Wind influence on surface current variability in the Ibiza Channel from HF Radar

Surface current variability is investigated using 2.5 years of continuous velocity measurements from an high frequency radar (HFR) located in the Ibiza Channel (Western Mediterranean Sea). The Ibiza Channel is identified as a key geographical feature for the exchange of water masses but still poorly documented. Operational, quality controlled, HFR derived velocities are provided by the Balearic Islands Coastal Observing and Forecasting System (SOCIB). They are assessed by performing statistical comparisons with current-meter, ADCP, and surface lagrangian drifters. HFR system does not show significant bias, and its accuracy is in accordance with previous studies performed in other areas. The main surface circulation patterns are deduced from an EOF analysis. The first three modes represent almost 70 % of the total variability. A cross-correlation analysis between zonal and meridional wind components and the temporal amplitudes of the first three modes reveal that the first two modes are mainly driven by local winds, with immediate effects of wind forcing and veering following Ekman effect. The first mode (37 % of total variability) is the response of meridional wind while the second mode (24 % of total variability) is linked primarily with zonal winds. The third and higher order modes are related to mesoscale circulation features. HFR derived surface transport presents a markedly seasonal variability being mostly southwards. Its comparison with Ekman-induced transport shows that wind contribution to the total surface transport is on average around 65 %.     

Comparative analysis of the response of various land covers to an exceptional rainfall event in the central Spanish Pyrenees,October 2012

The hydrological and geomorphological effects of an exceptional rainstorm event that occurred in the central Spanish Pyrenees during 19–21 October 2012 were studied in five experimental catchments under various land covers: (i) subhumid badlands; (ii) dense forest; (iii) an abandoned farmland area recolonized by shrubs and forest patches; and (iv) subalpine grasslands. Hydrographs and sedigraphs demonstrated that vegetation cover is a major factor affecting the control of floods even during exceptional rainstorms, at least at the spatial scale at which the phenomenon was studied (catchment sizes: 0.3–2.8 km2) and under dry catchment conditions. The combined precipitation over the two days (c. 250 mm) was the greatest for any two‐day event recorded since 1950 in the central‐western Pyrenees for all but one of the stations in the study. Five pulses of most intense rainfall were recorded. The forested catchment did not react to the two most intense rainfall pulses, because of the very low antecedent level of the water table. The main peak flow occurred only when at least a part of the catchment was saturated. The abandoned farmland catchment had two small peak discharges at the beginning of the event, which were produced by infiltration excess overland flow from eroded areas close to the main stream. During the third most intense rainfall period a large part of this catchment contributed to runoff and a relatively high peak discharge was produced. The badland catchment reacted immediately from the beginning of the rainstorm, yielding very high discharges accompanied by high suspended sediment concentrations. The subalpine catchment showed a hydrograph mirroring the hyetograph, with brief but intense hydrological responses to increased precipitation, because of the marked gradients and the presence of bare rock in the headwaters. A high volume of bedload was carried during the peak discharge. Copyright © 2013 John Wiley & Sons, Ltd.     

The Araguainha impact: a South American Permo–Triassic catastrophic event

The Araguainha meteorite impact was certainly one of the most catastrophic events in the history of the South American continent. The impact occurred around 250 Ma ago, when the region was covered by the estuarine waters of the Parana Basin in central parts of Brazil. The impacting body of approximately 2–3 km in diameter was sufficiently large to excavate a 2 km-thick sedimentary sequence of the Parana Basin and to expose a 4 km-wide core of basement crystalline rocks in the central part of the crater. The huge scar left by the meteorite collision is 40 km in diameter, the largest and best preserved impact crater on the continent. Combined field observations and remote sensing analysis demonstrates that the Araguainha impact structure preserves all morphological/structural features of large lunar craters, being thus an important analogue to study large extraterrestrial craters. The catastrophic energy released upon impact, close to 10⁶ megatons of TNT, must have been disastrous for marine organisms living in the Parana Basin. Ongoing studies are currently evaluating the link between the Araguainha impact and the Permian–Triassic mass extinction, which is the greatest of the mass extinctions in Earth history.     

