Hydrothermal fluid flow associated to the extensional evolution of the Adriatic rifted margin: Insights from the pre- to post-rift sedimentary sequence (SE Switzerland,N ITALY)     

Nicolò Incerpi  Luca Martire  Gianreto Manatschal  Stefano M. Bernasconi  Axel Gerdes  György Czuppon  László Palcsu  Garry D. Karner  Christopher A. Johnson  Patricio H. Figueredo 《Basin Research》2020,32(1):91-115

This paper investigates the hydrothermal fluid circulation that was linked to the extensional evolution of the Adriatic rifted margin during the Jurassic opening of the Alpine Tethys. Remnants of this rifted margin are spectacularly preserved in SE Switzerland and N Italy. Five study areas were chosen ranging from the former proximal to the most distal part of the margin. We demonstrate an intimate link between Jurassic extensional tectonics and fluid activity affecting the pre- to early post-rift sedimentary sequences. Nature, composition and origin of fluids are constrained by a multidisciplinary approach based on field observations and including geochemical (O-C, Sr, He isotopes, U-Pb datings, fluid inclusion microthermometry) and petrological methods. Several fluid-related diagenetic products and processes such as dolomitization, veining, hydraulic brecciation, and silicification can be recognized. It appears that different paleogeographic settings and different stratigraphic levels document distinct phases of fluid activity. The fluids reached temperatures of up to 150°C near paleo-seafloor. They were enriched in ¹⁸O, had high ⁸⁷Sr/⁸⁶Sr and low ³He/⁴He ratios, documenting a strong interaction between seawater and a granitic basement. Many lines of evidence point to the occurrence of over-pressured fluids and long-lasting fluid circulation due to fault-valve mechanisms and high thermal gradients. Two main stages with different fluid chemistry can be recognized: (1) a carbonate-rich stage that developed during the stretching phases and was linked to high-angle normal faulting, and (2) a silica-rich stage occurring during late rift exhumation of crustal and mantle rocks in the distal domains in the presence of detachment faults and high thermal gradients. This paper provides, for the first time, a large and robust characterization of fluid–rock interactions occurring during rifting along an almost complete section across a magma-poor rifted margin.  相似文献   

Source and genesis of sulphate and phosphate–sulphate minerals in a quartz‐sandstone cave environment     

Francesco Sauro  Nicola Tisato  Jo De Waele  Stefano M. Bernasconi  Tomaso R. R. Bontognali  Ermanno Galli 《Sedimentology》2014,61(5):1433-1451

Gypsum (CaSO₄·2H₂O), alunite (KAl₃(SO₄)₂(OH)₆), and rare phosphate–sulphate sanjuanite Al₂(PO₄)(SO₄)(OH) 9(H₂O) and rossiantonite (Al₃(PO₄)(SO₄) 2(OH)₂(H₂O)₁₄) have recently been identified as secondary mineral deposits in different quartz‐sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz‐sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to perhaps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteorological, hydrological and geographical data are used to investigate the origin of sulphates and phospho–sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz‐sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sulphate source, with potential contributions of marine non‐sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H₂S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sulphates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co‐occur with silica speleothems of biological origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz‐sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments.  相似文献   

A Comparative Evaluation of Automated Solar Filament Detection     

M. A. Schuh  J. M. Banda  P. N. Bernasconi  R. A. Angryk  P. C. H. Martens 《Solar physics》2014,289(7):2503-2524

We present a comparative evaluation for automated filament detection in Hα solar images. By using metadata produced by the Advanced Automated Filament Detection and Characterization Code (AAFDCC) module, we adapted our trainable feature recognition (TFR) module to accurately detect regions in solar images containing filaments. We first analyze the AAFDCC module’s metadata and then transform it into labeled datasets for machine-learning classification. Visualizations of data transformations and classification results are presented and accompanied by statistical findings. Our results confirm the reliable event reporting of the AAFDCC module and establishes our TFR module’s ability to effectively detect solar filaments in Hα solar images.  相似文献   

Inversion of Stokes vector profiles in terms of a 3-component model     

P. N. Bernasconi  S. K. Solanki 《Solar physics》1996,164(1-2):277-290

Various spectropolarimetric observations show peculiar Stokes profiles that reveal the coexistence of at least two magnetic components in the same resolution element. An example is given by observations of the full Stokes vector in a complex active region performed with the ZIMPOL I Stokes polarimeter. In order to deduce the physical parameters of the observed regions from such measured profiles, we have extended an existing inversion code, so that it can now fit the data with models composed of up to three different atmospheric components. Two of these components are magnetic and may possess different field strengths, field geometries, temperature stratifications, and velocity fields. The third component describes the field free atmosphere surrounding the magnetic features.The so extended inversion code has then been applied to the ZIMPOL I data. In this paper we present and discuss sample fits. The code is able to reproduce the observed complex Stokes profiles with good accuracy and provides physical parameters for both of the coexisting magnetic atmospheres. Inversion tests with a 2-component model (with one magnetic and one non-magnetic component) applied to the same profiles do not reproduce the measurements sufficiently well.  相似文献   

The marinkas kwela alkali-granite intrusive,a probable porphyry molybdenum system of cambrian age,namibia     

A. Bernasconi 《Mineralium Deposita》1981,16(3):447-454

The geology, geochemistry and alteration of the Marinkas Kwela prospect in Namibia, is described. The prospect is located within the Kuboos-Bremen line of intrusions largely composed of alkali-rich rocks and lesser carbonatites of Cambrian age. The body as it is presently known is entirely formed by alkali granite and partially surrounded by syenite. The mineralogy and chemistry of the Marinkas Kwela prospect are remarkably similar to that of the Werner Bjerge massif of East Greenland where Mo mineralisation is known to occur. The prospect is also characterised by a conspicuous hydrothermal alteration predominantly composed of quartz and sericite with lesser carbonate and pyrite. Also anomalous values of Mo, Cu, Pb and Zn occur in an annular distribution over the prospect.  相似文献   

