Tephrochronology of recent events in the Andean Range (northern Patagonia): spatial distribution and provenance of lacustrine ash layers in the Nahuel Huapi National Park

Tephrochronology is a powerful tool for dating sedimentary sequences, especially in Patagonia, where a large number of active volcanoes have produced frequent historical eruptions. Short lacustrine sedimentary sequences were extracted from the lakes Moreno Oeste and Ton?ek (Nahuel Huapi National Park). Seventeen volcanic ash layers were identified in both cores, ²¹⁰Pb and ¹³⁷Cs were used for dating techniques, and historical volcanic records were employed for correlation. White pumice and glass shards from the tephras were characterised by measuring major and trace element contents by instrumental neutron activation analysis. Two volcanic sources – the Cordón Caulle and the Calbuco volcanoes – were recognised as the prevailing systems that impacted the area in the past 800–1000 a. The Calbuco volcano was mainly responsible for the nine tephras identified in the Lake Ton?ek sequence. Four of these nine tephras also contained material from the Cordón Caulle complex, and could be interpreted as composite tephras or possible reworked ones. The Calbuco volcano showed predominant influences in the Lake Moreno Oeste sequence. Six of the eight tephras identified in such a sequence were from the Calbuco volcano, and three of these six contained material from the Cordón Caulle complex. The other two were from the Cordón Caulle complex, with contributions from the Calbuco volcano in one of them. These findings show that the high frequency of volcanic events in the study region demands an accurate characterisation of the products generated by each source, as well as an evaluation of their spatial distribution, to obtain a consistent framework for dating recent environmental changes. Copyright © 2010 John Wiley & Sons, Ltd.     

Diffuse and concentrated recharge evaluation using physical and tracer techniques: results from a semiarid carbonate massif aquifer in southeastern Spain

In the high-permeability, semiarid carbonate aquifer in the Sierra de Gádor Mountains (southeastern Spain), some local springs draining shallow perched aquifers were of assistance in assessing applicability of the atmospheric chloride mass balance (CMB) for quantifying total yearly recharge (R _T) by rainfall. Two contrasting hydrological years (October through September) were selected to evaluate the influence of climate on recharge: the average rainfall year 2003–2004, and the unusually dry 2004–2005. Results at small catchment scale were calibrated with estimated daily stand-scale R _T obtained by means of a soil water balance (SWB) of rainfall, using the actual evapotranspiration measured by the eddy covariance (EC) technique. R _T ranged from 0.35 to 0.40 of rainfall in the year, with less than a 5% difference between the CMB and SWB methods in 2003–2004. R _T varied from less than 0.05 of rainfall at mid-elevation to 0.20 at high elevation in 2004–2005, with a similar difference between the methods. Diffuse recharge (R _D) by rainfall was quantified from daily soil water content field data to split R _T into R _D and the expected concentrated recharge (R _C) at catchment scale in both hydrological years. R _D was 0.16 of rainfall in 2003–2004 and 0.01 in 2004–2005. Under common 1- to 3-day rainfall events, the hydraulic effect of R _D is delayed from 1 day to 1 week, while R _C is not delayed. This study shows that the CMB method is a suitable tool for yearly values complementing and extending the more widely used SWB in ungauged mountain carbonate aquifers with negligible runoff. The slight difference between R _T rates at small catchment and stand scales enables results to be validated and provides new estimates to parameterize R _T with rainfall depth after checking the weight of diffuse and concentrated mechanisms on R _T during moderate rainfall periods and episodes of marked climatic aridity.     

Re-testing the JET-X Flight Module No. 2 at the PANTER facility

The Joint European X-ray Telescope (JET-X) was the core instrument of the Russian Spectrum-X- γ space observatory. It consisted of two identical soft X-ray (0.3–10 keV) telescopes with focusing optical modules having a measured angular resolution of nearly 15 arcsec. Soon after the payload completion, the mission was cancelled and the two optical flight modules (FM) were brought to the Brera Astronomical Observatory where they had been manufactured. After 16 years of storage, we have utilized the JET-X FM2 to test at the PANTER X-ray facility a prototype of a novel X-ray polarimetric telescope, using a Gas Pixel Detector (GPD) with polarimetric capabilities in the focal plane of the FM2. The GPD was developed by a collaboration between INFN-Pisa and INAF-IAPS. In the first phase of the test campaign, we have re-tested the FM2 at PANTER to have an up-to-date characterization in terms of angular resolution and effective area, while in the second part of the test the GPD has been placed in the focal plane of the FM2. In this paper we report the results of the tests of the sole FM2, using an unpolarized X-ray source, comparing the results with the calibration done in 1996.     

