HP–LT Variscan metamorphism in the Cubito-Moura schists (Ossa-Morena Zone, southern Iberia)

Multi-equilibrium thermobarometry shows that low-grade metapelites (Cubito-Moura schists) from the Ossa–Morena Zone underwent HP–LT metamorphism from 340–370 °C at 1.0–0.9 GPa to 400–450 °C at 0.8–0.7 GPa. These HP–LT equilibriums were reached by parageneses including white K mica, chlorite and chloritoid, which define the earliest schistosity (S₁) in these rocks. The main foliation in the schists is a crenulation cleavage (S₂), which developed during decompression from 0.8–0.7 to 0.4–0.3 GPa at increasing temperatures from 400–450 °C to 440–465 °C. Fe³⁺ in chlorite decreased greatly during prograde metamorphism from molar fractions of 0.4 determined in syn-S₁ chlorites down to 0.1 in syn-S₂ chlorites. These new data add to previous findings of eclogites in the Moura schists indicating that a pile of allochtonous rocks situated next to the Beja-Acebuches oceanic amphibolites underwent HP–LT metamorphism during the Variscan orogeny. To cite this article: G. Booth-Rea et al., C. R. Geoscience 338 (2006).     

Geology,mineral chemistry and tourmaline B isotopes of the Córrego Bom Sucesso area,southern Serra do Espinhaço,Minas Gerais,Brazil: Implications for Au–Pd–Pt exploration in quartzitic terrain

The Palaeo-Mesoproterozoic metasiliciclastic rocks of the southern Serra do Espinhaço, Minas Gerais, Brazil, are host to historically important alluvial deposits of diamonds and gold. Detrital gold grains often comprise Au–Pd–Pt intermetallic compounds, with low Ag contents, which contain inclusions of tourmaline and titaniferous hematite (up to ~ 6 wt.% TiO₂). The latter minerals connect the alluvial mineralisation to the rutile–hematite–quartz veins and tourmalinisation observed in the quartzitic country rocks of the alluvial gravel. The quartzite (Sopa-Brumadinho Formation of lacustrine to fan-deltaic origin) is affected by pervasive B metasomatism with F-bearing tourmaline replacing the recrystallised quartz fabric. The tourmaline belongs to the alkali group, with Mg/(Mg + Fe) and X/(X + Na) ratios in the ranges from 0.5 to 0.7 and 0.18 to 0.29, respectively, where X represents vacancies in the X site. Boron-isotopic values of tourmaline vary from ~ 1 to − 10.4‰ δ¹¹B. The B-isotope range, in conjunction with the Na–Mg-rich tourmaline composition, and the widespread occurrence of tourmalinite in the Sopa-Brumadinho Formation suggest a derivation from non-marine evaporitic brines. Brines are capable of transporting otherwise immobile Ti and explain, under oxidising conditions, the fractionation of Ag from Pd to precipitate palladiferous gold with extremely high Pd/Ag ratios. Zirconium-in-rutile and Ti-in-quartz temperatures for a variety of hematite-rich veins suggest episodic vein emplacement over a temperature range from around 500 °C to ~ 350 °C. Cross-cutting relationships and episodic vein emplacement indicate a late-Brasiliano age.     

The relation of meridional pressure gradients to North Atlantic deep water volume transport in an ocean general circulation model

We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic.     

Influence of size on the constitutive equations of concrete or rock dowels

The numerical fracture analysis of non‐homogeneous rock or concrete dowels subjected to shear and compression is described in detail. The method of analysis allows the consideration of scale and rate effects due to material non‐homogeneity and fracture. The proposed approach is verified by comparing numerical predictions with experimental results reported in the literature for a series of small rock samples, since experimental evidence for large bodies is not yet available (2007). Results generated by Monte Carlo simulation using the so‐called discrete element method to model the dowels suggest that a simple three parameters law can be used to predict the relationship between tangential stress at the base and lateral distortion. It is observed that the larger the size of the cubes, the smaller both the peak tangential stress and the rupture distortion. Size effects are also evaluated in samples with vertical restraint. The influence of loading rate is likewise numerically assessed for two sample sizes. The effect is compatible with experimental evidence available for concrete using small samples. Copyright © 2008 John Wiley & Sons, Ltd.     

Concurrent imaging of chlorophyll fluorescence,Chlorophyll a content and green fluorescent proteins‐like proteins of symbiotic cnidarians

Research on photosynthetic cnidarians has been mainly focused on the symbiosis established between the cnidarian host and its dinoflagellates endosymbionts from genus Symbiodinium. Despite the potential of imaging techniques for assessing the spatial distribution of key parameters of cnidarian photobiology, such as photochemical activity, chlorophyll a content or green fluorescent proteins (GFPs), to our best knowledge, no study has ever attempted to simultaneous map these three features. In this study, we developed a modified imaging pulse amplitude fluorometer by applying excitation light of different wavelengths and selectively detecting short spectral bands through bandpass filters. The imaging system was used to sequentially excite and quantify chlorophyll variable fluorescence (maximum quantum yield of photosystem II, F_v/F_m), Chl a content (normalized difference vegetation index) and relative content of GFPs. The spatial distribution of these photophysiological parameters was mapped both horizontally, across the surface of the soft corals Sarcophyton cf. glaucum and Sinularia flexibilis and the zoanthid Protopalythoa sp., and vertically, throughout a vertical section of S. cf. glaucum. Results showed bleached areas within each individual coral colony and registered photophysiological changes with S. cf. glaucum tissue depth. Analysis of Protopalythoa sp. polyps’ expansion revealed differential surface patterns of NDVI and GFP concentration, and a negative relation between these latter parameters within each polyp. This novel non‐invasive approach allowed a high‐resolution characterization of the spatial relationship between these key parameters through the analysis of image information on a pixel‐by‐pixel basis, which has great potential for investigating the physiological state of symbiotic associations.     

