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1.
Satellite ocean color images were used to determine the space-time variability of the Amazon River plume from 2000–2004. The relationship between sea-surface salinity (SSS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) absorption coefficient for dissolved and detrital material (adg) (r2=0.76, n=30, rmse=0.4) was used to identify the Amazon River plume low-salinity waters (<34 psu). The plume's spatial information was extracted from satellite bi-weekly time series using two metrics: plume area and plume shape. These metrics identified the seasonal variability of plume dimensions and dispersion patterns. During the study period, the plume showed the largest areas from July to August and the smallest from December to January. The mean annual amplitude and the mean, maximum and minimum plume areas were 1020×103 km2, 680×103 km2, 1506×103 km2 and 268×103 km2, respectively. Three main shapes and dispersion pattern periods were identified: (1) flow to the northeastern South American coast, in a narrow band adjacent to the continental shelf, from January to April; (2) flow to the Caribbean region, from April to July; and (3) flow to the Central Equatorial Atlantic Ocean, from August to December. Cross-correlation techniques were used to quantify the relationship between the plume's spatial variability and environmental forcing factors, including Amazon River discharge, wind field and ocean currents. The results showed that (1) river discharge is the main factor influencing plume area variability, (2) the wind field regulates the plume's northwestward flow velocity and residence time near the river mouth, and (3) surface currents have a strong influence over river plume dispersion patterns.  相似文献   

2.
Stormwater river plumes are important vectors of marine contaminants and pathogens in the Southern California Bight. Here we report the results of a multi-institution investigation of the river plumes across eight major river systems of southern California. We use in situ water samples from multi-day cruises in combination with MODIS satellite remote sensing, buoy meteorological observations, drifters, and HF radar current measurements to evaluate the dispersal patterns and dynamics of the freshwater plumes. River discharge was exceptionally episodic, and the majority of storm discharge occurred in a few hours. The combined plume observing techniques revealed that plumes commonly detach from the coast and turn to the left, which is the opposite direction of Coriolis influence. Although initial offshore velocity of the buoyant plumes was ∼50 cm/s and was influenced by river discharge inertia (i.e., the direct momentum of the river flux) and buoyancy, subsequent advection of the plumes was largely observed in an alongshore direction and dominated by local winds. Due to the multiple day upwelling wind conditions that commonly follow discharge events, plumes were observed to flow from their respective river mouths to down-coast waters at rates of 20–40 km/d. Lastly, we note that suspended-sediment concentration and beam-attenuation were poorly correlated with plume salinity across and within the sampled plumes (mean r2=0.12 and 0.25, respectively), while colored dissolved organic matter (CDOM) fluorescence was well correlated (mean r2=0.56), suggesting that CDOM may serve as a good tracer of the discharged freshwater in subsequent remote sensing and monitoring efforts of plumes.  相似文献   

3.
We studied the circulation on the coastal domain of the Amazon Shelf by applying the hydrodynamic module of the estuarine and coastal ocean model and sediment transport. The first barotropic experiment aimed to explain the major bathymetric effects on tides and those generated by anisotropy in sediment distribution. We analyzed the continental shelf response of barotropic tides under realistic bottom stress parametrization (C d ), considering sediment granulometry obtained from a faciologic map, where river mud deposits and reworked sediments areas are well distinguished, among others classes of sediments. Very low C d values were set in the fluid mud regions off the Amapá coast (1.0 10???4), in contrast to values around 3.5 10???3 for coarser sediment regions off the Pará coast. Three-dimensional experiments represented the Amazon River discharge and trade winds, combined to barotropic tide influences and induced vertical mixing. The quasiresonant response of the Amazon Shelf to the M2 tide acts on the local hydrodynamics by increasing tidal admittance, along with tidal forcing at the shelf break and extensive fluid mud regions. Harmonic analysis of modeled currents agreed well with the analysis of the AMASSEDS observational data set. Tidal-induced vertical shear provided strong homogenization of threshold waters, which are subject to a kind of hydraulic control due to the topographic steepness. Ahead of the hydraulic jump, the low-salinity plume is disconnected from the bottom and acquires negative vorticity, turning southeastward. Tides act as a generator mechanism and topography, via hydraulic control, acts as a maintainer mechanism for the low-salinity frontal zone positioning. Tidally induced southeastward plume fate is overwhelmed by northwestward trade winds so that they, along with background circulation, probably play the most important role on the plume fate and variability over the Amazon Shelf.  相似文献   

