排序方式： 共有15条查询结果，搜索用时 15 毫秒
1.
Two Hercynian duplexes are developed in Viséan limestones in the Basse Normandie quarry. The lower duplex is completely exposed in a subvertical quarry wall; the partially exposed upper duplex lies immediately above the lower duplex. The duplexes are both located in the footwall of the Hydrequent thrust which emplaced Devonian clastic sediments above the Viséan limestones. The lower duplex exposes all the internal thrusts, a reference bed of chalky limestone, the roof and floor thrusts, and the duplex tip. The duplex has been graphically restored to its pre-deformation geometry by line-length and area balancing and its resultant geometry is close to the model of Boyer & Elliott. The lower duplex shortened by two different mechanisms, an initial phase of layer-parallel shortening which produced no cleavage, followed by thrust imbrication. The average contraction of the front portion of the duplex was ?49% (natural strain) of which ?27% is layer-parallel shortening and ?22% is thrust imbrication. However, locally the bulk shortening increases from zero at the duplex tip to over ?120% in a down-dip direction. The area balancing provides the most accurate estimates of bulk shortening; line-length balance calculations give minimum estimates only. An area balance on the whole of the lower duplex gives a bulk shortening of ?84%. An area balance of the upper duplex yields an average contraction of ?75% and the total contraction produced by both duplexes is ?92%. 相似文献
2.
Jonathan A. Warrick David M. Rubin Peter Ruggiero Jodi N. Harney Amy E. Draut Daniel Buscombe 《地球表面变化过程与地形》2009,34(13):1811-1821
A new application of the autocorrelation grain size analysis technique for mixed to coarse sediment settings has been investigated. Photographs of sand‐ to boulder‐sized sediment along the Elwha River delta beach were taken from approximately 1·2 m above the ground surface, and detailed grain size measurements were made from 32 of these sites for calibration and validation. Digital photographs were found to provide accurate estimates of the long and intermediate axes of the surface sediment (r2 > 0·98), but poor estimates of the short axes (r2 = 0·68), suggesting that these short axes were naturally oriented in the vertical dimension. The autocorrelation method was successfully applied resulting in total irreducible error of 14% over a range of mean grain sizes of 1 to 200 mm. Compared with reported edge and object‐detection results, it is noted that the autocorrelation method presented here has lower error and can be applied to a much broader range of mean grain sizes without altering the physical set‐up of the camera (～200‐fold versus ～6‐fold). The approach is considerably less sensitive to lighting conditions than object‐detection methods, although autocorrelation estimates do improve when measures are taken to shade sediments from direct sunlight. The effects of wet and dry conditions are also evaluated and discussed. The technique provides an estimate of grain size sorting from the easily calculated autocorrelation standard error, which is correlated with the graphical standard deviation at an r2 of 0·69. The technique is transferable to other sites when calibrated with linear corrections based on photo‐based measurements, as shown by excellent grain‐size analysis results (r2 = 0·97, irreducible error = 16%) from samples from the mixed grain size beaches of Kachemak Bay, Alaska. Thus, a method has been developed to measure mean grain size and sorting properties of coarse sediments. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
3.
Christopher H. Gammons Lica Milodragovich Jodi Belanger-Woods 《Environmental Geology》2007,53(3):611-622
Diurnal water samples were collected simultaneously at four locations along High Ore Creek (Montana, USA), a small stream
with near-neutral pH that contains elevated concentrations of Zn, Mn, Cd, and As from abandoned mines near its headwaters.
During the same time period, two sets of synoptic samples were collected by workers moving in opposite directions along the
stream. Large diurnal fluctuations in Zn concentration were found at three of the 24-h monitoring stations, but not at the
outlet to a settling pond. Because the concentrations of Zn were dropping at most locations in the creek during the day (in
response to the daily cycle of day-time attenuation and night-time release), the synoptic sampler who moved upstream obtained
a data set that led to the conclusion that Zn load increased with distance downstream. The sampler who moved in a downstream
direction obtained the opposite results. Thus, failure to take short-term diurnal cycling into account can lead to incorrect
conclusions regarding spatial or temporal trends in water quality within a watershed. 相似文献
4.
