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1.
In the frame of the R&D activities performed on the Boom Clay for assessing the suitability of deep clayey formations for radioactive waste disposal, the transferability of the scientific results obtained on the Boom Clay in Mol to the whole Campine Basin is investigated. Boreholes were drilled at different locations (e.g. Mol, Doel, Essen) and cores were sampled over the entire thickness of the Boom Clay formation on which the migration parameters for iodide and tritiated water (HTO) are determined.At Essen, the transport parameters in the Boom Clay can be considered as homogeneous in the range from 159 m to 241 m Below Drilling Table. The average hydraulic conductivity is (5.4 ± 1.7) × 10−12 m/s. The average ηR value for iodide is 0.25 ± 0.03 and 0.42 ± 0.05 for HTO. For HTO, this high value is mainly due to a higher value in the Putte Member (0.46 ± 0.03) compared to the other members (0.39 ± 0.02). The apparent diffusion coefficient is (1.3 ± 0.1) × 10−10 m2/s for HTO and (1.1 ± 0.2) × 10−10 m2/s for iodide. The expected effect of ionic strength (increasing with depth) on the ηR value of iodide is of the same size as the measurement error, what might explain why it was not observed.On a lateral (horizontal) level, the hydraulic conductivity at the Essen-1 borehole (5.4 × 10−12 m/s) lies between that of Boom Clay in Mol-1 (2.5 × 10−12 m/s) and that of Boom Clay in Doel-2b (1.4 × 10−11 m/s). For iodide, the higher ηR value in Essen-1 and Doel-2b (ηR ≈ 0.25) than in Mol-1 (ηR ≈ 0.16) can partly be explained by a higher ionic strength of the pore water. Apart from the Putte Member at Essen-1, the HTO porosities of the Terhagen Member and the Transition zone in Essen are in the range of the average porosities observed in Mol and Doel (ηR ≈ 0.37–0.39). For both iodide and HTO, the value of the apparent diffusion coefficient Dapp is similar in Mol-1 and in Doel-2b, with a clearly higher value for HTO than for iodide. In Essen-1, the apparent diffusion coefficients for iodide and HTO are nearly equal, and slightly smaller than the iodide value in Mol-1/Doel-2b. Accordingly, the HTO apparent diffusion coefficient is considerably smaller in Essen-1 than in Mol-1/Doel-2b.  相似文献   

2.
With transmission electron microscopy (TEM) we observed nanometer-sized pores in four ultracataclastic and fractured core samples recovered from different depths of the main bore hole of the San Andreas Fault Observatory at Depth (SAFOD). Cutting of foils with a focused ion beam technique (FIB) allowed identifying porosity down to the nm scale. Between 40 and 50% of all pores could be identified as in-situ pores without any damage related to sample preparation. The total porosity estimated from TEM micrographs (1–5%) is comparable to the connected fault rock porosity (2.8–6.7%) estimated by pressure-induced injection of mercury. Permeability estimates for cataclastic fault rocks are 10? 21–10? 19 m2 and 10? 17 m2 for the fractured fault rock. Porosity and permeability are independent of sample depth. TEM images reveal that the porosity is intimately linked to fault rock composition and associated with deformation. The TEM-estimated porosity of the samples increases with increasing clay content. The highest porosity was estimated in the vicinity of an active fault trace. The largest pores with an equivalent radius > 200 nm occur around large quartz and feldspar grains or grain-fragments while the smallest pores (equivalent radius < 50 nm) are typically observed in the extremely fine-grained matrix (grain size < 1 μm). Based on pore morphology we distinguish different pore types varying with fault rock fabric and alteration. The pores were probably filled with formation water and/or hydrothermal fluids at elevated pore fluid pressure, preventing pore collapse. The pore geometry derived from TEM observations and BET (Brunauer, Emmett and Teller) gas adsorption/desorption hysteresis curves indicates pore blocking effects in the fine-grained matrix. Observations of isolated pores in TEM micrographs and high pore body to pore throat ratios inferred from mercury injection suggest elevated pore fluid pressure in the low permeability cataclasites, reducing shear strength of the fault.  相似文献   

