The timing and extent of the eruption of the Siberian Traps large igneous province: Implications for the end-Permian environmental crisis     

Marc K. Reichow  M.S. Pringle  A.I. Al'Mukhamedov  M.B. Allen  V.L. Andreichev  M.M. Buslov  C.E. Davies  G.S. Fedoseev  J.G. Fitton  S. Inger  A.Ya. Medvedev  C. Mitchell  V.N. Puchkov  I.Yu. Safonova  R.A. Scott  A.D. Saunders 《Earth and Planetary Science Letters》2009,277(1-2):9-20

We present new high-precision ⁴⁰Ar/³⁹Ar ages on feldspar and biotite separates to establish the age, duration and extent of the larger Siberian Traps volcanic province. Samples include basalts and gabbros from Noril'sk, the Lower Tunguska area on the Siberian craton, the Taimyr Peninsula, the Kuznetsk Basin, Vorkuta in the Polar Urals, and from Chelyabinsk in the southern Urals. Most of the ages, except for those from Chelyabinsk, are indistinguishable from those found at Noril'sk. Cessation of activity at Noril'sk is constrained by a ⁴⁰Ar/³⁹Ar age of 250.3 ± 1.1 Ma for the uppermost Kumginsky Suite.The new ⁴⁰Ar/³⁹Ar data confirm that the bulk of Siberian volcanism occurred at 250 Ma during a period of less than 2 Ma, extending over an area of up to 5 million km². The resolution of the data allows us to confidently conclude that the main stage of volcanism either immediately predates, or is synchronous with, the end-Permian mass extinction, further strengthening an association between volcanism and the end-Permian crisis. A sanidine age of 249.25 ± 0.14 Ma from Bed 28 tuff at the global section and stratotype at Meishan, China, allows us to bracket the P–Tr boundary to 0.58 ± 0.21 myr, and enables a direct comparison between the ⁴⁰Ar/³⁹Ar age of the Traps and the Permo–Triassic boundary section.Younger ages (243 Ma) obtained for basalts from Chelyabinsk indicate that volcanism in at least the southern part of the province continued into the Triassic.  相似文献   

Study of water tension differences in heterogeneous sandy soils using surface ERT   总被引：1，自引：0，他引：1  

Marc Descloitres  Olivier Ribolzi  Yann Le Troquer  Jean Pierre Thibaux 《Journal of Applied Geophysics》2008,64(3-4):83-98

Herbaceous vegetation in the Sahel grows almost exclusively on sandy soils which preferentially retain water through infiltration and storage. The hydrological functioning of these sandy soils during rain cycles is unknown. One way to tackle this issue is to spatialize variations in water content but these are difficult to measure in the vadose zone. We investigated the use of Electrical Resistivity Tomography (ERT) as a technique for spatializing resistivity in a non-destructive manner in order to improve our knowledge of relevant hydrological processes. To achieve this, two approaches were examined. First, we focused on a possible link between water tension (which is much easier to measure in the field by point measurements than water content), and resistivity (spatialized with ERT). Second, because ERT is affected by solution non-uniqueness and reconstruction smoothing, we improved the accuracy of ERT inversion by comparing calculated solutions with in-situ resistivity measurements. We studied a natural microdune during a controlled field experiment with artificial sprinkling which reproduced typical rainfall cycles. We recorded temperature, water tension and resistivity within the microdune and applied surface ERT before and after the 3 rainfall cycles. Soil samples were collected after the experiment to determine soil physical characteristics. An experimental relationship between water tension and water content was also investigated. Our results showed that the raw relationship between calculated ERT resistivity and water tension measurements in sand is highly scattered because of significant spatial variations in porosity. An improved correlation was achieved by using resistivity ratio and water tension differences. The slope of the relationship depends on the soil solution conductivity, as predicted by Archie's law when salted water was used for the rain simulation. We found that determining the variations in electrical resistivity is a sensitive method for spatializing the differences in water tension which are directly linked with infiltration and evaporation/drainage processes in the vadose zone. However, three factors complicate the use of this approach. Firstly, the relation between water tension and water content is generally non-linear and dependent on the water content range. This could limit the use of our site-specific relations for spatializing water content with ERT through tension. Secondly, to achieve the necessary optimization of ERT inversion, we used destructive resistivity measurements in the soil, which renders ERT less attractive. Thirdly, we found that the calculated resistivity is not always accurate because of the smoothing involved in surface ERT data inversion. We conclude that further developments are needed into ERT image reconstruction before water tension (and water content) can be spatialized in heterogeneous sandy soils with the accuracy needed to routinely study their hydrological functioning.  相似文献   

