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1.
This study is the first quantification of the combined impact of diffusive and advective paleoclimatic phenomena to explain the weak vertical thermal flux anomaly in the upper part of the Anglo-Paris intracratonic sedimentary basin in northern France. The aim of the research is to understand the mechanisms at the origin of the thermal flux anomaly at the level of the Meso-Cenozoic sediment pile. Based on a temperature profile representative of the basin, transient thermo-hydraulic simulations were performed along a representative vertical cross-section of about 400 km within the Lower Cretaceous multi-layer aquifer. Four paleoclimatic scenarios are the combination of two paleotemperature climatic forcings and two hydrodynamic regimes, one of them taking into account the interruption of the recharge linked to permafrost development. The simulation results clearly show the transient nature of the basin’s thermal regime. Then, for the reference well, the majority of the thermal flux anomaly can be explained by advective and paleoclimatic mechanisms with a decrease in geothermal flux simulated up to a little over 30 mW/m2, depending on the scenarios. Decrease in heat flux because of basin-scale subsurface flows in the Lower Cretaceous is around 15 mW/m2. There are several ways forward from this first simple model, including simulation of development of permafrost and also the integration of vertical flows in the basin by use of a three-dimensional model to better explain the data.  相似文献   

2.
Priya  Kumari  Nadimpalli  Raghu  Osuri  Krishna K. 《Natural Hazards》2021,109(2):1655-1674

The timely prediction of thunderstorms (TS) is always a challenging task for operational and research community. The present study is aimed to address the credibility of the high grid-spacing and downscaling approach for improved simulation of TS. Fourteen TS are simulated with different domain configurations using weather research and forecasting (WRF) model. Two nested domains with 9–3 km (known as DD3), and 6–2 km (DD2), and 3 km single domain (SD3) are considered for simulations. Results indicate that the high-resolution DD2 has improved 2-m temperature (T2), 2-m relative humidity (RH2), and 10-m wind speed (WS10) at different stages of TS. The average mean error of T2 and RH2 in the DD2 experiment is 0.7 °C, ??6% during the mature stage, and 0.2 °C, ??4% at dissipating stage. The error in SD3 and DD3 is relatively higher (9–17% for T2 and 20–60% for RH2). Better horizontal and vertical representation of thermodynamic variables in DD2 run reinforces the atmosphere to initiate and intensify the convection in the right place. The DD2 could show slightly higher instability (convective available potential energy, CAPE, 3188 J kg?1) as compared with DD3 (3164 J kg?1) and SD3 (3020 J kg?1). The model is biased to simulate early TS activity. DD2 run could simulate the formation, mature and dissipation stages with fewer timing errors (??1.35 h, ??1.5 h, and ??2.6 h, respectively) than other experiments. The critical success index of the DD2 run is higher for all the rainfall thresholds; however, it is more than 0.2 up to 2.5 mm h?1. The results highlight that high resolution nested configuration yields better simulation skills than the single domain configuration.

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3.
Wang  Fei  Konietzky  Heinz  Frühwirt  Thomas  Dai  Yajie 《Acta Geotechnica》2020,15(8):2259-2275

