Observed precipitation in the Paraná-Plata hydrological basin: long-term trends, extreme conditions and ENSO teleconnections

The Paraná-Plata basin is the second largest hydrological basin in South America and is of great importance for the countries of the region (Argentina, Bolivia, Brazil, Paraguay and Uruguay). The present study focuses on the long-term trends in basin-scale precipitation with special emphasis on the role of distribution changes in extreme large-scale precipitation events and on the characteristics and evolution of ENSO teleconnections over the last 50 years. First, we defined a Paraná-Plata basin total precipitation index (PTPI) as the precipitations spatially averaged over the hydrological basin. On interannual time scales, such an index is mainly representative of anomalous monsoon precipitations in the northern part of the basin and large convective precipitation anomalies in the center of the basin (Paraguay-southern Brazil-Uruguay-northern Argentina) typical of the canonical ENSO teleconnection pattern. Our major findings clearly highlight a positive trend in yearly averaged PTPI mainly from the late 1960s to the early 1980s with a strong dependence from month-to-month. The largest precipitation increase is observed from November to May in southern Brazil and Argentina. A close examination of PTPI distributions during the two halves of the period 1950–2001 shows that the changes in the mean state from 1950–1975 to 1976–2001 result from significant changes in each calendar month mean state and in the tails of the PTPI anomaly distributions in May with lesser and weaker large-scale dry events and stronger large-scale wet events. Further studies will be needed to assess whether the observed trend in large-scale extreme precipitation conditions can be related to natural climate variability or anthropogenic activities and whether it is associated to changes in local/regional extreme events. The stronger wet conditions in different months seem to be associated to changes in ENSO characteristics (amplitude, propagation, spatial structure, ...) since the 1982–1983 El Niño. Indeed, spatial ENSO teleconnections (stronger in November and April–May) have greatly evolved from 1950–1975 to 1976–2001. Moreover, we demonstrate that there is a strong modulation and displacement of the teleconnection patterns from one event to another, impeding the definition of robust statistical relationship between ENSO and precipitation in the Paraná-Plata basin (except maybe over a very limited area near the common border between Paraguay, Argentina and Brazil). Finally, the non-antisymmetrical patterns of precipitation between El Niño and La Niña conditions and the non-linear relationship between precipitation and either Niño3.4 or Niño1+2 sea surface temperature indices show that linear statistical forecast systems are actually of very limited use for impact predictions on society on a local or regional scale.     

40Ar–39Ar geochronology across Archean and Paleoproterozoic terranes from southeastern Guiana Shield (north of Amazonian Craton,Brazil): Evidence for contrasting cooling histories

A ⁴⁰Ar/³⁹Ar geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapá Block and the Carecuru Domain. In the Amapá Block, an Archean continental block involved in the Transamazonian orogenesis (2.26–1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 ± 3 to 2047 ± 20 Ma, and biotite ages spread mainly between 2079 ± 18 and 2033 ± 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 ± 17 Ma, and biotite ages of 1928 ± 19 Ma and 1833 ± 13 Ma.These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U–Th–Pb monazite and Pb–Pb zircon geochronological records and petro-structural observations, the new ⁴⁰Ar/³⁹Ar data delineate contrasting cooling and exhumation histories for the tectonic domains. In the Amapá Block, the data suggest nearly vertical T–t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.     

Microbial diversity on the Tatahouine meteorite

Abstract— Biological processes can alter the chemistry and mineralogy of meteorites in a very short time, even in cold or hot deserts. It is thus important to assess the diversity of microorganisms that colonize meteorites in order to better understand their physiological capabilities. Microscopy observations of Tatahouine meteorite fragments that were exposed for 70 years in the Sahara desert showed that they were colonized by morphologically diverse biomorphs. A molecular diversity study based on 16S rRNA gene amplification of DNA supported the conclusion that a huge taxonomic diversity of prokaryotes colonized the Tatahouine meteorite in less than 70 years in the Tatahouine sand. Eleven different bacterial divisions were evidenced, among which Cytophaga‐Flexibacter‐Bacteroides (CFB), Cyanobacteria, and Alpha‐Proteobacteria were dominantly represented. Crenarcheota were also detected. Most of the Tatahouine meteorite phylotypes were related to sequences identified in the surrounding Tatahouine more generally to sequences detected in soils. Some of them, in particular many of the archaeal phylotypes, were detected in arid regions in association with desert varnish. The results suggest that the diversity of the clone library generated from the meteorite fraction was reduced compared with that of the Tatahouine sand clone library, which can be explained as the result of partial colonization of the meteorite and/or a specific selection of colonizing bacteria by the substrate. We discuss the possibility that several groups detected in this study may play a prominent role in the various alteration processes detected at the surface of the Tatahouine meteorite.     

From noise correlation to resonant ultrasound spectroscopy in rock acoustics

This article introduces an alternative experimental procedure for measuring the elastic properties of a solid material at laboratory scale, using both the principles of passive seismic interferometry and resonance ultrasound spectroscopy. We generate noise into the studied sample with a pneumatic air blow gun, and we cross‐correlate the signals recorded with two passive piezoelectric sensors put in soft contact with the sample surface. Resonance phenomena are induced in the sample, but in contrast with conventional resonance ultrasound spectroscopy experiments, we have no control over the injected frequencies that are sent all together within the noise spectrum. The spectrum of the correlogram is a good approximation of the resonance spectrum of the vibrating sample and can be inverted in terms of the elastic moduli of the constituent material of the sample. The experimental procedure is validated on samples made of standard materials (here, aluminium and Plexiglas) by consistently comparing the inverted elastic velocities with the velocities independently measured with the conventional technique of ultrasonic pulse transmission. Moreover, we got similar positive results on dry rock samples, such as Vilhonneur limestone. These encouraging preliminary results open up promising prospects for monitoring fluid substitution in rock samples using the technique described in this paper.     