Extensive impact melting on the H-chondrite parent asteroid during the cataclysmic bombardment of the early solar system: Evidence from the achondritic meteorite Dar al Gani 896

DaG 896 is an olivine-rich microporphyritic rock of komatiitic composition. Both the olivine composition (Fa_17.5±2.1, [Mn/Mg] = 0.0061) and the bulk oxygen isotopic composition (δ17O = +2.55, δ18O = +3.50) indicate that DaG 896 is a sample of the H-chondrite parent body. The bulk chemistry shows an H-chondritic distribution of lithophile elements, whereas chalcophile and siderophile elements are strongly depleted, indicating formation through whole-rock melting (or nearly so) of H-chondrite material, nearly complete loss of the metal plus sulfide component, and crystallization without significant igneous fractionation. Superheated, severely shocked chondritic relics (∼10 vol%), typically in the form of corroded lithic fragments <100 μm in size intimately distributed within the igneous lithology, indicate that melting was triggered by a highly energetic impact, which possibly induced shock pressures of ∼80-100 GPa. The relatively young 3.704 ± 0.035 Ga 40Ar-39Ar crystallization age is consistent with the impact melting origin, as magmatism in the asteroid belt was active only in the first hundred million years of solar system history.Based on textural data and thermodynamic crystallization modelling, we infer that DaG 896 crystallized from a liquidus temperature of ∼1630°C under relatively slow cooling rates (∼10°C h⁻¹) to ∼1300°C, before quenching. The two-stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor. Metal-silicate fractionation may have been accomplished, at least at the centimeter-scale of the studied meteorite sample, through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism. Alternatively, DaG 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater, as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid: for metal particles 1 to 10 mm in size, typically observed in partially differentiated impact melt rocks, Stokes’ Law indicates a settling velocity >1 m h ⁻¹ during the first few hours of crystallization on asteroidal bodies of >25 km radius.The ∼3.7 Ga age of DaG 896 nearly overlaps with the slightly older resetting ages of H-chondrites (all impact melts) available from the literature, indicating that the H-chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system. Such an age overlap may also indicate early disruption of the initial H-chondrite parent asteroid.The close similarity between the bulk composition and degassing age of DaG 896 and silicate inclusions in IIE iron meteorites is further evidence in support of a common origin by impact melting and metal-silicate segregation on the H-chondrite parent asteroid. Our new high-precision oxygen isotopic measurements of H-chondrites (Δ17O = 0.77 ± 0.04) should be extended to IIEs to verify this possible petrogenetic link.     

Distribution of colloidal trace metals in the San Francisco Bay estuary

The size distribution of trace metals (Al, Ag, Cd, Cu, Fe, Mn, Ni, Sr, and Zn) was examined in surface waters of the San Francisco Bay estuary. Water samples were collected in January 1994 across the whole salinity gradient and fractionated into total dissolved (<0.2 μm colloidal (10 KDa–0.2 μm) and < 10 kDa molecular weight phases. In the low salinity region of the estuary, concentrations of colloidal A1, Ag, and Fe accounted for ≥84% of the total dissolved fraction, and colloidal Cu and Mn accounted for 16–20% of the total. At high salinities, while colloidal Fe was still relatively high (40% of the dissolved), very little colloidal Al, Mn, and Cu (<10%) and no colloidal Ag was detectable. Colloidal Zn accounted for <3% of the total dissolved along the estuary, and colloidal Ni was only detectable (<2%) at the river endmember. All of the total dissolved Cd and Sr throughout the estuary consisted of relatively low molecular weight (<10 kDa) species. The relative affinity of metals for humic substances and their reactivity with particle surfaces appear to determine the amounts of metal associated with colloids. The mixing behavior of metals along the estuary appears to be determined by the relative contribution of the colloidal phase to the total dissolved pool. Metals with a small or undetectable colloidal fraction showed a nonconservative excess (Cd, Cu, Ni, and Mn) or conservative mixing (Sr) in the total dissolved fraction, relative to ideal dilution of river water and seawater along the estuary.

The salt-induced coagulation of colloidal A1, Fe, and Cu is indicated by their highly nonconservative removal along the salinity gradient. However, colloidal metals with low affinity for humic substances (Mn and Zn) showed conservative mixing behavior, indicating that some riverine colloids are not effectively aggregated during their transport to the sea. While colloidal metal concentrations correlated with dissolved organic carbon, they also covaried with colloidal Al, suggesting that colloids are a mixture of organic and inorganic components. Furthermore, the similarity between the colloidal metal:A1 ratios with the crustal ratios indicated that colloids could be the product of weathering processes or particle resuspension. Distribution coefficients for colloidal particles (K_c) and for large, filter-retained particles (K_d) were of the same magnitude, suggesting similar binding strength for the two types of particles. Also, the dependence of the distribution coefficients on the amount of suspended particulate matter (the so-called particle concentration effect) was still evident for the colloids-corrected distribution coefficient (K_p+c) and for metals (e.g., Ni) without affinity for colloidal particles.     

