Analyses of bottom simulating reflections offshore Arauco and Coyhaique (Chile)

Two seismic sections offshore Arauco and Coyhaique, Chile, have been analysed to better define the seismic character of hydrate-bearing sediments. The velocity analysis was used to estimate the gas-phase concentration, which can serve to correlate hydrate presence to the geological features. The velocity model allowed us to recognise the hydrate layer above the bottom simulating reflector (BSR), and the free gas layer below it. The velocity field is affected by strong lateral variation, showing maximum (above the BSR) and minimum (below the BSR) values in the southern sector. Here, highest gas hydrate and free gas concentrations were calculated (15% and 2.7% of total volume respectively). The estimated geothermal gradient ranges from 35 to 95°C/km. In the northern sector, the highest gas hydrate and free gas concentrations are 15% and 0.2% of total volume respectively, and the geothermal gradient is uniform and equal to about 30°C/km.     

Warm and cold dry months and associated circulation in the humid and semi-humid Argentine region

This paper studies the climatic conditions of warm and cold dry months in the humid and semi-humid Argentine region and some aspects of the regional circulation related to these cases. The climatic analysis of warm (temperatures above percentile 80) and cold (temperatures below percentile 20) dry months is based on precipitation and temperature data registered at reference stations over a period of at least 70 years, while the associated circulation is derived from daily data of geopotential height at 500 hPa from NCEP-DOE Reanalysis 2 database. The reference station for the center of the country registered a greater number of warm dry months during both the warm season (October–March) and the cold season (April–September), whereas the reference stations in the north-east and center-east showed differences depending on the time of the year, with more cold dry months during the April–September season and more warm dry months in the October–March season. A classification of daily fields of geopotential height anomalies at 500 hPa was used to analyze the atmospheric circulation related to warm and cold dry months. The circulation patterns were obtained by applying principal component analysis and cluster analysis. Findings show that some mid-level circulation patterns occur with a significant different frequency during the warm dry months or the cold dry months studied. Finally, cases of spatially extended precipitation-deficit conditions (hereinafter generalized droughts) were studied, noting dominant patterns that are coherent with the previous results.     

Dry spells in the River Plata Basin: an approximation of the diagnosis of droughts using daily data

This study addresses the dry spells observed in the La Plata Basin using daily data from 94 observation stations during sampling periods from 1900 to 2005. Dry days were defined as having less than 0.3?mm of accumulated precipitation. This definition allowed for the assessment of the dryness in the La Plata Basin and a comparison with other regions. The main purpose of this study was to analyse dry spells, especially extreme cases (meteorological droughts), and assess them on a daily basis. Trends and low frequency of droughts were analysed using a general framework to detect and compare properties of dry states based on daily and annual time scales. The trends were estimated using two different methods. Overall, the trends showed a decrease, especially in the eastern basin region during the period of 1972?C1996. The results showed sporadic decreases in dry events and events of extreme dryness (droughts). Spectral structure permits an inference of low-frequency maxima and confirmed an inter-annual 2- to 3-year period of variability in drought occurrence for most of the basin. Furthermore, probabilistic distribution functions of dry spells at basin stations were analysed to confirm that they followed a geometric?Cbinomial distribution. Additional tests were used to determine whether there was a second threshold, using the Weibull and gamma adjustment models. In order to study spatial homogeneity, the field of dry spell maxima in the basin was generated using a vector array based on the occurrence date and length of the maximum spell. Due to the dependence of spell length on the annual cycle, the longest spells were observed from April to the beginning of winter in the Argentine northwest region and in the northern and western regions of the basin. The intensity of droughts decreased in the Pampas and Mesopotamia regions. The drought of 1988 was considered to be the longest dry spell in the basin. The water deficits from this drought resulted in Argentinean economic losses of more than four billion US dollars during 1988.     

Obtaining Suction Distribution Within Vadose Zone of Highway Pavement System in Southwestern Nigeria Using Physico-Empirical Approach

Geotechnical and Geological Engineering - The stability of the pavement system on old Ogbomoso-Ilorin road in southwestern Nigeria basement complex was investigated. This was carried out by...     