Proterozoic anorogenic magmatism in the Central Amazonian Province,amazonian araton: Geochronological,petrological and geochemical aspects

Summary In the Central Amazonian Province, the anorogenic granites are older in the eastern block (1.88 Ga, U-Pb; 1.8 - 1.6 Ga Rb-Sr and K-Ar) and in the central block (1.75 -1.7 Ga, Rb-Sr) than in the western one (1.55 Ga, U-Pb). The country rocks are of Archaean age in the eastern block, and in the western block they are of Lower Proterozoic age (Trans-Amazonian Event). There is a minimum difference of 200 Ma between the last metamorphic event and the formation of the anorogenic granites. Metaluminous to peraluminous subsolvus granites are largely dominant but hypersolvus granites, sometimes peralkaline, as well as syenites and quartz-syenites also occur. Wiborgites and pyterlites are found only in the western block but rapakivi-like textures are described in the province as a whole. Mineralizations include large deposits of Sn, as well as small occurrences of F, Zr, REE, Y, and W. The granites are generally rich in Si, K, Fe, Zr, Ga, Nb, Y, and REE and show very high K/Na, Fe/(Fe + Mg) and Ga/Al₂O₃ ratios. They are geochemically similar to A-type and within-plate granites and more particularly to the Proterozoic rapakivi granites of the Fennoscandian shield and the metaluminous granites of the North American anorogenic province. Petrographic, geochemical and Sr isotopic data indicate crustal sources for the granite magmas. Differences in the sources should explain the contrast observed in some of the granites. The model of crustal anatexis induced by underplating or intrusion of mantle-derived basic magmas is preferred to explain the generation of the crustal granitic magmas.

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Proterozoischer, anorogener Magmatismus in der zentralen Amazonas-Provinz, Amazonas Kraton: Geochronologie, petrologische und geochemische Aspekte

Zusammenfassung In der zentralen Amazonas-Provinz sind die anorogenen Granite im östlichen (1.88 Mia, U-Pb; 1.8-1.6 Mia, Rb-Sr und K-Ar) und im zentralen Block (1.75-1.7 Mia,Rb-Sr) älter als im westlichen Block (1.55 Mia, U-Pb). Die Nebengesteine sind im östlichen Block archaiischen Alters, während sie im westlichen Block im unteren Proterozoikum (Transamazonas Event) gebildet worden sind. Dies ergibt eine minimale Altersdifferenz von 200 Mio zwischen dem letzten metamorphen Ereignis und der Intrusion der anorogenen Granite. Es dominieren metaluminöse bis peraluminöse Subsolvus-Granite, jedoch treten auch Hypersolvus-Granite, stellenweise Peralkaline, wie auch Syenite und Quarz-Syenite auf. Wiborgite und Pyterlite kommen lediglich im westlichen Block vor, Rapakivi-ähnliche Texturen werden jedoch aus der gesamten Provinz beschrieben. An Mineralisationen treten große Sn-Lagerstätten und kleinere Vorkommen von F, Zr, SEE, Y und W auf. Die Granite sind generell reich an Si, K, Fe, Ga, Nb, Y und SEE und zeigen sehr hohe K/Na, Fe/(Fe + Mg) und Ga/Al₂O₃ Verhältnisse. Sie zeigen geochemische Ähnlichkeiten mit A-Typ und Intraplatten-Graniten, besonders jedoch mit den proterozoischen Rapakivi Graniten des fennoskandischen Schildes und mit den metaluminösen Graniten der nordamerikanischen anorogenen Provinz. Petrographische, geochemische und Sr-Isotopendaten deuten krustale Ausgangsgesteine der granitischen Magmen an. Unterschiede im Ausgangsmaterial sollen die verschiedenen Granittypen erklären. Als Modell für die Entstehung krustaler granitischer Magmen wird krustale Anatexis, hervorgerufen durch underplating oder Intrusion von aus dem Mantel stammenden, basischen Magmen angenommen.

     

Roman road pollution assessed by elemental and lead isotope geochemistry in East Belgium

The ability of inorganic geochemistry to record environmental change and especially human impact has been evidenced by several studies across Europe, especially in peat, where it is possible to record the impact of agriculture, mining and other industries. However, despite the numerous investigations on the impact of ancient human activities such as ore mining and smelting, little attention has been paid to geochemistry as a tool to solve problems of palaeopollution in the surroundings of archaeological sites. This paper presents geochemical evidence of the impact of a possible early Roman road built in SE Belgian peatland. Increased Zn and Pb concentrations suggest that Pb–Zn ores were transported on the road. Lead isotope analyses suggest that these ores are locally derived, being compatible with those found in the nearby Pb–Zn ore deposits from East Belgium. Present results provide direct evidence that East Belgian Pb–Zn ores were already being mined during Roman times, i.e. earlier than previously suspected (i.e. 14th century) and that Zn appears to be relatively immobile here. On a broader scale, it also demonstrates that such an early road already had an impact on the environment in terms of metal pollution. This paper enlarges on the range of possibilities offered by geochemistry in the field of geoarchaeology.