Holocene climate and environmental history of East Greenland inferred from lake sediments     

Amanda R. Lusas  Brenda L. Hall  Thomas V. Lowell  Meredith A. Kelly  Ole Bennike  Laura B. Levy  William Honsaker 《Journal of Paleolimnology》2017,57(4):321-341

Prediction of future Arctic climate and environmental changes, as well as associated ice-sheet behavior, requires placing present-day warming and reduced ice extent into a long-term context. Here we present a record of Holocene climate and glacier fluctuations inferred from the paleolimnology of small lakes near Istorvet ice cap in East Greenland. Calibrated radiocarbon dates of organic remains indicate deglaciation of the region before ~10,500 years BP, after which time the ice cap receded rapidly to a position similar to or less extensive than present, and lake sediments shifted from glacio-lacustrine clay to relatively organic-rich gyttja. The lack of glacio-lacustrine sediments throughout most of the record suggests that the ice cap was similar to or smaller than present throughout most of the Holocene. This restricted ice extent suggests that climate was similar to or warmer than present, in keeping with other records from Greenland that indicate a warm early and middle Holocene. Middle Holocene magnetic susceptibility oscillations, with a ~200-year frequency in one of the lakes, may relate to solar influence on local catchment processes. Following thousands of years of restricted extent, Istorvet ice cap advanced to within 365 m of its late Holocene limit at ~AD 1150. Variability in the timing of glacial and climate fluctuations, as well as of sediment organic content changes among East Greenland lacustrine records, may be a consequence of local factors, such as elevation, continentality, water depth, turbidity, and seabirds, and highlights the need for a detailed spatial array of datasets to address questions about Holocene climate change.  相似文献   

UV survey of δ Scuti stars: Final results and open problems     

Laura E. Pasinetti Fracassini  Livio Pastori 《Astrophysics and Space Science》1996,240(2):205-213

The results obtained from our IUE observations of some Scuti stars and in particular 71 Tau and 69 Tau are reviewed. 71 Tau is the second brightest X-ray source of the Hyades cluster; on the contrary 69 Tau, very similar to 71 Tau in the optical range, does not show X emissions. The complex and in somewhat controversial scenario obtained from this survey is also discussed. In particular we consider the X-ray emission detected in some Scuti with respect to the chromospheric activity level as measured through the MgII line and to other stellar parameters.  相似文献   

The role of superheating in the formation of Glass Mountain obsidians (Long Valley,CA) inferred through crystallization of sanidine     

Laura E. Waters  Benjamin J. Andrews 《Contributions to Mineralogy and Petrology》2016,171(10):79

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Spatial and Temporal Variability in Estuary Habitat Use by American Alligators     

Ikuko Fujisaki  Kristen M. Hart  Michael S. Cherkiss  Frank J. Mazzotti  Jeff S. Beauchamp  Brian M. Jeffery  Laura A. Brandt 《Estuaries and Coasts》2016,39(5):1561-1569

Estuarine habitat occupied by Alligator mississippiensis, a primarily freshwater species, is spatially and temporally heterogeneous largely due to a salinity gradient that fluctuates. Using long-term night light survey data, we examined seasonal patterns in alligators’ habitat use by size classes in midstream and downstream estuary zones of Shark River, Everglades National Park, in southern Florida. We observed predominantly large-sized alligators (total length?≥?1.75 m); observations of alligators in the small size classes (0.5 m?≤?total length?<?1.25 m) were rare especially in the higher-salinity downstream zone. The density of alligators in the downstream zone was lower than that of the midstream zone during the dry season when salinity increases due to reduced precipitation. Conversely, the density of the large size alligators was higher in the downstream zone than in the midstream zone during the wet season, likely because of reduced salinity. We also found a significant declining trend over time in the number of alligators in the dry season, which coincides with the reported decline in alligator relative density in southern Florida freshwater wetlands. Our results indicated high adaptability of alligators to the fluctuating habitat conditions. Use of estuaries by alligators is likely driven in part by physiology and possibly by reproductive cycle, and our results supported their opportunistic use of estuary habitat and ontogenetic niche shifts.  相似文献   

Time-dependent ground movements induced by shield tunneling in soft clay: a parametric study     

Elisabetta Cattoni  Chiara Miriano  Laura Boco  Claudio Tamagnini 《Acta Geotechnica》2016,11(6):1385-1399

In this work, the effects of coupled hydromechanical (consolidation) processes associated with shield tunneling excavation in soft clays are investigated with particular attention to the prediction of ground movements at the ground surface. A series of 2d FE analyses have been carried out in parametric form in order to investigate the effects of tunnel excavation velocity relative to the soil consolidation rate and the hydraulic boundary conditions at the tunnel boundary. The shield advancement process has been simulated with a simplified procedure incorporating both volume loss and ovalization of the tunnel section. In order to investigate the relative importance of soil consolidation during the excavation process, different characteristic times for the tunnel face advancement and for the consolidation process around the tunnel have been considered, for the two limiting conditions of fully permeable liner and impervious liner. The potential damage induced by the tunnel excavation on existing structures, based on computed ground surface distortions and horizontal deformations, has been found to vary significantly with time during the consolidation process. The results of the simulations allowed to obtain useful information on the minimum tunnel face advancement speed for which the assumption of fully undrained conditions for the soil during the excavations is acceptable, as well as on the speed range for which solving the fully coupled hydromechanical problem is necessary.  相似文献   

