长江口拦门沙河段潮滩表层沉积物分布特征(英文)

Sediment samples with high spatial resolution (432 samples in total) and flow data were collected on the tidal flats in the mouth-bar region of the Yangtze Estuary. The data was collected in July 2005, July 2006 and May 2007. The samples were analyzed with a particle sizer, resulting in the sediment distribution. The grain sizes and related parameters were analyzed. The results were presented in a ternary diagram. The sediment mainly consisted of sand, silty sand, sandy silt, sand-silt-clay, silt and clayey silt. And sand skeletons and clay matrices were found. At Nanhui Shoal, silt skeletons could be identified as well. Furthermore, the results were discussed per shoal. Although some depth dependencies were found per shoal, no general relation was found. The results are as follows: sediment located at these tidal flats of the Yangtze Estuary was mainly composed of sand, silty sand and silt. The median grain size in sediment was relatively complex with a range from 2.5 φ to 8 φ. The distributions of sorting coefficients ranging from 1 to 2 were in agreement with median sizes. It was suggested that sediment of the tidal flats was coarser and better sorted or finer and worse sorted. The skewness in sediment distribution varied from 0.1 to 0.8. In addition, the distributions of sorting coefficient and skewness in sediment at Chongming Eastern Shoal, Hengsha Eastern Shoal and Jiuduan Shoal were of similar characteristics because there were closely positive correlated relationships among these parameters. However, due to the location difference between Nanhui Southern Shoal and Eastern Shoal, the values of sorting coefficient and skewness had relatively large distinctions. The tracks of sediment transport could be described based on the distributions of sediment, which might reveal sediment transport controlled by two dominant hydrodynamic factors of current and wave. It was appreciable that coarser sediment with lower sorted coefficient was affected by dominant ebb current action and intense wave action resulted from rapidly dissipated wave energy. Moreover, due to the effects of obstructed branches, guided current and broken wave actions of the Deep Water Channel Project, grain-size in sediment located at two sides of the groyne was of uneven distribution characteristics.     

Recalibration of the yellow Rhizocarpon growth curve in the Cordillera Blanca (Peru) and implications for LIA chronology

A new lichen dating method and new moraine observations enabled us to improve the chronology of glacier advances in the Cordillera Blanca (Peru) during the Little Ice Age (LIA). Our results reveal that an early LIA glacial advance occurred around AD 1330 ± 29. However, a second major glacial advance at the beginning of the 17th century overlapped the earlier stage for most glaciers. Hence, this second glacial stage, dated from AD 1630 ± 27, is considered as the LIA maximum glacial advance in the Cordillera Blanca. During the 17th–18th centuries, at least three glacial advances were recorded synchronously for the different glaciers (AD 1670 ± 24, 1730 ± 21, and 1760 ± 19). The moraines corresponding to the two first stages are close to the one in 1630 suggesting a slow recession of about 18% in the total length of the glacier. From the LIA maximum extent to the beginning of the 20th century, the 24 glaciers have retreated a distance of about 1000 m, corresponding to a reduction of 30% in their length. This rate is comparable to that observed during the 20th century. Estimates of palaeo-Equilibrium Line Altitudes show an increase in altitude of about 100 m from the LIA maximum glacial extension at the beginning of the 17th century to the beginning of the 20th century. Because long time series are not available for precipitation and temperature, this glacial retreat is difficult to explain by past climate changes. However, there is a fair correspondence between changes in glacier length and the δ¹⁸O recorded in the Quelccaya ice core at a century timescale. Our current knowledge of tropical glaciers and isotope variations leads us to suggest that this common tropical signal reflects a change from a wet LIA to the drier conditions of today. Finally, a remarkable synchronicity is observed with glacial variations in Bolivia, suggesting a common regional climatic pattern during the LIA.     