4.
Marine circulation above the northern Brazilian continental shelf is subject to energetic forcing factors of various origins: high water buoyancy fluxes induced by the Amazon River freshwater discharge, a strong coastal current associated with a mesoscale current (North Brazil Current (NBC)), a forcing by semidiurnal tide and by Northeast or Southeast trade winds according to the season. Using a three-dimensional (3-D) hydrodynamic numerical model (MOBEEHDYCS), and realistic bathymetry and coastline of the northern Brazilian shelf, this paper aims at studying the influence of some specific physical processes on the morphology of the Amazon plume. The very large volume discharge (180 000 m3/s on average) and the weak effect of Coriolis force are additional characteristics of the studied system, which induce a particular dynamics. The various forcing factors are successively introduced into the model in order to simulate and to determine their respective influences upon the plume extent and the hydrodynamics at the shelf scale. Simulation reveal that the coastal current is at the origin of the permanent northwestward Amazon plume extension while wind effect can either reinforce or moderate this situation. The tide intervenes also to modify the position of the salinity front: a horizontal migration of salinity front is observed under its action.  相似文献   

5.
《Continental Shelf Research》1999,19(9):1143-1159
The Oder river discharge into the Pomeranian Bight of the Baltic Sea was investigated in a combined study using satellite data, numerical modelling and shipborne measurements. The aim was to understand the dynamical processes forming the freshwater distribution patterns during the prevailing winds. From an analysis of typical distribution patterns of the river discharge in relation to the main wind directions and in comparison to seasonal wind statistics, the two main transport directions were determined. The prevailing westerly winds produce an onshore transport and a downwind coastal jet which transports the river water along the Polish coast, in certain cases over a distance of 300 km to the Gdansk Bay. During a period of stable westerly winds in June 1994, the calculated time scale for a water transport over 250 km corresponded to the observed time of 12 d. In spring, the period of maximum river runoff, easterly winds dominate and transport occurs along the German coast into the Arkona Sea. The river water is guided by upwelling processes in front of the Polish coast. During occasional north-easterly winds stable plumes form in front of the Swine river mouth; this occurred in May 1991 for several days. The numerical model showed that the stability of the plume is caused by an interaction between the alignment of the coast, the large-scale circulation in the north, the buoyancy of the freshwater and the Coriolis effect. The underlying anticyclonic eddy is indicated by warm rings in a high resolution Landsat Thematic Mapper scene. From the different datasets the range of the spatial and temporal scales of a stable plume were determined. The volume varied between 0.14 and 0.9 km3, and the suspended matter and chlorophyll load between 1120 and 7200 t and 2.8 and 18 t, respectively. These values are important for ecological budget calculations in turnover process studies.  相似文献   

6.
The 2017 Guptkashi earthquake occurred in a segment of the Himalayan arc with high potential for a strong earthquake in the near future. In this context, a careful analysis of the earthquake is important as it may shed light on source and ground motion characteristics during future earthquakes. Using the earthquake recording on a single broadband strong-motion seismograph installed at the epicenter, we estimate the earthquake’s location (30.546° N, 79.063° E), depth (H?=?19 km), the seismic moment (M0?=?1.12×1017 Nm, M w 5.3), the focal mechanism (φ?=?280°, δ?=?14°, λ?=?84°), the source radius (a?=?1.3 km), and the static stress drop (Δσ s ~22 MPa). The event occurred just above the Main Himalayan Thrust. S-wave spectra of the earthquake at hard sites in the arc are well approximated (assuming ω?2 source model) by attenuation parameters Q(f)?=?500f0.9, κ?=?0.04 s, and fmax?=?infinite, and a stress drop of Δσ?=?70 MPa. Observed and computed peak ground motions, using stochastic method along with parameters inferred from spectral analysis, agree well with each other. These attenuation parameters are also reasonable for the observed spectra and/or peak ground motion parameters in the arc at distances ≤?200 km during five other earthquakes in the region (4.6?≤?M w ?≤?6.9). The estimated stress drop of the six events ranges from 20 to 120 MPa. Our analysis suggests that attenuation parameters given above may be used for ground motion estimation at hard sites in the Himalayan arc via the stochastic method.  相似文献   

7.