Internal vein texture and vein evolution of the epithermal Shila-Paula district,southern Peru

The epithermal Shila-Paula Au–Ag district is characterized by numerous veins hosted in Tertiary volcanic rocks of the Western Cordillera (southern Peru). Field studies of the ore bodies reveal a systematic association of a main E–W vein with secondary N55–60°W veins—two directions that are also reflected by the orientation of fluid-inclusion planes in quartz crystals of the host rock. In areas where this pattern is not recognized, such as the Apacheta sector, vein emplacement seems to have been guided by regional N40°E and N40°W fractures. Two main vein-filling stages are identified. stage 1 is a quartz–adularia–pyrite–galena–sphalerite–chalcopyrite–electrum–Mn silicate–carbonate assemblage that fills the main E–W veins. stage 2, which contains most of the precious-metal mineralization, is divided into pre-bonanza and bonanza substages. The pre-bonanza substage consists of a quartz–adularia–carbonate assemblage that is observed within the secondary N45–60°W veins, in veinlets that cut the stage 1 assemblage, and in final open-space fillings. The two latter structures are finally filled by the bonanza substage characterized by a Fe-poor sphalerite–chalcopyrite–pyrite–galena–tennantite–tetrahedrite–polybasite–pearceite–electrum assemblage. The ore in the main veins is systematically brecciated, whereas the ore in the secondary veins and geodes is characteristic of open-space crystallization. Microthermometric measurements on sphalerite from both stages and on quartz and calcite from stage 2 indicate a salinity range of 0 to 15.5 wt% NaCl equivalent and homogenization temperatures bracketed between 200 and 330°C. Secondary CO₂-, N₂- and H₂S-bearing fluid inclusions are also identified. The age of vein emplacement, based on ⁴⁰Ar/³⁹Ar ages obtained on adularia of different veins, is estimated at around 11 Ma, with some overlap between adularia of stage 1 (11.4±0.4 Ma) and of stage 2 (10.8±0.3 Ma). A three-phase tectonic model has been constructed to explain the vein formation. Phase 1 corresponds to the assumed development of E–W sinistral shear zones and associated N60°W cleavages under the effects of a NE–SW shortening direction that is recognized at Andean scale. These structures contain the stage 1 ore assemblage that was brecciated during ongoing deformation. Phase 2 is a reactivation of earlier structures under a NW–SE shortening direction that allowed the reopening of the preexisting schistosity and the formation of scarce N50°E-striking S2-cleavage planes filled by the stage 2 pre-bonanza minerals. Phase 3 coincides with the bonanza ore emplacement in the secondary N45–60°W veins and also in open-space in the core of the main E–W veins. Our combined tectonic, textural, mineralogical, fluid-inclusion, and geochronological study presents a complete model of vein formation in which the reactivation of previously formed tectonic structures plays a significant role in ore formation.     

Life inhabits rocks: clues to rock erosion from electron microscopy of pisolite at a UNESCO heritage site in Brazil

Rock erosion is attracting increasing attention from scientists worldwide. The area encompassing the Saint John Baptist Church, Saint John Village, XVII century ruins in Rio Grande do Sul at the UNESCO World Heritage Site is considered a Brazilian treasure. However, the risk of damage to this site from rock erosion has recently increased tremendously. Generally, the rocky construction such as fence, wall and tomb stone, seems strong but is actually extremely sensitive to erosion caused by lichens, fungi, molds and bacteria. Because of biological erosion and massive exposure, the fresh rock is dominated by clays and microorganisms. Water-adsorbing clays and microorganisms influence the mechanisms of the rock erosion. In this study, the formation of bio-clay-minerals in porous structure of pisolite was demonstrated using electron microscopy. Bacterial clay mineralization can deform the rock structure and even produce organic materials. Biological activity could easily corrode rocky constructions around the Saint John Baptist Church site. The rocks are pisolitic laterites possibly formed in Tertiary over the Kretaceous Parana flood Basalts. Samples inhabited by lichens and fungi were collected from a collapsed wall in the ancient church. The zonal reddish-brown pisolites are 4 mm in diameter in a matrix of clays associated with porous and empty spaces. Elemental distribution maps from X-ray fluorescence microscopy show iron-rich spherules of pisolite, whereas the matrix is composed of Al, Si, Mn, and Sr; thus producing goethite and kaolinite. Transmission electron microscopic observation showed that various types of bacteria inhabit the spherule and are associated with clay minerals and graphite. STEM elemental analysis confirmed the bio-clay-mineralization with Al, Si, S, and Fe, around bacterial cells. The results presented here will improve our understanding of nm-scale bio-mineralization and bio-erosion in lateritic rocks. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Supergene enrichment of copper deposits since the onset of modern hyperaridity in the Atacama Desert,Chile

Supergene enrichment of Cu deposits in the Atacama Desert has played a critical role in making this the prime Cu-producing province of the world. Previously, this has been believed to have occurred exclusively over a long period from the middle Eocene to the late Miocene, which ended when climatic conditions changed from arid to hyperarid. Here, we report U-series disequilibrium ages in atacamite-bearing supergene assemblages that provide a new conceptualization on both the supergene enrichment process and the onset of extreme hyperaridity in the Atacama Desert. ²³⁰Th–²³⁴U ages of gypsum intergrown with atacamite in supergene veins from Cu deposits cluster at ~240 ka (Chuquicamata), 130 ka (Mantos Blancos, Spence), and 80 ka (Mantos de la Luna, Michilla). When coupled with previous data, these results indicate that supergene enrichment of Cu deposits did not cease after the onset of hyperaridity. We propose that supergene enrichment in the Atacama region developed in two main stages. The main phase, caused by downward circulation of meteoric waters in a semi-arid setting, was active from 45 until ~9 Ma, with a last pulse ca. 5 Ma in the southern Atacama Desert. During this phase, atacamite-bearing supergene assemblages were not preserved because atacamite requires saline water for its formation and rapidly dissolves when contacted by meteoric water. This was followed by a second stage starting at ~2–1.5 Ma and continuing until at least the late Pleistocene, when deep formation waters derived from the basement passed up through and modified the pre-existing supergene Cu oxide minerals. Atacamite has then been preserved in the prevailing hyperarid climate.     

