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121.
Alexandre Raphael Cabral Michael Wiedenbeck Francisco Javier Rios Ant?nio Augusto Seabra Gomes Jr Orlando Garcia Rocha Filho Richard David Jones 《Mineralium Deposita》2012,47(4):411-424
Talc mineralisation occurs as hematite–talc schist between soft hematite ore and dolomitic itabirite at Gongo Soco, Quadrilátero
Ferrífero of Minas Gerais, Brazil. The hematite–talc schist and soft hematite have a prominent tectonic foliation of tabular
hematite. Tabular hematite without preferential orientation is superimposed on the tectonic foliation. The talcose schist
is enriched in F and has a constant Fe/S ratio. Electron-microprobe analyses indicate trace amounts of S in different generations
of hematite. The whole-rock Fe/S ratio possibly represents sulfate S from hematite-hosted fluid inclusions. Fluid inclusions
in foliation-overprinting hematite and chlorite geothermometry from talcose rocks suggest, respectively, temperatures from
<200°C to ~300°C. Tourmaline, a rarely observed mineral in the hematite–talc schist, belongs to the alkali group and falls
in the dravite compositional field. Boron-isotope determinations of tourmaline crystals, using secondary ion mass spectrometry,
vary from −20‰ to −12‰ δ11B. This compositional isotopic range and the tourmaline chemical composition suggest a meta-evaporitic origin. A non-marine
evaporitic setting is the most likely source of acidic, highly oxidising fluids, which resulted in the abundant F-bearing
talc and the presence of otherwise immobile Ti in hematite. Oxidising brines were channelled along shear zones and converted
dolomitic itabirite into the Gongo Soco soft hematite and the talc mineralisation. The latter is envisaged as the hydrothermal
wall-rock alteration of dolomitic itabirite, which gave rise to the soft hematite ore. 相似文献
122.
Leandro R. Alejano Javier González José Muralha 《Rock Mechanics and Rock Engineering》2012,45(6):1023-1035
A relevant parameter for estimating discontinuity shear strength is the basic friction angle, usually derived from different types of tilt tests. However, the tilt tests described in the literature produce varying basic friction angle values. From a large number of different types of tilt tests on different kinds of rocks, it was possible to conclude that the mechanisms of sliding along cylinder generatrixes and planar surfaces are quite different, and that tests based on sliding along generatrixes are not appropriate for determining reliable basic friction angle values for discontinuity planes. Tests on small specimens are also not recommended, for geometry reasons and because ensuring reliable stress conditions is difficult. To quantify the natural variability in tilt testing, large specimens of the same granite were tested. The results revealed coefficients of variation for the basic friction angle in the range of 5–10?%, a variability which is no greater than that found for other rock mechanics parameters. This observation enables to forward some recommendations concerning the appropriate number of tests needed to obtain reliable results. 相似文献
123.
Duncan A. Christie Jos�� A. Boninsegna Malcolm K. Cleaveland Antonio Lara Carlos Le Quesne Mariano S. Morales Manfred Mudelsee David W. Stahle Ricardo Villalba 《Climate Dynamics》2011,36(7-8):1505-1521
The Andes Cordillera acts as regional ??Water Towers?? for several countries and encompasses a wide range of ecosystems and climates. Several hydroclimatic changes have been described for portions of the Andes during recent years, including glacier retreat, negative precipitation trends, an elevation rise in the 0° isotherm, and changes in regional streamflow regimes. The Temperate-Mediterranean transition (TMT) zone of the Andes (35.5°?C39.5°S) is particularly at risk to climate change because it is a biodiversity hotspot with heavy human population pressure on water resources. In this paper we utilize a new tree-ring network of Austrocedrus chilensis to reconstruct past variations in regional moisture in the TMT of the Andes by means of the Palmer Drought Severity Index (PDSI). The reconstruction covers the past 657?years and captures interannual to decadal scales of variability in late spring?Cearly summer PDSI. These changes are related to the north?Csouth oscillations in moisture conditions between the Mediterranean and Temperate climates of the Andes as a consequence of the latitudinal position of the storm tracks forced by large-scale circulation modes. Kernel estimation of occurrence rates reveals an unprecedented increment of severe and extreme drought events during the last century in the context of the previous six centuries. Moisture conditions in our study region are linked to tropical and high-latitude ocean-atmospheric forcing, with PDSI positively related to Ni?o-3.4 SST during spring and strongly negatively correlated with the Antarctic Oscillation (AAO) during summer. Geopotential anomaly maps at 500-hPa show that extreme dry years are tightly associated with negative height anomalies in the Ross?CAmundsen Seas, in concordance with the strong negative relationship between PDSI and AAO. The twentieth century increase in extreme drought events in the TMT may not be related to ENSO but to the positive AAO trend during late-spring and summer resulting from a gradual poleward shift of the mid-latitude storm tracks. This first PDSI reconstruction for South America demonstrates the highly significant hindcast skill of A. chilensis as an aridity proxy. 相似文献
124.
