Thermal structure and energy budget in a small high mountain lake: La Caldera,Sierra Nevada,Spain

This work examines the diel change of energy storage and its associated patterns of thermal stratification during the ice‐free period in a high mountain lake (La Caldera Lake, Sierra Nevada, Spain), in response to meteorological conditions. Bihourly data have been implemented to a standard methodology of surface heat exchange calculations in lakes. Strong variations have been observed on the diverse components of the energy budget at different time scales, ranging from diel to seasonal. Additionally, time‐series analyses have been applied to reveal the underlying periodicities involved in relation to the different variables studied. The results obtained from this study provided realistic conditions for the environmental modelling of such processes, which are very sensitive in time scale.     

Facies and palaeosol analysis in a progradational distributive fluvial system from the Campanian–Maastrichtian Bauru Group,Brazil

The Upper Cretaceous Bauru Group in south‐east Brazil consists of alluvial strata whose characteristics and distribution indicate a fluvial system developed in a semi‐arid to arid climate. Sections exposed within a 90 000 km² study area in Minas Gerais State (in south‐eastern Brazil) were evaluated using facies and palaeosol analysis to formulate depositional and pedogenic models that may account for geomorphic and climate features. From east to west, the study succession records a gradual decrease in grain size, an increase in the width/thickness ratio in channels, a decrease in the lateral and vertical connectivity of channel deposits, and an increase in overbank deposits. The fluvial architecture indicates a braided channel belt, ephemeral ribbon–channels, and an unconfined fluvial facies from east to west in the study area. The lateral and vertical distribution of facies, stratigraphic architecture and palaeocurrent data suggest proximal, medial and distal portions of a progradational distributive fluvial system. The sedimentary dynamics were marked by the building and abandonment of channels related to processes of aggradation, vegetation growth and palaeosol generation. Macromorphological and micromorphological analyses have identified pedological and mineralogical features that indicate an arid to semi‐arid climate with a provenance from the north‐eastern part of the basin (Alto Paranaiba Uplift). From the proximal to the distal portions of the distributive fluvial system, the palaeosol development is different. In the proximal portion, the palaeosols are absent or poorly developed, allowing a possible general comparison with the present soil order: Inceptisols and Aridisols. In the medial portion of the fluvial system, the palaeosols are well‐developed and characterized by Bt, Btk, C and Ck horizons (Alfisols, Aridisols, Inceptisols and Entisols). Poorly drained to well‐drained palaeosols from the base to the top in the distal plain (Aridisols and Inceptisols) are associated with geomorphic and hydromorphic changes in the fluvial system due to progradational evolution. The genetic relationship between the fluvial architecture and the palaeosols enhances the understanding that the sedimentation and pedogenesis that occurs in different portions of the distributive fluvial system are related to the tectonic and climatic evolution of the basin.     

Free-surface formulation of mantle convection—II. Implication for subduction-zone observables

Viscous and viscoelastic models for a subduction zone with a faulted lithosphere and internal buoyancy can self-consistently and simultaneously predict long-wavelength geoid highs over slabs, short-wavelength gravity lows over trenches, trench-forebulge morphology, and explain the high apparent strength of oceanic lithosphere in trench environments. The models use two different free-surface formulations of buoyancy-driven flows (see, for example, Part I): Lagrangian viscoelastic and pseudo-free-surface viscous formulations. The lower mantle must be stronger than the upper in order to obtain geoid highs at long wavelengths. Trenches are a simple consequence of the negative buoyancy of slabs and a large thrust fault, decoupling the overriding from underthrusting plates. The lower oceanic lithosphere must have a viscosity of less than to²⁴ Pa s in order to be consistent with the flexural wavelength of forebulges. Forebulges are dynamically maintained by viscous flow in the lower lithosphere and mantle, and give rise to apparently stiffer oceanic lithosphere at trenches. With purely viscous models using a pseudo-free-surface formulation, we find that viscous relaxation of oceanic lithosphere, in the presence of rapid trench rollback, leads to wider and shallower back-arc basins when compared to cases without viscous relaxation. Moreover, in agreement with earlier studies, the stresses necessary to generate forebulges are small (∼ 100 bars) compared to the unrealistically high stresses needed in classic thin elastic plate models.     

57Fe-Mössbauer spectroscopy on natural eosphorite-childrenite-ernstite samples

Samples of the eosphorite-childrenite [(Mn²⁺, Fe²⁺)AlPO₄(OH)₂H₂O] series from Divino das Laranjeiras and Araçuaí (Minas Gerais State) and Parelhas (Rio Grande do Norte State) pegmatites have been investigated by X-ray diffraction, microprobe analysis and Mössbauer spectroscopy at 295 and 77 K. The Mössbauer spectra of ernstite [(Mn²⁺, Fe³⁺)AlPO₄(OH)_2-xO_x] showed the existence of ferric ions in both A and B sites, whereas ferrous ions seem to be located exclusively in the A site. Nonoxidised samples show ferrous ions located in both sites, and no Fe³⁺ could be detected. The interpretation of the Mössbauer spectra of both, oxidised and nonoxidised samples, is difficult because the hyperfine parameters of these minerals are rather similar, rendering it difficult to make proper site assignments.     

