Integrating neo- and paleolimnological approaches to refine interpretations of environmental change     

Jasmine E. Saros 《Journal of Paleolimnology》2009,41(2):243-252

With the development of quantitative transfer functions to relate community structure to physicochemical variables, reconstructions of past environmental conditions have been possible and have enhanced our understanding of various ecosystem processes. There are cases, however, in which this approach is not applicable, or does not provide enough information for the questions being asked. In these cases, some alternatives are to conduct experiments or to examine the distribution of species at a finer spatial resolution. These two approaches have been used as alternatives to or in conjunction with the development of transfer functions. In this review, I discuss the ways in which these two approaches are now being used in paleolimnological studies to enhance our understanding of the ecology of the species found in sediment records and thus refine interpretations of environmental change. My focus is primarily on studies that deal with establishing clearer relationships between environmental variables and the growth or distribution of organisms. I present examples of how these approaches have been integrated in a variety of studies, including those designed to: (1) refine and enhance reconstructions that are based on transfer functions; (2) develop new paleolimnological tools to reconstruct environmental change; (3) explore mechanistic links in the relationships between organisms and commonly reconstructed environmental variables; and (4) pose and test hypotheses based on patterns in the sediment record. These cases demonstrate that the use of these approaches was essential to clarify species-environment relationships as well as lake responses to disturbance. As in all disciplines, however, there are many challenges in this area of research. In particular, the quantitative integration of these approaches with the sediment record is a major challenge, due to disparate spatial and temporal scales. This research can also be quite labor-intensive, and provides information on fewer taxa than in the calibration set approach. It also requires interdisciplinary training and/or collaboration in fields that have historically been less integrated, hence they may require greater effort. These issues may hinder the use of these approaches because of the perceived difficulty. I discuss these challenges and address possible solutions.  相似文献   

Diatom-inferred lake level from near-shore cores in a drainage lake from the Experimental Lakes Area,northwestern Ontario,Canada   总被引：1，自引：0，他引：1  

Kathleen R. Laird  Brian F. Cumming 《Journal of Paleolimnology》2009,42(1):65-80

We inferred late Holocene lake-level changes from a suite of near-shore gravity cores collected in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area, northwestern Ontario. Results were reproduced across all cores. A gravity core from the deep central basin was very similar to the near-shore cores with respect to trends in the percent abundance of the dominant diatom taxon, Cylcotella stelligera. The central basin, however, does not provide a sensitive site for reconstruction of lake-level changes because of the insensitivity of the diatom model at very high percentages of C. stelligera and other planktonic taxa. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several depth transects in Lake 239. The lake-level reconstructions during the past ~3,000 years indicate that lake depth varied on average by ±2 m from present-day conditions, with maximum rises of ~3–4 m and maximum declines of ~3.5–5 m. The diatom-inferred depth record indicates several periods of persistent low levels during the nineteenth century, from ~900 to 1100 AD, and for extended periods prior to ~1,500 years ago. Periods of inferred high lake levels occurred from ~500 to 900 AD and ~1100 to 1650 AD. Our findings suggest that near-shore sediments from small drainage lakes in humid climates can be used to assess long-term fluctuations in lake level and water availability.  相似文献   

Experimental and numerical analyses of vibrations induced by high-speed trains on the Córdoba–Málaga line     

P. Galvín  J. Domínguez 《Soil Dynamics and Earthquake Engineering》2009

This paper has two main purposes. One is to present and analyse soil and structural vibration data obtained experimentally during certification testing of the high-speed train line between Córdoba and Málaga (Spain) that was opened on December 2007. The second is to show the capabilities of a three-dimensional boundary element method (BEM)/finite element method (FEM) numerical approach for the analysis of train induced vibrations. The model can represent local soil conditions, discontinuities such as underpasses, as well as structures placed next to the rail track. Vibrations in those structures can be computed taking into account, in a rigorous way, dynamic soil–structure interaction and local soil properties. Experimental and numerical results at several points near the track are compared. Results for an overhead contact support structure are also evaluated. The comparison of numerically predicted and recorded results shows that the model is reliable for predicting the amplitude of vibrations produced in the soil and nearby structures by high-speed trains.  相似文献   

Improvement and Application of Riedel Shear Systerm     

Hui Deng  Guoying Li  Haifeng Yang  Honglei Wen  Can Zhang 《地球科学进展》2019,34(8):868-878