The knowledge about thermo-mechanical properties of granite is still limited to some extent. Individual measurements are necessary to obtain reliable properties for specific granite types. A reliable numerical model of thermal cracking behaviours of granite exposed to extreme high temperatures (e.g. 800–1000 °C) is missing. In this study, the impact of temperature up to 1000 °C on physical, mechanical, and thermal properties as well as thermo-mechanical coupled behaviour of Eibenstock granite was investigated by laboratory testing and numerical simulations. The physical properties including mineral composition, density, P-wave velocity, and open porosity are measured to be temperature dependent. Uniaxial compression and Brazilian tests were carried out to measure uniaxial compressive strength (UCS), Young’s modulus, stress–strain relationship, and tensile strength of Eibenstock granite before and after thermal treatment, respectively. Thermal properties including specific heat, thermal conductivity, thermal diffusivity, and linear thermal expansion coefficient are also measured and found to be temperature dependent, especially the expansion coefficient which shows a steep increase around 573 °C as well as at 870 °C. The numerical simulation code FLAC3D was used to develop a numerical scheme to simulate the thermal-induced damage of granite at high temperatures. Statistical methods combined with real mineral composition were used to characterize the heterogeneity of granite. The numerical model is featured with reliable temperature-dependent parameters obtained from laboratory tests. It can well reproduce the laboratory results in form of thermal-induced micro- and macrocracks, as well as the stress–strain behaviour and the final failure pattern of Eibenstock granite after elevated temperatures up to 1000 °C. The simulation results also reveal that the thermal-induced microcracks are randomly distributed across the whole sample. Although most thermal-induced damages are tensile failures, shear failure begins to develop quickly after 500 °C. The obvious UCS reduction in granite due to heating is mainly caused by the increase in shear failure. The simulation also shows that the dominant impact of α–β quartz transition is widening pre-existing cracks rather than the formation of new microcracks.

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4.
The structure of mantle convection and spatial fields of superlitho static pressure and vertical and horizontal stresses in the Earth’s mantle are studied in a 2D numerical model with non-Newtonian viscosity and heat sources. The model demonstrates a jump-like motion of subduction zones and reveals abrupt changes in the stress fields depending on the stage of slab detachment. The stresses decrease dramatically in the areas without slabs. The horizontal stresses oxx, superlitho static pressure, and vertical stresses ozz in the part of the mantle lacking intense near-vertical flows are approximately equal, varying within ± 6, ± 8, and ± 10 MPa, respectively. However, these fields are stronger in the areas of descending slabs, where the values of the above parameters are about an order of magnitude higher (± 50 MPa).This result agrees with the current views of the oceanic slabs as the most important gent of mantle convection. We have found significant differences among the oxx, ozz, and pressure fields. The pressure field reveals both the vertical and horizontal features of slabs and plumes, clearly showing their long thermal conduits with broader heads. The distributions of oxx are sensitive to the near-horizontal parts of the flows, whereas the ozz fields reveal mainly their vertical substructures. The model shows the presence of relatively cold remnants of slabs in the lower mantle above the thermal boundary layer. Numerous hot plumes penetrating through these high-viscosity remnants, as well as the new descending slabs, induce intense stress fields in the lower mantle, which are strongly inhomogeneous in space and time.  相似文献   

5.
《Quaternary Science Reviews》2007,26(15-16):2019-2029
Three potential mechanisms behind centennial-scale Holocene cooling events are studied in simulations performed with the coupled climate model ECBilt–CLIO: (1) internal variability, (2) solar forcing, and (3) freshwater forcing. In experiments with constant preindustrial forcings, three centennial-scale cooling events occur spontaneously in 15,000 years. These rare events represent an unstable internal mode of variability that is characterised by a weaker thermohaline circulation, a more southward location of the main site of deep-water formation, expanded sea-ice cover and cooling of 10 °C over the Nordic Seas. This mode is visited more frequently when the climate is cooled by abruptly reducing the solar constant by 5 or 3 Wm−2. Prescribing a solar forcing of the same magnitude, but following a sinusoidal function with a period of 100 or 1000 years, does not result in any centennial-scale cooling events. The latter forcing does however result in more frequent individual cold years in the North Atlantic region that are related to local weakening of the deep convection and sea-ice expansion. Adding realistic freshwater pulses to the Labrador Sea is also able to trigger centennial-scale cooling events with temperature anomalies resembling proxy evidence for the cooling event at 8.2 kyr BP, suggesting that freshwater forcing is a valid explanation for early Holocene cooling events.  相似文献   