Long-term morphodynamic evolution and the equilibrium mechanism of a flood channel in the Yangtze Estuary (China)

The stability of flood channels has attracted considerable attention because of their complicated interactions with the prevailing hydrodynamics and importance in ship navigation. This research examines long-term morphodynamic evolution in the Yangtze Estuary from 1861 to 2002 and the equilibrium mechanism of the Xinqiao Channel in the Yangtze Estuary by digitizing 15 selected maritime charts and calculating the volume of the channel. Although the total period of channel development is much longer than the historical data used in this paper, three stages are identified during the study period: the first embryonic stage (66 years), the second formation stage (33 years) and the third equilibrium stage (45 years). Variations in coastline location, channel volume, and hydrodynamics in the channel during the three stages indicate that the channel equilibrium was reached and maintained when the channel direction was aligned with the direction of offshore tidal wave propagation. Variations in river and sediment discharges affect erosion and deposition in the channel and thus channel geometry. However, future reduction in sediment supply by 10–33% due to the ongoing river engineering projects would increase the volume of the Xinqiao Channel only by 1–3%. It seems unlikely that the above change in sediment discharge will disrupt the equilibrium of the Xinqiao Channel.     

Mazama gouazoubira (Cervidae) diet during the dry season in the arid Chaco of Córdoba (Argentina)

Grey brocket deer diet selection was studied during a period of water scarcity in the arid Chaco of Córdoba province, Argentina. Sampling took place during the dry season in and around Chancaní Provincial Reserve (Pocho department). Forty-eight fresh fecal samples were collected from 85 sampling plots located at random. Plant cover and abundance were used to estimate food availability. Diet composition was determined using microscopic analysis of brocket feces and resource selection was calculated using Ivlev’s Selectivity Index. Thirty-three plant species were identified in the diet. Fruits were found in high proportion (12.4%). Woody plants made up 67.7% of the diet; the most consumed species were Castela coccinea (23.1%), Maytenus spinosa (10.2%), Condalia microphylla (9.5%), Schinus fasciculatus (8.5%) and Ximena americana (7.4%). Mazama gouazoubira selects woody and succulent plants, while herbs are consumed according to availability and grasses in a proportion that is below what is available.     

Cretaceous evolution of the Andean margin between 36°S and 40°S latitude through a multi‐proxy provenance analysis of Neuquén Basin strata (Argentina)

During the Cretaceous, the Neuquén Basin transitioned from an extensional back‐arc to a retroarc foreland basin. We present a multi‐proxy provenance study of Aptian to Santonian (125–84 Ma) continental sedimentary rocks preserved in the Neuquén Basin used to resolve changes of sediment drainage pattern in response to the change in tectonic regime. Sandstone petrology and U–Pb detrital zircon geochronology constrain the source units delivering detritus to the basin; apatite U–Pb and fission track dating further resolve provenance and determine the age and patterns of exhumation of the source rocks. Sandstone provenance records a sharp change from a mixed orogenic source during Aptian time (ca. 125 Ma), to a magmatic arc provenance in the Cenomanian (ca. 100 Ma). We interpret this provenance change as the result of the drainage pattern reorganisation from divergent to convergent caused by tectonic basin inversion. During this inversion and early stages of contraction, a transient phase of uplift and basin erosion, possibly due to continental buckling, caused the pre‐Cenomanian unconformity dividing the Lower from Upper Cretaceous strata in the Neuquén Basin. This phase was followed by the development of a retroarc foreland basin characterised by a volcanic arc sediment provenance progressively shifting to a mixed continental basement provenance during Turonian‐Santonian (90–84). According to multi‐proxy provenance data and lag times derived from apatite fission track analysis, this trend is the result of a rapidly exhuming source within the Cordillera to the west, in response to active compressional tectonics along the western margin of South America, coupled with the increasing contribution of material from the stable craton to the east; this contribution is thought to be the result of the weak uplift and exhumation of the foreland due to eastward migration of the forebulge.     

