Tidal propagation in estuaries is affected by friction and fresh water discharge, besides changes in the depth and morphology
of the channel. Main distortions imply variations in the mean water level and asymmetry. Tidal asymmetry can be important
as a mechanism for sediment accumulation and turbidity maximum formation in estuaries, while mean water level changes can
affect navigation depths. Data from several gauges stations from the Amazon estuary and the adjacent coast were analyzed and
a 2DH hydrodynamic model was configured in a domain covering the continental shelf up to the last section of the river where
the tidal signature is observed. Based on data, theoretical and numerical results, the various influences in the generation
of estuarine harmonics are presented, including that of fresh water discharge. It is shown that the main overtide, M4, derived from the most important astronomic component in the Amazon estuary, M2, is responsible for the tidal wave asymmetry. This harmonic has its maximum amplitude at the mouth, where minimum depths
are found, and then decreases while tide propagates inside the estuary. Also, the numerical results show that the discharge
does not affect water level asymmetry; however, the Amazon river discharge plays an important role in the behavior of the
horizontal tide. The main compound tide in Amazon estuary, Msf, generated from the combination of the M2 and S2, can be strong enough to provoke neap low waters lower than spring ones. The results show this component increasing while
going upstream in the estuary, reaching a maximum and then slightly decaying. 相似文献
Magnetic and non-magnetic mineral analyses were conducted on a lacustrine sequence from Lago Verde in the tropical coast along the Gulf of Mexico that covers the last 2000 years. The site witnessed the transformation of the environment since the early Olmec societies until forest clearance in the last century. Through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes, and compare this model with the archeological record and inferred climatic changes in the northern hemisphere of tropical America. Volcanic activity has played a major influence on sediment magnetic properties, as a purveyor of Ti-magnetites/Ti-maghemites, and as a factor of instability in the environment. Anoxic reductive conditions are evident in most of Lago Verde's sedimentary record.
Direct observations of magnetic minerals and ratios of geochemical (Fe, Ti), and ferrimagnetic (χf) and paramagnetic (χp) susceptibility (χ) data, are used as parameters for magnetite dissolution (χp/χ, Fe/χf), and precipitation (χf/Ti) of magnetic minerals. Intense volcanic activity and anoxia are recorded before A.D. 20, leading to the formation of framboidal pyrite. Increased erosion, higher evaporation rates, lower lake levels, anoxia and reductive diagenesis in non-sulphidic conditions are inferred for laminated sediments between A.D. 20–850. This deposit matches the period of historical crisis and multiyear droughts that contributed to the collapse of the Maya civilization. Dissolution of magnetite, a high organic content and framboidal pyrite point to anoxic, sulphidic conditions and higher lake levels after A.D. 850. Higher lake levels in Lago Verde broadly coincide with the increased precipitation documented during the Medieval Warm Period (A.D. 950–1350) in the northern tropical and subtropical regions of the American continent. For the Little Ice Age (A.D. 1400–1800), the relatively moist conditions inferred are in concordance with the glacial advances recorded in central Mexico. Higher erosion rates reflect destruction of the rainforest over the last 40 years. 相似文献
Current velocity and hydrographic profiles obtained for the first time in a Chilean glacial fjord were combined with under-way surface temperature and salinity measurements to describe the formation of tidal intrusion fronts and plume-like fronts. These fronts formed within several hundred meters from each other in the vicinity of a shallow sill, maximum depth of approximately 3 m, in a glacial fjord off the Strait of Magellan in the Chilean Patagonia. Measurements were obtained in mid-December of 2003 and 2004, during late austral spring, under active glacier melting and calving. The glacial fjord is approximately 18 km long from the face of the glacier to the connection with the Strait of Magellan and typically less than 1 km wide throughout the system. Between the glacier face and the 3-m sill, depths are typically less than 100 m, and seaward of the sill, depths increase to more than 200 m. Velocity and salinity data obtained during flood periods revealed that water with oceanic salinity was aspirated to near-surface levels from depths of approximately 30 m as flood flows accelerated from approximately 10 cm s−1, seaward of the sill, to approximately 60 cm s−1 at the sill crest. The upwelled water was then slightly diluted by mixing at the sill crest before plunging down to the basin between the glacier and the sill. The plunging of salty water over the sill created dramatic tidal intrusion fronts only a few tens of meters from the sill crest and pumping of salt with every flood period. During ebb periods, the low salinity waters derived from the glacier and a small river near the glacier converged at the sill crest. After some mixing, the buoyant waters were released within a thin layer (∼3 m deep) lead by a plume-like front that remained coherent for a few hundred meters seaward of the sill. The main findings of this study were that tidal intrusion and plume fronts were observed within 2 km from each other, and that tidal pumping was the predominant mechanism for salt fluxes into the system. 相似文献