6.
Among the various methods used to determine vertical water seepage in unsaturated soils, thermal convection presents significant advantages: temperature measurements are simple to perform and record, and a wide range of time scales can be considered. The authors analysed the data recorded by the meteorological stations of the Seine river basin, at three different depths: 20, 50 and 100 cm. As the measurement sensitivity was limited to 0.1 K, long series of data needed to be stacked in order to obtain sufficient precision to quantify the convective component of heat transfer, in a predominantly conductive context. For the period from 1984 to 2001, it was possible to determine the average recharge at each station, and the recharge variation between groups of three-year periods. By interpolating these data over the whole basin, a global assessment has been made and compared to the exported flow rate at the river mouth: the resulting value of 94 mm yr?1 lies between the lowest annual rate, 52 mm yr?1, and the mean total exported value of 252 mm yr?1.  相似文献   

7.
In the Caledonides of northwest Scotland, two independent geothermometers (Fe‐Mg exchange and quartz c‐axis fabric opening angle) are used to characterize the thermal structure of the lower part of the Scandian (435–420 Ma) orogenic wedge within the Moine, Ben Hope and Naver‐Sgurr Beag thrust sheets. Traced from west (foreland) to east (hinterland), Fe‐Mg exchange thermometry yields peak or near‐peak temperatures ranging from 484 ± 50 °C to 524 ± 50 °C in the immediate hangingwall of the Moine thrust to 601 ± 50 °C in the immediate hangingwall of the Ben Hope thrust, to 630 ± 50 °C in the Naver thrust sheet. Preserved metamorphic facies and textural relationships are consistent with thermometric estimates. Deformation temperatures calculated from quartz c‐axis fabric opening angles across two similar orogen‐perpendicular transects also yield systematic increases (Glen Golly – Ben Klibreck, 520–630 °C; Ullapool‐Contin, 465–632 °C) traced towards the Naver and Sgurr Beag thrusts. In addition, deformation temperatures show a pronounced increase along the leading edge of the Moine thrust sheet moving south towards the Assynt window, which is interpreted to reflect deeper exhumation of the thrust plane above the Assynt footwall imbricate stack. Because temperatures calculated from metamorphic assemblages are within error of the quartz fabric‐derived deformation temperatures that are of demonstrably Scandian age, the metamorphic sequence between the Moine and Naver‐Sgurr Beag thrusts is interpreted to have developed during the Scandian orogeny. Integration of our results with previous 2D thermal‐mechanical studies allows development of new conceptual thermal‐kinematic models of Scandian orogenesis that may be broadly applicable to other collisional systems. Furthermore, it highlights the critical nature of coupling between orogen kinematic and thermal evolution.  相似文献   

8.
The aim was to study density-driven groundwater flow and analyse groundwater mixing because of seasonal changes in groundwater temperature. Here, density-driven convection in groundwater was studied by numerical simulations in a subarctic climate, i.e. where the water temperature was <4 °C. The effects of soil permeability and groundwater temperature (i.e. viscosity and density) were determined. The influence of impermeable obstacles in otherwise homogeneous ground was also studied. An initial disturbance in the form of a horizontal groundwater flow was necessary to start the convection. Transient solutions describe the development of convective cells in the groundwater and it took 22 days before fully developed convection patterns were formed. The thermal convection reached a maximum depth of 1.0 m in soil of low permeability (2.71 · 10?9 m2). At groundwater temperature close to its density maximum (4 °C), the physical size (in m) of the convection cells was reduced. Small stones or frost lenses in the ground slightly affect the convective flow, while larger obstacles change the size and shape of the convection cells. Performed simulations show that “seasonal groundwater turnover” occurs. This knowledge may be useful in the prevention of nutrient leakage to underlying groundwater from soils, especially in agricultural areas where no natural vertical groundwater flow is evident. An application in northern Sweden is discussed.  相似文献   

