Initiation of the Mekong River delta at 8 ka: evidence from the sedimentary succession in the Cambodian lowland     

Toru Tamura  Yoshiki Saito  Sotham Sieng  Bunnarin Ben  Meng Kong  Im Sim  Sokuntheara Choup  Fumio Akiba 《Quaternary Science Reviews》2009,28(3-4):327-344

Modern deltas are understood to have initiated around 7.5–9 ka in response to the deceleration of sea-level rise. This episode of delta initiation is closely related to the last deglacial meltwater events and eustatic sea-level rises. The initial stage of the Mekong River delta, one of the world's largest deltas, is well recorded in Cambodian lowland sediments. This paper integrates analyses of sedimentary facies, diatom assemblages, and radiocarbon dates for three drill cores from the lowland to demonstrate Holocene sedimentary evolution in relation to sea-level changes. The cores are characterized by a tripartite succession: (1) aggrading flood plain to natural levee and tidal–fluvial channel during the postglacial sea-level rise (10–8.4 ka); (2) aggrading to prograding tidal flats and mangrove forests around and after the maximum flooding of the sea (8.4–6.3 ka); and (3) a prograding fluvial system on the delta plain (6.3 ka to the present). The maximum flooding of the sea occurred at 8.0 ± 0.1 ka, 2000 years before the mid-Holocene sea-level highstand, and tidal flats penetrated up to 20–50 km southeast of Phnom Penh after a period of abrupt ～5 m sea-level rise at 8.5–8.4 ka. The delta progradation then initiated as a result of the sea-level stillstand at around 8–7.5 ka. Another rapid sea-level rise at 7.5–7 ka allowed thick mangrove peat to be widely deposited in the Cambodian lowland, and the peat accumulation endured until 6.3 ka. Since 6.3 ka, a fluvial system has characterized the delta plain, and the fluvial sediment discharge has contributed to rapid delta progradation. The uppermost part of the sedimentary succession, composed of flood plain to natural-levee sediments, reveals a sudden increase in sediment accumulation over the past 600–1000 years. This increase might reflect an increase in the sediment yield due to human activities in the upper to middle reaches of the Mekong, as with other Asian rivers.  相似文献   

Evidence for relativistic features in the X-ray spectrum of Mrk 335   总被引：1，自引：0，他引：1  

A. L. Longinotti  S. A. Sim  K. Nandra  M. Cappi 《Monthly notices of the Royal Astronomical Society》2007,374(1):237-247

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Hydrological and geomorphological criteria to evaluate the dispersion risk of waste sludge generated by the Aznalcollar mine spill (SW Spain)   总被引：2，自引：0，他引：2  

G. Benito  Alfonso Benito-Calvo  Francesc Gallart  Juan Pedro Martín-Vide  David Regües  Ernest Bladé 《Environmental Geology》2001,40(4-5):417-428

Following the Aznalcóllar pyrite mine disaster (Seville, Spain) which caused the spilling of some 4.5?hm³ of acid water, the floodplains of the rivers Agrio and Guadiamar were rapidly cleaned of waste sludge. However, despite the efficiency of cleaning activities, there is still evidence of a fine superficial layer of sludge and some soil contamination, with the consequent risk of remobilisation of the pollutants by water erosion. There is much concern that these contaminated sediments may affect the precious ecosystems of the Doñana National Park and the Guadalquivir marshlands. This report describes the evaluation of the risk of mobilisation of the waste sludge through (1) detailed geomorphological analysis, indicating potential areas of erosion-sedimentation on the floodplains of the rivers Agrio and Guadiamar, and predicted dynamics of the waste sludge, and (2) evaluation of the potential dispersion of the waste sludge provoked by future flood events, including hydraulic calculations to model channel flow and the analysis of the texture of the sludge to obtain critical transport and sedimentation values. Findings suggest that the waste sludge is likely to be transported and deposited within the Doñana National Park during future flood events.  相似文献   

International Association of Geomorphologists Newsletter No. 18 (3/2001)     

