FEM solution of the Navier-Stokes equations for vortex shedding behind a cylinder: experiments with the four-node element     

Philip M. Gresho  Robert L. Lee  Craig D. Upson 《Advances in water resources》1981,4(4):175-184

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Measurement of the effect of particle size variation on the detrital remanent magnetization to anhysteretic remanent magnetization ratio in some abyssal sediments     

Craig Amerigian 《Earth and Planetary Science Letters》1977,36(3):434-442

Deposition experiments have been carried out to measure the effect of particle size variation on the relationship between detrital remanent magnetization (DRM), anhysteretic remanent magnetization (ARM), and geomagnetic field intensity in sediments.Foraminiferal ooze from a box core taken in the Columbian Basin south of Jamaica was separated into several particle size ranges and redeposited in the laboratory in known magnetic fields. The intensity and alternating field (AF) demagnetization characteristics of the DRM and a subsequently applied ARM were compared for the various particle size ranges.The results show a variation of DRM/ARM ratios with particle size. The DRM intensities and directions indicate that particle sizes greater than 38 μm do not contribute significantly to the DRM of the total sediment. ARM intensities for larger particle sizes and particle size analysis of the whole sediment indicate that the fraction greater than 38 μm does make a significant contribution to the total ARM of a sample. Use of the DRM/ARM ratio in experimental measurements of magnetic paleointensity indicates that the method is unsatisfactory for sediments having a significant fraction of magnetic particles larger than 38 μm. It is also shown that, for sediments having a significant fraction of high-coercivity magnetic grains, the relative orientation of the ARM and DRM will affect ARM intensities, making necessary the use of corrected ARM and DRM intensities for ratio calculations.  相似文献   

Formal subdivision of the Holocene Series/Epoch: a Discussion Paper by a Working Group of INTIMATE (Integration of ice‐core,marine and terrestrial records) and the Subcommission on Quaternary Stratigraphy (International Commission on Stratigraphy)     

M. J. C. Walker  M. Berkelhammer  S. Björck  L. C. Cwynar  D. A. Fisher  A. J. Long  J. J. Lowe  R. M. Newnham  S. O. Rasmussen  H. Weiss 《第四纪科学杂志》2012,27(7):649-659

This discussion paper, by a Working Group of INTIMATE (Integration of ice‐core, marine and terrestrial records) and the Subcommision on Quaternary Stratigraphy (SQS) of the International Commission on Stratigraphy (ICS), considers the prospects for a formal subdivision of the Holocene Series/Epoch. Although previous attempts to subdivide the Holocene have proved inconclusive, recent developments in Quaternary stratigraphy, notably the definition of the Pleistocene–Holocene boundary and the emergence of formal subdivisions of the Pleistocene Series/Epoch, mean that it may be timely to revisit this matter. The Quaternary literature reveals a widespread but variable informal usage of a tripartite division of the Holocene (‘early’, ‘middle’ or ‘mid’, and ‘late’), and we argue that this de facto subdivision should now be formalized to ensure consistency in stratigraphic terminology. We propose an Early–Middle Holocene Boundary at 8200 a BP and a Middle–Late Holocene Boundary at 4200 a BP, each of which is linked to a Global Stratotype Section and Point (GSSP). Should the proposal find a broad measure of support from the Quaternary community, a submission will be made to the International Union of Geological Sciences (IUGS), via the SQS and the ICS, for formal ratification of this subdivision of the Holocene Series/Epoch. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Glacial stratigraphy of East and Central Jutland     

Christian  Kronborg  Henry  Bender  Regnar  Bjerre  Rud  Friborg  Hans O.  Jacobsen  Lars  Kristiansen  Poul  Rasmussen  Poul R.  Sørensen Gunnar  Larsen 《Boreas: An International Journal of Quaternary Research》1990,19(3):273-287