3.
Inversion of local earthquake travel times and joint inversion of receiver functions and Rayleigh wave group velocity measurements were used to derive a simple model for the velocity crustal structure beneath the southern edge of the Central Alborz (Iran), including the seismically active area around the megacity of Tehran. The P and S travel times from 115 well-located earthquakes recorded by a dense local seismic network, operated from June to November 2006, were inverted to determine a 1D velocity model of the upper crust. The limited range of earthquake depths (between 2 km and 26 km) prevents us determining any velocity interfaces deeper than 25 km. The velocity of the lower crust and the depth of the Moho were found by joint inversion of receiver functions and Rayleigh wave group velocity data. The resulting P-wave velocity model comprises an upper crust with 3 km and 4 km thick sedimentary layers with P wave velocities (Vp) of ～5.4 and ～5.8 km s?1, respectively, above 9 km and 8 km thick layers of upper crystalline crust (Vp ～6.1 and ～6.25 km s?1 respectively). The lower crystalline crust is ～34 km thick (Vp ～ 6.40 km s?1). The total crustal thickness beneath this part of the Central Alborz is 58 ± 2 km.  相似文献   

4.
A new IRSL dataset is presented for the age and setting of a critical Late Glacial Maximum tephra isochron marker. The rhyolitic tephra, known as the Kawakawa Tephra, occurs as a 14 cm thick layer within a 5.9 m thick loess section overlying alluvial gravels in the Rangitikei River valley, SW North Island of New Zealand. Ages range from 21 at the base to 5 ka near the top of the loess and bracket an age of 17.0 ± 2.2 for the tephra. The new IRSL ages are in agreement with published and unpublished luminescence ages from other localities of loess, sand and ash above and below the tephra and of the tephra itself, that indicate an age of ca. 19 ka for the Kawakawa Tephra. This age is considerably younger than the generally accepted 14C 27.1 ka cal yrs BP age of the Kawakawa Tephra and highlights an unresolved discrepancy between the two dating systems.  相似文献   

5.
In the present study, the uppermost 3 cm of muddy, seabed sediment was collected from a deep-sea sediment core, drilled from a water depth of 4 km, near the deepest site of the Mediterranean Sea, outside Pylos, Greece. The core was divided into 7 layer samples, each 3–4 mm thick, in order to get an independent age assessment for each one using luminescence dating; from polymineral coarse grains in the range 30–60 μm. Between 11 and 22 aliquots were measured for each sample using the optically stimulated luminescence, single-aliquot regenerative-dose analysis. The estimated mean equivalent dose values had an uncertainty less than 3% and resulted in optical ages ranging from 3.5 to approximately 5 ka, with uncertainties lying between 5.5 and 7.4%. Multiple-aliquot, thermoluminescence-based, additive-dose, total bleaching approach provided equivalent doses with typical errors of 10–15% and ages in the range of 3.6 and 9.4 ka, with uncertainties up to 17%. Single-aliquot optical ages are shown to be relatively successful, due to their consistency with AMS 14C radiocarbon ages, obtained from Planktonic foraminifera from the same core. Luminescence dates for the topmost 1.5 cm indicate a substantial mix and post burial disturbance of the surface sediment. Below the topmost 1.5 cm, both luminescent approaches indicate ages which increase smoothly with depth. The concordant OSL and TL age estimates for the sample O5, in conjunction with specific luminescence properties and its major-element geochemical chemistry content, suggest that it was heated during the Santorini volcanic eruption. Deposition rates of 8.6–18.9 cm over 10 ka below the sample O5, provided by optical ages, exhibit an excellent level of agreement with the accumulation rate of 7–18 cm over 10 ka at the sea bottom, already reported for the site under study.  相似文献   