For some strain in the strike slip zone is difficult to be determined accurately in the classification scheme of the 7 elements of the ellipse of the strike slip strain ellipse (PDZ, R, R', P, T, Y and local contraction), the properties of the strain elements in all directions in the strained ellipse were reunderstood by analyzing the causal relationship between the mechanical state and the strain properties in a certain direction. A new model of the Riedel shear system was proposed, which contained 4 pure strain zones (pure right walk slip, pure left walking slip, pure extrusion, pure Stretch Strain Zone) and 4 complex strain zones (both stretching and right walking, both stretching and left walking, both extrusion and left walking, both extrusion and right walking slip strain zone). The closer the crack was to the direction of a single strain band, the stronger the corresponding property. The model included all the elements of the traditional model and made up for its vacancy. It proved the rationality of the left (right) row left (right) order and the double tectonic belt rule. At the same time, three research examples of Bohai Sea were cited to illustrate the application of the model: (1) The synchronous and perpendicular spatial-temporal relationship between compressive and extensional stress fields in strike-slip zones, as well as the unified but not contradictory dialectical relationship between them, were illustrated through the example of the co-controlled cycle formation of northeastern depression in Laizhou Bay Depression; (2) The Bohai Sea walk was enumerated. The research results of the slip transition zone illustrated the strain trend law of the extensional or compressive transition zone in the strike-slip zone; (3) The relationship between the strike of the effective fracture and the extensional strain zone in the present strike-slip stress field was illustrated by giving an example of the effective fracture research results of a buried hill structure in the Bohai Sea.  相似文献   

Multi-scale study of permeability evolution of a bentonite clay owing to pollutant transport: Part II. Application to an MG-bentonite     

F. Bouchelaghem  N. Jozja 《Engineering Geology》2009,108(3-4):286-294

This paper is concerned with the application of a multi-scale model of permeability evolution to the simulation of permeation tests of lead nitrate solutions in a compacted natural bentonite. In a previous work, the weathered hydraulic conductivity has been measured for different pollutant concentrations. Textural and structural evolutions induced by heavy metal sorption have also been investigated, therefore giving us information about the various pore spaces that may be encountered during the process of pollutant leaching. Such knowledge has been exploited during the construction of the numerical microstructures. First of all, pure water permeability tests have been simulated, this has allowed us to ascertain the representation of compacted clay before pollutant permeation. For both Prrenjas and Wyoming clays, permeation tests after pollutant permeation have then been simulated, using connected and non-connected macro-voids configurations. Different conclusions have been drawn based on simulation results for the two different clays investigated, which are supported by experimental evidence.  相似文献   

Multi-scale modelling of the permeability evolution of fine sands during cement suspension grouting with filtration     

Fatiha Bouchelaghem 《Computers and Geotechnics》2009

Fluid flow during permeation grouting of fine sands with a microcement-based grout is studied by assuming that the heterogeneous medium composed of the initial granular skeleton, filtered cement and the interstitial fluid phase can be replaced by a continuous equivalent medium at the macroscopic level. Consequently, the method of Homogenization of Periodic Structures (HPS) is used to identify the effective permeability tensor evolution under the effect of cement filtration. The expression of the macroscopic permeability tensor derived through the HPS procedure is shown to depend on the permeating fluid viscosity and the geometrical arrangement of the sand grains and cement deposit within the microstructure. Numerical computations are made using various two-dimensional and three-dimensional microstructures, and the model results are confronted with grouting experiments performed on small scale columns in the laboratory.  相似文献   

Archaeometric characterisation and origin of black coatings on prehistoric pottery     

《Chemie der Erde / Geochemistry》2019,79(3):453-466

Black or dark grey vessel surfaces with characteristic metallic lustre are recognised from various archaeological contexts throughout Europe. This feature is commonly attributed to the application of graphite-bearing coatings onto ceramic vessels. However, recent experimental studies have shown that a very similar visual effect can alternatively be achieved by polishing and, subsequently, firing in a reducing atmosphere (so-called smudging). In this study, experimentally manufactured ceramics as well as samples of prehistoric ware (dated to the Neolithic Age, late Bronze Age and early Iron Age) have been analysed to find the distinguishing features between graphite-based and non-mineral black coatings. In the low-magnification BSE images the well-preserved graphite-coated surfaces are characterized by a distinctive spotty pattern with numerous dark grey angular fragments of monomineralic or nearly monomineralic metamorphic rocks scattered in the clay groundmass. Provided that polished thin sections are skilfully prepared, individual graphite plates can be easily recognized in the near-surface layer of the sherds using polarized reflected light microscopy. A relatively homogeneous appearance is typical of the low-magnification BSE images of surfaces coated by smudging. At higher magnifications, presence of blurred darkish stains (presumably organic-derived, as indicated by elevated levels of biogenic elements) is characteristic of this kind of non-mineral black coatings. SEM-EDS and optical microscopy have been supplemented by micro-Raman spectroscopy, which allows differentiation between various carbonaceous phases.  相似文献   

