Heating-induced changes in the anisotropy of magnetic susceptibility of impact breccias, Chicxulub Crater (Mexico)     

Jaime Urrutia-Fucugauchi  Margarita Delgadillo-Peralta  Ligia Pérez-Cruz  Miriam Velasco-Villarreal 《Studia Geophysica et Geodaetica》2012,56(3):769-787

Initial results of a thermal treatment study on the anisotropy of magnetic susceptibility (AMS) of impact breccias from Chicxulub crater are used to investigate the nature of the magnetic fabrics. Chicxulub impact breccias are heterogeneous materials, with carbonate, basement and melt clasts within carbonate-rich or melt-rich matrix. Samples studied come from the carbonate-rich basal unit Lower Suevite in the Yaxcopoil-1 borehole impactite sequence (core depth interval: 885?C895 m). The Lower Suevite is characterized by mixed prolate and oblate ellipsoids with shallow to steep principal susceptibility axes, which had been related to emplacement as an excavation flow with ground-surge components during the early cratering stages. Thermal treatment results in changes in the fabrics with a tendency to oblate fabrics. Stepwise thermal treatment up to 700°C reveals different behaviors for the oblate, neutral and prolate fabrics marked by changes in AMS parameters and principal susceptibility axis orientations. A sample with oblate fabrics and vertical minimum axes showed an increase of magnetic susceptibility at high temperatures, indicating formation of secondary magnetite and fabric enhancement. A sample with neutral ellipsoid showed heating-induced changes towards oblate fabrics and vertical minimum susceptibility axes. Samples characterized by prolate ellipsoids with horizontal maximum axes showed no directional changes. In a sample with apparent intermediate or inverse fabrics, vertical maximum axes showed changes to horizontal inclinations, with the intermediate and maximum axes switching positions. Changes induced by stepwise thermal treatment appear useful to characterize the fabrics of impact lithologies. Further investigation of heating-induced effects in mineralogy, grain size and textural changes is, however, required to relate the different behaviors observed after stepwise thermal treatment with the magnetic mineralogy and emplacement mode of the breccias.  相似文献   

The age and composition of the pre-Cenozoic basement of the Jalisco Block: implications for and relation to the Guerrero composite terrane     

Victor A. Valencia  Kevin Righter  Jose Rosas-Elguera  Margarita López-Martínez  Marty Grove 《Contributions to Mineralogy and Petrology》2013,166(3):801-824

The Jalisco Block is thought to be part of the Guerrero terrane, but the nature and age of the underlying crystalline basement are largely unknown. We have collected a suite of schists, granitoids, and weakly metamorphosed marine sediments from various parts of the Jalisco Block including Atenguillo and Ameca, Mascota and San Sebastián, Cuale, Puerto Vallarta, Punta Mita, Yelapa, and Tomatlán. The schists range in age from 135 to 161 Ma, with many exhibiting Proterozoic and Phanerozoic zircon ages. The granitoids range in age from 65 to 90 Ma, and are calc-alkaline compositionally—similar to granitoids from the Puerto Vallarta and Los Cabos batholiths. The Jalisco granitoids also experienced similar uplift rates to granitoids from the regions to the north and south of the Jalisco Block. The marine sediments yield a maximum depositional age of 131 Ma, and also contain a significant zircon population with ages extending back to the Archean. Granitoids from this study define two age groups, even after the effects of thermal resetting and different closure temperatures are considered. The 66.8-Ma silicic ash flow tuff near Union de Tula significantly expands the extent of this Cretaceous–Paleocene age ash flow tuff unit within the Jalisco Block, and we propose calling the unit “Carmichael silicic ash flow tuff volcanic succession” in honor of Ian Carmichael. The ages of the basement schists in the Jalisco Block fully overlap with the ages of terranes of continental Mexico, and other parts of the Guerrero terrane in the south, confirming the autochthonous origin of the Jalisco Block rather than exotic arc or allochthonous origin. Geologic data, in combination with geochronologic and oxygen isotopic data, suggest the evolution of SW Mexico with an early 200–1,200-Ma passive margin, followed by steep subduction in a continental arc setting at 160–165 Ma, then shallower subduction by 135 Ma, and finally, emplacement of granitoids at 65–90 Ma.  相似文献   

Gamma-ray emission from individual classical novae     

Jordi Gómez-Gomar  Margarita Hernanz  Jordi José  & Jordi Isern 《Monthly notices of the Royal Astronomical Society》1998,296(4):913-920

Classical novae are important producers of radioactive nuclei, such as ⁷Be, ¹³N, ¹⁸F, ²²Na and ²⁶Al. The disintegration of these nuclei produces positrons (except for ⁷Be) that through annihilation with electrons produce photons of energies 511 keV and below. Furthermore, ⁷Be and ²²Na decay producing photons with energies of 478 and 1275 keV, respectively, well in the γ-ray domain. Therefore, novae are potential sources of γ-ray emission. We have developed two codes in order to analyse carefully the γ-ray emission of individual classical novae: a hydrodynamical one, which follows both the accretion and the explosion stages, and a Monte Carlo one, able to treat both production and transfer of γ-ray photons. Both codes have been coupled in order to simulate realistic explosions. The properties of γ-ray spectra and γ-ray light curves (for the continuum and for the lines at 511, 478 and 1275 keV) have been analysed, with a special emphasis on the difference between carbon–oxygen and oxygen–neon novae. Predictions of detectability of individual novae by the future SPI spectrometer on board the INTEGRAL satellite are made. Concerning ²⁶Al, its decay produces photons of 1809 keV but this occurs on a time-scale much longer than the typical time interval between nova outbursts in the Galaxy, making it undetectable in individual novae. The accumulated emission of ²⁶Al from many Galactic novae has not been modelled in this paper.  相似文献   