6.
Explosion deep seismic sounding data sections of high quality had been obtained with RV Meteor in the Reykjanes Iceland Seismic Project (RRISP77 [Angenheister, G., Gebrande, H., Miller, H., Goldflam, P., Weigel, W., Jacoby, W.R., Pálmason, G., Björnsson, S., Einarsson, P., Pavlenkova, N.I., Zverev, S., Litvinenko, I.V., Loncarecic, B., Solomon, S., 1980. Reykjanes Ridge Iceland Seismic Experiment (RRISP 77). J. Geophys. 47, 228–238]) which close an information gap near 62°N. Preliminary results were presented by Weigel [Weigel, W., 1980. Aufbau des Reykjanes Rückens nach refraktionsseismischen Messungen. In: Weigel, W. (Ed.), Reykjanes Rücken, Island, Norwegischer Kontinentalrand. Abschlusskolloquium, Hamburg zur Meteor-Expedition, vol. 45. DFG, Bonn, pp. 53–61], and here we report on the data and results of interpretation. Clear refracted phases to 90 km distance permit crustal and uppermost mantle structure to be modelled by ray tracing. The apparent P-wave velocities are around 4.5, 6–6.5, 7–7.6 and 8.2–8.7 km/s, but no wide-angle reflections have been clearly seen. Accompanying sparker reflection data reveal thin sediment ponds in the axial zone and up to 400 m thick sediments at 10 Ma crustal age. Ray tracing reveals the following model below the sediments: (1) a distinct, 1–2 km thick upper crust (layer 2A) with Vp increasing with age (to 10 Ma) from <3.4 to 4.9 km/s and with a vertical gradient of 0.1–0.2 km/s/km, (2) a lower crust or layer 3 beginning at depths of 2 (axis) to 4 km (10 Ma age) below sea level with 6.1–6.8 km/s and similar vertical gradients as above, (3) the lower crust bottoms at 5.2–9.5 km depth below sea level (0–10 Ma) with a marked discontinuity, underneath which (4) Vp rises from about 7.5–7.8 km/s (0–10 Ma) with a positive vertical gradient of, again, 0.1–0.2 km/s/km such that 8 km/s would be reached at 12 km and deeper near the axis. Our preferred interpretation is that the mantle begins at the distinct discontinuity (“Moho”), but a deeper “Moho” of Vp ≈ 8 km/s cannot be excluded. From Iceland southward to 60°N several experiments show a decrease of crustal thickness from 14 to 8 km. Velocity trends with age across the ridge reflect cooling and filling of cracks, and thickness trends probably suggest volcanic productivity variations as previously suggested.Gravity inversion concentrates on a profile across the ridge with the above seismic a priori information; with 0.2–0.5 km depth uncertainty it leads to a good fit (±2.5 mGal where seismic data exist). Best fitting densities are (in kg/m3) for sediments, 2180; upper crust, 2450–2570; lower crust, 2850–2940; mantle lithosphere, 3215–3240 with a deficit for an asthenospheric wedge of no more than −100 kg/m3. The morphological ridges and troughs superimposed on the SE ridge flank are partly correlated, partly anti-correlated with the Bouguer anomaly and suggest that variable crustal density variations accompany the morphology variations.  相似文献   

7.
The Late Jurassic Kimmeridge Clay Formation (KCF) is an economically important, organic-rich source rock of Kimmeridgian–Early Tithonian age. The main rock types of the KCF in Dorset, UK, include grey to black laminated shale, marl, coccolithic limestone, and dolostone, which occur with an obvious cyclicity at astronomical timescales. In this study, we examine two high-resolution borehole records (Swanworth Quarry 1 and Metherhills 1) obtained as part of a Rapid Global Geological Events (RGGE) sediment drilling project. Datasets examined were total organic carbon (TOC), and borehole wall microconductivity by Formation Microscanner (FMS). Our intent is to assess the rhythmicity of the KCF with respect to the astronomical timescale, and to discuss the results with respect to other key Late Jurassic geological processes. Power spectra of the untuned data reveal a hierarchy of cycles throughout the KCF with ～ 167 m, ～ 40 m, 9.1 m, 3.8 m and 1.6 m wavelengths. Tuning the ～ 40 m cycles to the 405-kyr eccentricity cycle shows the presence of all the astronomical parameters: eccentricity, obliquity, and precession index. In particular, ～ 100-kyr and 405-kyr eccentricity cycles are strongly expressed in both records. The 405-kyr eccentricity cycle corresponds to relative sea-level changes inferred from sequence stratigraphy. Intervals with elevated TOC are associated with strong obliquity forcing. The 405-kyr-tuned duration of the lower KCF (Kimmeridgian Stage) is 3.47 Myr, and the upper KCF (early part of the Tithonian Stage, elegans to fittoni ammonite zones) is 3.32 Myr. Two other chronologies test the consistency of this age model by tuning ～ 8–10 m cycles to 100-kyr (short eccentricity), and ～ 3–5 m cycles to 36-kyr (Jurassic obliquity). The ‘obliquity-tuned’ chronology resolves an accumulation history for the KCF with a variation that strongly resembles that of Earth's orbital eccentricity predicted for 147.2 Ma to 153.8 Ma. There is evidence for significant non-deposition (up to 1 million years) in the lowermost KCF (baylei–mutabilis zones), which would indicate a Kimmeridgian/Oxfordian boundary age of 154.8 Ma. This absolute calibration allows assignment of precise numerical ages to zonal boundaries, sequence surfaces, and polarity chrons of the lower M-sequence.  相似文献   