Likely effects of climate change on groundwater availability in a Mediterranean region of Southeastern Spain

Groundwater resources are typically the main fresh water source in arid and semi‐arid regions. Natural recharge of aquifers is mainly based on precipitation; however, only heavy precipitation events (HPEs) are expected to produce appreciable aquifer recharge in these environments. In this work, we used daily precipitation and monthly water level time series from different locations over a Mediterranean region of Southeastern Spain to identify the critical threshold value to define HPEs that lead to appreciable aquifer recharge in this region. Wavelet and trend analyses were used to study the changes in the temporal distribution of the chosen HPEs (≥20 mm day^?1) over the observed period 1953–2012 and its projected evolution by using 18 downscaled climate projections over the projected period 2040–2099. The used precipitation time series were grouped in 10 clusters according to similarities between them assessed by using Pearson correlations. Results showed that the critical HPE threshold for the study area is 20 mm day^?1. Wavelet analysis showed that observed significant seasonal and annual peaks in global wavelet spectrum in the first sub‐period (1953–1982) are no longer significant in the second sub‐period (1983–2012) in the major part of the ten clusters. This change is because of the reduction of the mean HPEs number, which showed a negative trend over the observed period in nine clusters and was significant in five of them. However, the mean size of HPEs showed a positive trend in six clusters. A similar tendency of change is expected over the projected period. The expected reduction of the mean HPEs number is two times higher under the high climate scenario (RCP8.5) than under the moderate scenario (RCP4.5). The mean size of these events is expected to increase under the two scenarios. The groundwater availability will be affected by the reduction of HPE number which will increase the length of no aquifer recharge periods (NARP) accentuating the groundwater drought in the region. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluating restoration of man‐made slopes: a threshold approach balancing vegetation and rill erosion

The management of reclaimed slopes derived from industrial and civil activities (e.g. surface mining and road construction) requires the development of practical stability analysis approaches that integrate the processes and mechanisms that rule the dynamics of these ubiquitous emerging ecosystems. This work describes a new modelling approach focused on stability analysis of water‐limited reclaimed slopes, where interactive relationships between rill erosion and vegetation regulate ecosystem stability. Our framework reproduces two main groups of possible trends along the temporal evolution of reclaimed slopes: successful trends, characterized by widespread vegetation development and the effective control of rill erosion processes; and gullying trends, characterized by the progressive loss of vegetation and a sharp logistic increase in erosion rates. Furthermore, this analytical approach allows the determination of threshold values for the state variables (i.e. vegetation cover and rill erosion) that drive the system's stability, facilitating the identification of critical situations that require specific human intervention (e.g. revegetation or, in very problematic cases, revegetation combined with rill network destruction) to ensure the long‐term sustainability of the restored ecosystem. The application of our threshold analysis framework in Mediterranean‐dry reclaimed slopes derived from surface coal mining (the Teruel coalfield in central‐eastern Spain) showed a good field‐based performance. Therefore, we believe that this model is a valuable contribution for the management of water‐limited reclaimed systems, including those associated with rill erosion, as it provides a tool for the evaluation of restoration success and can play an important role in decision‐making during ecosystem restoration in severely disturbed landscapes. Copyright © 2011 John Wiley & Sons, Ltd.     

Tectonic implications of the earthquake swarm of 1997 in the Michoacan Triangle, Mexico

An earthquake swarm occurred during February and March 1997 in the vicinity of the Tancitaro Volcano, in the southern part of the tectonically complex Michoacan Triangle. A study of these events provides an opportunity to map the active faults in the area and to learn if the orientation and the sense of motion on these faults are consistent with the mapped faults and the alignment of cinder cones in the region. The foci of 230 earthquakes, which could be located, are distributed between 10 and 18 km depth, and show an alignment in, roughly, a NE direction. The focal mechanisms and seismic moments of the 27 best-recorded events were determined by waveform modeling of P and S waves. These mechanisms show two distinct patterns. More than 50% of the solutions are left-lateral strike–slip mechanisms with a normal component. The preferred fault plane strikes NE. Another group of events, probably caused by triggered seismicity on the Chapala–Oaxaca fault zone, shows left-lateral strike–slip mechanisms with a large-thrust component on NW-trending faults. S wave splitting shows 1–2.5% crustal-anisotropy. The direction of the anisotropy coincides with the NE alignment of events, and the preferred nodal plane. This is also the alignment of cinder cones, suggesting that preexisting fractures and cracks are responsible for the seismicity and anisotropic behavior of the crust. The resulting stress orientation, NE compression, is the one expected for the fore-arc region. We conclude that although Michoacan Triangle lies in the Trans-Mexican Volcanic Belt, it does not form part of this stress province where the stress orientation is NS extension.