Paul Williams  Selma Simes De Castro  Janet Hooke  Wang Ying  Vít Vilímek  Karl‐Heinz Schmidt  Anto Raukas  Tsuguo Sunamura  Algimantas Cesnulevicios  Irasema Alcntara‐Ayala  James P. Terry  Milos Stankoviansky  Carol P. Harden 《地球表面变化过程与地形》2001,26(12):1353-1360

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Quaternary reactivation of flexural-slip folds by diapiric activity: example from the western Ebro Basin (Spain)   总被引：1，自引：0，他引：1  

A. M. Casas  I. Gil  B. Leránoz  H. Millán  L. Simón 《International Journal of Earth Sciences》1994,83(4):853-867

In the western sector of the Ebro Basin two types of structures deform the Quaternary terraces and pediments developed by the Ebro River and its tributaries: (1) folds up to 10 km long in the lower levels of fluvial terraces and (2) normal listric faults that produce tilting and rollover anticlines of the Quaternary deposits. Both types of structures are linked to the geometrical and lithological features of the Tertiary beds that underlie the Quaternary deposits. Quaternary folds, whose axes are parallel to the strike of the Tertiary beds, are the result of reactivation of Tertiary large-scale (60 km long) folds due to diapirism of their gypsum cores, where the gypsum units reach a maximum thickness of 3 000 m. This reactivation produced flexural-slip in some beds on the limbs of the folds, bringing about the folding of the overlying Quaternary terraces. The mechanism of Quaternary folding involves layer-parallel shear in alternating Tertiary units and folding linked to detachments and reverse layer-parallel faults. Normal listric faults are widespread throughout the studied area. They are partly parallel to low dipping Tertiary beds and are the result of a NNE-SSW tectonic extension, compatible with minor structures and focal mechanisms of recent earthquakes. The relationship between the two kinds of Quaternary structures indicates that diapirism of the gypsum cores of the anticlines was activated by extensional tectonics.  相似文献   

A systematic deviation from Na-K-Ca geothermometer below 75°C and above 10−4 atm Pco2     

Tomáš Pačes 《Geochimica et cosmochimica acta》1975,39(4):541-544

If the temperature of ground water is below 75°C and the partial pressure of CO₂ in the aquifer is above 10^?4 atm, a chemical steady-state between water and felsic rocks (rather than chemical equilibrium) may be maintained. The temperature of water in the aquifer may be estimated using a modified form of the Na-K-Ca geothermometer from, . where the departure of the steady-state from equilibrium, $\text{I}$, is a function of : .  相似文献   

Thermal decomposition of NH<Subscript>4</Subscript><Superscript>+</Superscript>-vermiculite from Santa Olalla (Huelva,Spain) and its relation to the metal ion distribution in the octahedral sheet     

J. L.?Pérez-RodríguezEmail author  J.?Poyato  M. C.?Jiménez de Haro  L. A.?Pérez-Maqueda  A.?Lerf 《Physics and Chemistry of Minerals》2004,31(7):415-420

The investigation of the NH₃ loss in the NH₄⁺-vermiculite (Santa Olalla) by thermogravimetry, evolved gas analysis, chemical analysis, X-ray diffraction and IR spectroscopy is reported here. The mass loss during heating takes place in two steps at about 650 and 825 °C. Additionally, the releases of H₂O and NH₃ occurs simultaneously. The experimental results indicate that the protons remaining in the interlayer space after NH₃ removal trigger the H₂O release. X-ray diffraction shows that during the decomposition of NH₄⁺-vermiculite there are two domains with different interlayer spaces at ~9 and ~10 Å. As the decomposition proceeds, the intensity of the 9 Å peak increases at the expense of the second one. The change in the IR-stretching modes of the structural OH groups during heating indicates that the OH groups surrounded by 3Mg²⁺ or 2Mg²⁺Fe²⁺ are released at lower temperatures than those with environments like 2Mg²⁺Fe³⁺, 2Mg²⁺Al³⁺ or more complex ones.  相似文献   

Influence of harbour constructions on morphosedimentary changes in the Tinto-Odiel estuary mouth (south-west Spain)     

J.?A.?MoralesEmail author  J.?Borrego  M.?Ballesta 《Environmental Geology》2004,46(2):151-164