The Caulerpa racemosa invasion: a critical review   总被引：1，自引：0，他引：1  

Klein J  Verlaque M 《Marine pollution bulletin》2008,56(2):205-225

Caulerpa racemosa var. cylindracea is a marine Chlorophyta introduced into the Mediterranean Sea from south-western Australia. Since 1990, it has been invading the Mediterranean Sea and the Canary Islands, raising ecological problems. Although this invasion event can be considered as one of the most serious in the history of species introduced into the Mediterranean Sea, C. racemosa has not triggered as much attention as the famous "killer alga"Caulerpa taxifolia. The aim of the present study was: (i) to summarize the current state of knowledge with regard to the distribution, the various biological and ecological characteristics of the introduced C. racemosa and its impact on the Mediterranean coastal environment; (ii) to discuss the various hypotheses regarding the explanation for its rapid and successful spread; (iii) to investigate the disparity in the treatment of C. racemosa and Caulerpa taxifolia invasions; and (iv) to outline future research needs.  相似文献   

The Correlation Structure of Matheron's Classical Variogram Estimator Under Elliptically Contoured Distributions     

Marc G. Genton 《Mathematical Geology》2000,32(1):127-137

The classical variogram estimator proposed by Matheron can be written as a quadratic form of the observations. When data have an elliptically contoured distribution with constant mean, the correlation between the classical variogram estimator at two different lags is a function of the spatial design matrix, the covariance matrix, and the kurtosis. Several specific cases are studied closely. A subclass of elliptically contoured distributions with a particular family of covariance matrices is shown to possess exactly the same correlation structure for the classical variogram estimator as the multivariate independent Gaussian distribution. The consequences on variogram fitting by generalized least squares are discussed.  相似文献   

The South Eastern Durance fault permanent network: Preliminary results     

Philippe Volant  Catherine Berge-Thierry  Pierre Dervin  Marc Cushing  Georgianna Mohammadioun  Franck Mathieu 《Journal of Seismology》2000,4(2):175-189

The Durance fault area is located in South EasternFrance. This fault system is characterized byhistorical earthquakes (one every century, since 1509,with a magnitude between 5.0 and 5.3). This is theonly fault in France with such a periodic historicalseismic activity. In order to study an active fault ina moderate seismic context, the IPSN (Institute forNuclear Safety and Protection) decided to install apermanent network in 1992, surrounding the fault area.Such a permanent seismic network has been installed inthe french Pyrenees in the Arette area (Gagnepain etal., 1980). While the Arette network covers a regionaffected by several major faults, our network isdevoted to the study of the specific Durance fault.Major historical earthquakes are clearly associatedwith this structure. From an instrumental point ofview, few earthquakes have been recorded since 1962with the national network. Our network shows a smallseismic activity, with the epicenters well alignedalong the fault direction. Moreover, focal mechanismscomputed for two events agree with the regionalmicrostructural studies (Cushing et al., 1997).Finally, a study of the shear wave splittingunderlines preferential S wave polarization for twostations. The H/V ratio on noise microtremors has beencomputed for each station in order to check theirpositions in term of site effects. It does not exhibitany amplification effect (except for two stations).The comparisons with H/V ratio on earthquake datasetshow the important biases we can obtain with realearthquakes.  相似文献   

Major and trace element compositions of georgiaites: Clues to the source of North American tektites     

Edward F. ALBIN  Marc D. NORMAN  Michael RODEN 《Meteoritics & planetary science》2000,35(4):795-806

Abstract— Electron microprobe and laser ablation, inductively‐coupled plasma mass spectrometer analyses of 24 georgiaites show that these tektites are all Si‐rich (79–83 wt% SiO₂) glasses with variable major and trace element abundances (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to but not identical with average upper continental crust. For example, georgiaites are light rare earth element enriched with small negative Eu anomalies (Eu/Eu*=0.73‐0.86) and La‐Th‐Sc systematics are intermediate between that of Archean and post‐Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide‐oxide plots (e.g., FeO‐MgO). Large variations in refractory element abundances and ratios compared to the variation in SiO₂ favors mixing over volatilization as a cause of the compositional variation. If all the tektites formed as a result of a single impact, then triangular arrays in oxide‐oxide variation diagrams require at least three source components. These components include a Si‐rich material, probably a quartz‐rich sand that was predominant in the formation of georgiaites. Two relatively silica‐poor and Fe‐rich components have compositional characteristics similar to shales and greywackes. The La‐Th‐Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.  相似文献   

Groundwater components in the alluvial aquifer of the alpine Rhone River valley, Bois de Finges area, Wallis Canton, Switzerland     

