1,721,135 research outputs found
Recent advances in karst research: From theory to fieldwork and applications
No full textKarst landscapes and karst aquifers, which are composed of a variety of soluble rocks such as salt, gypsum, anhydrite, limestone, dolomite and quartzite, are fascinating areas of study. As karst rocks are abundant on the Earth's surface, the fast evolution of karst landscapes and the rapid flow of water through karst aquifers present challenges from a number of different perspectives. This collection of 25 papers deals with different aspects of these challenges, including karst geology, geomorphology and speleogenesis, karst hydrogeology, karst modelling, and karst hazards and management. Together these papers provide a state-of-the-art review of the current challenges and solutions in describing karst from a scientific perspective
A model comparison of karst aquifer evolution for different matrix-flow formulations
No full textThe evolution of permeability and flow in a karst aquifer is studied by numerical simulations. The aquifer considered consists of a large central fracture, a network of finer fissures, and a porous rock matrix. Enlargement of both the central fracture and the fissures by chemical dissolution is possible, hence the conductivities in the fracture and the fissure system can increase with time. No dissolution is allowed in the porous rock matrix, which has a constant conductivity. How is driven by a simple fixed head boundary condition representative for the initial phase of karstification. A systematic parameter study is carried out by varying the initial width of the fissure network and the conductivity of the rock matrix, while keeping the initial width of the central fracture fixed. Key parameters such as flowrates, breakthrough times, and conductivities for the different models are compared. If either the conductivity of the rock matrix is high enough or the initial width of the fissures is large enough to carry flow, breakthrough times of the aquifer are significantly reduced, when compared to a model with low matrix conductivity and small fissures. However, due to the dissolutional widening of fissures the evolution of the aquifer is distinctively different for models with rock matrix simulated by a porous medium or a fissure network. (C) 2003 Elsevier B.V. All rights reserved
Ice-ocean mass balance during the Late Pleistocene glacial cycles in view of CHAMP and GRACE satellite missions
No full textDuring the last glacial cycles, global sea level dropped several times by about 120 m and large ice sheets covered North America, northern Europe and Antarctica during the glacial stages. The changes in the ice-ocean mass balance have displaced mantle material mainly via viscous flow, and the perturbation of the equilibrium figure of the Earth by glacial isostatic adjustment is still observable today in time-dependent changes of gravitational and rotational observations. Contemporary ice-ocean mass balance from volume changes of polar ice caps also contributes to secular variations of the Earth's gravitational field. In the near future, several satellite gravity missions will significantly improve the accuracy of the observed time-dependent gravitational field. In view of the expected improvements in the observations, we predict glacially induced perturbations of the gravitational field, induced by Late Pleistocene and contemporary ice volume changes, for a variety of radial mantle viscosity profiles. We assess the degree of uncertainty for the glacially induced contributions to gravitational and rotational parameters, both in the spectral and the spatial domain. Predictions of power spectra for the glacially induced free-air gravity and geoid anomalies are about one order of magnitude lower than the observed values, and uncertainties arising from different plausible viscosity profiles are around 0.15-0.4 mGal and 0.2-1.5 m, respectively. Uncertainties from different ice models are of secondary importance for the predicted power spectra. Predicted secular changes in geoid anomalies in formerly glaciated areas are mainly controlled by the viscosity profile and contemporary ice volume changes. We also show that the simple three-layer viscosity profiles currently employed for the majority of postglacial rebound studies represent a limited subset for model predictions of the time-dependent gravitational field
Karst aquifer evolution in a changing water table environment
No full text[1] A vertical cross section through a karst aquifer is modeled by means of the finite element method to study the evolution of fractures and flow in the aquifer. The karst aquifer receives a constant recharge along the top boundary by precipitation and drains toward a resurgence assumed to be the base level in a valley. Flow is allowed both in the permeable rock matrix and the fracture network, and the fractures are enlarged with time by chemical dissolution. Hence during the early evolution of the karst aquifer the conductivity increases over several orders of magnitude, and the initially high water table drops to a steady state base level niveau. As a consequence, fractures above the final water table change from phreatic to vadose flow conditions. A systematic parameter study is carried out to investigate the aquifer evolution over a wide range of parameters, such as recharge rate, initial fracture width and density, and initial calcium concentration. The numerical models cover a wide range of drainage patterns, from phreatic water table caves to deep bathyphreatic caves to vadose river caves. The models suggest that a single theoretical approach is capable of explaining most common cave passage patterns
Geodetic signatures of a Late Pleistocene Tibetan ice sheet
No full textThe extent of the Tibetan Plateau glaciation during the last glacial cycle is not well known. Estimates range from a limited glaciation restricted to the mountain ranges to a large regional ice sheet covering the entire plateau. We test the hypothesis of a large ice sheet by calculating the secular changes in geoid, gravity, and uplift, which would result today from regional ice sheet models that melted in late-glacial times. Predictions of the secular variations both in size (2000 km x 5000 km) and in amplitude (>0.2 mm/year for the geoid anomaly, >0.4 muGal/year for the free-air gravity anomaly) are large enough to be detected by satellite-gravity missions. Smaller-scale features such as variations in ice thickness could potentially be resolved by absolute gravity measurements over a 10-year period. However, non-glacial contributions to present-day mass imbalances such as tectonic uplift and groundwater movement can obscure the glacial signal. Especially the variability of groundwater storage induces a secular geoid rate as large as the predicted glacial rates. (C) 2004 Elsevier Ltd. All rights reserved
