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
Climatic, oceanographic and ecological changes that control the formation and deposition of sediment in shallow and deep depositional environments commonly occur with periodicities of a few 10 000 years. Consequently, in order to interpret sedimentary sequences in the geological past, high time resolution is required. This is best obtained by cyclostratigraphy. Three sections have been studied in the Oxfordian of north-eastern Spain: one represents a shallow, siliciclastic-carbonate platform with repetitive subaerial exposures, one an intraplatform basin with sponge bioherms, and one a swell where iron ooids and glauconite formed. The platform section displays a well-defined stacking pattern of depositional sequences; the deeper-water sections are well dated by ammonites. The correlation between the three sections is a best-fit solution integrating biostratigraphy, sequence stratigraphy and cyclostratigraphy. It is concluded that the small-scale depositional sequences formed in tune with the 100-ka orbital eccentricity cycle. An additional factor was differential subsidence that ruled basin morphology. 相似文献
A vast majority of the recent literature on spatial hedonic analysis has been concerned with residential property values,
with only very few examples of studies focused on commercial property prices. The dearth of studies can be attributed to some
of the challenges faced in the analysis of commercial properties, in particular the scarcity of information compared to residential
transactions. In order to address this issue, in this paper we propose the use of cokriging and housing prices as ancillary
information to estimate commercial property prices. Cokriging takes into account the spatial autocorrelation structure of
property prices, and the use of more abundant information on housing prices helps to improve the accuracy of property value
estimates. A case study of Toledo in Spain, a city for which commercial activity stemming from tourism is one of the key elements
of the economy in the city, demonstrates that substantial accuracy and precision gains can be obtained from the use of cokriging. 相似文献
We present field and seismic evidence for the existence of Coniacian–Campanian syntectonic angular unconformities within basal foreland basin sequences of the Austral or Magallanes Basin, with implications for the understanding of deformation and sedimentation in the southern Patagonian Andes. The studied sequences belong to the mainly turbiditic Upper Cretaceous Cerro Toro Formation that includes a world‐class example of conglomerate‐filled deep‐water channel bodies deposited in an axial foredeep depocentre. We present multiple evidence of syntectonic deposition showing that the present internal domain of the fold‐thrust belt was an active Coniacian–Campanian wedge‐top depozone where deposition of turbidites and conglomerate channels of Cerro Toro took place. Cretaceous synsedimentary deformation was dominated by positive inversion of Jurassic extensional structures that produced elongated axial submarine trenches separated by structural highs controlling the development and distribution of axial channels. The position of Coniacian‐Campanian unconformities indicates a ca. 50–80 km advance of the orogenic front throughout the internal domain, implying that Late Cretaceous deformation was more significant in terms of widening the orogenic wedge than all subsequent Andean deformation stages. This south Patagonian orogenic event can be related to compressional stresses generated by the combination of both the collision of the western margin of Rocas Verdes Basin during its closure, and Atlantic ridge push forces due to its accelerated opening, during a global‐scale plate reorganization event. 相似文献
The objective of this study is to understand and predict the alteration of porous rock by crystallization of salts. Samples
of different rocks have been tested according to the EN 12370 standard test. Two parameters are proposed to evaluate the alteration
of a rock during these tests. The alteration index AI represents the cycle in which the first damages occur. The alteration
velocity AV represents the alteration rate at the end of the experiment, when the decay has become regular. These parameters
can be estimated with the help of a microstructural study of the rocks. The most relevant intrinsic parameters of the stones
for this estimation are capillary coefficient, evaporation coefficient, tensile strength and P-waves velocity. An evaluation
of the alteration pattern is also proposed depending on the eventual heterogeneities and anisotropies of the rocks. The influence
of the dimension and shape of the samples is also discussed. 相似文献
This paper presents results of interdisciplinary palaeoenvironmental research carried out on a sediment sequence from the
Nutria Mansa 1 archaeological site. This late Holocene site is located in the Pampean plains, Argentina. Siliceous microfossils
were recorded (i.e. phytoliths, diatoms, chrysophytes, sponges) and micro-charcoal was analyzed. In addition, fauna identified
at the site were correlated with regional palaeoclimatic evidence. Based on the microfossil associations, we concluded that
the human occupations occurred beside a fluvial and brackish-freshwater lacustrine setting. Grass communities were located
on its margins that developed under a warm temperate climate with evidence for dryness and marked seasonality in part of the
record. Although some stratigraphic and faunal evidence indicates a possible temperature increase, these fluctuations were
not sufficient to extirpate mammals of arid and cold environments. 相似文献