Effect of vegetation with different evolution degree on soil organic matter in a semi-arid environment (Cabo de Gata-Níjar Natural Park, SE Spain)

The organic matter of the surface horizons of soils developed below scrub vegetation in a Mediterranean semi-arid area of great environmental interest (Cabo de Gata-Níjar Natural Park, SE Spain) has been studied. The study mainly concentrates on examining the influence of two vegetation types, one evolved (according to its successional stage), and the other clearly degraded as a result of prior removal of vegetation. In spite of the homogeneity in the results obtained from the analysis of the organic matter from the soils studied, a relationship may be established between vegetation biotype and characteristics and evolution of the soil organic matter. The evolved vegetation results in the presence in the soil of a somewhat more evolved and stable organic matter (demonstrated by certain chemical and microbiological aspects), resulting in a greater degree of humification, thus favouring the protection of the soil and the ecosystem as a whole. Hence, the presence of degraded vegetation might lead to soil degradation, something that is unsustainable in semi-arid areas that are particularly fragile in nature.     

Geomorphological and sedimentological analysis of a catastrophic flash flood in the Arás drainage basin (Central Pyrenees, Spain)

On August 7th, 1996, an intense and short-duration convective storm occurred over the 18.6-km² Arás drainage basin (Central Pyrenees, Spain). This high relief basin is composed of three subbasins, Aso, Betés and La Selva, and feeds the Arás alluvial fan, in the Gállego river valley. This alluvial fan had been drained by an artificial channel (about 125 m³/s at bank-full capacity). More than 30 check dams in its feeder channel, the Arás barranco, had been previously filled by earlier sediments. The heaviest rain was over the Betés subbasin (total rainfall 178.4 mm; maximum rainfall intensity of 153 mm/h for a 10-min time interval was estimated). Most of the rainfall fell in a 70-min period. This storm resulted in high runoff, causing catastrophic damage and significant geomorphic changes in the drainage basin, especially in the Betés subbasin. The high discharge, concentrated in the Arás barranco, destroyed most of the check dams, flushing out a great amount of debris. Major channel trenching and widening occurred in this barranco. When the confined sediment-laden flash flood reached the basin mouth, it sheet-flooded the southern sector of the Arás fan depositing a massive amount of debris. On this fan 87 people lost their lives and the direct physical damage has been estimated at 55 million dollars. Two stages in the development of the flood have been differentiated from the sedimentological and morphological analysis of the flooded fan lobe. A first stage (peak discharge) of sheet-flooding deposited a coarse boulder lobe, burying the artificial channel at the fan head and causing a darnming effect on the water flood. During the second stage (discharge decline) the flood made its way through the fan head, incising the previous debris accumulation and splitting into two main flow paths.     

Curvature analysis as a tool for subsidence-related risk zones identification in the city of Tuzla (BiH)

Shallow Miocene salt deposits located below the city of Tuzla (BiH) have been exploited during the last 60 years by means of wells extracting salt-saturated groundwater brines. The massive extraction activities have led to severe subsidence accommodated by collapse and strain localization. Surface topography and geomorphology have been influenced and modified by several faults and fractures.A series of sequential topographical survey data collected during the last 50 years have been used to obtain the total subsidence envelope surface. In order to analyze the salt dissolution-related morphological development of the Tuzla topography and to identify the location of faults and fractures in the deformed area, we applied curvature analysis to the subsidence surface by means of two different analytical methods: the directional, two-dimensional curvature and the analytic Gaussian curvature. The comparison of the curvature maps with those of surface fractures visible in the area shows a good spatial agreement between the directional curvature and fracture intensity, whereas the subsurface normal faults are more evident in the Gaussian curvature maps.     

