Climate variability in the Spanish Pyrenees during the last 30,000 yr revealed by the El Portalet sequence     

P. González-Sampériz  B.L. Valero-Garcés  G. Jalut  C. Martí-Bono  A. Navas  J.J. Dedoubat 《Quaternary Research》2006,66(1):38-52

Palynological, sedimentological and stable isotopic analyses of carbonates and organic matter performed on the El Portalet sequence (1802 m a.s.l., 42°48′00?N, 0°23′52?W) reflect the paleoclimatic evolution and vegetation history in the central-western Spanish Pyrenees over the last 30,000 yr, and provide a high-resolution record for the late glacial period. Our results confirm previous observations that deglaciation occurred earlier in the Pyrenees than in northern European and Alpine sites and point to a glacial readvance from 22,500 to 18,000 cal yr BP, coinciding with the global last glacial maximum. The patterns shown by the new, high-resolution pollen data from this continental sequence, chronologically constrained by 13 AMS ¹⁴C dates, seem to correlate with the rapid climate changes recorded in Greenland ice cores during the last glacial-interglacial transition. Abrupt events observed in northern latitudes (Heinrich events 3 to 1, Oldest and Older Dryas stades, Intra-Allerød Cold Period, and 8200 cal yr BP event) were also identified for the first time in a lacustrine sequence from the central-western Pyrenees as cold and arid periods. The coherent response of the vegetation and the lake system to abrupt climate changes implies an efficient translation of climate variability from the North Atlantic to mid latitudes.  相似文献   

Optimization of nonconventional wells under uncertainty using statistical proxies     

Vincent Artus  Louis J. Durlofsky  Jérôme Onwunalu  Khalid Aziz 《Computational Geosciences》2006,10(4):389-404

The determination of the optimal type and placement of a nonconventional well in a heterogeneous reservoir represents a challenging optimization problem. This determination is significantly more complicated if uncertainty in the reservoir geology is included in the optimization. In this study, a genetic algorithm is applied to optimize the deployment of nonconventional wells. Geological uncertainty is accounted for by optimizing over multiple reservoir models (realizations) subject to a prescribed risk attitude. To reduce the excessive computational requirements of the base method, a new statistical proxy (which provides fast estimates of the objective function) based on cluster analysis is introduced into the optimization process. This proxy provides an estimate of the cumulative distribution function (CDF) of the scenario performance, which enables the quantification of proxy uncertainty. Knowledge of the proxy-based performance estimate in conjunction with the proxy CDF enables the systematic selection of the most appropriate scenarios for full simulation. Application of the overall method for the optimization of monobore and dual-lateral well placement demonstrates the performance of the hybrid optimization procedure. Specifically, it is shown that by simulating only 10% or 20% of the scenarios (as determined by application of the proxy), optimization results very close to those achieved by simulating all cases are obtained.  相似文献   

Study of geothermal water intrusion due to groundwater exploitation in the Puebla Valley aquifer system, Mexico   总被引：1，自引：1，他引：1  

E. Leticia Flores-Márquez  Gabriel Jiménez-Suárez  Raymundo G. Martínez-Serrano  René E. Chávez  Daniel Silva Pérez 《Hydrogeology Journal》2006,14(7):1216-1230

Significant intrusion of geothermal water into fresh groundwater takes place in the Puebla Valley aquifer system, Mexico. The decline in the potentiometric surface due to the overexploitation of the groundwater induces this intrusion. This hydrological system comprises three aquifers located in Plio-Quaternary volcanic sediments and Mesozoic calcareous rocks. The hydraulic balance of the aquifer shows that the annual output exceeds the natural inputs by 12 million m³. Between 1973 and 2002, a drop in the potentiometric surface, with an 80 m cone of depression, was identified in a 5-km-wide area located southwest of the city of Puebla. Chemical analyses performed on water samples since 1990 have shown an increase in total dissolved solids (TDS) of more than 500 mg/L, coinciding with the region showing a cone of depression in the potentiometric surface. A three-dimensional flow and transport model, based on the hydrogeological and geophysical studies, was computed by using the MODFLOW and MT3D software. This model reproduces the evolution of the aquifer system during the last 30 years and predicts for 2010 an additional drawdown in the potentiometric surface of 15 m, and an increase in the geothermal water intrusion.  相似文献   

Risk assessment of groundwater pollution using sensitivity analysis and a worst-case scenario analysis   总被引：5，自引：0，他引：5  

Marijke Huysmans  Tamás Madarász  Alain Dassargues 《Environmental Geology》2006,50(2):180-193