Marc Schürch  François-D. Vuataz 《Hydrogeology Journal》2000,8(5):549-563

Source, type, and quantity of various components of groundwater, as well as their spatial and temporal variations were determined by different hydrochemical methods in the alluvial aquifer of the upper Rhone River valley, Bois de Finges, Wallis Canton, Switzerland. The methods used are hydrochemical modeling, stable-isotope analysis, and chemical analysis of surface water and groundwater. Sampling during high- and low-water periods determined the spatial distribution of the water chemistry, whereas monthly sampling over three years provided a basis for understanding seasonal variability. The physico-chemical parameters of the groundwater have spatial and seasonal variations. The groundwater chemical composition of the Rhone alluvial aquifer indicates a mixing of weakly mineralized Rhone River water and SO₄-rich water entering from the south side of the valley. Temporal changes in groundwater chemistry and in groundwater levels reflect the seasonal variations of the different contributors to groundwater recharge. The Rhone River recharges the alluvial aquifer only during the summer high-water period. Electronic Publication  相似文献   

Localized algal blooms induced by river inflows in a canyon type reservoir     

Vidal  Javier  Marcé  Rafael  Serra  Teresa  Colomer  Jordi  Rueda  Francisco  Casamitjana  Xavier 《Aquatic Sciences - Research Across Boundaries》2012,74(2):315-327

The local response of the phytoplankton community to river inflow processes was investigated with modeling and field analyses in a long and narrow, stratified reservoir in mid-summer. The river water had high concentrations of phosphorus and nitrogen (ammonium and nitrate) and temperature had large variations at diurnal scales. As a consequence of the large variation in river temperature, the level of neutral buoyancy (the depth where the river water spreads laterally in the reservoir) oscillated between the surface (overflows) during the day, and the depth of the metalimnion (interflows) during the night. The reservoir remained strongly stratified, which favoured the presence of cyanobacteria. It is shown that under these conditions, nutrient-rich river water injected during overflows into the surface layers promoted the occurrence of localized algal blooms in the zones where the overflow mixed with the quiescent water of the reservoir. A series of hydrodynamic simulations of the reservoir were conducted both with synthetic and realistic forcing to assess the importance of river temperatures and wind-driven hydrodynamics for algal blooms. The simulations confirmed that the river inflow was the main forcing mechanism generating the localized bloom.  相似文献   

Spatial and temporal infiltration dynamics during managed aquifer recharge     

Racz AJ  Fisher AT  Schmidt CM  Lockwood BS  Los Huertos M 《Ground water》2012,50(4):562-570

Natural groundwater recharge is inherently difficult to quantify and predict, largely because it comprises a series of processes that are spatially distributed and temporally variable. Infiltration ponds used for managed aquifer recharge (MAR) provide an opportunity to quantify recharge processes across multiple scales under semi-controlled conditions. We instrumented a 3-ha MAR infiltration pond to measure and compare infiltration patterns determined using whole-pond and point-specific methods. Whole-pond infiltration was determined by closing a transient water budget (accounting for inputs, outputs, and changes in storage), whereas point-specific infiltration rates were determined using heat as a tracer and time series analysis at eight locations in the base of the pond. Whole-pond infiltration, normalized for wetted area, rose rapidly to more than 1.0 m/d at the start of MAR operations (increasing as pond stage rose), was sustained at high rates for the next 40 d, and then decreased to less than 0.1 m/d by the end of the recharge season. Point-specific infiltration rates indicated high spatial and temporal variability, with the mean of measured values generally being lower than rates indicated by whole-pond calculations. Colocated measurements of head gradients within saturated soils below the pond were combined with infiltration rates to calculate soil hydraulic conductivity. Observations indicate a brief period of increasing saturated hydraulic conductivity, followed by a decrease of one to two orders of magnitude during the next 50 to 75 d. Locations indicating the most rapid infiltration shifted laterally during MAR operation, and we suggest that infiltration may function as a "variable source area" processes, conceptually similar to catchment runoff.  相似文献   

Trace metal pollution and its influence on the community structure of soft bottom molluscs in intertidal areas of the Dar es Salaam coast, Tanzania   总被引：1，自引：0，他引：1  

Rumisha C  Elskens M  Leermakers M  Kochzius M 《Marine pollution bulletin》2012,64(3):521-531

The influence of trace metal pollution on the community structure of soft bottom molluscs was investigated in intertidal areas of the Dar es Salaam coast. Significant enrichment of As, Mn, Mo, Sb, and Zn in sediments was recorded. Redundancy analysis indicated that trace metal pollution contributed 68% of the variation in community structure. Monte Carlo permutation test showed that As and Sb contributed significantly to variation in species composition. T-value biplots and van Dobben circles showed that the gastropods Acteon fortis, Assiminea ovata, and Littoraria aberrans, were negatively affected by As and Sb, while the bivalve Semele radiata and the gastropod Conus litteratus were only negatively affected by As. Bioaccumulation of As, Cd, Cu, Mo and Zn occurred in the bivalve Mactra ovalina and the gastropod Polinices mammilla. This calls for regular monitoring and management measures.  相似文献