High rates of geomorphological processes in a tropical area: the Remparts River case study (Réunion Island, Indian Ocean)

Reunion Island is characterized by rapid landscape evolution resulting from its cyclonic tropical climate. However, local active surface processes are not well understood. The relationships between climatic events, large scale landslides and torrential transport of sediment by the rivers remain unclear. The Remparts River is an appropriate area for studying such geomorphological processes, as it deeply incises the active Piton de la Fournaise volcano. In this study, different approaches are used to analyze the morphological evolution of the river from the sediment production areas to the outlet over the last 40 years. Recurrent events of huge mass wasting occur at Mahavel Cliff, upstream of one of the river tributaries, the most recent producing around 50×10⁶ m³ of sediment in 1965. Combined analyses of the sequence of cyclonic events, major mass wasting events and aerial photography interpretation over the last 40 years led to the proposal of a functional model of river system responses to these events. The river system can be divided into three compartments, each affected by three classes of geomorphological events. The sedimentary response (erosion and/or aggradation) of each compartment to a triggering event, such as cyclonic rainfall and/or seasonal rise of water discharge, is controlled both by the magnitude of the climatic event and by the state of the compartment resulting from previous evolution. A set of five aerial photographs and a satellite image showing the evolution of the studied area during the last 40 years are examined in detail in the light of the functional model. Observations confirm a rapid and complex evolution of the river bed (erosion and aggradation), and provide information about the dynamics of the sediment transfer from the production areas to the ocean. Analysis of two distinct topographic datasets bracketing the last major cyclone Dina in 2002 allows the estimation of the river sediment budget resulting from this event. The net volume of aggraded sediments in the river bed is estimated at around 8×10⁶ m³.With no major collapse event recorded at Mahavel Cliff, sediment transfer due to the flood associated with the 3-day cyclone Dina event is responsible for this significant increase in river bed sediment volume. This quantification shows that several million cubic meters of sediment may take only a few years to spread over more than 5 km downstream. The river bed has now reached its highest elevation since the 1965 landslide, with potential consequences for natural hazards in the area of the outlet at the city of Saint-Joseph.     

1200 years of fire impact on biogeochemistry as inferred from high resolution diatom analysis in a kettle lake from the Picea mariana-moss domain (Quebec, Canada)

The consequences of fire on water chemistry are important considering that major changes in the frequency and intensity of forest fires are anticipated as a result of global warming. Due to the important differences in succesionnal vegetative trends after fire between mixed-wood and coniferous-dominated forests in Quebec (Canada), we undertook a long-term paleoecological study of the impact of fires on the biogeochemistry of Lac à la Pessière, a small lake located in a conifer-dominated boreal forest ecoregion (Picea mariana-moss domain). The paleolimnological study was carried out using diatom assemblages (class:Bacillariophyceae) to reconstruct changes in environmental variables of limnological interest [pH, total phosphorus (TP), total nitrogen (TN), dissolved organic carbon (DOC) and, epilimnetic carbon dioxide (CO₂)] potentially associated with fire over the last 1200 calendar years. Diatom composition and related reconstructed limnological variables were compared before and after fire events. No significant changes were systematically observed in lake chemistry associated with fire events. However, diatom-inferred epilimnetic CO₂showed a clear decreasing trend over the last 400 cal. yrs BP. The results suggest that fire-induced changes in lake chemistry are limited in catchments dominated by black spruce (Picea mariana). We hypothesize that this fact result of excess moisture associated to the thick humus layer, which likely limits the mobilization of nutrients and major ions even during a fire event.     

Rock glacier dynamics in a marginal periglacial high mountain environment: Flow, movement (1991–2000) and structure of the Argualas rock glacier, the Pyrenees

The Argualas rock glacier is located on the southern side of the Central Pyrenees (Argualas massif). Vertical electric sounding, debris surface analysis and a topographic survey were performed on this rock glacier between 1991 and 2000. High precision measurement records were obtained using a total station. Horizontal and vertical movements of the rock glacier were measured by means of sixteen steel rods. Horizontal and vertical angles and distances to each rod were measured from three stations on a bedrock ridge. Total and annual average displacements were derived from the emergence value and the horizontal displacement of each rod. Then the horizontal and vertical displacement rates in different sectors of the rock glacier were compared. The inferred surface deformation was characterized by both extensional and compressive flows as well as thinning of the frozen body. The displacement rates showed temporal variations related to atmospheric thermal changes, pointing to a high sensibility of the rock glacier.