Statistical distribution and spectral analysis of rainfallanomalies for Barcelona (NE Spain)

Summary  Rainfall anomaly patterns are obtained for the city of Barcelona from a statistical and a spectral point of view. The time series consists of monthly rainfall amounts recorded over 128 years without interruption. Monthly positive and negative anomalies, obtained as the difference between monthly amounts and monthly threshold values, are used for both types of analyses. The threshold levels are derived form the deciles of theoretical monthly rainfall distributions, which have been previously modelled by the gamma distribution. Positive and negative anomalies of the monthly rain amounts are investigated for these threshold levels. The statistical analysis is applied to each decile considered, yielding empirical exponential laws that can be used to forecast the cumulative number of episodes of consecutive months with either positive or negative anomalies equalling or exceeding a fixed length. A set of linear laws, relating the expected rainfall amount cumulated during an episode of a fixed length, is also deduced. It is worthy of mention that, independently of the decile considered, all the exponential and linear laws have satisfactory regression coefficients. At the same time, it has also been possible to establish the evolution of the coefficients of these laws with respect to the different deciles considered. The exponential laws for episodes of positive and negative anomalies are the starting point, together with two hypotheses, to model probabilities of repeated long episodes over an arbitrary number of years and their return periods in terms of the Poisson distribution model. Moreover, power spectra are derived for anomalies relative to the 50% decile at monthly and seasonal scale. The spectral estimates obtained are then compared with theoretical spectra deduced from possible Markovian or random behaviour of the time series of anomalies. Finally, the significant spectral peaks are discussed and compared with other significant spectral components deduced for some areas of the Mediterranean domain. Received November 11, 1999 Revised February 28, 2000     

Generation, mobilization and crystallization of impact-induced alkali-rich melts in granitic target rocks: Evidence from the Araguainha impact structure, central Brazil

This paper provides important insights into the generation, extraction and crystallization of clast-laden impact melt rocks from the Araguainha impact structure, central Brazil. Despite the mixed nature of the Araguainha target rocks (comprising a 2 km thick sequence of sedimentary rocks and underlying granitic basement), the exposed melt bodies are characterised by an alkali-rich granitic matrix embedding mineral and rock fragments derived only from the target granite. The melt rocks occur in the form of a massive impact melt sheet overlying the eroded central uplift structure, and as melt veins in the granite of the core of the central uplift. Bulk-rock major and trace element data (including platinum group elements) indicate that the precursor melts were generated locally, principally by partial melting of the target granite, without any contribution from the sedimentary sequence or the projectile. The dense network of melt veins was formed in isolation, by selective melting of plagioclase and alkali feldspar within the granite target. Plagioclase and alkali feldspar melted discretely and congruently, producing domains in the matrix of the melt veins, which closely match the stoichiometry of these minerals. The compositionally discrete initial melt phases migrated through a dense network of microfractures before being assembled into larger melt veins. Freezing of the melt veins was substantially fast, and the melt components were quenched in the form of alkali-feldspar and plagioclase schlieren in the matrix of the melt veins. The overlying impact melt rock is, in contrast, characterised by a granophyric matrix consisting of albite, sanidine, quartz, biotite and chlorite. In this case, melt components appear to have been more mobile and to have mixed completely to form a granitic parental melt. We relate the melting of the minerals to post-shock temperatures that exceeded the melting point of feldspars.     

Andean orogenic signature in the Quaternary sandy barriers of Southernmost Brazilian Passive Margin-Paradigm as a source area

Multiple source provenance of sediments and submarine fan formation are herein investigated based on Quaternary sandy barriers of the Brazilian Southernmost Coastal Plain.LA-ICP-MS dating on 1625 detrital zircons from marine and aeolian facies sands of four successive lagoon-barrier systems are analyzed.The characterization of Archean to Pleistocene zircons into a younger Andean(22 ± 4 Ma to 1 ± 1 Ma,48 from 1625),Mesozoic to Paleozoic,and Mesoproterozoic(479 from 1622) populations suggest that the main feeding of sediments to the coastal plain occurred through the La Plata River drainage system.The significant contribution of sediments is transported from the mouth of La Plata River northward by longshore circulation(littoral drift).Minor contributions are also recognized as.a farther source associated with the Patagonia drainage and nearby source related to the Uruguay/Rio Grande do Sul Shield and the Paraná Basin,drained by the Camaqu? and Jacuí rivers.The latter one is recognized by the contribution from Neoproterozoic to Early Paleozoic,and some Paleoproterozoic and Archean zircon grains.The definition of the sources of clastic sediments allows inferences about the origination of Rio Grande Fan where both the cold Falkland and the warm Brazil currents played a major role.     

Macrobenthos and multi-molecular markers as indicators of environmental contamination in a South American port (Mar del Plata,Southwest Atlantic)

We evaluated benthic habitat quality along a presumed contamination gradient in the Mar del Plata port (Southwestern Atlantic) by coupling biological and chemical proxies in a multidisciplinary approach. Organic matter and photosynthetic pigment contents were higher in silty-clay bottoms of the inner port sites. Levels of all fecal steroids decreased from the inner sites to the port inlet. High concentrations of coprostanol in the inner sites seemed to derive from a permanent population of sea lions rather than from sewage outfalls due to coprostanol/epicoprostanol ratio (IV) values <2.5. PAHs levels were also higher in the inner sector, related to both biomass combustion and petroleum combustion associated to local marine traffic. High disturbance and low ecological status were reflected in low benthic diversity and high AMBI values in the inner sites.