We assess the sheltering effect of Posidonia oceanica meadows on drag forces exerted on shells of the fan mussel Pinna nobilis. We examine a range of shell sizes under four unidirectional flow speeds (0.05–0.34 m s−1) and two oscillating regimes. Three meadow densities are evaluated and a control without vegetation. We found that the attenuating
effect of the meadow on drag forces experienced by bivalves is determined by the form of the hydrodynamic energy, e.g., as
unidirectional flow or wave action. In tidal currents, the meadow protects most sizes of bivalves, with a higher efficiency
for dense meadows, while in wave dominant zones the meadow reduces drag forces for bivalves with shell areas below a threshold
of 0.019 m2, whereas larger animals experience increased drag forces within the meadow independent of meadow density. Reduction of shoot
density in seagrass meadows might therefore not affect the effectiveness of the canopy to reduce drag forces on associated
species like the fan mussel in wave-dominated areas while increased storm frequency could result into losses of larger individuals
during periods of high wave action. 相似文献
40Ar/39Ar geochronological data on hornblende, biotite and K-feldspar provide constraints on the cooling path experienced by a high-grade metamorphic complex from the Mühlig–Hofmannfjella and Filchnerfjella (6–8°E), central Dronning Maud Land, Antarctica, during the late Neoproterozoic-early Palaeozoic Pan–African orogeny. Hornblende ages yield c. 481 Ma, biotite ages range from c. 466 Ma to c. 435 Ma, whereas K-feldspar ages of the gneisses are c. 437 Ma. The 40Ar/39Ar data suggest initial cooling at a rate of ~10 °C/Myr between 481 and 465 Ma, followed by a lower cooling rate of ~6 °C/Myr during the subsequent c. 30 million years. The K-feldspar 40Ar/39Ar ages place a lower time limit on the duration of the exhumation, by the time of thermal relaxation to a stable continental geotherm. The 40Ar/39Ar data reflecting cooling indicate tectonic exhumation related to orogenic collapse during a later phase of the Pan–African orogeny. 相似文献
Combining a geological model with a geomechanical model, it generally turns out that the geomechanical model is built from units that are at least a 100 times larger in volume than the units of the geological model. To counter this mismatch in scales, the geological data model's heterogeneous fine-scale Young's moduli and Poisson's ratios have to be upscaled to one equivalent homogeneous coarse-scale rigidity. This coarse-scale rigidity relates the volume-averaged displacement, strain, stress, and energy to each other, in such a way that the equilibrium equation, Hooke's law, and the energy equation preserve their fine-scale form on the coarse scale. Under the simplifying assumption of spatial periodicity of the heterogeneous fine-scale rigidity, homogenization theory can be applied. However, even then the spatial variability is generally so complex that exact solutions cannot be found. Therefore, numerical approximation methods have to be applied. Here the node-based finite element method for the displacement as primary variable has been used. Three numerical examples showing the upper bound character of this finite element method are presented. 相似文献
In the linear digital filter theory for calculation of Hankel transforms it is possible to find explicit series expansions for the filter coefficients. A method is presented for optimizing the Hankel filters calculated in this way. For a certain desired accuracy of computation, the sampling density and filter length are minimized by choosing the parameters determining the filter characteristics according to the analytical properties of the input function. A new approach to the calculation of the filter coefficients has been developed for these optimized filters. The length of the filters may be further reduced by introducing a shift in the sampling scheme. 相似文献
The ocean captures a large part of the anthropogenic carbon dioxide emitted to the atmosphere. As a result of the increase in CO2 partial pressure the ocean pH is lowered as compared to pre-industrial times and a further decline is expected. Ocean acidification has been proposed to pose a major threat for marine organisms, particularly shell-forming and calcifying organisms. Here we show, on the basis of meta-analysis of available experimental assessments, differences in organism responses to elevated pCO2 and propose that marine biota may be more resistant to ocean acidification than expected. Calcification is most sensitive to ocean acidification while it is questionable if marine functional diversity is impacted significantly along the ranges of acidification predicted for the 21st century. Active biological processes and small-scale temporal and spatial variability in ocean pH may render marine biota far more resistant to ocean acidification than hitherto believed. 相似文献