The present study focuses on the nutrient sources and gradients in Paranaguá Bay (Southern Brazil), where nutrient inputs are related to losses from fertilizer loading in Paranaguá harbour and the discharge of untreated waste water. The input of dissolved inorganic nutrients to the bay from the harbour and city, as well as from river and atmospheric deposition, amounted to 642 t year−1 DIN-N and 92 t year−1 PO4-P. Harbour losses accounted for 6 % of total DIN-N and 39 % of total PO4-P loads to the bay, whereas sewage inputs from the city were responsible for 21 % and 22 %, respectively. River inputs made up 68 % of DIN-N, mainly in the form of nitrate, and 35 % of PO4-P loads, while atmospheric wet deposition was estimated to be in the order of 5 % of DIN-N and 4 % of PO4-P loads. Local maxima in nutrient levels deriving from highly concentrated sewage discharge were observed in front of the harbour and city of Paranaguá, but the plumes are diluted rapidly due to short residence times. DIN concentrations are negatively correlated with salinity, indicating the importance of freshwater input as a main factor controlling nitrogen distribution. Elevated phosphate levels in the stratified middle section of the bay may result both from harbour emissions and phosphate remobilization from sediments. Generally lower DIN and PO4 concentrations during the warmer rainy season are supposed to be due to intensified assimilation rates especially in the middle section of the bay where dense phytoplankton blooms are observed. The bay as a whole cannot be classified as being seriously eutrophic, albeit eutrophication symptoms prevail in some restricted locations in front of Paranaguá harbour.

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8.
To investigate how salinity changes with abrupt increases and decreases in river discharge, three surveys were conducted along six sections around the Yellow River mouth before, during and after a water regulation event during which the river discharge was increased from ∼200 to >3000 m3 s−1 for the first 3 days, was maintained at >3000 m3 s−1 for the next 9 days and was decreased to <1000 m3 s−1 for the final 4 days. The mean salinity in the Yellow River estuary area during the event varied ∼1.21, which is much larger than its seasonal variation (∼0.50) and interannual variation (∼0.05). Before the event, a small plume was observed near the river mouth. During the event, the plume extended over 24 km offshore in the surface layer in the direction of river water outflow. After the event, the plume diminished in size but remained larger than before the event. The downstream propagation of the plume (as in a Kelvin wave sense) was apparent in the bottom layer during the second survey and in both the surface and bottom layers during the third survey. The plume sizes predicted by the formulas from theoretical studies are larger than those we observed, indicating that factors neglected by theoretical studies such as the temporal variation in river discharge and vertical mixing in the sea could be very important for plume evolution. In addition to the horizontal variation of the plume, we also observed the penetration of freshwater from the surface layer into the bottom layer. A comparison of two vertical processes, wind mixing and tidal mixing, suggests that the impact of wind mixing may be comparable with that of tidal mixing in the area close to the river mouth and may be dominant over offshore areas. The change in Kelvin number indicates an alteration of plume dynamics due to the abrupt change in river discharge during the water regulation event.  相似文献   

9.
《Continental Shelf Research》1999,19(14):1833-1848
A well-defined front in temperature and salinity separates the stratified Clyde Sea water from the vertically well mixed water of the North Channel. The detailed structure of the front was observed in autumn 1990 by a combination of, repeated crossings of the front using a ship-borne ADCP and a towed undulating CTD system, and the deployment of a fixed mooring system with temperature, salinity and velocity sensors for a period of 12 days. The results show that the front was situated on the Great Plateau near a contour of log10(H/U32)=2.7∼3.7 where H is the water depth and U2 the amplitude of M2 tidal velocity. The temperature structure in the Clyde Sea was inverted and the Clyde Sea surface temperature was lower than that of the vertically well mixed water in the North Channel. Since the salinity gradient was stronger than the temperature gradient with fresher water on the surface, the density structure was predominantly controlled by salinity. There were indications of warm and saline bottom water upwelling on the mixed side of the front during spring tides. This upwelling disappeared and the salinity and temperature structure at the front was more diffuse during the neap tide period. A jet-like along-front residual current was observed flowing to the northwest in the surface layer with a counter flow to the southeast in the bottom layer. The vertical difference in velocity was about 9 cm s−1 and was approximately consistent with the shear determined from the thermal wind relation. Both cross- and along-front components of the current observed at the mooring station varied in response to the advection of the front, although both components had large variations with periods of less than one day and several days. The front was advected past the mooring system by a mean flow from the North Channel to the inner basin, while oscillating 3–5 km back and forth with the tidal currents. From the velocity at a current meter mooring and CTD data, the front was estimated to have moved up to 20 km during the observational period and the cross frontal velocity was inferred to be 3–4 cm s−1.  相似文献   