Stalagmite growth and palaeo-climate: the numerical perspective
No full textThe growth of stalagmites can be approximated by a simple mathematical model, which depends on growth rate and equilibrium radius. These two parameters are controlled by the climate. Temperature variations derived from ice and deep-sea core data, together with models for changes in precipitation and soil cover, are used to derive stalagmite stratigraphies, which reflect the palaeo-climate variations imposed. In general, stalagmite growth is strongly correlated to temperature and the amount of carbon dioxide available in the soil. Furthermore, precipitation is correlated to the stalagmite diameter. However, several assumptions need to be made: (i) A functional relationship between temperature on the one hand and precipitation and soil cover on the other needs to be established. (ii) The kinetics of calcite dissolution and precipitation needs to be assigned, either under soil and epikarst conditions open to the atmosphere or under fractured rock conditions closed from the atmosphere. These assumptions are difficult to access from field data, and therefore a stalagmite stratigraphy can be ambiguous and not easily converted back into an unknown palaeo-climate signal. (C) 2003 Elsevier B.V. All rights reserved
Predictions of secular geoid changes from Late Pleistocene and Holocene Antarctic ice-ocean mass imbalance
No full textEstimates on ice volume changes in Antarctica, from both contemporary mass imbalance during the last hundred to thousand years and ice volume changes during the last glacial cycles, are difficult to obtain. Future satellite-gravity missions monitoring secular changes in gravity as a proxy of mass changes of the Antarctic Ice Sheet will therefore provide an additional important constraint on the Antarctic Ice Sheet history. We assess the contributions from both past ice volume changes during the last glacial cycles and present ice imbalances over Antarctica, using a variety of published ice models. Model predictions of secular changes of the geoid anomaly based on plausible mantle viscosity profiles enable us to discuss the importance of past and present ice volume changes and their characteristic signatures. While uncertainties arising from the viscosity profile are of secondary importance for model predictions of the secular change of the geoid anomaly, we expect significant bias of the observed signal from uncertainties in the timing of the end of deglaciation, which is in agreement with previous studies. Hence, the benefit of inferring present-day Antarctic ice imbalance from satellite-gravity missions will depend on improvements to ice models for the last glacial cycle
Modelling unsaturated flow in an evolving karst aquifer
No full textA two-dimensional cross-section of a karst aquifer, in which chemical dissolution enlarges fractures with time, is studied. The karst aquifer is recharged by precipitation and drains towards a resurgence. The initial aquifer has low conductivities both in the rock matrix and the fracture network, and the initial water table is high. As the enlargement of fractures by dissolution increases the fracture conductivity, the water table drops, until it reaches a steady-state along the level of the resurgence. Several parameterisations are discussed for flow in the unsaturated zone above the water table. It is shown that different approaches result in similar cave passage patterns, with a large water-table cave draining the recharge towards the resurgence. However, flow patterns in the unsaturated zone can be very different for the different parameterisations. (C) 2003 Elsevier Science B.V. All rights reserved
Numerical models for mixing corrosion in natural and artificial karst environments
No full text[1] The enlargement of initially small fractures in a karst aquifer by chemical dissolution is studied. Flow in the aquifer is driven by head differences between sinks and resurgences, and flow depends on the permeability of small fissures and fractures in the aquifer. Enlargement of fractures is controlled by the chemical composition of the recharge, as water undersaturated with respect to calcite is able to dissolve material from the fracture walls. As fractures are enlarged with time, permeability within the aquifer increases significantly, and flow becomes very heterogeneous. Two different processes are considered: enlargement due to normal corrosion, where water is undersaturated with respect to calcite, and enlargement due to mixing corrosion, where two solutions saturated with respect to calcite but with different carbon dioxide concentrations mix and the resulting solution becomes undersaturated again. The importance of mixing corrosion is discussed for two boundary conditions: A natural karst aquifer is modeled with fixed recharge boundary conditions representing sinking streams, and an artificial karst aquifer is simulated with fixed head boundary conditions representing a reservoir. In both cases, mixing corrosion is important, especially if recharge is characterized by an almost saturated chemistry. Mixing corrosion significantly changes the evolving passage pattern, as dissolution due to mixing of solutions is possible deep in the aquifer. Mixing corrosion also reduces breakthrough times of the aquifer and can result in dramatic leakage underneath dam sites, even if the impounded water is almost saturated with respect to calcite
Glacial isostatic adjustment in Fennoscandia with a three-dimensional viscosity structure as an inverse problem
No full textGlacial isostatic adjustment data are commonly used to invert for the radial viscosity structure of the mantle. However, the effects of lateral variations in mantle viscosity in such inversions are not yet accounted for. Here we analysed synthetic sea-level data for the Fennoscandian region, which are derived from a three-dimensional (3D) earth model with realistic lateral and vertical viscosity variations deduced from seismological and geological information. The inversion of the 3D synthetic data for a best-fitting 1D viscosity profile reveals that (i) lateral lithospheric thickness variations can be detected with 1 D model predictions, if the data are grouped into regional subsets, (ii) combined lateral variations in lithospheric thickness and asthenospheric viscosity are not properly resolved with 1D model predictions, and (iii) the spatial and temporal distribution of observational data strongly affects the resulting 1D viscosity profile. (C) 2002 Elsevier Science B.V. All rights reserved
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