Il problema inverso del trasporto delle coordinate per geodetiche brevi e medie     

Carlo Bernasconi 《Pure and Applied Geophysics》1958,41(1):28-32

Riassunto L'Autore elabora formule che servono per rappresentare l'intorno diWeingarten sulla sfera osculatrice, con ciò il problema inverso del trasporto delle coordinate sull'ellissoide viene ricondotto alla risoluzione di un triangolo sferico. Dimostra altresì che le formule della proiezione si potrebbero ottenere, almeno fino al secondo ordine, mediante la teoria dei minimi quadrati.

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Summary The Author elaborates the formulas giving the rapresentation ofWeingarten area on the osculatrix sphere; by means of these formulas the inverse problem of coordinate transport on the ellipsoid is reduced to the solution of a spheric triangle. The Author moreover proves that one can refine, at least to the second order, the same formulas making use of the least squares' theory.

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Sulla variazione della densità nell'interno della terra     

Carlo Bernasconi 《Pure and Applied Geophysics》1958,41(1):19-27

Riassunto L'Autore considera dapprima un geoide di rotazione di cui esprime la densità mediante sviluppo in serie di potenze del raggio vettore con i coefficienti dipendenti dalla latitudine . Risolve l'equazione di Poisson relativa al geoide, dimostra che la funzione potenziale si sviluppa in serie di potenze pari di e mette in evidenza la influenza dei termini dipendenti dalla latitudine nel rapporto fra i momenti di inerzia e nella variazione della gravità in superficie. Considera poi il geoide sferico e determina, in due esempi, l'andamento della densità, basandosi nel presupposto che l'energia del campo di gravità sia la più alta possibile compatibilmente allo stato attuale della Terra; ne risulta una densità centrale compresa fra 15 e 19.

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Summary The Author first considers a rotational geoid the density of which he introduces through potential series development of radius vector with coefficients as functions of latitude . He solves Poisson equation of the geoid, demonstrates that its potential function develops through potential series even of , points out the influence of the terms deriving from the latitude in the ratio from the inertia moments and the variation in surface gravity. Then he considers a spheric geoid and, by two demonstrations, he establishes the density, basing his theory on the assumption that gravity field energy is the highest one can trace in the present condition of the Earth; the result give a central density ranging from 15 to 19.

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Questioni di ottica geometrica risolte con metodi meccanici     

Carlo Bernasconi 《Pure and Applied Geophysics》1952,22(3-4):162-165

Riassunto Dall'analogia fra il principio ottico diFermat e quello meccanico della minima azione, l'A. ricava un metodo per determinare con mezzi meccanici: 1° la traiettoria di un raggio luminoso passante per due punti dati e separati da un lente; 2°) le superficie di lenti stigmatiche soddisfacenti a condizioni prefissate, in particolare a quelle diAbbe, Herschel.... Si accenna infine a possibili applicazioni nel campo delle lenti sferiche.

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Summary From the analogy between the optical principle ofFermat and the mechanical one of minimum action, the Author deduces a method for obtaining with mechanical devices: 1) the optical path of a light beam passing through two given points and separated by a lens; 2) the surfaces of stigmatic lenses fulfilling prefixed conditions, particularly those ofAbbe, Herschell.... Further, it is hinted at possible applications in the field of spherical lenses.

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Linearized inversion of vertically fractured media     

M. M. Beretta  G. Bernasconi and  G. Drufuca 《Geophysical Journal International》2000,143(3):965-968

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20 My of nitrogen fixation during deposition of mid-Cretaceous black shales on the Demerara Rise,equatorial Atlantic Ocean     

Philip A. Meyers  Stefano M. Bernasconi  Jong-Gwon Yum 《Organic Geochemistry》2009,40(2):158-166

Thick sequences of dark colored, organic carbon rich, finely laminated Santonian–Cenomanian claystones and homogeneous Albian siltstones were recovered from Ocean Drilling Program Sites 1257, 1258 and 1260 on the Demerara Rise in the western equatorial Atlantic Ocean. Total organic carbon (TOC) concentrations vary from 2 to over 20 wt% in the sequences of “black shales” that were deposited over a period of ～20 million years. Similarly long periods of elevated marine productivity implied by the high TOC concentrations are uncommon in the geological record and must have required unusual paleoceanographic conditions. The importance of nitrogen fixing bacteria to sustaining the amplified export production of organic matter is indicated by δ¹⁵N values that remain between ?4‰ and 0‰, a range that is notably less positive than the average of +5‰ for modern ocean sediments. Although containing mostly marine organic matter, the black shales have TOC/TN molar ratios between 20 and 40 that mimic those of land plant organic matter. The anomalously large TOC/TN ratios suggest selective organic matter degradation, probably associated with low oxygen conditions in the water column, that favored preservation of nitrogen poor forms of organic matter relative to nitrogen rich components. Deposition of black shales on the Demerara Rise was likely a consequence of the mid-Cretaceous warm and wet greenhouse climate that strengthened thermohaline stratification of this part of the Atlantic Ocean, which in turn encouraged bacterial nitrogen fixation, enhanced primary production, magnified organic matter export, and ultimately established anoxic conditions at the seafloor that improved preservation of organic matter for much of the 20 My period represented by these thick sequences.  相似文献