This paper illustrates how sensitivity analysis and a worst-case scenario analysis can be useful tools in risk assessment of groundwater pollution. The approach is applied to a study area in Hungary with several known groundwater pollution sources and nearby drinking water production wells. The main concern is whether the contamination sources threaten the drinking water wells of the area. A groundwater flow and transport model is set up to answer this question. Due to limited data availability, the results of this model are associated with large uncertainty. Sensitivity analysis and a worst-case scenario analysis are applied to estimate this uncertainty and build confidence in the model results.  相似文献   

Reconstructing hydrological variability from testate amoebae analysis in Carpathian peatlands     

C. Schnitchen  D. J. Charman  E. Magyari  M. Braun  I. Grigorszky  B. Tóthmérész  M. Molnár  Zs. Szántó 《Journal of Paleolimnology》2006,36(1):1-17

Peatlands offer the potential for high resolution records of water balance over Holocene timescales, yet this potential is under-exploited in many areas of the world. Within Europe, peatlands are mostly confined to areas north of 55° N, but several areas of southern and eastern Europe contain small peatlands which may be suitable for palaeoclimatic reconstruction. In this paper we test the potential of peatlands in the Carpathian region for deriving quantified estimates of water table changes using testate amoebae analysis. A training set for palaeohydrological reconstruction from testate amoebae assemblages was obtained by collecting surface samples from 13 peatlands, including 9 from Hungary and 4 from Transylvania (Romania). Using a simple measure of mean annual water tables estimated from staining of PVC tape, we found that some peatlands were heavily influenced by runoff and groundwater, and were therefore not suitable as modern analogues of ombrotrophic climatically sensitive sites. The relationship between the testate amoebae assemblages in the modern samples and the environmental variables was explored using CCA. The CCA biplot showed that the most important variables are depth to water table and moisture content, confirming that hydrology is a key control on taxon distribution. pH was a secondary gradient. A transfer function for % moisture and depth to water table was established and applied to fossil assemblages from a sequence from Fenyves-tető, Transylvania, Romania. The reconstructed water table shows a number of variations which have parallels with other palaeoclimatic records from Europe and the North Atlantic prominent phases of higher water tables are associated with the periods 8000–8300 cal BP, 3000–2500 cal BP and after 600 cal BP. We suggest that these were periods of particular intensification of westerly airflow which affected eastern Europe as well as western and central Europe.  相似文献   

Interpretation of subhorizontal crustal reflections by metamorphic and rheologic effects in the eastern part of the Pannonian Basin     

Károly Posgay  Tamás Bodoky  Zoltán Hajnal  Tivadar M. Tóth  Tamás Fancsik  Endre Heged&#;s  Attila Cs. Kovács  Ern&#; Takács 《Geophysical Journal International》2006,167(1):187-203

The geologic origin of subhorizontal reflections, often observed in crustal seismic sections, was investigated by establishing metamorphic facies and strength of rocks in depth, and correlating these properties to seismic reflection sections from eastern Hungary. Estimation of the depths of metamorphic mineral stability zones utilized the principles developed by Fyfe et al. and known geothermal data of the area. The strength versus depth profile was derived by relating local seismic P -wave interval velocities to Meissner et al. 's activation energy. The results show that the series of subhorizontal reflections, observed in the Pannonian Basin, are a consequence of combined metamorphic and rheologic changes in depths. The synthesis of the integrated data set suggests that the retrograde alteration of the pre-Tertiary basement above the percolation threshold was made possible by the softening effect of shear zones and their water-conducting capacity. The subhorizontal reflections of highest energy, of the consolidated crust below the percolation threshold, originate in the depths of greenschist, amphibolite and granulite metamorphic mineral facies, which were formed in geothermal and pressure conditions similar to those existing today. These results imply the overprint of earlier (Variscan) metamorphic sequences of the crust by more recent retrograde metamorphic processes.  相似文献   

Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu–Hf and Sm–Nd geochronology     

Susanne Skora  Lukas P. Baumgartner  Nancy J. Mahlen  Clark M. Johnson  Sébastien Pilet  Eric Hellebrand 《Contributions to Mineralogy and Petrology》2006,152(6):703-720

Garnets from the Zermatt-Saas Fee eclogites contain narrow central peaks for Lu + Yb + Tm ± Er and at least one additional small peak towards the rim. The REE Sm + Eu + Gd + Tb ± Dy are depleted in the cores but show one prominent peak close to the rim. These patterns cannot be modeled using Rayleigh fractionation accompanied by mineral breakdown reactions. Instead, the patterns are well explained using a transient matrix diffusion model where REE uptake is limited by diffusion in the matrix surrounding the porphyroblast. Observed profiles are well matched if a roughly linear radius growth rate is used. The secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism. The model predicts anomalously low ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁴⁷Sm/¹⁴⁴Nd ratios in garnets where growth rates are fast compared to diffusion of the REE, and these results have important implications for Lu–Hf and Sm–Nd geochronology using garnet.  相似文献   