10.
Abstract

Since 1995, hydrologists of the HiBAm (Hydrology and Geochemistry of the Amazon Basin) Research Program carried out several hundred discharge measurements in the Amazon basin. Implementation of modern discharge measurement techniques using ultrasonic devices (ADCP), give evidence of a systematic error linked to the displacement of the river bottom due to high water velocity close to the bottom. This error leads to an underestimation of discharge value. It was possible to establish a correlation between the water velocity close to the river bottom and the error between real position and position computed by ADCP when the boat returns to its starting point after a two-way crossing of the river. When there is no bottom displacement, i.e. during low flow period, this return position error is weak (less than 50 m). This has allowed quantification of river bed load speed, or bottom displacement speed. A correction method was developed on the basis of this correlation. This method, systematically applied to ADCP discharge measurements obtained at Óbidos hydrometric station, allowed all measured discharges to be corrected, especially for 1997 and 1999 floods. Another method, based on the analysis of real trajectory of the boat (obtained from topographic measurement or GPS positioning) compared with the ADCP computed trajectory, is under study.  相似文献   

11.
We analyzed variation of channel–floodplain suspended sediment exchange along a 140 km reach of the lower Amazon River for two decades (1995–2014). Daily sediment fluxes were determined by combining measured and estimated surface sediment concentrations with river–floodplain water exchanges computed with a two‐dimensional hydraulic model. The average annual inflow to the floodplain was 4088 ± 2017 Gg yr?1 and the outflow was 2251 ± 471 Gg yr?1, respectively. Prediction of average sediment accretion rate was twice the estimate from a previous study of this same reach and more than an order of magnitude lower than an estimate from an earlier regional scale study. The amount of water routed through the floodplain, which is sensitive to levee topography and increases exponentially with river discharge, was the main factor controlling the variation in total annual sediment inflow. Besides floodplain routing, the total annual sediment export depended on the increase in sediment concentration in lakes during floodplain drainage. The recent increasing amplitude of the Amazon River annual flood over two decades has caused a substantial shift in water and sediment river–floodplain exchanges. In the second decade (2005–2014), as the frequency of extreme floods increased, annual sediment inflow increased by 81% and net storage increased by 317% in relation to the previous decade (1995–2004). Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

12.
In June 2003, we conducted a two-part field exercise to examine biogeochemical characteristics of water in the lower Mississippi river during the 4 days prior to discharge and in the Mississippi river plume over 2 days after discharge. Here we describe the fates of materials immediately after their discharge through Southwest Pass of the Mississippi delta into the northern Gulf of Mexico. Changes in surface water properties immediately after discharge were much larger and more rapid than changes prior to discharge. Total suspended matter (TSM) declined, probably due to sinking, dissolved macronutrients were rapidly diminished by mixing and biological uptake, and phytoplankton populations increased dramatically, and then declined. This decline appeared to begin at salinities of approximately 10 and was nearly complete by 15. A large increase in dissolved organic carbon (DOC) occurred over approximately the same salinity range. Weak winds (<2 m s−1) during and preceding this cruise apparently led to the formation of an extensive but thin freshwater lens from the river. This lens spread widely without much mixing, and the bloom of phytoplankton that occurred between discharge and a salinity of 10 was probably a freshwater community seeded from the lower river. Phytoplankton bloomed for a period of about 1–2 days, then declined dramatically, apparently releasing large amounts of DOC. Macronutrients from the river were utilized by the river phytoplankton community in the extensive freshwater lens. This contrasted with the more typical situation in which river nutrients stimulate a marine phytoplankton bloom at salinities in the mid-20s. We concluded that the direct effects of dissolved and particulate bio-reactive materials discharged by the Mississippi river were spatially restricted at this time to low-salinity water, at least as surface phenomena. After being transported through the lower river essentially unaltered, these materials were biogeochemically processed within days and tens of km. More generally, the mixing rate of plume water with receiving oceanic water has profound effects on the food web structure and biogeochemical cycling in the plume.  相似文献   