9.
Based on the Crust2.0 model and the topography data of Chinese continent and its adjacent regions, a three-dimensional finite element model is constructed in terms of the spherical coordinate system. In our numerical model, the average annual ground temperature from 195 meteorological stations and temperature of upper mantle derived from the seismic velocities are adopted as the top and bottom boundary conditions, respectively. The observed thermal conductivity and heat production from P wave velocity based on empirical formula are employed in our numerical model as well. The comparison between the calculated and observed surface heat flow proved that our results are reliable. The temperature beneath the Precambrian cratons is lower than that of other areas for 100–300 °C also. The typical temperature rang at the Moho is estimated to be 800–1000 °C beneath the Tibetan plateau and 500–700 °C beneath the Precambrian cratons (such as Indian plate, Sichuan basin, South China, North China and Tarim), respectively. The thermal state in the eastern part of Sino-Korean craton at the depth deeper than 60 km indicates that it was destructed. The thermal structure in center of Tibetan plateau (especially beneath Qiangtang area) supports the proposed flow of lower crustal or upper mantle material to the east. Generally, the distribution of volcanoes in Chinese continent is consistent with the high temperature areas in the crust or upper mantle. There are many obvious thermal transition zones across the orogenic belts. The thermal transition zone between eastern and western parts in the crust of Chinese continent is consistent with the north–south seismic zone.  相似文献   

10.
The Proterozoic Athabasca basin and underlying basement host numerous unconformity-related uranium deposits that were formed from extensive fluid circulation near the basement-cover interface. Although it is generally agreed that the mineralizing fluids were basinal brines, it is still unclear what driving forces were responsible for the circulation of the basinal fluids. Because different fluid flow driving forces are associated with different thermal profiles, knowing the basin-scale distribution of paleo-fluid temperatures can help constrain the fluid flow mechanism. This study uses fluid inclusions entrapped in quartz overgrowths and authigenic illite in sandstones from three drill cores (WC-79-1, BL-08-01, and DV10-001) in the central part of the Athabasca basin as thermal indicators of paleo-fluids in the basin. A total of 342 fluid inclusions in quartz overgrowths were studied for microthermometry. The homogenization temperatures (Th) range from 50° to 235 °C, recording the minimum temperatures in various diagenetic stages. Temperatures estimated from illite geothermometry (121 points) range from 212° to 298 °C, which are systematically higher than (partly overlapping) the Th values, suggesting that illite was precipitated in hotter fluids following the formation of quartz overgrowths. Neither the fluid inclusion Th values nor the illite temperatures show systematic increase with depth in individual drill cores. This, together with the high illite temperatures that cannot be explained by burial at a normal geothermal gradient (35 °C/km), is interpreted to indicate that basin-scale fluid convection took place during the diagenetic history of the basin. Prolonged fluid convection is inferred to be responsible for delivering uranium (extracted from the basin or the upper part of the basement) to the unconformity, where uranium mineralization took place due to redox reactions associated with fluid-rock interaction or structurally controlled fluid mixing.  相似文献   

11.
Based on a numerical model of the Northeast German Basin (NEGB), we investigate the sensitivity of the calculated thermal field as resulting from heat conduction, forced and free convection in response to consecutive horizontal and vertical mesh refinements. Our results suggest that computational findings are more sensitive to consecutive horizontal mesh refinements than to changes in the vertical resolution. In addition, the degree of mesh sensitivity depends strongly on the type of the process being investigated, whether heat conduction, forced convection or free thermal convection represents the active heat driver. In this regard, heat conduction exhibits to be relative robust to imposed changes in the spatial discretization. A systematic mesh sensitivity is observed in areas where forced convection promotes an effective role in shorten the background conductive thermal field. In contrast, free thermal convection is to be regarded as the most sensitive heat transport process as demonstrated by non-systematic changes in the temperature field with respect to imposed changes in the model resolution.  相似文献   