Glaciostratigraphic investigations at one key locality (Haldum), 9 major and about 160 minor localities in East and Central Jutland, Denmark, together with laboratory work, have led to the establishment of a stratigraphy consisting of 10 till units, usually separated by meltwater deposits. The stratigraphy is in some degree supported by thermoluminescence datings. The complete sequence includes one till unit with associated meltwater deposits of Menapian age, three till units with intercalated meltwater deposits of Elsterian age, marine sediments deposited during the Holsteininan, and three till units with intercalated of Elsterian age, marine sediments deposited during the Holsteinian, and three till units with intercalated glaciofluvial sedimants of Saalian age. Eemian deposits are present above this level, and the whole sequence is capped by till and meltwater deposits related to three glacial advances during the Weichselian.  相似文献   

Biogeochemical and hydrographic controls on chromophoric dissolved organic matter distribution in the Pacific Ocean     

Chantal M. Swan  David A. Siegel  Norman B. Nelson  Craig A. Carlson  Elora Nasir 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(12):2175-2192

Recent in situ observations of chromophoric dissolved organic material (CDOM) in the Pacific Ocean reveal the biogeochemical controls on CDOM and indicate predictive potential for open-ocean CDOM in diagnosing particulate organic matter (POM) remineralization rates within ocean basins. Relationships between CDOM and concentrations of dissolved oxygen, nutrients and inorganic carbon in the subthermocline waters of the Pacific reflect the relative influences of water mass ventilation and water-column oxidative remineralization. Apparent in situ oxygen utilization (AOU) accounts for 86% and 61% of variance in CDOM abundance, respectively, in Antarctic Intermediate Water and North Pacific Intermediate Water. In the deep waters of the Pacific below the zone of remineralization, AOU explains 26% of CDOM variability. The AOU–CDOM relationship results from competing biogeochemical and advective processes within the ocean interior. Dissolved organic carbon (DOC) is not statistically linked to the CDOM or AOU distributions, indicating that the majority of CDOM production occurs during the remineralization of sinking POM and thus potentially provides key information about carbon export. Once formed in the ocean interior, CDOM is relatively stable until it reaches the surface ocean where it is destroyed by solar bleaching. Susceptibility to bleaching confers an additional tracer-like quality for CDOM in water masses with active convection, such as mode waters that appear as subsurface CDOM minima. In the surface ocean, atypically low CDOM abundance highlights a region of unusually extreme oligotrophy: the subtropical South Pacific gyre. For these hyper-oligotrophic waters, the present CDOM observations are consistent with analysis of in situ radiometric observations of light attenuation and reflectance, demonstrating the accuracy of the CDOM spectrophotometric observations. Overall, we illustrate how CDOM abundance in the ocean interior can potentially diagnose rates of thermohaline overturning as they affect regional biogeochemistry and export. We further show how relative surface ocean CDOM abundances are driven in large part by processes occurring in the deep layers of the ocean. This is particularly significant for the interpretation of the global surface distribution of CDOM using satellite remote sensing.  相似文献   

Observations of Fire–Atmosphere Interactions and Near-Surface Heat Transport on a Slope     

Craig B. Clements  Daisuke Seto 《Boundary-Layer Meteorology》2015,154(3):409-426

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Three-dimensional benchmark for variable-density flow and transport simulation: matching semi-analytic stability modes for steady unstable convection in an inclined porous box     

Clifford I. Voss  Craig T. Simmons  Neville I. Robinson 《Hydrogeology Journal》2010,18(1):5-23

This benchmark for three-dimensional (3D) numerical simulators of variable-density groundwater flow and solute or energy transport consists of matching simulation results with the semi-analytical solution for the transition from one steady-state convective mode to another in a porous box. Previous experimental and analytical studies of natural convective flow in an inclined porous layer have shown that there are a variety of convective modes possible depending on system parameters, geometry and inclination. In particular, there is a well-defined transition from the helicoidal mode consisting of downslope longitudinal rolls superimposed upon an upslope unicellular roll to a mode consisting of purely an upslope unicellular roll. Three-dimensional benchmarks for variable-density simulators are currently (2009) lacking and comparison of simulation results with this transition locus provides an unambiguous means to test the ability of such simulators to represent steady-state unstable 3D variable-density physics.  相似文献   

Assessment of channel dynamics,in-stream structures and post-project channel adjustments in North Carolina and its implications to effective stream restoration   总被引：3，自引：3，他引：0  