The Removal of Dissolved Metals by Hydroxysulphate Precipitates during Oxidation and Neutralization of Acid Mine Waters, Iberian Pyrite Belt   总被引：4，自引：0，他引：4  

J. Sánchez España  E. López Pamo  E. Santofimia Pastor  J. Reyes Andrés  J. A. Martín Rubí 《Aquatic Geochemistry》2006,12(3):269-298

This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD) waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters were selected to cover the spectrum of pH (2.2–3.5) and chemical composition (e.g., 319–2,103 mg/L Fe; 2.85–33.3 g/L SO₄⁼) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH < 3.5 (stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO₄⁼ anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous free cations (Pb²⁺, Zn²⁺, Cu²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely scavenged during the oxidation stage at pH < 3.5. In waters with high activities (> 10⁻¹) of the SO ₄⁼ ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH < 5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60–80% for Pb, to less than 20% for Zn, Co, Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB.  相似文献   

Synchrotron IR study of hydrous ringwoodite (γ-Mg2SiO4) up to 30 GPa     

E. M. Chamorro Pérez  I. Daniel  J.-C. Chervin  P. Dumas  J. D. Bass  T. Inoue 《Physics and Chemistry of Minerals》2006,33(7):502-510

High-pressure synchrotron infrared (IR) absorption spectra were collected between 650 and 4,000 cm⁻¹ at ambient temperature for hydrous Mg-ringwoodite (γ-Mg₂SiO₄) up to 30 GPa. The main feature in the OH⁻ stretching region is an extremely broad band centred at 3,150 cm⁻¹. The hydrogen bond is strong for most protons and the most probable site for protonation is the tetrahedral edge. With increasing pressure, this band shifts downward while decreasing its integrated intensity until disappearance at a pressure of 25 GPa. Only one band at 2,450 cm⁻¹ and an absorption plateau persist with a maximum wavenumber of 3,800 cm⁻¹. This behaviour is reversible upon pressure release. We interpret this as a second-order phase transition occurring in hydrated Mg-ringwoodite at high pressure (beyond ∼ 25 GPa). This result is compatible with the observation by Kleppe et al. (Phys Chem Miner 29:473–476, 2002a) who suggested the presence of Si–O–Si linkages and/or partial increase in the coordination of Si. Beyond the phase transition, the protons are delocalized and their environment on the ringwoodite structure is probably quite different from that at low pressure. Data obtained in situ at high pressures and temperatures are needed to better understand the effect of protonation on the structure and to better constrain this phase transition.  相似文献   

Mean flow-transient perturbation interaction in the Southern Hemisphere: a simulation using a variable-resolution GCM     

A. Carril  C. Menéndez  M. Nuñez  H. Le Treut 《Climate Dynamics》2002,18(8):661-673

The ability of an atmospheric general circulation model to reproduce fundamental features of the wintertime extratropical Southern Hemisphere (SH) circulation is evaluated with emphasis on the daily variability of the SH mean flow and the mean flow-transient perturbations interaction. Two 10-year simulations using a new version of the LMDZ GCM with a stretched grid scheme centered at 45 °S and forced by climatological SST are performed: a high (144Ꮡ) and low (64Ꭹ) horizontal resolution runs. The performance of both simulations was determined by comparing several simulated fields (zonal wind, temperature, kinetic energy, transient eddy momentum and heat fluxes, Eliassen-Palm fluxes, Eady growth rate and baroclinic conversion term) against the European Centre for Medium Range Weather Forecast reanalyses (ERA). High and low-resolution simulations are similar in many respects; in particular, both experiments reproduce the main patterns of the southern extratropical large-scale circulation satisfactorily. Increasing resolution does not improve universally some spurious aspects of the low resolution simulation (e.g. the cold bias in the high polar troposphere, the debilitated subtropical jet, the low baroclinic conversion rate). Those aspects present little sensitivity to the model resolution. The interaction between transient eddies and zonal mean flow are examined. The low-resolution experiment is able to qualitatively represent the acceleration/deceleration of the mean flow by transient perturbations, south/north of 30 °S with an accuracy similar to that of the high-resolution experiment. Although both experiments represent the baroclinic structure of the mean flow satisfactorily, the model underestimates some transient properties due to the underestimation of the baroclinic conversion term in middle latitudes. Such misrepresentation does not improve with increasing resolution and is related to the relatively weak meridional temperature gradient and the inadequate geographical distribution of the eddy heat fluxes. In particular, the eddy kinetic energy is always underestimated. Eddy kinetic energy does not improve convincingly with increasing resolution, suggesting that the adequate representation of the storm tracks is highly influenced by the physical parametrizations.  相似文献