12.
《Applied Geochemistry》2006,21(2):253-268
The Dalaman and Köyceğiz thermal springs are from karstic limestones belonging to Upper Cretaceous to Burdigalian Beydağları autochthon and Carboniferous to Lutetian Lycian nappes. They have measured temperatures of 24– 41 °C, specific electrical conductivities of 14,310–45,600 μS/cm, and are dominated by Na (1550–8500 mg/kg) and Cl (2725–15,320 mg/kg). The heat source of the geothermal systems of the area is tectonic related and the occurrence of the thermal springs is related to the young normal faults. Meteoric waters and seawaters recharge the reservoir rocks, are heated at depth with increasing geothermal gradient, and move up to the surface through the fractures and faults by convection trend and emerge as thermal springs. While thermal waters move up to the surface, they mix with different proportions of seawater and cold fresh waters. The seawater contribution to the thermal waters varies from 24% to 78%. Lake waters in the area are connected with thermal waters. Consequently, their chemical composition is influenced by the chemistry of thermal waters. Chemical equilibrium modelling based on measured outlet temperatures and measured pH shows that all the waters are oversaturated with respect to quartz and K-mica and undersaturated with respect to Al(OH)3, anorthite, gypsum, siderite and SiO2(a). Albite, alunite, aragonite, Ca-montmorillonite, calcite, chalcedony, chlorite, dolomite, Fe(OH)3(a), fluorite, gypsum, illite, K-feldspar, kaolinite and sepiolite minerals are mostly oversaturated or undersaturated. Mineral saturation studies of the thermal springs indicate that dolomite, chalcedony and quartz are most likely to cause scaling at outlet conditions. Assessments from various chemical geothermometers, and Na–K–Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature is around 65–90 °C. The temperatures obtained from quartz, quartz-steam loss, Mg/Li geothermometers and mineral equilibrium diagrams give the most reasonable results.  相似文献   

13.
The Uatumã silicic large igneous province (SLIP) has covered about 1,500,000 km2 of the Amazonian craton at ca. 1880 Ma, when the Columbia/Nuna supercontinent has been assembled. Paleomagnetic and geochronological data for this unit were obtained for the Santa Rosa and Sobreiro Formations in the Carajás Province, southwestern Amazonian craton (Central-Brazil Shield). AF and thermal demagnetizations revealed northern (southern) directions with high upward (downward) inclinations (component SF1), which passes a ‘B’ reversal test, and is carried by magnetite and SD hematite with high-blocking temperature. This component is present on well-dated 1877.4 ± 4.3 Ma (U-Pb zrn - LA-ICPMS) rhyolitic lava flows, providing the SF1 key paleomagnetic pole (Q = 6) located at 319.7°E, 24.7°S (A95 = 16.9°). A second southwestern (northeastern) direction with low inclination (Component SF2) was obtained for a well-dated 1853.7 ± 6.2 Ma (U-Pb zrn - LA-ICPMS) dike of the Velho Guilherme Suite. This component also appears as a secondary component in the host rhyolites of the Santa Rosa Fm and andesites of the Sobreiro Fm at the margins of the dike previously dated. Its primary origin is confirmed by a positive baked contact test, where a Velho Guilherme dike crosscuts the 1880 Ma andesite from the Sobreiro Formation. The corresponding SF2 key pole is located at 220.1°E, 31.1°S (A95 = 5°) and is classified with a reliability criterion Q = 7. The large angular distance between the almost coeval (difference of ~ 25 Ma) SF1 and SF2 poles implies high plate velocities (~ 39.3 cm/yr) which are not consistent with modern plate tectonics. The similar significant discrepancy of paleomagnetic poles with ages between 1880 and 1860 Ma observed in several cratons could be explained by a true polar wander (TPW) event. This event is the consequence of the reorganization of the whole mantle convection, and is supported by paleomagnetic reconstructions at 1880 Ma and 1860 Ma and also by geological/geochronological evidence.  相似文献   