8.
The diffusion behavior of HA and Nd in the presence of HA in compacted bentonite was investigated experimentally by means of the through-diffusion method. Breakthrough of HA is observed in 1 and 0.1 mol dm−3 NaCl solution and is more significant with a lower dry density such as 1.2 Mg m−3. The one dimensional diffusion model taking parallel complexation equilibrium into account was fitted to the experimentally obtained breakthrough curves and concentration profiles, and the diffusion parameters, such as effective diffusivity and rock capacity factor, were evaluated. The obtained effective diffusivity, around 10−11 m2 s−1, for HA and Nd–HA is comparable to the previously reported value. Using these parameters, predictive calculations were performed to evaluate the effect of HA concentration and sorption distribution coefficient. It is indicated that the effect of sorption distribution coefficient is significant only for a short period and that relatively low HA concentrations might bring higher diffused mass depending on the diffusion behavior of dominant species.  相似文献   

9.
Cement-based materials play an important role in multi-barrier concepts developed worldwide for the safe disposal of hazardous and radioactive wastes. Cement is used to condition and stabilize the waste materials and to construct the engineered barrier systems (container, backfill and liner materials) of repositories for radioactive waste. In this study, bulk-X-ray absorption spectroscopy (XAS) was used to investigate the uptake mechanism of Nd on the crystalline C–S–H phase 11 Å tobermorite in the presence of Zn (co-absorbing metal), and vice versa, as potential competitor under strongly alkaline conditions (pH = 12.5–13.3). The Zn and Nd concentration in all samples was 50,000 ppm, whereas the reaction times varied from 1 to 6 months.Extended X-ray absorption fine structure (EXAFS) data of the Nd LIII-edge indicate that the local structural environment of Nd consists of ∼7–8 O atoms at 2.42 Å, ∼7–8 Si at ∼3.67 Å and ∼5–6 Ca at ∼3.8 Å, and that this environment remains unchanged in the presence and absence of Zn. In contrary, Zn K-edge EXAFS data exhibit distinct differences in the presence and absence of Nd as co-absorbing element. Data analysis indicates that Zn is tetrahedrally coordinated (∼4 O at ∼1.96 Å) and the obtained structural data in the simultaneous presence of Nd and Zn are consistent with the formation of mixed Zn surface complexes and Zn bound in the interlayer remaining in these positions also with prolonged reaction times (up to 6 months). However, without the co-absorbing element Nd, strong structural changes in the uptake mechanisms of Zn are observable, e.g., after 3 month reaction time Zn–Zn backscattering pairs can be observed. These findings suggest that Nd has an influence on the incorporation of Zn in the tobermorite structure. In addition, the results of this study indicate that competitive uptake of metal cations with similar sorption behaviour by C–S–H phases can take place, deserving further attention in future assessments of the safe disposal of radioactive wastes in cement-based repositories.  相似文献   

10.
We have compiled a dataset containing information on patterns of burial and erosion that occurred during the last ca. 30 Ma in the Campine area, north-eastern Belgium. This area coincides with the occurrence of the Rupelian Boom Clay which is considered as one of the potential host formations for radwaste disposal. Patterns and estimates of erosion and sedimentation, based on geological and geomorphological data, from several locations within the Boom Clay outcrop and subcrop zone, show increasing burial in northern and north-eastern direction over the last 30 Ma. More southern areas have experienced erosion and denudation during some geological stages, usually in the order of several 10 m, up to 100 m at most at two discrete locations. During the Quaternary, erosion is a widespread phenomenon in the Campine area, except in the Roer Valley Graben. The obtained estimates of erosion and burial are compared and discussed in the context of the overall geodynamic evolution of north-western Europe.  相似文献   