13.
Distribution of parameters characterizing soil response during the 1999 Chi-Chi, Taiwan, earthquake (M w = 7.6) around the fault plane is studied. The results of stochastic finite-fault simulations performed in Pavlenko and Wen (2008) and constructed models of soil behavior at 31 soil sites were used for the estimation of amplification of seismic waves in soil layers, average stresses, strains, and shear moduli reduction in the upper 30 m of soil, as well as nonlinear components of soil response during the Chi-Chi earthquake. Amplification factors were found to increase with increasing distance from the fault (or, with decreasing the level of “input” motion to soil layers), whereas average stresses and strains, shear moduli reduction, and nonlinear components of soil response decrease with distance as ~ r ?1 . The area of strong nonlinearity, where soil behavior is substantially nonlinear (the content of nonlinear components in soil response is more than ~40–50% of the intensity of the response), and spectra of oscillations on the surface take the smoothed form close to E(f) ~ f ?n , is located within ~20–25 km from the fault plane (~ 1/4 of its length). Nonlinearity decreases with increasing distance from the fault, and at ~40–50 km from the fault (~ 1/2 of the fault length), soil response becomes virtually linear. Comparing soil behavior in near-fault zones during the 1999 Chi-Chi, the 1995 Kobe (M w = 6.8), and the 2000 Tottori (Japan) (M w = 6.7) earthquakes, we found similarity in the behavior of similar soils and predominance of the hard type of soil behavior. Resonant phenomena in upper soil layers were observed at many studied sites; however, during the Chi-Chi earthquake they involved deeper layers (down to ~ 40–60 m) than during lesser-magnitude Kobe and Tottori earthquakes.  相似文献   

14.
Based on geochronological, petrological, stratigraphical, and sedimentological data, this paper describes the deposits left by the most powerful Holocene eruption of Chachimbiro compound volcano, in the northern part of Ecuador. The eruption, dated between 3640 and 3510 years BC, extruded a ～650-m-wide and ～225-m-high rhyodacite dome, located 6.3 km east of the central vent, that exploded and produced a large pyroclastic density current (PDC) directed to the southeast followed by a sub-Plinian eruptive column drifted by the wind to the west. The PDC deposit comprises two main layers. The lower layer (L1) is massive, typically coarse-grained and fines-depleted, with abundant dense juvenile fragments from the outgassed dome crust. The upper layer (L2) consists of stratified coarse ash and lapilli laminae, with juvenile clasts showing a wide density range (0.7–2.6 g cm?3). The thickness of the whole deposit ranges from few decimeters on the hills to several meters in the valleys. Deposits extending across six valleys perpendicular to the flow direction allowed us to determine a minimum velocity of 120 m s?1. These characteristics show striking similarities with deposits of high-energy turbulent stratified currents and in particular directed blasts. The explosion destroyed most of the dome built during the eruption. Subsequently, the sub-Plinian phase left a decimeter-thick accidental-fragment-rich pumice layer in the Chachimbiro highlands. Juvenile clasts, rhyodacitic in composition (SiO2?=?68.3 wt%), represent the most differentiated magma of Chachimbiro volcano. Magma processes occurred at two different depths (～14.4 and 8.0 km). The hot (～936 °C) deep reservoir fed the central vent while the shallow reservoir (～858 °C) had an independent evolution, probably controlled by El Angel regional fault system. Such destructive eruptions, related to peripheral domes, are of critical importance for hazard assessment in large silicic volcanic complexes such as those forming the Frontal Volcanic Arc of Ecuador and Colombia.  相似文献   

15.
Abstract

Water quality of the Uruguay River was evaluated with multi-parametric (temperature, turbidity, conductivity, pH, dissolved oxygen) and sediment trap data (particle flux, total organic carbon and nitrogen contents) and correlated to precipitation, river discharge and El Niño Southern Oscillation (ENSO) indices for the period 2006–2011. Hydro-meteorological parameters averaged 24–85% variability with coincident precipitation (200–400 mm month-1), discharge (7000–28 000 m3 s-1) and turbidity peaks (50–80 NTU) in the austral spring, and absolute maxima during the El Niño 2009 episode. Spectral analysis of discharge and sea-surface temperature anomaly (SSTA) showed consistent variance maxima at approx. 3 and 1.5 years. Deseasonalized discharge was significantly correlated to SSTA. During river floods, pH decreased (from 7.5 to 6.6) and particle dynamics peaked (turbidity: 15–80 NTU; vertical fluxes: 20–200 g m-2 d-1; total solid load: <1000 to 100 000 t d-1),whereas TOC remained stable (3.2 ± 0.8%) and C/N ratios increased (10–12) due to the higher contribution of terrestrial detritus.  相似文献   