Quantifying the uncertainty associated with monitoring protocols is essential to prevent the misclassification of ecological status and to improve sampling design. We assessed the Posidonia oceanica multivariate index (POMI) bio-monitoring program for its robustness in classifying the ecological status of coastal waters within the Water Framework Directive. We used a 7-year data set covering 30 sites along 500 km of the Catalonian coastline to examine which version of POMI (14 or 9 metrics) maximises precision in classifying the ecological status of meadows. Five factors (zones within a site, sites within a water body, depth, years and surveyors) that potentially generate classification uncertainty were examined in detail. Of these, depth was a major source of uncertainty, while all the remaining spatial and temporal factors displayed low variability. POMI 9 matched POMI 14 in all factors, and could effectively replace it in future monitoring programs. 相似文献
125.
Gustavo M. Cruz-Bello Hallie Eakin Helda Morales Juan F. Barrera 《Natural Hazards》2011,58(1):103-116
This paper analyses the responses related to land use of coffee growers in Chiapas, Mexico to the impact of Hurricane Stan
(October 2005). A multi-temporal analysis of the effect on land cover was performed through the combination of unsupervised
classification of SPOT multispectral images and visual interpretation of panchromatic images (8 months previous to the hurricane,
and 2, 14, and 40 months after the hurricane). The information provided by this geographic analysis was interpreted in light
of information gathered though household surveys. Although the hurricane wrecked havoc across the region, the main impact
in the study area was in the riparian zones where the extent of the loss experienced in terms of coffee harvest and soil was
such that, even 14 months after the event, households with land in those areas were struggling to recover. Nevertheless, after
40 months, the zones that had suffered total soil loss began to support soil and vegetation, indicating the possibility of
replanting coffee in those areas. Although the hurricane occurred when the coffee sector was particularly fragile as a result
of the preceding several years of poor prices, the impact did not trigger extensive land use change. The surveys showed, however,
that people are now more informed of the risk of living and farming on the river margins and are now performing soil conservation
practices and planting trees to reduce risk. 相似文献
126.
Lourdes L��pez-Merino Ana Moreno Manel Leira Javier Sigr�� Pen��lope Gonz��lez-Samp��riz Blas L. Valero-Garc��s Jos�� Antonio L��pez-S��ez Manola Brunet Enric Aguilar 《Journal of Paleolimnology》2011,46(3):453-467
We present a study of two short sediment cores recovered from Lago Enol, in the Picos de Europa National Park, Cantabrian Mountains, northern Iberia. We inferred past climate conditions and anthropogenic impacts using geochemical and biological (pollen and diatoms) variables in the dated sequences, in conjunction with temperature and precipitation data collected since 1871 at meteorological stations in the region. The record provides evidence of environmental changes during the last 200 years. At the end of the Little Ice Age (~1800?C1875 AD) the region was characterized by an open landscape. Long-term use of the area for mixed livestock grazing in the mountains, and cultivation of rye during the nineteenth century, contributed to the expansion of grassland at the expense of forest. Warmer temperatures since the end of the nineteenth century are inferred from a change in diatom assemblages and development of the local forest. Socioeconomic transformation during the twentieth century, such as livestock changes related to dairy specialization, planting of non-native trees, mining activities, and management of the national park since its creation in 1918, caused profound changes in the catchment and in the lake ecology. The last several decades (~1970?C2007 AD) of the Lago Enol sediment record are strikingly different from previous periods, indicating lower runoff and increasing lake productivity, particularly since AD 2000. Today, the large number of tourists who visit the area cause substantial impacts on this ecosystem. 相似文献
127.
Black dimensional stones: geology, technical properties and deposit characterization of the dolerites from Uruguay 总被引:1,自引:1,他引:0
Manuela Morales Demarco Pedro Oyhantçabal Karl-Jochen Stein Siegfried Siegesmund 《Environmental Earth Sciences》2011,63(7-8):1879-1909
Dimensional stones with a black color occupy a prominent place on the international market. Uruguayan dolerite dikes of andesitic and andesitic–basaltic composition are mined for commercial blocks of black dimensional stones. A total of 16 dikes of both compositions were studied and samples collected for geochemical and petrographical analysis. Color measurements were performed on different black dimensional stones in order to compare them with the Uruguayan dolerites. Samples of the two commercial varieties (Absolute Black and Moderate Black) were obtained for petrophysical analysis (e.g. density, porosity, uniaxial compressive strength, tensile strength, etc.). Detailed structural analyses were performed in several quarries. Geochemistry and petrography determines the intensity of the black color. The Uruguayan dolerite Absolute Black is the darkest black dimensional stone analyzed in this study. The petrophysical properties of Uruguayan dolerites make them one of the highest quality black dimensional stones. Structural analyses show that five joint sets have been recognized: two sub-vertical joints, one horizontal and two diagonal. These joint sets are one of the most important factors that control the deposits, since they control the block size distribution and the amount of waste material. 相似文献
128.