11.
Outcrops of the Cretaceous Upper sandstone formation some 375 km to the East of Addis Ababa on the motor Highway to Harar was paleomagnetically investigated. About seventy core samples were collected at various stratigraphic levels from 250–300 meters thick sedimentary formation. After standard sample preparations in the laboratory the resulting specimens were subjected to routine paleomagnetic demagnetization protocol. In the first steps of demagnetizations process the recent and viscous magnetizations were removed by heating until a temperature of level of 300 °C. Further demagnetization of the samples resulted in the isolation of the final magnetization with stable line segments that is directed towards the origin, which is interpreted as Characteristic Remanent Magnetization (ChRM). Rock – magnetic experiments have identified goethite (αFeOOH), hematite (αFe2O3), detritial hematite, and magnetite as the magnetic mineral phases carrying the remanence. The ChRM identified resulted in an average value of (Ds = 0.5°, Is = ?0.7°, α95 = 4.3°, N = 34) for the red sandstones while an average value of (Ds = 335.8°, Is = ?31.8°, α95 = 4.7°, N = 14) for the limestone intercalations. The former ChRM in the red sandstone is determined to be secondary while the latter ChRM is known to be primary. Comparison of these directional results and their pole equivalents with the African plate Apparent Polar Wander Path curve established by Besse and Courtillot (2003) give ages of between 115–130 Million years for the limestone intercalation and ages of 30 million years for red sandstone unit. These are interpreted respectively as estimates of the age of deposition and a later remagnetization respectively.  相似文献   

12.
In this work we present the hydrogeophysical imaging of a key sector of the Quaternary Po foreland basin (northern Italy), focussing on the reconstruction of clastic aquifers and aquitards in a complex tectono-sedimentary subsurface architecture. The study area includes the relic reliefs of Casalpusterlengo and Zorlesco, two smooth morphological features involving uplifted and gently folded Pleistocene marine to alluvial sediments, plausibly linked to the buried Northern Apennines thrust and fold belt. The geophysical data include 35 Direct Current Vertical Electrical Soundings collected over a 37 km2 wide area, acquired with Schlumberger array and maximum half-spacing of 500 m. 1-D resistivity-depth profiles were computed for each VES. An integrated hydrostratigraphic approach was applied, to constrain the interpretation of the geophysical data along several cross-sections, including the comparison of resistivity soundings to stratigraphic logs, borehole electric logs and the pore-water properties.The resistivity interfaces, traceable with the same laterally continuous vertical polarity, were used to develop an electrostratigraphic model in order to portray the stacking of electrostratigraphic units down to 200 m below ground surface. Their vertical associations show a general upward increase of electrical resistivity. This assemblage mimics the regional coarsening upwards depositional trend, from the conductive units of the Plio-Pleistocene marine-to-transitional depositional systems to the resistive units of the Middle–Late Pleistocene fluvial and alluvial plain depositional systems. Middle Pleistocene depositional systems host an alternation of North-dipping, high-to-intermediate permeability aquifer systems (70–180 Ωm, thickness of 5–70 m) separated by low permeability aquitards (20–50 Ωm, thickness up to 40 m). These units pinch out against the Casalpusterlengo and Zorlesco relic reliefs, where they cover the uplifted and folded regional aquitard (20–50 Ωm) formed by Pliocene-Lower Pleistocene clays to sandy silts with gravel lenses in agreement with borehole data. In the deepest part of the local stratigraphy, a broad low-resistivity anomaly (< 10 Ωm) was clearly mapped through the study area. By comparison with electrical borehole logs in deep oil-wells, it could be interpreted as the fresh–saltwater interface due to the presence of connate waters and brines hosted by the marine-to-transitional shales.  相似文献   

13.
《Marine pollution bulletin》2009,58(6-12):883-888
Mussels (Mytilus galloprovincialis) were transplanted to seven stations around a large shipyard for 126 days to evaluate tributyltin (TBT) contamination. Although the application of TBT-based paints to ships is totally banned in Korea, butyltin compounds were found to accumulate in mussels following transplantation. Concentrations of TBT and total butyltins in transplanted mussels near the shipyard were in the range of 40–350 ng Sn/g and 74–530 ng Sn/g on a dry weight basis, respectively. Obviously, low TBT concentrations (6.0–53 ng Sn/g dw) were determined in mussels at four stations outside the shipyard. A negative gradient of TBT concentrations and TBT portion to total butyltin concentrations were found in both the surface water and transplanted mussels according to distance from the shipyard. In addition, TBT concentrations in surface water and transplanted mussels showed significant correlation (r2 = 0.71; p < 0.001). These results indicate that the shipyard still releases fresh TBT into surrounding waters even after TBT regulation in Korea, and mussel transplantation is useful in evaluating TBT contamination in shipyard area.  相似文献   

