Evidence of rainfall variations in Southern Brazil from trace element ratios (Mg/Ca and Sr/Ca) in a Late Pleistocene stalagmite

Trace element ratios in the Bt2 stalagmite from Botuverá cave, Southern Brazil, are explored as a proxy for changes in the local rainfall recharge during the last 116 ky. BP Mg/Ca and Sr/Ca ratios, measured with an electron microprobe, are significantly positively correlated with one another throughout the entire record, and vary in a way that is very consistent with variations of δ¹⁸O in the same speleothem during the last glacial period. We suggest that prior calcite precipitation in the vadose zone of the cave system is the main factor affecting the incorporation of Mg and Sr into calcite of the stalagmite. This interpretation is supported by trace element correlation patterns and by results from a hydrochemistry study performed in a cave located in the same region and in a similar environmental setting. Therefore, we conclude that higher (lower) Mg/Ca and Sr/Ca values are associated with lower (higher) levels of recharge into the karstic aquifer, as such conditions lead to an increase (decrease) in the volume of calcite precipitated in the unsaturated zone above the cave during dry (wet) climate periods.Trace element variations point to generally dryer (wetter) conditions during lower (high) phases of summer insolation in the southern hemisphere. These periods coincide with decreased (increased) activity of the South American summer monsoon, as revealed by δ¹⁸O stalagmite records. In addition trace element variations show that rather wet conditions persisted throughout most of the last glacial period from approximately 70 to 17 ky BP. We suggest that during this period the glacial boundary conditions, especially ice volume buildup in the northern hemisphere, played an important role for monsoon rainfall intensification in the region.     

Anomalous Diffuse CO2 Emission prior to the January 2002 Short-term Unrest at San Miguel Volcano, El Salvador, Central America

On January 16, 2002, short-term unrest occurred at San Miguel volcano. A gas-and-steamash plume rose a few hundred meters above the summit crater. An anomalous microseismicity pattern, about 75 events between 7:30 and 10:30 hours, was also observed. Continuous monitoring of CO₂ efflux on the volcano started on November 24, 2001, in the attempt to provide a multidisciplinary approach for its volcanic surveillance. The background mean of the diffuse CO₂ emission is about 16 g m^-2 d^-1, but a 17- fold increase, up to 270 g m^-2 d^-1, was detected on January 7, nine days before the January 2002 short-term unrest at San Miguel volcano. These observed anomalous changes on diffuse CO₂ degassing could be related to either a sharp increase of CO₂ pressure within the volcanic-hydrothermal system or degassing from an uprising fresh gas-rich magma within the shallow plumbing system of the volcano since meteorological fluctuations cannot explain this observed increase of diffuse CO₂ emission.     

Paleosubsidence and active subsidence due to evaporite dissolution in the Zaragoza area (Huerva River valley, NE Spain): processes, spatial distribution and protection measures for transport routes

The lowest 17-km long reach of the Huerva River valley, down to its confluence with the Ebro River in Zaragoza city, flows across salt-bearing evaporites of the Ebro Tertiary Basin (NE Spain). Upstream, the horizontally lying Miocene evaporites are interfingered with non-soluble distal alluvial fan facies (shales and sandstones). The proportion of soluble facies in the Huerva River valley increases in a downstream direction towards the basin depocenter. On the basis of the type and magnitude of the paleosubsidence features, the valley has been divided into four reaches. Along reach I, undeformed terrace deposits less than 4 m thick rest on insoluble detrital bedrock. In reaches II and III, dissolution at the alluvium–bedrock boundary has generated local thickening, deformation and paleocollapse structures, which only affect the alluvial mantle. In reach IV, terrace deposits thicken to over 60 m resulting from a large-scale synsedimentary subsidence. In this sector, subsidence locally affects to both the alluvium and the underlying bedrock. This indicates that dissolution acts at the rockhead beneath the alluvial cover (alluvial karst) and within the evaporitic substratum (interstratal karst). The development of an intraevaporitic karst in reach IV is attributed to gypsum and salt dissolution. Irregular terrace gravel bodies (gravel pockets) embedded in a fine-grained matrix associated with paleocollapse structures have been interpreted as liquefaction–fluidization structures resulting from ground acceleration and suction induced by catastrophic collapses. Subsidence is currently active in the region affecting areas with a thin alluvial cover in reaches III and IV. The low subsidence activity in most of Zaragoza city is explained by the presence of thickened (around 50 m) and indurated alluvial deposits. In the surrounding area, numerous buildings in Cadrete and Santa Fe villages have been severely damaged by subsidence. Natural and human-induced subsidence favours the development of slope movements in the gypsum scarp overlooking Cadrete village. Several transport routes including the Imperial Canal (irrigation canal) and the recently completed Madrid–Barcelona high-speed railway are affected by human-induced sinkholes. The paleocollapse structures exposed in the trenches of this railway and a ring road under construction point to hazardous locations underlain by cavities and collapse structures where special protection measures should be applied. Rigid structures are recommended beneath the high-speed railway with sufficient strength to span the larger sinkholes with no deformation. Electronic monitoring devices linked to a warning system can detect subtle subsidence-induced deformations in carriageways or railways. This research demonstrates that the study of the paleokarst helps to understand the processes involved in the present-day subsidence phenomena and their general spatial distribution.     