SHRIMP U–Pb zircon geochronology of Neoproterozoic crustal granitoids (Southern Brazil): A case for discrimination of emplacement and inherited ages

SHRIMP U–Pb zircon studies on two post-collisional granitic plutons and reassessment of the data previously reported for two anatectic gneissic granites are used to assess the late Neoproterozoic history of the Florianópolis Batholith, southern Brazil. The results, supported by SEM backscattered and cathodoluminescence imagery, identify inherited zircon populations and confirm the long-lived, crustal recycling processes responsible for the accretion of the batholith. The study casts new lights on the timing of the processes involved in the generation and modification of the internal structure of distinct zircon populations, and enables discrimination to be made between inherited cores and melt-precipitated overgrowths. New dating of two post-tectonic plutons (samples 1 and 2) revealed crystals showing magmatic-textured cores sharply bounded by melt-precipitated overgrowths. The U/Pb isotopic results from both samples spread along concordia by ca. 40 m.y. (sample 1) to 100 m.y. (sample 2), clustering in two closely spaced (bimodal), partially overlapping peaks. Melt-precipitated rims and homogeneous new grains, dated at ca. 600 Ma, furnish the crystallisation age of the plutons. The magmatic textured cores and xenocrysts dated at ca. 630–620 Ma are interpreted as inherited restitic material from supposedly short-lived (meta)granitic sources. The reassessment of previous SHRIMP data of two banded anatectic granitoids (samples 3 and 4) revealed more complex morphological patterns, in which the overgrown inherited cores are sharply bounded against large melt-precipitated rims, dated at ca. 600 Ma and 592±2 Ma, respectively. Major populations of magmatic-textured inherited cores dated at 2006±3 Ma and 2175±13 Ma characterise samples 3 and 4, respectively. The latter additionally shows metamorphic and magmatic inherited cores with a large range of ages (ca. 2900–620 Ma), suggesting partial melting of metasedimentary components. The main magmatic Paleoproterozoic core populations are interpreted as inherited restite from partial melting of the adjacent (meta)tonalitic gneiss and amphibolitic country-rock (paleosome). The recognition of the (melt-precipitated) Neoproterozoic overgrowths and new crystals, and the restite provenance of the cores, supplants a previous interpretation of Paleoproterozoic magmatism (cores) and Neoproterozoic (solid-state) metamorphic overprint. As a major consequence of the former interpretation, the unit was mistakenly considered part of major Paleoproterozoic gneissic remnant within the Neoproterozoic Florianópolis Batholith/arc.     

Uberabasuchus terrificus sp. nov., a New Crocodylomorpha from the Bauru Basin (Upper Cretaceous), Brazil

A new fairly complete and articulated skull of a Peirosauridae crocodylomorph from Bauru Basin (Late Cretaceous), Brazil, is described. The fossil is from a level of clayish sandstone within Serra do Veadinho sequence, Peirópolis, Uberaba County, Minas Gerais State. The sedimentary strata of Serra do Veadinho belong to the Marília Formation (Serra da Galga Member), Bauru Group, considered to be Campanian-Maastrichtian in age. The species -Uberabasuchus terrificus sp. nov. - is a peirosaurid with moderately narrow snout, large round orbits protected by supraorbital bones of triangular shape and an antorbital fenestra bounded posteriorly by a deep groove. This fossil resembles Peirosaurus tormini Price, 1955 in the size pattern of premaxillary teeth and by showing a similar wedge-like maxillary process in the premaxilla. It also shares some morphological features with the other species of the Peirosauridae, namely the crocodylomorph Lomasuchus palpebrosus Gasparini, Chiappe and Fernandez, 1991 from Argentina. Their common features comprise a moderately narrow snout and the deep lateral groove at the premaxilla and maxilla articulation for the reception of a large mandibular tooth. However, the nasal participates in the external nares and does not divide the nasal aperture, producing a “beak-like” structure at the extremity of the snout which is unique among peirosaurids. The stratigraphic setting suggests that the specimen was buried when a flash flood overflowed the shallow channels of a braided fluvial system.

Parsimony analysis of 183 morphological characters is performed for 23 crocodylomorphs. Analysis of the morphological data matrix resulted in three most parsimonious trees (374 steps, CI = 0.679; RI = 0.826). The new species is closely related to Mahajangasuchus and both, in addition to Peirosaurus and Lomasuchus, compose the Peirosauridae.     