14.
《Continental Shelf Research》2006,26(17-18):2141-2156
A predominant sigmoidal clinoform deposit extends from the Yangtze River mouth southwards 800 km along the Chinese coast. This clinoform is thickest (∼40 m) between the 20 and 30 m isobaths and progressively thins offshore, reaching water depths of 60 and 90 m and distances up to 100 km offshore. Clay mineral, heavy metal, geochemical and grain-size analyses indicate that the Yangtze River is the primary source for this longshore-transported clinoform deposit. 210Pb chronologies show the highest accumulation rates (>3 cm/yr) occur immediately adjacent to the Yangtze subaqueous delta (north of 30 °N), decreasing southward alongshore and eastward offshore. The interaction of strong tides, waves, the China Coastal Current, winter storms, and offshore upwelling appear to have played important roles in trapping most Yangtze-derived sediment on the inner shelf and transporting it to the south.  相似文献   

15.
At the appropriate times, silica diffusion in clay is possibly the rate determining process for the dissolution of vitrified waste disposed of in a clay layer. For testing this hypothesis, combined glass dissolution/silica diffusion experiment are performed. SON68 glass coupons doped with the radioactive tracer 32Si are sandwiched between two cores of humid Boom Clay, heated to 30 °C. Due to glass dissolution, 32Si is released and diffuses into the clay. At the end of an experiment, the mass loss of the glass coupon is measured and the clay core is sliced to determine the diffusion profile of the 32Si released from the glass in the clay.Both mass loss and the 32Si diffusion profile in the clay are described well by a model combining glass dissolution according to a linear rate law with silica diffusion in the clay. Fitting the experiments to this model leads to an apparent silica diffusion coefficient in the clay between 7 × 10−13 m2/s and 1.2 × 10−12 m2/s. Previously determined values from diffusion experiments at 25 °C are around 6 × 10−13 m2/s (In-Diffusion experiments) and 2 × 10−13 m2/s (percolation experiments). The maximal glass dissolution rate for glass next to clay is around 1.6 × 10−7 g glass/m2 s (i.e. 0.014 g glass/m2 day). In undisturbed clay, the measured silica concentration is around 5 mg/L. Combining these values with the previously measured (In-Diffusion experiments) product of accessible porosity and retardation factor, leads in two ways to a silica glass saturation concentration in clay between 8 and 10 mg Si/L.Another candidate for the rate determining process of the dissolution of vitrified waste disposed in a clay layer is silica precipitation. Although silica precipitation due to glass dissolution has been shown experimentally at 90 °C, extending the model with silica precipitation does not lead to much better fits, nor could meaningful values of a possible precipitation rate be obtained.  相似文献   

16.
The study analyses the long-term biophysical and demographic changes in Dal lake, located in the heart of Srinagar city, Kashmir India, using a repository of historical, remote sensing, socio-economic and water quality data supported by the extensive field observations. The lake faces multiple pressures from the unplanned urbanization, high population growth, nutrient load from intensive agriculture and tourism. The data showed that the lake has shrunk from 31 km2 in 1859–24 km2 in 2013. Significant changes were observed in the land use and land cover (LULC) within the lake (1859–2013) and in the vicinity of the lake (1962–2013). Analysis of the demographic data indicates that the human population within the lake has shown more than double the national growth rate. Additionally, 7 important water quality parameters from 82 well distributed sites across the lake were analyzed and compared with the past data to determine the historical changes in the water quality from 1971 to 2014. The changes in the LULC and demography have adversely affected the pollution status of this pristine lake. Ortho-phosphate phosphorous concentration has increased from 16.75 μg L−1 in 1977–45.78 μg L−1 in 2014 and that of the nitrate-nitrogen from 365 μg L−1 to 557 μg L−1, indicating nutrient enrichment of the lake over the years. Built-up area within the lake has increased 40 times since 1859, which, together with the changes in the population and settlements, have led to the high discharge of untreated nutrient-rich sewage into the lake. Similarly the expansion of floating gardens within the lake and agriculture lands in the catchment has contributed to the increased nutrient load into the lake due to the increasing use of fertilizers. The information about the existing land cover, demography and water quality was integrated and analyzed in GIS environment to identify the trophic status of the lake. The analysis indicated that 32% of the lake falls under sever degradation, 48% under medium degradation while as 20% of the lake waters are relatively clean. It is believed that the results provide improved knowledge and insights about the lake health and causal factors of its degradation necessary for effectively restoring its ecological and hydrological functionality.  相似文献   