16.
An ～22-m-thick saucer-shaped sill occurs near Mahad and is exposed as a curvilinear, miniature ridge within the Deccan Traps. The sill has variable dips (42–55°). It has a 7.1-km long axis and 5.3 km short axis (aspect ratio of 1.4) and is larger than the MV sill of the Golden Valley sill complex, South Africa and the Panton sill, Australia. The sill has distinct glassy upper and lower chilled margins with a coarse-grained highly jointed core. The samples from the margin are invariably fractured and iron stained because of deuteric alteration. The rock from the sill is plagioclase-phyric basalt. At least three thick sill-like apophyses emanate from the base of the main sill. The apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet. We interpret such features as ‘nascent saucer-shaped sills’ that did not inflate to form nested sills. Geochemically, the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range. Critical trace element ratios and primitive mantle normalised trace and REE patterns indicate that the sills have geochemical affinities to the Poladpur chemical type and that the pahoehoe flow they intrude belongs to the Bushe Formation. Calculated magmatic overpressures during sill emplacement range from 8.4 to 11.3 MPa (for Young’s modulus E?=?5 GPa) and 16.7 to 22.5 MPa (for E=10 GPa) and depth to magma chamber ranges from 8.5 to 11.5 km (E?=?5 GPa) and 17.1 to 22.9 km (E?=?10 GPa), consistent with petrological and gravity modelling. The volume of the Mahad sill is approximately 276 km3 and is constant irrespective of the variations in the values of host-rock Young’s modulus. In 1980, Cox (J Petrol 21:629–650, 1980) proposed a conceptual model of the crust–mantle section beneath the Karoo CFB which is considered as the fundamental model for flood basalt volcanism. Our paper confirms the presence of a sill plus the inferred substructure beneath Mahad that are compatible with predictions of that model. In LIPS, saucer-shaped sills are formed in areas experiencing extensional tectonics where processes such as the Cook–Gordon delamination and Dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills. A similar process is envisaged for the formation of the Mahad sill.  相似文献   

17.
Concentrations of dissolved226Ra in Winyah Bay, South Carolina, and in the adjacent Atlantic Ocean are augmented by the desorption of radium from sediments in the low-salinity area of the estuary and diffusion from bottom sediments. Desorption of226Ra is reflected by lower concentrations in suspended sediments from higher-salinity regions of the estuary. Bottom sediments from the high-salinity region have lower226Ra/230Th activity ratios than those from the low-salinity end.The shape of the dissolved226Ra vs. salinity profile is influenced by the river discharge. During average-discharge conditions, desorption of226Ra from suspended and bottom sediments increases the dissolved226Ra concentrations by a factor of 3.5 as the water passes through Winyah Bay. High river discharge produces an initial increase of dissolved226Ra by a factor of 2 to 3 and apparently reflects only desorption from suspended sediments. By driving the salt wedge down the estuary and reducing the zone of contact of salt water with bottom sediments, the high-flow conditions sharply reduce the flux of226Ra from bottom sediments.  相似文献   