Internal structure of a collapsed terrain: The Lújar syncline and its significance for the fold- and sheet-structure of the Alborán Domain (Betic Cordilleras, Spain)

Detailed structural analysis of the Sierra de Lújar in the western Alpujarras region (Betic Cordilleras, S Spain), a very representative area of the terrain known as the Alborán Domain, has revealed the existence of a very large N-vergent recumbent syncline which involves the whole mountain massif and neighbouring areas. The Lújar syncline and, probably, the associated recumbent anticline which crops out southeast of Sierra de Lújar show a great variation in the orientation of the hinge line. Although having a curved shape, the hinge line is contained in a plane whose attitude coincides with the main attitude of the axial-plane crenulation foliation (Sc), suggesting that it is a sheath fold.The strongly deformed overturned limb of the syncline is cut by two low-angle normal faults displacing towards the north. Similarity in the kinematics between the faults and the fold, and the association between the faults and the high-strain zone in the overturned limb of the fold, suggest that they are related. Regional constraints on the age of the crenulation cleavage and the low-angle normal faults indicate that they formed during the early Miocene late-orogenic extensional event in the Alborán Domain.We propose an alternate explanation for the structure of the Alpujarras region in which the Lújar syncline forms part of a recumbent syncline–anticline pair that extends along much of the Alpujarride outcrop in the southern Betic Cordillera. In several places, the fold is disrupted by low-angle normal faults, and it is overlain by an upper Alpujárride extensional sheet mainly composed of medium- to high-grade metamorphic rocks. We suggest that all these structures arose from the extensional deformation under decreasing temperature conditions of a previously thickened and metamorphosed orogenic crust.     

Mineralogy, geochemistry and uses of the mordenite–bentonite ash-tuff beds of Los Escullos, Almería, Spain

The mordenite ore deposit of Los Escullos has a surface area of 10⁶ m² with an average thickness of 5 m and estimated reserves of 7,500,000 tons of mordenite–bentonite. It is made up of horizontal layers of interbedded epiclastic tuffs with volcanic bentonitised materials which have been subjected to hydromagmatic activity. The layers are essentially composed of bentonite and mordenite with lesser amounts of quartz, cristobalite, biotite, plagioclase, chlorite, amphiboles, titanomagnetite, ilmenite and calcite. The harder layers display a higher proportion of plagioclase crystals and are enriched in Al₂O₃, CaO, Fe₂O₃, TiO₂, P₂O₅, Cu, Zn, Co, Cr, Ni and V, while the more altered layers contain larger contents of SiO₂, K₂O and Y. The amount of sodium increases (from 2% to 4%) relative to depth. Alteration processes resulted in a reduction in the contents of CaO, K₂O and MnO and increase in Na₂O and MgO. The beds of volcanic ash-tuffs have been devitrified by hydrothermal solutions giving rise to bentonites and sodium- and silica-rich residual fluids which have partly crystallized as mordenite and cristobalite. The raw material (mordenite–bentonite) can be improved removing biotite (magnetic separation) and plagioclase and quartz (by floating methods); however, the mordenite–bentonite mineral assemblage is practically impossible to separate due to the size of the crystals (average 0.5 μm under SEM–EDAX). In turn, this upgraded raw material has very useful properties (total area=520 m²/g and cation exchange capacity=70 meq/100 g) which may make it suitable for use in absorption processes (e.g. deodorization, cationic exchange), catalysis and molecular sieving.     

