Age constraints to the relationships between magmatism, metamorphism and tectonism in the Aracena metamorphic belt, southern Spain

 The Aracena metamorphic belt (AMB), southwest Iberian peninsula, is characterized by the following geological elements: (a) a high-temperature/low-pressure (HT/LP) metamorphic belt a few kilometres wide and more than 200 km long; (b) a linear belt of oceanic amphibolites with a low-pressure inverted metamorphic gradient; (c) crustal-scale ductile shear zones; and (d) mafic, noritic intrusions of high-Mg andesite (boninite) composition. The relationships between these elements led to the proposal of a model of ridge subduction for this sector of the Hercynian belt of Europe. This interpretation is supported by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously. The results of a geochronological study (Ar–Ar, Rb–Sr and Sm–Nd) clearly support a Late Paleozoic tectonic evolution at an active continental margin. The time evolution of the metamorphism in the oceanic domain, ranging from 342.6±0.6 Ma in the west to 328.4±1.2 Ma in the east, over a distance of 70 km along the metamorphic belt, support a tectonic model of triple-junction migration responsible for the creation at depth of a slab-free window with decisive consequences for the thermal evolution of the region. The origin of the linear metamorphic belt of HT/LP regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by the creation of a triple-junction at the continental margin. Received: 9 March 1998 / Accepted: 9 December 1998     

Representivity of incompletely sampled fall deposits in estimating eruption source parameters: a test using the 12–13 January 2011 lava fountain deposit from Mt. Etna volcano,Italy

The Southeast Crater (SEC) of Mt. Etna, Italy, is renowned for its high activity, mainly long-lived eruptions consisting of sequences of individual paroxysmal episodes which have produced more than 150 eruptive events since 1998. Each episode typically forms eruption columns followed by tephra fallout over distances of up to about 100 km from the vent. One of the last sequences consisted of 25 lava fountaining events, which took place between January 2011 and April 2012 from a pit-vent on the eastern flank of the SEC and built a new scoria cone renamed New Southeast Crater. The first episode on 12–13 January 2011 produced tephra fallout which was unusually dispersed toward to the South extending out over the Mediterranean Sea. The southerly deposition of tephra permitted an extensive survey at distances between ~1 and ~100 km, providing an excellent characterization of the tephra deposit. Here, we document the stratigraphy of the 12–13 January fallout deposit, draw its dispersal, and reconstruct its isopleth map. These data are then used to estimate the main eruption source parameters. The total erupted mass (TEM) was calculated by using four different methodologies which give a mean value of 1.5?±?0.4?×?10⁸ kg. The mass eruption rate (MER) is 2.5?±?0.7?×?10⁴ kg/s using eruption duration of 100 min. The total grain-size (TGS) distribution, peaked at ?3 phi, ranges between ?5 and 5 phi and has a median value of ?1.4 phi. Further, for the eruption column height, we obtained respective values of 6.8–13.8 km by using the method of Carey and Sparks (1986) and 3.4?±?0.3 km by using the methods of Wilson and Walker (1987), Mastin et al. (2009), and Pistolesi et al. (2011) and considering the mean value of MER from the deposit. We also evaluated the uncertainty and reliability of TEM and TGS for scenarios where the proximal and distal samples are not obtainable. This is achieved by only using a sector spanning the downwind distances between 6 and 23 km. This scenario is typical for Etna when the tephra plume is dispersed eastward, i.e., in the prevailing wind direction. Our results show that, if the analyzed deposit has poorer sample coverage than presented in this study, the TEM (3.4?×?10⁷ kg) is 22 % than the TEM obtained from the whole deposit. The lack of the proximal (<6 km) deposit may cause more significant differences in the TGS estimations.     

Code‐based record selection methods for seismic performance assessment of buildings

The recent concerns regarding the seismic safety of the existing building stock have highlighted the need for an improvement of current seismic assessment procedures. Alongside with the development of more advanced commercial software tools and computational capacities, nonlinear dynamic analysis is progressively becoming a common and preferable procedure in the seismic assessment of buildings. Besides the complexity associated with the formulation of the mathematical model, major issues arise related with the definition of the seismic action, which can lead to different levels of uncertainty in terms of local and global building response. Aiming to address this issue, a comparative study of different code‐based record selection methods proposed by Eurocode 8, ASCE41‐13 and NZS1170.5:2004 is presented herein. The various methods are employed in the seismic assessment of four steel buildings, designed according to different criteria, and the obtained results are compared and discussed. Special attention is devoted to the influence of the number of real ground motion records selected on the estimation of the mean seismic response and, importantly, to the efficiency that is achieved when an additional selection criteria, based on the control of the spectral mismatch of each individual record with respect to the reference response spectrum, is adopted. The sufficiency of the methods with respect to the pairs of M–R of the selected group of records and the robustness of the scaling procedure are also examined. The paper closes with a study which demonstrates the suitability of a simplified probability‐based approach recently proposed for estimating mean seismic demands. Copyright © 2015 John Wiley & Sons, Ltd.     