Javier Sellanes Germán Zapata-Hernández Silvio Pantoja Gerdhard L. Jessen 《Estuarine, Coastal and Shelf Science》2011
We analyzed C and N stable isotope ratios of benthic fauna and their potential food sources at an intertidal methane seep site and a control site without emanation at Mocha Island (central Chile). The objective was to trace the origin of the main food sources used by the local heterotrophic fauna, based on the hypothesis that chemosynthetic production could be partially fueling the local food web at the seep site. Food sources sampled at both sites included macroalgae, particulate organic matter and bacteria-like filaments found growing over the red algae Gelidium lingulatum within the areas of active methane release. 相似文献
129.
Javier Cotin Manuel García-Tarrasón Carolina Sanpera Lluis Jover Xavier Ruiz 《Estuarine, Coastal and Shelf Science》2011
The Ebro catchment, the largest river basin in Spain, includes various heavily industrialized areas. Among these is the Flix site, where a chemical plant has been operating since the beginning of the 20th century. This extended operational period, together with the construction of a dam next to the factory around 1960, resulted in the accumulation of large amounts of heavily polluted sediments in the adjacent riverbed, many of which are contaminated by mercury. Pollutants from Flix are carried downstream by the Ebro River to its delta. In order to assess the transfer of mercury to the complex river estuary ecosystem, we studied the ecology of the tern community living there as these birds segregate into a range of habitats. For this purpose, first we used stable isotope analysis (SIA) (δ34S, δ13C, δ15N) of eggs to determine the trophic ecology and habitat partitioning of several tern species (Common, Sandwich, Little, Gull-Billed and Whiskered Tern) breeding sympatrically, in order to link their foraging ecology with habitat types. Next we measured mercury concentrations in eggs to monitor the input of this metal into the diverse habitats. With the exception of the Little Tern, the other terns used a restricted habitat range in the Ebro Delta, as shown by C and S isotopes; the Gull-Billed and Whiskered Tern foraged in freshwater habitats, while the Common and Sandwich Tern used marine habitats. This restricted feeding behavior of the Gull-Billed and Common Tern contrasts with previous reports in other breeding sites. The Little Tern, which showed a wide range of isotopic values, was found to be an opportunistic forager but fed mainly in saltpans, a feeding habitat not reported previously for this species in this area. We found that mercury concentrations are related to foraging habitat and diet, and are unexpectedly higher in species feeding on demersal prey in marine habitats and also higher in birds feeding in saltpans than in those feeding in freshwater habitats. The mercury concentrations found in the Little and Common Tern eggs sampled in “Punta de la Banya” may be sufficiently high to endanger breeding success. 相似文献
130.
Juan Pablo Corella Adel El Amrani Javier Sigró Mario Morellón Eugenio Rico Blas Lorenzo Valero-Garcés 《Journal of Paleolimnology》2011,46(3):469-485
We present a high-resolution, multiproxy reconstruction of the depositional history of Lake Arreo, northern Spain, for the
last 60 years. We conducted sedimentological, geochemical and diatom analyses in short cores and made a detailed comparison
with regional instrumental climate data (1952–2007), limnological monitoring of the lake (1992–2008) and recent land use changes
that affect the lake catchment. Chronology is based on “floating” discontinuous varve counts and 137Cs and 14C dates. Four periods were identified in the Lake Arreo recent history: (1) prior to 1963, varved facies intercalated with
fine turbidite deposits, and diatom assemblages dominated by Cyclotella taxa indicate predominantly meromictic conditions, (2) from 1964 to 1978, permanent anoxia persisted in bottom waters, as shown
by similar facies and diatom assemblages as before, though detrital layers were coarser, (3) from 1979 to 1994, sediment delivery
to the lake increased and laminated, clastic facies were deposited, and (4) from 1995 to 2008, dominance of massive facies
and an increase in Fragilaria tenera and Achnanthes minutissima reflect relatively lower lake levels, less frequent bottom anoxia with more frequent water column mixing, similar to modern
conditions. The period 1952–1979 was a time of meromixis and varved facies deposition, and was characterized by higher rainfall
and less intense agricultural pressure in the watershed. There were two short humid periods (1992–1993 and 1996–1998) when
monitoring data show more anoxic weeks per year and relatively higher lake levels. Increased cultivation of small landholdings
in 1963, and particularly after 1979, caused a large increase in sediment delivery to the lake. The inferred lake evolution
is in agreement with monitoring data that suggest a transition from dominantly meromictic conditions prior to 1993–1994 to
a predominantly monomictic pattern of circulation since then, particularly after 2000. The synergistic effects of intensive
water extraction for irrigation and lower rainfall since 1979, and particularly since 1994, brought the long period of meromictic
conditions in Lake Arreo to an end. Water balance and sediment delivery to the lake are dominant factors that control the
limnological and mixing conditions in Lake Arreo and they must be considered in management and restoration plans. 相似文献