When studying the heliospheric current sheet (HCS) local structure, it is not unusual to find wide HCS crossings. In this paper, we present one crossing that appears to have a complex internal structure composed of three parallel sheets and several possible HCS crossings that are consecutive and are on the order of minutes. Depending on their origin, different scenarios can explain multiple current sheets such as complex structures of helmet streamer at the Corona flowing into the solar wind, local waviness in the HCS structure, local oscillations of the HCS, and inverted magnetic fields or planar magnetic structures (PMS) close to the HCS. Distinguishing among these scenarios using just one observational point is very difficult because all of them are 3D structures. Nevertheless, we think that nearly parallel sheets are more likely in the first and in the last scenarios, i.e. multiple helmet streamer structure and PMS. In order to make the distinction between them, we have studied the possible reversal in the Qe·B sign for every event. Our results suggest that the existence of not-wide HCS composed of multiple parallel sheets cannot be rejected. 相似文献
The goals of this study were to map the spatial distribution of sediment production and to estimate the probability of this production at the waterline based on a high potential of silting. The RUSLE-GIS model and Monte Carlo simulation were used. A sensitivity analysis of stochastic factors was performed by calculating the simple correlation coefficient. This procedure was applied to the Estrada Nova catchment, located in the city of Belém, northern Brazil, which has been subject to channel improvements and the construction of a detention basin. The results indicate that, following the urbanization and drainage improvements, there was a reduction in the annual sediment production probability, which is consistent with the dynamics in land use. The erodibility was the most sensitive factor in the sedimentation estimates. The methodology was considered an alternative to estimate sediment production in an urban catchment. 相似文献
Rising in the Andes, the Madeira River drains the southwestern part of the Amazon basin, which is characterized by high geographical, biological and climatic diversity. This study uses daily records to assess the spatio-temporal runoff variability in the Madeira sub-basins. Results show that inter-annual variability of both discharge and rainfall differs between Andean and lowland tributaries. High-flow discharge variability in the Andean tributaries and the Guaporé River is mostly related to sea surface temperature (SST) in the equatorial Pacific in austral summer, while tropical North Atlantic (TNA) SST modulates rainfall and discharge variability in the lowlands. There also is a downward trend in the low-flow discharge of the lowland tributaries which is not observed in the Andes. Because low-flow discharge values at most lowland stations are negatively related to the SST in the tropical North Atlantic, these trends could be explained by the warming of this ocean since the 1970s.
EDITOR A. CastellarinASSOCIATE EDITOR A. Viglione 相似文献
Although the trajectory and geometry of clinoforms in different types of basins have been described in many studies, few studies discuss the influence of halokinesis on clinoforms in salt-related basins. In this study, we analyse the Lower Cretaceous clinoforms in the Tiddlybanken Basin, Norwegian Barents Sea to evaluate the impact of salt mobilization on the geometry and trajectory of clinoforms as well as its implications on sediment partitioning. To accomplish this objective, we use a multidisciplinary approach consisting of seismic and well-interpretation, 3D structural restoration, and forward stratigraphic modelling. The results show that salt mobilization affects prograding clinoforms by: (a) causing lateral variations in progradation rates, resulting in complex palaeogeography, (b) increasing slope angles, which affect the equilibrium of the clinoform profile and can trigger slope-readjustment processes and (c) producing lateral and temporal variations in accommodation space, leading to different clinoform trajectories, stacking patterns and reservoir distribution along the basin. Forward stratigraphic modelling shows that in salt-related basins and other tectonically active basins, the isolated use of conventional methods for clinoform analysis might lead to potential interpretation pitfalls such as misinterpretation of trajectories and overestimation of foreset angles, which can have negative consequences for exploration models. 相似文献