The Odiel-Tinto estuary is one of the most significant estuarine systems along the mesotidal Huelva Coast. The dynamics of this coastal sector are controlled by the interaction of ebb-tidal currents and the prevailing southwesterly waves. The main sediment supply is provided by an intensive west-to-east longshore current, transporting sand from Portuguese cliffs and discharge from the River Guadiana. The tidal range is mesotidal (2.0 m) and the mean significant wave height is 0.6 m with an average period of 3.6 s. This estuary mouth comprises three different barriers separated by the main estuary channel, and another marginal one that drains a significant part of the system. Saltés Island can be found between these two channels, which is composed of sandy barriers separated by muddy salt marshes that form a protected natural park. East of the main channel is Mazagón Beach, one of the most important tourist resorts on this coast. Recent papers have demonstrated that Saltés Island evolved as a chenier plain over the last 3000 years. The cause of this evolution was a special wave refraction scheme caused by two ebb-tidal delta systems located at the end of the two tidal channels. Three important harbour infrastructures have been constructed at the estuary mouth: a) a jetty at the end of Saltés Island, finished in 1977; b) a second and shorter jetty bordering the marginal channel constructed in 1984, and; c) a dock for sailing activities built on Mazagón Beach. These three structures have modified the natural dynamic scheme. The first consequence was to inhibit the functioning of the chenier plain, affecting the natural environment; the second was to intensively erode Mazagón Beach, endangering this tourist site.This paper analyses the causal relationships that exist between the harbour infrastructures and the resultant modifications.  相似文献   

Comparative analysis of continuous radon sensors in underground environments     

R.?Vi?as  A.?Eff-DarwichEmail author  V.?Soler  J.A.?Castro-Almazan  M.?C.?Martín-Luis  J.?Coello  M.?L.?Quesada  J.?de la?Nuez 《Environmental Geology》2004,46(8):1108-1117

Four different continuous radon (²²²Rn)-detection systems have been compared in underground environments, namely three subhorizontal tunnels excavated for groundwater exploitation. Within observational uncertainties, all sensors detected the same concentrations of radon in the air of the tunnels, regardless of the methodology used to measure this radioactive gas. In this sense, the choice of continuous long-term radon monitoring sensors in underground tunnels is constrained by factors such as robustness of the instrumentation, power supply and cost, rather than the sensitivity of the detection methodology. This is particularly important for the monitoring of radon in the context of seismo-volcanic surveillance, where the harsh environmental conditions of the monitoring sites and the absence of electrical power supply are key factors to take into account.  相似文献   

Changes in groundwater chemistry due to metallurgical activities in an alluvial aquifer in the Moa area (Cuba)   总被引：2，自引：1，他引：1  

R.?RodríguezEmail author  L.?Candela 《Environmental Geology》2004,46(1):71-82

The aim of this work is to evaluate the changes in groundwater chemistry in an alluvial aquifer in the Moa area. Surface and ground water, metallurgical waste and various geological material samples were collected in order to evaluate groundwater composition. The results show that the alluvial aquifer is polluted with SO₄^2-, Mg²⁺and heavy metals. According to its major components in the alluvial aquifer, two types of groundwater are identified: magnesi-bicarbonated and sulphate-magnesic. Maximum SO₄^2– and Mg²⁺ contents are more than 1000 mg/L, and are four times higher than the acceptable levels for human consumption of water. The high values of Cr(VI), Ni(II), Mn(II) Fe_(total), SO₄^2– and Mg²⁺ in alluvial aquifers are due to polluted recharge from metallurgical waste from the tailing dam. This recharge is favoured by the preferential flow due to desiccation cracks in metallurgical waste. Geochemical modelling showed that potentially toxic heavy metals might exist largely in the forms of MSO₄^2– and M²⁺ in pore water of SAL metallurgical waste. All samples were supersaturated in goethite and hematite. Results from batch testing indicate that the heavy metals have two origins: natural, due to the existing ultramaphic rocks and laterites, and anthropogenic, by metallurgical waste rich in sulphate and (oxy)hydroxide minerals. These results highlight the need to locate and evaluate a new water source to supply the population of the city of Moa.  相似文献