Horizontal patterns of water temperature and salinity in an estuarine tidal channel: Ria de Aveiro

This work presents results from two complementary and interconnected approaches to study water temperature and salinity patterns in an estuarine tidal channel. This channel is one of the four main branches of the Ria de Aveiro, a shallow lagoon located in the Northwest coast of the Iberian Peninsula. Longitudinal and cross-sectional fields of water temperature and salinity were determined by spatial interpolation of field measurements. A numerical model (Mohid) was used in a 2D depth-integrated mode in order to compute water temperature and salinity patterns. The main purpose of this work was to determine the horizontal patterns of water temperature and salinity in the study area, evaluating the effects of the main forcing factors. The field results were depth-integrated and compared to numerical model results. These results obtained using extreme tidal and river runoff forcing, are also presented. The field results reveal that, when the river flow is weak, the tidal intrusion is the main forcing mechanism, generating saline and thermal fronts which migrate with the neap/spring tidal cycle. When the river flow increases, the influence of the freshwater extends almost as far as the mouth of the lagoon and vertical stratification is established. Results of numerical modelling reveal that the implemented model reproduces quite well the observed horizontal patterns. The model was also used to study the hydrology of the study area under extreme forcing conditions. When the model is forced with a low river flow (1 m³ s⁻¹) the results confirm that the hydrology is tidally dominated. When the model is forced with a high river flow (1,000 m³ s⁻¹) the hydrology is dominated by freshwater, as would be expected in such an area.     

Hydrothermal fluid evolution and structural control of the Guarim gold mineralisation, Tapajós Province, Amazonian Craton, Brazil

Fluid inclusion and structural studies were carried out at the Guarim gold deposit in the Palaeoproterozoic Tapajós province of the Amazonian craton. Guarim is a fault-hosted gold deposit cutting basement granitoids. It consists of a quartz vein, which is massive in its inner portions, grading laterally either to a massive or to cavity-bearing quartz vein associated with hydrothermal breccias. The wallrock alteration comprises chlorite, carbonate, white mica and sulphide minerals, with free gold occurring within quartz grains and spatially associated with sulphide mineral grains. Petrographic, microthermometric and Laser Raman investigations recognised CO₂-rich, mixed H₂O–CO₂, and H₂O fluid inclusions. The coexisting CO₂ and H₂O–CO₂ inclusions were interpreted as primary immiscible fluids that formed the gold-bearing vein. The H₂O inclusions were considered a product of later infiltration of fluids unrelated to the mineralising episode. The mineralising fluid has CO₂ ranging typically from 5–10 mol%, contains traces of N₂, has salinities of ∼5 wt% NaCl equiv., and densities varying between 0.85 and 0.95 g/cm³. The P–T estimations bracket gold deposition between 270–320 °C and 0.86–2.9 kb; ƒ_O2–ƒ_S2–pH estimates suggest a reduced, near-neutral character for the fluid. Variations in the physico-chemical properties, as demonstrated by the fluid inclusion study, resulted from a combination of fluid immiscibility and pressure fluctuation. This interpretation, combined with textural and structural evidence, suggests the emplacement of the mineralised vein in an active fault and at a rather shallow level (4–7 km). The geological and structural setting, deposit-scale textures and structures, wallrock alteration and physico-chemical fluid properties are compatible with those of epizonal to mesozonal orogenic lode gold deposits. Received: 3 March 2000 / Accepted: 21 October 2000     

Spatial patterns of hydrology, geomorphology, and vegetation on the floodplain of the Amazon river in Brazil from a remote sensing perspective

The spatial heterogeneity of hydrology and vegetation during high-water periods in geomorphically distinct reaches of the Amazon River in Brazil was determined based on semivariance statistics. The spatial statistics were derived from three classified Landsat Thematic Mapper images representing upstream to downstream geomorphic characteristics. In the upstream river reach, scroll-bar topography on the floodplain tends to channelize floodwater into floodplain drainage channels, thus reducing the diversity of water types by reducing opportunities for mixing of flooding river water with locally derived floodplain water. The highest diversity of vegetation types is along floodplain drainage channels, while the rest of the floodplain has a more homogeneous cover. In the middle reach of the river the diversity of wetland classes as measured by semivariance is higher than both upstream and downstream, perhaps because of exposure to more water types and landforms. The diversity of water types is high, because flooding river water flows onto the floodplain as diffuse, non-channelized overbank flow, as well as through drainage channels. The non-channelized overbank flow readily mixes with locally derived floodplain water. Floodplain landforms available for colonization by vegetation include scroll bars, swales, lake shores, lake deltas, and floodplain drainage channels. In the downstream reach where the floodplain is wide, relatively flat, and covered with huge lakes, the floodplain supports a moderately heterogeneous mix of vegetation communities. Where landforms are similar, the spatial distribution of the vegetation is similar to that of the middle reach of the river. In the downstream reach flooded forest comprised only 37% of the wetland vegetation. In contrast, in both the upstream and middle reaches, over 70% of the wetland vegetation was flooded forest. Agricultural clearing of the floodplain is more. common in downstream reaches and may account for the smaller percent of floodplain forest cover.