Jerry R. Miller  R. Craig Kochel 《Environmental Earth Sciences》2010,59(8):1681-1692

Site assessment and monitoring data were analyzed for 26 stream restoration projects in North Carolina where the channel was reconfigured. Post-project changes in channel capacity were highly variable from site to site, but more than 60% of the projects underwent, on average at a given site, at least a 20% change in channel capacity. An analysis of site and basin geomorphology revealed that large post-construction adjustments were associated with highly dynamic stream channels characterized by a combination of high sediment transport capacity, large sediment supply, and/or easily eroded bank materials. In-stream structures along dynamic, reconfigured channels also exhibited high incidences of damage. Thus, the design and construction of channels in a state of equilibrium, which do not exhibit excessive erosion or deposition along highly dynamic rivers is currently problematic. In light of these findings, a conceptual framework based on geomorphic parameters is put forth to assess the likelihood of project success early in the design process to (1) eliminate high risk sites from consideration of channel reconfiguration and (2) improve upon the implemented management strategies that are ultimately used. It is also argued that where space permits an enhanced natural channel, adjustment approach is likely to be more effective than projects based on natural channel design.  相似文献   

Scenarios on future land changes in the West African Sahel     

Eric F. Lambin  Sarah Ann Lise D’haen  Ole Mertz  Jonas Østergaard Nielsen  Kjeld Rasmussen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2014,114(1):76-83

In an attempt to anticipate possible futures of drylands of West Africa in the face of rapid socio-economic and environmental changes, we developed four scenarios based on recent survey data, the literature and our knowledge of the region. The four scenarios are inspired by those developed by the Millennium Ecosystem Assessment: (1) ‘downward spiral’ characterized by rapid climate change, expansion of agriculture and chaotic urban growth; (2) ‘integrated economy’ with integrated land management, food production for local markets and rural–urban exchanges; (3) ‘open doors’ characterized by large-scale out-migrations, land grabbing by foreign companies and development aid and (4) ‘climate change mitigation’ with an increase in biofuel crops, land management for carbon capture and development of off-farm activities. We conclude that the Sahel region is most likely moving away from being a highly climate-dependent region based on agriculture towards a more open and diversified economy. West African countries have to find a balance between the new opportunities and risks created by economic globalization.  相似文献   

Modern influences on chironomid distribution in western Ireland: potential for palaeoenvironmental reconstruction     

Aaron P. Potito  Craig A. Woodward  Michelle McKeown  David W. Beilman 《Journal of Paleolimnology》2014,52(4):385-404

Ireland provides a unique setting for the study of past climates, as its climate is dominated by westerly airflow from the North Atlantic and readily responsive to changes in North Atlantic circulation patterns. Although there has been substantial research on Ireland’s past environments, quantitative palaeolimnological research, especially chironomid-based research, has been lacking. In order to further develop chironomid-based palaeolimnological investigations, a calibration set was constructed to determine the dominant environmental controls on modern chironomids in western Ireland. Chironomid subfossils were collected from surface sediments of 50 lakes. The lakes were characterised with 36 environmental variables, including physical attributes, lake water characteristics, lake sediment characteristics and land cover within each catchment. In this exploratory study, no specific environmental variable was targeted and lakes were chosen to span gradients of latitude, elevation, depth and trophic status. Redundancy analysis showed that six environmental variables—mean July air temperature, lake depth, dissolved organic carbon, and percentage catchment land cover of agriculture, peat bog and scrubland—captured a large and statistically significant portion of the variance in the chironomid data. July temperature and agricultural land cover were the most dominant environmental variables, with July temperature proving the most suitable for inference model development. A classical weighted-averaging model was developed to estimate July air temperature, with a coefficient of determination (r _jack ² ) of 0.60 and root mean square error of prediction (RMSEP) of 0.57 °C. Results suggest that summer temperature is the dominant influence on chironomid distribution across a wide variety of lake types, and the relatively small RMSEP should allow for more accurate reconstructions of Ireland’s relatively subdued Holocene temperature fluctuations.  相似文献