18.
Two expeditions (October 1989 and May 1992) were carried out to two points of the main Amazon River channel and four tributaries. The Solimões and Madeira rivers, taking their origin in the Andes, are whitewater rivers. The Negro River is a typical acid, blackwater river. The Trombetas River flows through bauxite‐rich areas, and is characterized by low concentrations of dissolved humic substances. The 238U, 234U, 232Th and 230Th activities were recorded from dissolved, suspended particulate phases and river bank sediments. The latter were analysed for their 226Ra, 228Ra and 210Pb contents, and also subjected to leaching with 0·2 M hydroxylamine–hydrochloride solution to determine the concentrations of radionuclides bound to amorphous Fe hydroxides and Mn oxides and hydroxides. The dissolved U average concentration in the Amazon system is ten times lower than the mean world river concentration. The uranium concentration observed at Óbidos in the lower Amazon (0·095 µg L?1), where the U content in the river bank sediments and suspended matter is lowest, suggests U release from the solid phase during river transport. About 485 t of U are transported annually to the Amazon delta area in dissolved form, and 1943 t bound to suspended particulate matter. Total U and Th concentrations in the river bank sediments ranged from 1·59 to 7·14 µg g?1 and from 6·74 to 32 µg g?1, respectively. The highest concentrations were observed in the Trombetas River. The proportion extracted by means of the hydroxylamine solution (HL) was relatively high for U in the Trombetas river bank sediment (31%) and for Th in the Solimões sediment (30%). According to the alpha recoil effects, the 234U/238U activity ratios of the Andean river waters and downstream Amazon water (Óbidos) were >1, but were <1 in the Negro River (at Manaus). The activity ratios of dissolved U correlate with pH and also with the U activity ratios in the river bank sediment hydroxylamine extracts. As expected, the 234U/238U activity ratios in river bank sediments were <1 in the Andean rivers and in the downstream Amazon, but they were >1 in the Trombetas and Negro rivers. Such ratios probably result from the binding of dissolved uranium to solid sediment. The 228Th/232Th ratios of river bank sediments were close to unity (except for the Negro River, where it is lower), suggesting no significant Th exchanges between the river water and the sediment. The 226Ra/232Th activity ratios were <1, and the 226Ra/228Ra activity ratios generally were significantly higher than the activity ratios of their respective parents. This perhaps is the result of easier leaching of the 226Ra parent, 230Th, from solid material (owing to the alpha recoil effect) than of the 228Ra parent. Uranium and thorium isotopes were used as tools to evaluate the chemical weathering rate of rocks in the Amazon system, which was estimated to be 2·7 cm 1000 year?1 s?1. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

19.
Groundwaters feeding travertine‐depositing rivers of the northeastern segment of the Barkly karst (NW Queensland, Australia) are of comparable chemical composition, allowing a detailed investigation of how the rate of downstream chemical evolution varies from river to river. The discharge, pH, temperature, conductivity and major‐ion concentrations of five rivers were determined by standard field and laboratory techniques. The results show that each river experiences similar patterns of downstream chemical evolution, with CO2 outgassing driving the waters to high levels of calcite supersaturation, which in turn leads to widespread calcium carbonate deposition. However, the rate at which the waters evolve, measured as the loss of CaCO3 per kilometre, varies from river to river, and depends primarily upon discharge at the time of sampling and stream gradient. For example, Louie Creek (Q = 0·11 m3 s?1) and Carl Creek (Q = 0·50 m3 s?1) have identical stream gradients, but the loss of CaCO3 per kilometre for Louie Creek is twice that of Carl Creek. The Gregory River (Q = 3·07 m3 s?1), O'Shanassy River (Q = 0·57 m3 s?1) and Lawn Hill Creek (Q = 0·72 m3 s?1) have very similar gradients, but the rate of hydrochemical evolution of the Gregory River is significantly less than either of the other two systems. The results have major implications for travertine deposition: the stream reach required for waters to evolve to critical levels of calcite supersaturation will, all others things being equal, increase with increasing discharge, and the length of reach over which travertine is deposited will also increase with increasing discharge. This implies that fossil travertine deposits preserved well downstream of modern deposition limits are likely to have been formed under higher discharge regimes. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

20.
Recent developments of the Middle East catalog   总被引：8，自引：2，他引：6  
This article summarizes a recent study in the framework of the Global Earth model (GEM) and the Earthquake Model of the Middle East (EMME) project to establish the new catalog of seismicity for the Middle East, using all historical (pre-1900), early and modern instrumental events up to 2006. According to different seismicity, which depends on geophysical, geological, tectonic, and seismicity data, this region is subdivided to nine subregions, consisting of Alborz–Azerbaijan, Afghanistan–Pakistan, Saudi Arabia, Caucasus, Central Iran, Kopeh–Dagh, Makran, Zagros, and Turkey (Eastern Anatolia; after 30° E). After omitting the duplicate events, aftershocks, and foreshocks by using the Gruenthal method, and uniform all magnitude to Mw scale, 28,244 main events remain for the new catalog of Middle East from 1250 B.C. through 2006. The magnitude of completeness (Mc) was determined as 4.9 for five out of nine subregions, where the least values of Mc were found to be 4.2. The threshold of Mc is around 5.5, 5.0, 4.5, and 4.0, for the time after 1950, 1963, 1975, and 2000, respectively. The average of teleseismic depths in all regions is less than 15 km. Totally, majority of depth for Kopeh–Dagh and Central Iran, Zagros, and Alborz–Azerbaijan, approximately, is 15, 13, and 11 km and for Afghanistan–Pakistan, Caucasus, Makran, Turkey (after 30° E), and Saudi Arabia is about 9 km.  相似文献   

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