within a few km of the nearshore erosion zones. During high wave events, silt is transported over further distances and also partly offshelf in distinct plumes. The effectiveness of horizontal silt transport depends strongly on vertical mixing due to both surface wind stress and wave-enhanced bottom stress. High wave events coincident with strong winds (hence strong vertical mixing) are the most optimal conditions for sediment-transport. Excluding wave effects in the simulation shows that surface waves are the dominant factor in resuspending bed material on the Southern Californian shelves. The sensitivity experiments also show that the direct influence of additional ripple roughness on erosion and deposition is relatively weak. Switching off bed armoring locally results in increases of near-bottom concentrations by a factor of 20 for silt and a factor of 5 for sand as well as stronger spatial gradients in grain size. 相似文献
937.
Walton R. Kelly Samuel V. Panno Keith C. Hackley Hue-Hwa Hwang Adam T. Martinsek Momcilo Markus 《Applied Geochemistry》2010
Chloride concentrations in waterways of northern USA are increasing at alarming rates and road salt is commonly assumed to be the cause. However, there are additional sources of Cl− in metropolitan areas, such as treated wastewater (TWW) and water conditioning salts, which may be contributing to Cl− loads entering surface waters. In this study, the potential sources of Cl− and Cl− loads in the Illinois River Basin from the Chicago area to the Illinois River’s confluence with the Mississippi River were investigated using halide data in stream samples and published Cl− and river discharge data. The investigation showed that road salt runoff and TWW from the Chicago region dominate Cl− loads in the Illinois Waterway, defined as the navigable sections of the Illinois River and two major tributaries in the Chicago region. Treated wastewater discharges at a relatively constant rate throughout the year and is the primary source of Cl− and other elements such as F− and B. Chloride loads are highest in the winter and early spring as a result of road salt runoff which can increase Cl− concentrations by up to several hundred mg/L. Chloride concentrations decrease downstream in the Illinois Waterway due to dilution, but are always elevated relative to tributaries downriver from Chicago. The TWW component is especially noticeable downstream under low discharge conditions during summer and early autumn when surface drainage is at a minimum and agricultural drain tiles are not flowing. 相似文献
938.
Iron isotope compositions of carbonatites record melt generation, crystallization, and late-stage volatile-transport processes 总被引:2,自引:0,他引:2
Clark M. Johnson Keith Bell Brian L. Beard Aaron I. Shultis 《Mineralogy and Petrology》2010,98(1-4):91-110
Carbonatites define the largest range in Fe isotope compositions yet measured for igneous rocks, recording significant isotopic fractionations between carbonate, oxide, and silicate minerals during generation in the mantle and subsequent differentiation. In contrast to the relatively restricted range in δ56Fe values for mantle-derived basaltic magmas (δ56Fe?=?0.0?±?0.1‰), calcite from carbonatites have δ56Fe values between ?1.0 and +0.8‰, similar to the range defined by whole-rock samples of carbonatites. Based on expected carbonate-silicate fractionation factors at igneous or mantle temperatures, carbonatite magmas that have modestly negative δ56Fe values of ~ ?0.3‰ or lower can be explained by equilibrium with a silicate mantle. More negative δ56Fe values were probably produced by differentiation processes, including crystal fractionation and liquid immiscibility. Positive δ56Fe values for carbonatites are, however, unexpected, and such values seem to likely reflect interaction between low-Fe carbonates and Fe3+-rich fluids at igneous or near-igneous temperatures; the expected δ56Fe values for Fe2+-bearing fluids are too low to produced the observed positive δ56Fe values of some carbonatites, indicating that Fe isotopes may be a valuable tracer of redox conditions in carbonatite complexes. Further evidence for fluid-rock or fluid-magma interactions comes from the common occurrence of Fe isotope disequilibrium among carbonate, oxide, silicate, and sulfide minerals in the majority of the carbonatites studied. The common occurrence of Fe isotope disequilibrium among minerals in carbonatites may also indicate mixing of phenocyrsts from distinct magmas. Expulsion of Fe3+-rich brines into metasomatic aureols that surround carbonatite complexes are expected to produce high-δ56Fe fenites, but this has yet to be tested. 相似文献
939.
940.
Scott Gordon Bärbel Koribalski Sally Houghton Keith Jones 《Monthly notices of the Royal Astronomical Society》2000,315(2):248-262
The data reduction process for optical emission-line observations of galaxies using the TAURUS-2 Fabry–Perot interferometer mounted on the 3.9-m Anglo-Australian Telescope is described in detail. The initial steps (bias subtraction, flat-fielding, etc.) are the same as for calibration of CCD images, and the wavelength calibration is similar to that in optical spectroscopy. The final steps are specific to Fabry–Perot instruments, and include the fitting of several instrumental parameters and a phase correction to convert the raw ( x , y , z ) data cube into a useful position–velocity ( α , δ , v ) cube. Software has been written to assist with the latter steps of the data reduction. H α observations of NGC 1808, NGC 2442 and Circinus are used to demonstrate the reduction process. 相似文献