Hierarchy of Factors of Seismic Danger in Four Towns in Colima State,México

The RADIUS methodology is applied to study the hierarchy of the factors of seismic danger in four towns of Colima state, México. These towns are situated near the western coast of Pacific ocean, within zone of highest seismic risk. The 31 indicators and five factors (Hazard, Exposure, Vulnerability, External context and Emergency response and recovery capability) of seismic danger were calculated. Their comparative analysis showed that the social factors of seismic danger might compete and even prevail over the natural factors. The recent earthquake of 21 January 2003 (Intensity up to VII–VIII MM in Colima state) was used to discuss some problems of application of RADIUS methodology in various locations, such as in Latin America.     

The direct boundary element method: 2D site effects assessment on laterally varying layered media (methodology)

The Direct Boundary Element Method (DBEM) is presented to solve the elastodynamic field equations in 2D, and a complete comprehensive implementation is given. The DBEM is a useful approach to obtain reliable numerical estimates of site effects on seismic ground motion due to irregular geological configurations, both of layering and topography.The method is based on the discretization of the classical Somigliana's elastodynamic representation equation which stems from the reciprocity theorem. This equation is given in terms of the Green's function which is the full-space harmonic steady-state fundamental solution. The formulation permits the treatment of viscoelastic media, therefore site models with intrinsic attenuation can be examined. By means of this approach, the calculation of 2D scattering of seismic waves, due to the incidence of P and SV waves on irregular topographical profiles is performed. Sites such as, canyons, mountains and valleys in irregular multilayered media are computed to test the technique. The obtained transfer functions show excellent agreement with already published results.     

Implications for the early shield-stage evolution of Tenerife from K/Ar ages and magnetic stratigraphy

The combined use of field geology, radioisotopic dating and magnetic stratigraphy applied to the old shield volcanoes of Tenerife provides a reliable time framework for the early, shield-stage evolution of the island. The greater part of this new set of ages, obtained from sequences of lava flows is in agreement with the astronomical polarity time scale. This approach illustrates that previous K-Ar data collected without a comprehensive stratigraphy should be viewed with caution, and in some cases discarded altogether. The shield volcanoes of Tenerife encompass a relatively small number of magnetozones, an observation consistent with the relatively short periods of growth shown by the new ages (1-2 my). The island was constructed by the aggregation of three successive shields: the Roque del Conde (Central shield), between about 11.9 and 8.9 Ma, and the Teno (6.2-5.6 Ma) and Anaga (4.9-3.9 Ma) volcanoes. This new oldest subaerial age of Tenerife fits with the others obtained in the Canaries in a clear west to east monotonous age progression, one of the main restrictions for hotspot-related island chains.     

Chronology and causes of the extinction of the Lava Mouse, Malpaisomys insularis (Rodentia: Muridae) from the Canary Islands

Understanding late Holocene extinctions on islands requires accurate chronologies for all relevant events, including multiple colonisations by humans and the introduction of alien species. The most widely held hypothesis on the causes of Holocene island vertebrate extinctions incorporates human impacts, although climatic-related hypotheses cannot be excluded. Both hypotheses have been suggested to account for the extinction of the endemic Lava Mouse, Malpaisomys insularis from the Canary Islands. Here we present the first accelerator mass spectrometer (AMS) ¹⁴C ages from collagen of M. insularis bones from ancient owl pellets collected at Fuerteventura (Canary Islands, eastern Atlantic Ocean). These new dates contribute to an understanding of the extinction of this species. We are able to exclude climatic causes, predation by invasive species, and competition with the house mouse, Mus musculus. The arrival of Europeans in the Canary Islands correlates with the extinction of Malpaisomys. The introduction of rats, Rattus spp., together with their parasites and diseases, emerges as the most reasonable hypothesis explaining the extinction of M. insularis.