17.
SHRIMP zircon U–Pb dating, mineral chemical, element geochemical and Sr–Nd–Pb–Hf isotopic data have been determined for the Yulong monzogranite-porphyry in the eastern Tibet, China. The Yulong porphyry was emplaced into Triassic strata at about 39 Ma. The rocks are weakly peraluminous and show shoshonitic affinity, i.e., alkalis-rich, high K2O contents with high K2O / Na2O ratios, enrichment in LREE and LILE. They also show some affinities with the adakite, e.g., high SiO2 and Al2O3, and low MgO contents, depleted in Y and Yb, and enrichment in Sr with high Sr / Y and La / Yb ratios, and no Eu anomalies. The Yulong porphyry has radiogenic 87Sr / 86Sr (0.7063–0.7070) and unradiogenic 143Nd / 144Nd (εNd = − 2.0 to − 3.0) ratios. The Pb isotopic compositions of feldspar phenocrysts separated from the Yulong porphyry show a narrow range of 206Pb / 204Pb ratios (18.71–18.82) and unusually radiogenic 207Pb / 204Pb (15.65–15.67) and 208Pb / 204Pb (38.87–39.00) ratios. In situ Hf isotopic composition of zircons that have been SHRIMP U–Pb dated is characterized by clearly positive initial εHf values, ranging from + 3.1 to + 5.9, most between + 4 and + 5. Phenocryst clinopyroxene geothermometry of the Yulong porphyry indicates that the primary magmas had anomalously high temperature (> 1200 °C). The source depth for the Yulong porphyry is at least 100 km inferred by the metasomatic volatile phase (phlogopite–carbonate) relations. Detailed geochemical and Sr–Nd–Pb–Hf isotopic compositions not only rule out fractional crystallization or assimilation-fractional crystallization processes, but also deny the possibility of partial melting of subducted oceanic crust or basaltic lower crust. Instead, low degree (1–5%) partial melting of a metasomatized lithosphere (phlogopite–garnet clinopyroxenite) is compatible with the data. This example gives a case study that granite can be derived directly by partial melting of an enriched lithospheric mantle, which is important to understand the source and origin of diverse granites.  相似文献   

18.
Narrow bipolar events (NBEs) are a distinct class of intra-cloud lightning discharge. In this paper we present observations of 10 negative and 67 positive such events in East China. Positive NBEs occurred at 7–12 km altitude above mean sea level (MSL) with a mean altitude of 9.5 km, and negative NBEs occurred at 14–16 km altitude. Electrical/channel characteristics of these events were derived from NBE pulse waveforms based on the transmission-line model. On average, the peak current moment and the charge moment change of a NBE event is 15 kA km, and 0.12 C km, respectively. The mean time for the propagation of current front along the channel is 2.2 μs. The upper limit on channel length for NBEs in this study is 510–1060 m, the lower limit on discharge current amplitude is 12.5–43.2 kA, and the minimum charge transfer is 0.1–0.3 C.  相似文献   

19.
The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U–Th)/He dating we observe variable slip rates across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8 ± 0.8 to 9.7 ± 0.8 Ma and 11.2 ± 1.6 to 8.2 ± 1.2 Ma and (U–Th)/He zircon and apatite ages are between 10.4 ± 0.4 to 9.2 ± 0.3 Ma and 10.7 ± 1.0 to 8.9 ± 0.6 Ma, respectively. On Paros, ZFT and AFT ages range from 13.1 ± 1.4 Ma to 11.1 ± 1.0 Ma and 12.7 ± 2.8 Ma to 10.5 ± 2.0 Ma while the (U–Th)/He zircon ages are slightly younger between 8.3 ± 0.4 Ma and 9.8 ± 0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of ∼ 6–8 km Myr− 1 on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at ∼ 12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of ∼ 50 km between ∼ 16 and 8 Ma.  相似文献   

20.
Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average Vp/Vs ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.  相似文献   

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