Ocean acidification due to anthropogenic CO2 emissions is a dominant driver of long-term changes in pH in the open ocean, raising concern for the future of calcifying organisms, many of which are present in coastal habitats. However, changes in pH in coastal ecosystems result from a multitude of drivers, including impacts from watershed processes, nutrient inputs, and changes in ecosystem structure and metabolism. Interaction between ocean acidification due to anthropogenic CO2 emissions and the dynamic regional to local drivers of coastal ecosystems have resulted in complex regulation of pH in coastal waters. Changes in the watershed can, for example, lead to changes in alkalinity and CO2 fluxes that, together with metabolic processes and oceanic dynamics, yield high-magnitude decadal changes of up to 0.5 units in coastal pH. Metabolism results in strong diel to seasonal fluctuations in pH, with characteristic ranges of 0.3 pH units, with metabolically intense habitats exceeding this range on a daily basis. The intense variability and multiple, complex controls on pH implies that the concept of ocean acidification due to anthropogenic CO2 emissions cannot be transposed to coastal ecosystems directly. Furthermore, in coastal ecosystems, the detection of trends towards acidification is not trivial and the attribution of these changes to anthropogenic CO2 emissions is even more problematic. Coastal ecosystems may show acidification or basification, depending on the balance between the invasion of coastal waters by anthropogenic CO2, watershed export of alkalinity, organic matter and CO2, and changes in the balance between primary production, respiration and calcification rates in response to changes in nutrient inputs and losses of ecosystem components. Hence, we contend that ocean acidification from anthropogenic CO2 is largely an open-ocean syndrome and that a concept of anthropogenic impacts on marine pH, which is applicable across the entire ocean, from coastal to open-ocean environments, provides a superior framework to consider the multiple components of the anthropogenic perturbation of marine pH trajectories. The concept of anthropogenic impacts on seawater pH acknowledges that a regional focus is necessary to predict future trajectories in the pH of coastal waters and points at opportunities to manage these trajectories locally to conserve coastal organisms vulnerable to ocean acidification. 相似文献
Since 1979 the Laboratory of Geophysics, University of Aarhus, has been developing a new prospecting tool for obtaining information on the topmost 100 m of the earth. The method is an extension of the conventional geoelectric sounding method, but instead of direct current (DC) the AC-geoelectrical sounding method uses alternating current (AC) with frequencies in the range 100 Hz to 20,000 Hz. The use of alternating current adds an inductive contribution to the ordinary galvanic electric field, thus producing two different sorts of information about the underlying earth structure. These two sets of information are, in many cases, of complementary nature, which enables determination of the ground parameters much more accurately than would otherwise be possible from ordinary DC-geoelectrical soundings. Among these cases is the high resistivity equivalence which appears so frequently in Danish Quaternary deposits. 相似文献
In this paper the meteorological and hydrological conditions are analyzed which trigger shallow and deeper landslides in glacio-lacustrine deposits (varved or laminated clays) in the French Alps. The hydrological system of these landslides consists of a colluvial cover which supplies water into the fissures of the underlying varved clays. From these fissures water can infiltrate more or less horizontally into the silt layers of the varved clays. A combined reservoirs model was used to simulate the water fluctuations in the colluvial cover and the fissures. Both the water level in the fissures and the residence time of water in the fissures are mainly controlled by the amount of water storage in the colluvial cover. Simulations over the last 25 years show that almost each year the fissures are completely filled with water for several months a year.
Infiltration experiments in the field show that infiltration into the varved clays occurs mainly by horizontal infiltration into the silt laminae. Calculated infiltration rates from these fissures into the silt layers show that the mean yearly residence time of water in the fissures is not sufficient to fully saturate the clay mass each year. It is therefore concluded that the triggering of the landslide movements is mainly controlled by the development of positive pore water pressures in the fissure system and that the rise of pore water pressures induced by the matrix system of the varved clays only plays a minor role. The calculations also show that drainage of the colluvial cover is a very efficient measure to stabilize the deeper landslides. 相似文献