Fluvial architecture as a response to two-layer lithospheric subsidence during the Permian and Triassic in the Iberian Basin,eastern Spain

The stratigraphy of a sedimentary basin is mainly the result of the long-term response of a depositional surface to prolonged subsidence. However, the real nature of interrelations between fluvial architecture and subsidence is still unknown. Herein, we present new data on these relationships by combining the results of detailed sedimentological field work with data acquired through automated forward modelling and backstripping for the alluvial Permian and Triassic sediments of the SE Iberian Ranges. Using this methodology, we determined tectonic subsidence of the basin by means of backstripping analysis and crust and lithospheric mantle stretching factors (δ and β, respectively) using forward modelling technique. Results indicated that a configuration of two individual and independent layers during lithospheric subsidence for each tectonic phase fit better for this time of the studied basin evolution than the assumption of subsidence due to a single layer spanning the whole lithosphere.For this study, we simplified fluvial geometries as two main types: isolated (I) and amalgamated (A), with subtypes in each case. Different order bounding surfaces (b.s.) were distinguished in the field, although we only selected those affecting the whole basin under study. These included those b.s. of clear tectonic origin, ranging from individual basin boundary-fault pulses produced over periods of approximately 1 My to those arising from major tectonic events, such as the beginning of extension in the basin, causing major changes in basin geometry over periods of 3–5 My.The comparison of δ and β values and fluvial geometries for each identified tectonic phase in the basin evolution, revealed some possible relationship between subsidence and fluvial geometry: Sections showing the most varied fluvial architectural geometries, including ribbon and nested forms, were related to higher β and δ stretching factors values indicating tectonic phases of greater stretching and subsidence. When both stretching factors were similar and close to 1, fluvial geometry was basically reduced to amalgamated geometry type. Wider ranging of fluvial geometries was associated with stages of basin development in which crust and upper mantle activities differed, that is, showing larger differences of β and δ stretching factors values. The related slope changes are proposed as the main surface control of fluvial styles.Combination of subsidence with other possible controlling factors such as avulsion rate, climate or budget of sediments, gives rise to the definitive alluvial architecture of a basin.     

Allochthonous subsidies of organic matter across a lake–river–fjord landscape in the Chilean Patagonia: Implications for marine zooplankton in inner fjord areas

Ecosystems can act as both sources and sinks of allochthonous nutrients and organic matter. In this sense, fjord ecosystems are a typical interface and buffer zone between freshwater systems, glaciated continents, and the coastal ocean. In order to evaluate the potential sources and composition of organic matter across fjord ecosystems, we characterized particulate organic matter along a lake–river–fjord corridor in the Chilean Patagonia using stable isotope (δ¹³C) and lipid (fatty acid composition) biomarker analyses. Furthermore, estimates of zooplankton carbon ingestion rates and measurements of δ¹³C and δ¹⁵N in zooplankton (copepods) were used to evaluate the implications of allochthonous subsidies for copepods inhabiting inner fjord areas. Our results showed that riverine freshwater flows contributed an important amount of dissolved silicon but, scarce nitrate and phosphate to the brackish surface layer of the fjord ecosystem. Isotopic signatures of particulate organic matter from lakes and rivers were distinct from their counterparts in oceanic influenced stations. Terrestrial allochthonous sources could support around 68–86% of the particulate organic carbon in the river plume and glacier melting areas, whereas fatty acid concentrations were maximal in the surface waters of the Pascua and Baker river plumes. Estimates of carbon ingestion rates and δ¹³C in copepods from the river plume areas indicated that terrestrial carbon could account for a significant percentage of the copepod body carbon (20–50%) during periods of food limitation. Particulate organic matter from the Pascua River showed a greater allochthonous contribution of terrigenous/vascular plant sources. Rivers may provide fjord ecosystems with allochthonous contributions from different sources because of the distinct vegetation coverage and land use along each river’s watershed. These observations have significant implications for the management of local riverine areas in the context of any human project that may modify terrestrial habitats as well as the productivity, food webs, and community structure of rivers, lakes, fjords, and the coastal ocean in the Chilean Patagonia.