Climatic and halokinetic controls on alluvial–lacustrine sedimentation during compressional deformation,Andean forearc,northern Chile          下载免费PDF全文

Laura A. Evenstar  Adrian J. Hartley  Stuart G. Archer  Joyce E. Neilson 《Basin Research》2016,28(5):634-657

The Salar de Atacama forms one of a series of forearc basins developed along the western flank of the Central Andes. Exposed along the northwest margin of the basin, a salt‐cored range, the Cordillera de la Sal, records the Mid‐Miocene to recent sedimentological and structural development of this basin. Sediments of the Mid‐Miocene Vilama Formation record the complex interaction between regional/local climate change, halokinesis and compressional deformation. This study reveals how these factors have controlled the facies development and distribution within the Salar de Atacama. Detailed sedimentary logging, cross‐sections and present day geomorphology through the northern Cordillera de la Sal have been used to establish a lithostratigraphy, chronostratigraphy and the regional distribution of the Vilama Formation. The Vilama Formation documents an increase in aridity with a hiatus in sedimentation from Mid‐Miocene to 9 Ma with initial uplift of the Cordillera de la Sal. From 9 Ma to 8.5 Ma deposition of a meandering fluvial system is recorded followed by a rapid decrease in sedimentation till 6 Ma. From 6 to 2 Ma, the deposition of extensive palustrine carbonates and distal alluvial–mudflat–lacustrine demonstrates the existence of an extensive lake within the Salar de Atacama. Post 2 Ma, the lake decreased in size and braided alluvial gravels associated with alluvial fans were widespread through the region suggesting a final shift to hyperarid conditions. By comparing the Vilama Formation with similar age facies throughout northern Chile and southern Peru, several shifts in climate are recognized. Climate signatures within northern Chile appear to be largely diachronous with the last regional event in the Mid‐Miocene. Since that time, humid events have been restricted to either Precordillerian basins or the Central Atacama. Within the Central Atacama, the final switch to hyperarid conditions was not till the earliest Pleistocene, much later than previously estimated within the region.  相似文献   

Some additional results on vibrating, simply supported rectangular plates with free, rectangular cut-outs     

P. A. A. Laura  B. H. Valerga de Greco  J. C. Utjes 《Ocean Engineering》1986,13(5)

The present study deals with numerical experiments performed using the doublesine Fourier expansion and the Ritz method to determine natural frequencies of vibration of simply supported rectangular plates with free rectangular holes parallel to the plate sides.The problem is of basic structural interest in naval and ocean engineering systems since holes are frequently practiced in plate-type structural elements to allow for the passage of ducts, elevators, etc…  相似文献   

A low-cost fiber-optic system for monitoring the state of structural health of a mechanical cable     

Daniel Cortázar  Hilda A. Larrondo  Patricio A.A. Laura  Daniel R. Avalos 《Ocean Engineering》1996,23(2):193-199

Methods for evaluating the structural health of mechanical cables and detecting their imminent failure could prevent the loss of valuable equipment and, more importantly, the possible loss of human life.The non-destructive test methods available are: thorough visual examination and measurement of the external diameter; X-rays; (induced) wave propagation; acoustic emission; magnetostrictive sensors; infrared detection.A new method which employs a commercial optical fiber for detecting the breakage of individual wires in a rope is proposed in the present paper.  相似文献   

Theory of Wire Rope: by George A. Costello, Second Edition pp. 122, 1997. Springer: New York, N.Y.     

P. A. A. Laura  C. A. Rossit 《Ocean Engineering》2001,28(3):343

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Natural frequencies of a vibrating repaired panel in an ocean structure     

D.H. FelixD.V. Bambill  C.A. RossitR.E. Rossi  P.A.A. Laura 《Ocean Engineering》2003,30(8):955-963

The present work arose from the need of repairing steel plates badly damaged by corrosion in a portion of the structural element of the naval vehicle. The possibility of removing the portion and replacing it by a patch of a composite material was considered. Its dynamic behavior is altered by the introduction of the patch and the prediction of its new behavior is of great interest in many situations. This condition would appear in other real-life situations like as a localized orthotropic effect caused in the panel by a welding procedure or a metallurgical process. The first four natural frequency coefficients of the composite repaired panel with different types of boundary conditions are determined by means of a variational approach. The displacement function is approximated making use of complete sets of beam functions. The eigenvalues have been computed from (225×225) secular determinants. An independent solution is obtained using the finite element method and a reasonably good agreement with the analytical solution is encountered.  相似文献