Human impacts overwhelm the effects of sea-level rise on Basque coastal habitats (N Spain) between 1954 and 2004

According to coastal measurements, global mean sea-level has risen at a rate of 1.8 mm yr⁻¹ between 1950 and 2000, with large spatial variability at regional scales. Within the Bay of Biscay, trends computed from coastal tide gauges records have revealed that sea-level rise is accelerating over this period of time; this is in agreement with rates obtained from satellite imagery in the open ocean since 1993. The objectives of the present study are: (1) to assess the evidence of the relative sea-level rise on coastal morphology and habitats in the Gipuzkoan littoral zone (Basque coast, northern Spain) for the period 1954–2004, and (2) to evaluate the relative contribution of local anthropogenic versus sea-level rise impacts for explaining inter-supratidal habitat changes. A high-resolution airborne laser altimetry data (LIDAR) has been used to derive a Digital Terrain Model (DTM) of 15-cm vertical resolution. Coastal habitats were mapped for two periods, using historic airborne photography (1954) and high-resolution imagery (2004). Analysis of tide gauge records from Santander (northern Spain) has revealed that relative mean sea-level has been rising at a rate of 2.08 ± 0.33 mm yr⁻¹ from 1943 to 2004; this is consistent with sea-level trends from other measurements within the area (St. Jean de Luz and Bilbao), obtained over shorter periods of time, and with previous results obtained in the Bay of Biscay. Based upon this sea-level trend and by means of a LIDAR-based DTM, the results have indicated that the predicted change along the Gipuzkoan coast due to sea-level rise was of 11.1 ha within the 50-yr period. In contrast, comparison of historical and recent orthophotography has detected only 2.95 ha of change, originated possibly from sea-level rise, and 98 ha transformed by anthropogenic impacts. Hence, coastal changes due to sea-level rise might be overwhelmed by excessive human impacts, at the spatial and temporal scales of the analysis. This work highlights that local anthropogenic impact is the major threat to Basque coastal and estuarine habitats, compared with natural erosive processes and global climate change driving forces over recent times.     

Growth modelling indicates hurricanes and severe storms are linked to low coral recruitment in the Caribbean

This study set out to test the hypothesis that hurricanes and tropical storms limit the recruitment and subsequent survival of massive non-branching corals on the barrier reef off the coast of Belize in the Gulf of Honduras. Overall, the surface areas of 523 individual coral specimens were measured, and recruitment dates were then modelled. There was no significant difference in coral cover or coral biodiversity between any of the sites studied (p > 0.1). There were significant differences in non-branching coral recruitment in years when hurricanes impacted the area (p < 0.05) compared with years when no hurricanes impacted the area. There were significantly more non-branching massive corals recruited in non-hurricane years (mean 7.7) than in hurricane years (mean 3.8; p = 0.011). When years with tropical storms are added to the years with hurricanes, there was significantly lower coral recruitment (mean 4.7) relative to non-storm or hurricane years (mean 7.4; p = 0.019). These results show that hurricanes and severe storms limited the recruitment and survival of massive non-branching corals of the Mesoamerican barrier reef and on patch reefs near the Belize coast in the Caribbean, and suggests that marine park managers may need to assist coral recruitment in years where there are hurricanes or severe storms.     

Structural study of synthetic mica–montmorillonite by means of 2D MAS NMR experiments

Syn-1, is a synthetic mica–montmorillonite interstratified mineral that forms one of the standard clay samples in the Clay Minerals Society Source Clays Project. However, there are still controversies regarding some structural aspects such as the interlayer composition or the location of the extra-aluminium determined by chemical analysis. The main objective of this paper is to shed light on those structural aspects that affect the reactivity of the interstratified minerals. For this purpose, we have used ¹ H–²⁹ Si and ¹ H–²⁷Al HETCOR MAS NMR to show that it is likely that the interlayer space of the beidellite part is composed of ammonium ions whereas ammonium and aluminium ions are responsible for the charge balance in the mica type layer.     

Proto-Adamastor ocean crust (920 Ma) described in Brasiliano Orogen from coetaneous zircon and tourmaline

Proto-Adamastor ocean bathed Rodinia and successor continental fragments from 1.0–0.9 Ga up to 0.75 Ga, and evolved into world Adamastor Ocean at 0.75–0.60 Ga. Mesoproterozoic oceanic crust is poorly preserved on continents, only indirect evidence registered in Brasiliano Orogen. We report first evidence of ophiolite originated in proto-Adamastor. We use multi-technique U-Pb-Hf zircon and δ¹¹B tourmaline isotopic and elemental compositions. The host tourmalinite is enclosed in metaserpentinite, both belonging to the Bossoroca ophiolite. Zircon is 920 Ma-old, ?_{Hf(920 Ma)} = +12, Hf_TDM = 1.0 Ga and has ‘oceanic’ composition (e.g., U/Yb < 0.1). Tourmaline is dravite with δ¹¹B = +1.8‰ (Tur 1), 0‰ (Tur 2), ?8.5‰ (Tur 3). These characteristics are a novel contribution to Rodinia and associated world ocean, because a fragment of proto-Adamastor oceanic crust and mantle evolved at the beginning of the Brasiliano Orogen.     