Hydrodynamics of oil jets without and with dispersant: Experimental and numerical characterization     

《Applied Ocean Research》2017

In this paper, we present the analysis of an underwater horizontal oil jet experimental measurement and Computational Fluid Dynamics (CFD) using the Reynolds Averaged Navier Stokes (RANS) equations. Two oil subsurface releases were conducted: one with crude oil and another with crude oil premixed with dispersant at the dispersant to oil ratio (DOR) of 1:20. The jet profile was captured by a camera at moderate resolution, and the instantaneous velocity was measured by a Vectrino Profiler. The velocity components, turbulence kinetic energy, and turbulence dissipation rate from the experiment agreed well with those from the CFD simulation using the k-epsilon turbulence model. The spread angle of the jet was found to be around 21° and 24° from the experiment measurement, for oil without dispersant and oil with dispersant, respectively. The latter is close to the angle of miscible jets at 23°. The jet profile of oil with dispersant had a smaller buoyancy than that without dispersant, which is probably due to the large water entrainment for the oil with dispersant jet. The cross sections of the jet for both cases gradually became flattened with distance, as the plume turned upward.  相似文献   

Seismic performance of bar-mat reinforced-soil retaining wall: Shaking table testing versus numerical analysis with modified kinematic hardening constitutive model     

I. Anastasopoulos  T. GeorgarakosV. Georgiannou  V. DrososR. Kourkoulis 《Soil Dynamics and Earthquake Engineering》2010

Reinforced-soil retaining structures possess inherent flexibility, and are believed to be insensitive to earthquake shaking. In fact, several such structures have successfully survived destructive earthquakes (Northridge 1994, Kobe 1995, Kocaeli 1999, and Chi-Chi 1999). This paper investigates experimentally and theoretically the seismic performance of a typical bar-mat retaining wall. First, a series of reduced-scale shaking table tests are conducted, using a variety of seismic excitations (real records and artificial multi-cycle motions). Then, the problem is analyzed numerically employing the finite element method. A modified kinematic hardening constitutive model is developed and encoded in ABAQUS through a user-defined subroutine. After calibrating the model parameters through laboratory element testing, the retaining walls are analyzed at model scale, assuming model parameters appropriate for very small confining pressures. After validating the numerical analysis through comparisons with shaking table test results, the problem is re-analyzed at prototype scale assuming model parameters for standard confining pressures. The results of shaking table testing are thus indirectly “converted” (extrapolated) to real scale. It is shown that: (a) for medium intensity motions (typical of M_s≈6 earthquakes) the response is “quasi-elastic”, and the permanent lateral displacement in reality could not exceed a few centimeters; (b) for larger intensity motions (typical of M_s≈6.5–7 earthquakes) bearing the effects of forward rupture directivity or having a large number of strong motion cycles, plastic deformation accumulates and the permanent displacement is of the order of 10–15 cm (at prototype scale); and (c) a large number of strong motion cycles (N>30) of unrealistically large amplitude (A=1.0 g) is required to activate a failure wedge behind the region of reinforced soil. Overall, the performance of the bar-mat reinforced-soil walls investigated in this paper is totally acceptable for realistic levels of seismic excitation.  相似文献   

钱塘江双向声传播测流实验研究   总被引：1，自引：0，他引：1       下载免费PDF全文

张传正  朱小华  吴清松 《地球物理学报》2010,53(7):1741-1748

2009年4月和5月在钱塘江杭州段进行了双向声传播测流实验.两个声信号发送与接收站设置在钱塘江两岸,间距为3050 m.在声传播实验期间,沿着声传播路径还进行了12次走航ADP(Acoustic Doppler Profiler)流速观测.根据双向声传播的顺逆流传播时间差计算得到沿声传播路径的平均流速,与ADP走航观测结果基本一致,两者的均方差为0.03 m/s.通过ADP走航期间双向声传播时间差的平均值和ADP走航观测所得断面流量之间的线性关系,得到了整个观测期间断面流量的时间序列.实验较好地捕捉到了因钱塘江涌潮而引起的流量变化,流量的最小值为 -3406 m³/s,最大为3509 m³/s.第一和第二实验阶段钱塘江的净流量分别为1329 m³/s和979 m³/s.以上结果表明用双向声传播测流方法可实现对河道流速和流量连续的监测.  相似文献