Amy?L?Ryser Daniel?G?StrawnEmail author Matthew?A?Marcus Jodi?L?Johnson-Maynard Mickey?E?Gunter Gregory?M?ller 《Geochemical transactions》2005,6(1):1
Mining activities in the US Western Phosphate Resource Area (WPRA) have released Se into the environment. Selenium has several
different oxidation states and species, each having varying degrees of solubility, reactivity, and bioavailability. In this
study we are investigating the speciation of Se in mine-waste rocks. Selenium speciation was determined using bulk and micro-x-ray
absorption spectroscopy (XAS), as well as micro-x-ray fluorescence mapping. Rocks used for bulk-XAS were ground into fine
powders. Shale used for micro-XAS was broken along depositional planes to expose unweathered surfaces. The near edge region
of the XAS spectra (XANES) for the bulk rock samples revealed multiple oxidation states, with peaks indicative of Se(-II),
Se(IV), and Se(+VI) species. Micro-XANES analysis of the shale indicated that three unique Se-bearing species were present.
Using the XANES data together with ab initio fitting of the extended x-ray absorption fine structure region of the micro-XAS data (micro-EXAFS) the three Se-bearing species
were identified as dzharkenite, a di-selenide carbon compound, and Se-substituted pyrite. Results from this research will
allow for a better understanding of the biogeochemical cycling of Se in the WPRA. 相似文献
5.
Collen JD Baker JA Dunbar RB Rieser U Gardner JP Garton DW Christiansen KJ 《Marine pollution bulletin》2011,62(2):251-257
Anthropogenic lead (Pb) inputs to the atmosphere increased greatly over the past century and now dominate Pb supply to the oceans. However, the Pb content of sediments across the equatorial Pacific region is relatively unknown, and data exist only for deep sea sites where Pb deposition lags surface water inputs by up to a century. Here we present ICP-MS analyses of Pb of a core from a lagoon at Palmyra Atoll, northern Line Islands, that spans approximately the past 160 years. The non-bioturbated sediments of the euxinic lagoon, coupled with rapid rates of deposition, provide a unique fine-scale record of atmospheric Pb supply at a remote Pacific location. These first observations of historic Pb sedimentation in an atoll lagoon reveal a 63-fold increase in Pb flux to sediments during the past century and correlate directly with the North American consumption of leaded gasoline that began in 1926. 相似文献
6.
7.
Late Quaternary histories of two North American desert biomes—C4 grasslands and C3 shrublands—are poorly known despite their sensitivity and potential value in reconstructing summer rains and winter temperatures. Plant macrofossil assemblages from packrat midden series in the northern Chihuahuan Desert show that C4 grasses and annuals typical of desert grassland persisted near their present northern limits throughout the last glacial–interglacial cycle. By contrast, key C3 desert shrubs appeared somewhat abruptly after 5000 cal. yr BP. Bioclimatic envelopes for select C4 and C3 species are mapped to interpret the glacial–interglacial persistence of desert grassland and the mid‐to‐late Holocene expansion of desert shrublands. The envelopes suggest relatively warm Pleistocene temperatures with moist summers allowed for persistence of C4 grasses, whereas winters were probably too cold (or too wet) for C3 desert shrubs. Contrary to climate model results, core processes associated with the North American Monsoon and moisture transport to the northern Chihuahuan Desert remained intact throughout the last glacial–interglacial cycle. Mid‐latitude effects, however, truncated midsummer (July–August) moisture transport north of 35° N. The sudden expansion of desert shrublands after 5000 cal. yr BP may be a threshold response to warmer winters associated with increasing boreal winter insolation, and enhanced El Niño–Southern Oscillation variability. Published in 2006 by John Wiley & Sons, Ltd. 相似文献
8.