14.
Accuracies of Real-Time Kinematic Global Positioning (RTK-GPS) system and Total Station (TS) were investigated in GIS environment. In geostatistical evaluations, Kriging method was used with spherical, exponential, and Gaussian models. The survey results demonstrated that an area of 3.5 ha or smaller can be best explained with Gaussian model, while the larger areas require a spherical model. A vertical error of 60 cm and a horizontal error of 30 cm can be observed when the survey points outside the construction area are eliminated. The optimum area per survey point was calculated to be 20×20 m2 to increase the accuracy. This case study showed that an inaccurate survey can result cost over estimations up to 27%.  相似文献   

15.
Chemical and isotopic compositions of three hot springs and one cold spring in the Kirkgecit geothermal field, located 15 km southwest of Canakkale-Biga in the northwest of Turkey, were monitored five times during 2005 and 2007. The physico-chemical characteristics of the hot springs are average discharge 3–3.5 L/s, surface temperature 45–52°C, pH 8.9–9.3, and electrical conductivity (EC) 620–698 μS/cm. The cold spring has a temperature of 12–13°C, pH 7.5–8.3, and EC 653–675 μS/cm. The hot waters are Na-SO4 type, whereas the cold water is Ca-HCO3 type. Chemical geothermometers suggest that the reservoir temperature is around 80–100°C. The isotopic data (oxygen-18, deuterium and tritium) indicate that the thermal waters are formed by local recharge and deep circulation of meteoric waters.  相似文献   

16.
本文对8个初始模型和7个组合模型中沿断层的水热对流、断层产状、山体地形和沉积盆地与基岩热导率反差等四个影响因素对传导型地表热流分布的影响进行了计算机模拟研究。模型设计和参数的选值以西藏中北部一些地热区实测的传导-对流型热流为主要参考依据,但不直接涉及对流组分的校正,而着眼于更广泛的单因子和多因素的模型研究。分析中采用无量纲参数:α=(K1)/(K2)(K1和K2分别为基岩和沉积盆地的热导率),β=(q1)/(q2)(q1和q2分别为地表热流的垂向分量和模型的底部热流)以及γ=L/H(L和H分别为离模型左侧边界的距离和山体的高度),以求更广的普适性。对模拟结果的分析表明,上述四项影响因素依其重要性可排序为对流强度—断层倾角—介质热导率反差—地形效应。  相似文献   

17.
This study aims to model temperature distributions in an aquifer thermal extraction (ATE) system that contains a single extraction well in a thin confined aquifer. The aquifer is bounded by hot dry rocks with different thermomechanical properties and thicknesses. Based on the heat convection–conduction equation, a mathematical model is developed to describe the spatial and temporal temperature distributions of the ATE systems. The mechanisms of heat transfer in the model involve horizontal convection and thermal conduction in the aquifer, and vertical thermal conduction in both rocks. A semi‐analytical solution in dimensionless form is developed using the Laplace transform technique and its corresponding time‐domain result is computed by the modified Crump method. In addition, the steady‐state solution is obtained by applying the final value theorem. The simulation results from the semi‐analytical solution indicate that the aquifer temperature distributions are affected by aquifer thickness, the thermomechanical properties of the aquifer and rocks, geothermal gradient, outer boundary temperatures of the rocks, extraction rate, and operating time. The present solution can be used as a preliminary tool for assessing heat extraction efficiency in ATE systems. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

18.
A series of thermal conductivity measurements for various materials was performed in a large climate chamber. The size of the chamber allowed the preparation of relatively large samples in a controlled thermal environment. Three types of thermal sensors were used: (1) two needle probes; (2) a grid of temperature sensors, evenly distributed inside the sample; (3) two additional thermal probes, which were simplified versions of an instrument originally developed for measuring thermal properties of the ice/dust mixture expected to exist at the surface of a comet nucleus. They consist of a series of individual temperature sensors integrated into a glass fibre rod. Each of these sensors can be operated in an active (heated) or passive (only temperature sensing) mode. The following sample materials were used: fine-grained reddish sand, coarse-grained moist sand, gravels with various grain size distributions from < 1 cm up to about 6 cm, and for comparison and calibration pure water (with convection suppressed by adding agar-agar), compact ice, and compact granite. Of particular interest are the measurements with composite samples, like stones embedded in an agar-agar matrix. We describe the evaluation methods and present the results of the thermal conductivity measurements.  相似文献   