A Late Jurassic fossil assemblage in Gondwana: Biostratigraphy and correlations of the Tacuarembó Formation, Parana Basin, Uruguay

The Tacuarembó Formation has yielded a fossil assemblage that includes the best known body fossils, consisting of isolated scales, teeth, spines, and molds of bones, recovered from thin and patchy bonebeds, from the Botucatu Desert, Parana Basin, South America. The remains are preserved in the sandstones widespread around the city of Tacuarembó. We propose a new formalized nomenclature for the Tacuarembó Formation, naming its “Lower” and “Upper” members as the Batoví (new name) and Rivera (new rank) members, respectively. An assemblage zone is defined for the Batoví Member (fluviolacustrine and aeolian deposits). In this unit, the freshwater hybodontid shark Priohybodus arambourgi D’Erasmo is well represented. This species was previously recorded in Late Jurassic–Early Cretaceous units of the Sahara and the southern Arabian Peninsula. Globally considered, the fossil assemblage of this member (P. arambourgi, dipnoan fishes, Ceratosaurus-like theropods, and conchostracans) is indicative of a Kimmeridgian–Tithonian age, which in combination with the stratigraphic relationships of the Tacuarembó Formation with the overlying basalts of the Arapey Formation (132 My average absolute age) implies that the latter was deposited during the Kimmeridgian–Hauterivian interval.     

New data for the kinematic interpretation of the Alps–Apennines junction (Northwestern Italy)

Alps and Apennines are juxtaposed within an approximately 100 km-wide area covered by the Upper Eocene to Miocene successions of the Tertiary Piedmont Basin. The Upper Eocene–Oligocene evolution of this area was characterized to the north and west by the propagation of the SE-verging Southalpine thrust-fold belt that can be traced from the Po Plain subsurface until the Torino Hill-Saluzzese area, and to the south by a high-angle, broadly E–W oriented megashear zone that led to the juxtaposition of different crustal levels and controlled the development of a mosaic of partly independent sub-basins. Since the latest Oligocene the N-verging Apenninic tectonics prevailed in the collisional system and the Tertiary Piedmont Basin evolved as a wide thrust-top basin, bounded to the north by the N-verging Monferrato arc and characterized by a tectono-sedimentary evolution recording changes of subsidence and shift of depocentres in relation to crustal structures.     

Modeling the influence of hydroperiod and vegetation on the cross-sectional formation of tidal channels

The evolution of the cross section of a salt-marsh channel is explored using a numerical model. Deposition on the marsh platform and erosion and deposition in the channel affect the tidal prism flowing through the cross section, such that the model captures the evolution of the stage–discharge relationship as the channel and marsh platform evolve. The model also captures the growth of salt-marsh vegetation on the marsh platform, and how this vegetation affects flow resistance and the rate of sedimentation. The model is utilized to study the influence of hydroperiod and vegetation encroachment on channel cross section. Numerical results show that a reduction in hydroperiod due to the emergence of the marsh platform causes an infilling of the channel. Vegetation encroachment on the marsh surface produces an increase in flow resistance and accretion due to organic and mineral sedimentation, with important consequences for the shape of the channel cross section. Finally, modeling results indicate that in microtidal marshes with vegetation dominated by Spartina alterniflora, the width-to-depth ratio of the channels decreases when the tidal flats evolve in salt marshes, whereas the cross-sectional area remains proportional to the tidal peak discharge throughout channel evolution.     

Vestan lithologies mapped by the visual and infrared spectrometer on Dawn

We present global lithological maps of the Vestan surface based on Dawn mission's Visible InfraRed (VIR) Spectrometer acquisitions with a spatial sampling of 200 m. The maps confirm the results obtained with the data set acquired by VIR with a spatial sampling of 700 m, that the reflectance spectra of Vesta's surface are dominated by pyroxene absorptions that can be interpreted within the context of the distribution of howardites, eucrites, and diogenites (HEDs). The maps also partially agree with the ground and Hubble Space Telescope observations: they confirm the background surface being an assemblage of howardite or polymict eucrite, as well as the location of a diogenitic‐rich spot; however, there is no evidence of extended olivine‐rich regions in the equatorial latitudes. Diogenite is revealed on the Rheasilvia basin floor, indicating that material of the lower crust/mantle was exposed. VIR also detected diogenites along the scarp of Matronalia Rupes, and the rims of Severina and a nearby, unnamed crater, and as ejecta of Antonia crater. The diogenite distribution is fully consistent with petrological constraints; although the mapped distribution does not provide unambiguous constraints, it favors the hypothesis of a magma ocean.