Jodi C. Beattie Russell L. Elsberry 《Asia-Pacific Journal of Atmospheric Sciences》2016,52(5):413-435
The objective of this study is to describe how a monsoon depression in the western North Pacific, which typically has a diameter of 1000 km, may be transitioned into a tropical cyclone with an inner core of strong winds and deep convection on the order of 100 km. Our previous case study of the pre-Typhoon Man-Yi monsoon depression formation is extended to show that the same cross-equatorial airstream continued and led to enhanced equatorial westerlies on the equatorward side of the pre-Man-Yi circulation, and a surge in the trade easterlies was also present on the poleward side. As these surges in the near-equatorial flow are inertially unstable, inward-directed wave-activity fluxes then led to flux convergence over the eastern vorticity maximum of the monsoon depression, which resulted in a scale contraction to that of a pretropical cyclone seedling. Eight case studies of the transitions of monsoon depressions during 2009 are presented that document persistent inward-directed wave-activity fluxes over a vorticity maximum within the monsoon depression is a key feature of each transition. In some transitions, the same cross-equatorial airstream as led to the monsoon depression formation continues as the primary airstream, and in other transitions another airstream to the west or enhanced tropical easterlies become the primary airstream. Analysis of 10 non-transitioning monsoon depressions during 2009 indicated the airstream wave-activity flux did not persist after the formation of the monsoon depression. In another 11 non-transitioning monsoon depressions, the inward-directed wave-activity flux was small and no region of wave-activity flux convergence was associated with the vorticity maximum in the monsoon depression. 相似文献
9.
Patrick L. Barnard Maarten van Ormondt Li H. Erikson Jodi Eshleman Cheryl Hapke Peter Ruggiero Peter N. Adams Amy C. Foxgrover 《Natural Hazards》2014,74(2):1095-1125
The Coastal Storm Modeling System (CoSMoS) applies a predominantly deterministic framework to make detailed predictions (meter scale) of storm-induced coastal flooding, erosion, and cliff failures over large geographic scales (100s of kilometers). CoSMoS was developed for hindcast studies, operational applications (i.e., nowcasts and multiday forecasts), and future climate scenarios (i.e., sea-level rise + storms) to provide emergency responders and coastal planners with critical storm hazards information that may be used to increase public safety, mitigate physical damages, and more effectively manage and allocate resources within complex coastal settings. The prototype system, developed for the California coast, uses the global WAVEWATCH III wave model, the TOPEX/Poseidon satellite altimetry-based global tide model, and atmospheric-forcing data from either the US National Weather Service (operational mode) or Global Climate Models (future climate mode), to determine regional wave and water-level boundary conditions. These physical processes are dynamically downscaled using a series of nested Delft3D-WAVE (SWAN) and Delft3D-FLOW (FLOW) models and linked at the coast to tightly spaced XBeach (eXtreme Beach) cross-shore profile models and a Bayesian probabilistic cliff failure model. Hindcast testing demonstrates that, despite uncertainties in preexisting beach morphology over the ~500 km alongshore extent of the pilot study area, CoSMoS effectively identifies discrete sections of the coast (100s of meters) that are vulnerable to coastal hazards under a range of current and future oceanographic forcing conditions, and is therefore an effective tool for operational and future climate scenario planning. 相似文献
10.
The recognition of littoral sediment cells has proven valuable to coastal zone management in many areas of the world because it emphasises that sediment transport and deposition relate to physical processes rather than to administrative boundaries and that sedimentary units are self-contained. To date, cells identified for shoreline management planning are often extensive, extending for tens to hundreds of km. We suggest that the concept is also applicable to the management of atoll coastlines which, although spatially very small overall, consist of isolated segments with efficient intervening sediment sinks. As an example, Palmyra Atoll, a protected wildlife refuge in the northern Line Islands, is undergoing natural major changes resulting from sediment-related processes. Defining sediment cells as integral parts of management units here will aid decision-making necessary for the protection of specific areas, potentially giving savings in research effort and remedial costs and minimising disruption to other sensitive areas of the reserve. Similar benefits would apply to the management of the coastlines of other atolls. 相似文献