19.
15 abrupt warming transitions perturbed glacial climate in Greenland during Marine Isotope Stage 3 (MIS 3, 60–27 ka BP). One hypothesis states that the 8–16 °C warming between Greenland Stadials (GS) and Interstadials (GI) was caused by enhanced heat transport to the North Atlantic region after a resumption of the Atlantic Meridional Overturning Circulation (AMOC) from a weak or shutdown stadial mode. This hypothesis also predicts warming over Europe, a prediction poorly constrained by data due to the paucity of well-dated quantitative temperature records. We therefore use a new evidence from biotic proxies and a climate model simulation to study the characteristics of a GS–GI transition in continental Europe and the link to enhanced AMOC strength. We compare reconstructed climatic and vegetation changes between a stadial and subsequent interstadial – correlated to GS15 and GI14 (～55 ka BP) – with a simulated AMOC resumption using a three-dimensional earth system model setup with early-MIS 3 boundary conditions. Over western Europe (12°W–15°E), we simulate twice the annual precipitation, a 17 °C warmer coldest month, a 8 °C warmer warmest month, 1300 °C-day more growing degree days with baseline 5 °C (GDD5) and potential vegetation allowing tree cover after the transition. However, the combined effect of frequent killing frosts, <20 mm summer precipitation and too few GDD5 after the transition suggest a northern tree limit lying at ～50°N during GI14. With these 3 climatic limiting factors we provide a possible explanation for the absence of forests north of 48°N during MIS 3 interstadials with mild summers. Finally, apart from a large model bias in warmest month surface air temperatures, our simulation is in reasonable agreement with reconstructed climatic and vegetation changes in Europe, thus further supporting the hypothesis.  相似文献   

20.
《Applied Geochemistry》2000,15(4):425-438
The dissolution of silica and diffusion of reactive dissolved Si in the porewaters of river sediments are investigated using sediments of different physical and chemical properties. Three sediments are considered: (a) from sectioned cores taken from a river-bed, (b) fine organic-rich surface sediment (<5 cm depth) installed in a fluvarium channel and, (c) coarse river sediment of low organic matter content also installed in a fluvarium channel. Dissolution rates of silica are measured at 10°C using batches of suspended material. The derived dissolution rate constants show large differences between the sediments. The river bed-sediment cores had vertical concentration profiles of dissolved Si that are consistent with the diffusion and dissolution of biogenic silica. Experiments in a fluvarium channel enabled Si fluxes to be calculated from a mass-balance of the overlying solution. The results are consistent with the attainment of a steady-state concentration profile of dissolved Si in the sediment. There are no discernible effects of water velocity over the sediment between 5 and 11 cm s−1. However, at 20 cm s−1, the flux increases as a result of either entrainment of fine particles at the surface or advective effects in the surface sediment. A fluvarium experiment with the fine sediment (<125 μm) over 61 days, produced a concentration profile with the highest concentration of 1025 μmol dm−3 at a depth of 4–5 cm in the sediment. A FORTRAN program is used to model the results of the increase in dissolved Si in the overlying water and development of a concentration profile in the porewater. This leads to a sediment diffusion coefficient of 1.21×10−9 m2 s−1 at 8.8°C at the beginning of the experiment and rate constant k=13.1×10−7 s−1 at pH=7.82 and average temperature of 7.6°C for the entire experiment. Fluxes measured at the sediment–surface interface and calculated assuming steady-state profiles had developed are typically 0.01–0.04 μmol m−2 (of river bed) s−1. The approach enables the efflux of dissolved Si from bottom-sediments to be estimated from dissolution rates measured using suspensions of bed-sediment.  相似文献   

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