Phytoplankton functional groups: Short‐term variation in a tropical tidal‐forced estuarine system

Functional groups have become an important tool for characterizing communities of marine and estuarine environments. Their use also holds promise for a better understanding of the temporal dynamics of phytoplankton. This study aimed to evaluate the contributions of phytoplankton size fractions and functional groups characterizing short‐term variation throughout tidal cycles and between dry and rainy seasons in a tropical estuarine system. Camamu Bay is an oligotrophic estuarine system that is under strong influence from tropical shelf waters and is characterized by high salinity and low concentrations of dissolved nutrients. Surface‐water samples were collected at nine sampling sites distributed among the three hydrodynamic regions of the bay, and at a mooring, at 3‐hr intervals during tidal cycles (12 hr each) in both the rainy and the dry season. Although the abundances of the phytoplankton fractions (pico‐, nano‐, and micro‐) were higher in the rainy season and during periods of higher tide, they were not significantly higher. The phytoplankton community in the bay comprises three functional groups: GI = “colonial” (i.e., chain‐forming diatoms and filamentous cyanobacteria); GII = “GALD >40” (i.e., pennate and centric diatoms with MDL >40 µm), and GIII = “flagellates” (i.e., species with motility via flagella). Nanoflagellates were the most abundant form in the bay, while chain‐forming diatoms, in particular, contributed to the microphytoplankton fraction during both the rainy and dry seasons. Functional groups, as defined by cluster analysis, reflected ecological strategies compatible with the high hydrodynamics of Camamu Bay, which is characterized by processes of tidal‐forced intense mixing, mainly during periods of spring tides. The phytoplankton of the bay was found to possess a series of attributes (functional traits) that endow them with some resistance to sinking. Functional diversity indexes (FEve, FDiv, and FDis) indicated a stable community without significant short‐term variation due to low variability in the environmental conditions of the system during the study period.     

Sequence of facies at a Holocene storm-dominated regressive barrier at Praia de Leste,southern Brazil

Along the coast at Praia de Leste, southern Brazil (25° 41’ S latitude), a positive sedimentary budget and a lowering of the sea level following the postglacial maximum has given rise to a progradational strandplain barrier that is 3–5 km wide. Sand quarries that lie 3.5 km from the present-day coastline have provided material from exceptional outcrops of 75% of the sequence of Holocene barrier facies. Five facies associations have been identified, which correspond to the inner shelf, the lower, middle and upper shorefaces, and the foreshore. The architecture of the facies shows a regressive sequence that overlies an erosional surface and downlapping Pleistocene sediments. At Praia de Leste, the facies association has a thickness of 14 m and is deposited between 2 m above and 12 m below mean paleo sea level. The barrier corresponds to a coastal environment that is characterised by medium to low wave energy under the additional influence of episodic storm events. The sequence at the Praia de Leste barrier differs from the sequence of clastic shoreline facies found elsewhere in two main ways. The first difference is the high content of fine sediment and plant debris, released at around the same time as the formation of the barrier from large estuarine systems, and the second is the predominance on the middle and lower shorefaces of swaley cross-stratified sand facies with abundant plant debris. We also describe two further characteristics of the barrier at Praia de Leste. First, the swaley cross-stratified sand represents a component of onshore transport that resulted in the accumulation of sediment transported from the shelf. Second, the beach step has been well preserved, thereby allowing the continuous tracing of seaward-dipping, low-angle cross-stratification to a sigmoidal cross-stratified beach-step sandy facies. Finally, we have herein been able to improve the precision of the sequence of formation of the depth of the facies, by making reference to paleo sea level during the formation of the barrier, rather than to present-day mean sea level.     

Occurrence of selected estrogens in mangrove sediments

This paper presents results related to the occurrence and distribution of estrogens along the Brazilian coast. Three mangrove areas were chosen to evaluate the presence of estrogens in surface sediments of mangrove forests. The presence of estrogens was observed in all studied sites. 17-α-Ethinylestradiol (EE2), a synthetic estrogen, was the most common and has been found in higher concentration (0.45-129.78 ng/g) compared to 17-β-estradiol (E1) and estrone (E2) (both being natural estrogens). The concentrations of E1 and E2 ranged from 0.02 to 49.27 ng/g and 0.03 to 39.77 ng/g, respectively. Theoretically, under anaerobic conditions EE2 can be reduced to E1 even in environments such as sediments of mangrove forests, which are essentially anaerobic. Even if the concentrations of estrogens seem to be insignificant in some samples, the effects remain uncertain.