High Angular Resolution Studies of Coronal Structures with SOHO     

Margarita Karovska  Brian Wood  John W. Cook  James Chen  Russ Howard 《Astrophysics and Space Science》2000,273(1-4):7-15

Since January 1996, the Solar and Heliospheric Observatory (SOHO)has been providing unprecedented views of the extended solar coronato heliocentric distances of up to 32 solar radii. During the past threeyears we carried out studies of the morphology and dynamical evolutionof various structures in the solar corona using two instruments on boardSOHO: the Large Angle Spectrometric Coronagraph (LASCO) and the ExtremeUltraviolet Imaging Telescope (EIT). We have applied advanced image resolutionenhancement techniques to explore in detail the characteristics of thesmall-scale structures and/or the low contrast structures in the solarcorona. We describe here the results from these high-angular resolutionstudies, including of the kinematics of several Coronal Mass Ejections andpolar jets.  相似文献   

Holocene Climatic Optimum centennial-scale paleoceanography in the NE Aegean (Mediterranean Sea)     

Maria V. Triantaphyllou  Alexandra Gogou  Margarita D. Dimiza  Sofia Kostopoulou  Constantine Parinos  Grigoris Roussakis  Maria Geraga  Ioanna Bouloubassi  Dominik Fleitmann  Vassilis Zervakis  Dimitris Velaoras  Antonia Diamantopoulou  Angeliki Sampatakaki  Vassilis Lykousis 《Geo-Marine Letters》2016,36(1):51-66

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Experience in measuring the wind-velocity profile in an urban environment with a Doppler sodar     

V. P. Yushkov  M. A. Kallistratova  R. D. Kouznetsov  G. A. Kurbatov  V. F. Kramar 《Izvestiya Atmospheric and Oceanic Physics》2007,43(2):168-180

The experience of long-term acoustic remote measurements of vertical wind-velocity profiles at two sites in Moscow is reported. Equipment performances and measurement conditions are described. Acoustic measurement features characteristic of a large city with high traffic noise and spurious reflections from buildings are discussed. Criteria and techniques of rejecting noisy and false signals are described as well as the methods of statistical data processing suitable in the case of a signal-to-noise ratio rapidly varying in time and a significant number of rejected signals. Preliminary results of measurements are given.  相似文献   

Extrapolating SMBH correlations down the mass scale: the case for IMBHs in globular clusters     

Margarita Safonova  Prajval Shastri 《Astrophysics and Space Science》2010,325(1):47-58

Empirical evidence for both stellar mass black holes (M _•<10²  M _⊙) and supermassive black holes (SMBHs, M _•>10⁵  M _⊙) is well established. Moreover, every galaxy with a bulge appears to host a SMBH, whose mass is correlated with the bulge mass, and even more strongly with the central stellar velocity dispersion σ _c , the M _•–σ relation. On the other hand, evidence for “intermediate-mass” black holes (IMBHs, with masses in the range 100–10⁵ M _⊙) is relatively sparse, with only a few mass measurements reported in globular clusters (GCs), dwarf galaxies and low-mass AGNs. We explore the question of whether globular clusters extend the M _•–σ relationship for galaxies to lower black hole masses and find that available data for globular clusters are consistent with the extrapolation of this relationship. We use this extrapolated M _•–σ relationship to predict the putative black hole masses of those globular clusters where existence of central IMBH was proposed. We discuss how globular clusters can be used as a constraint on theories making specific predictions for the low-mass end of the M _•–σ relation.  相似文献   

Response of early Ruppia cirrhosa litter breakdown to nutrient addition in a coastal lagoon affected by agricultural runoff     

Margarita Menndez 《Estuarine, Coastal and Shelf Science》2009,82(4):608-614

The response of early Ruppia cirrhosa Petagna (Grande) litter decomposition to external nitrogen and phosphorus availability in La Tancada (Ebro River, NE Spain), a coastal lagoon that receives agricultural freshwater runoff from rice fields has been examined. Recently abscised dead R. cirrhosa stems were collected and 25 g of fresh weight was placed in litter bags with a mesh size of 100 μm and 1 mm. These bags were fertilised by adding nitrogen (N), a mixture of nitrogen plus phosphorus (N + P), or phosphorus (P), or were left untreated (CT). Macroinvertebrates were retrieved from the bags and the ash-free dry weight, and carbon, and N and P content of the remaining plant material were measured after 0, 3, 7, 14, 22 and 32 days. Litter decomposition rates, k (day⁻¹), were estimated using a simple exponential model. Litter decay was clearly accelerated by the addition of P in the fine (100 μm) litter bags (0.042), but when N was added alone (0.0099) the decomposition rate was lower than in the CT treatments (0.022). No significant difference was observed between the N (0.0099–0.018) and N + P (0.0091–0.015) treatments in either the fine or the coarse (1 mm) litter bags. These results could be attributed to the relatively high availability of external (environmental) and internal (detritus contents) N. No significant effect of macro invertebrates was observed in the CT treatment or under N or P or N + P addition. The ratio between the decomposition rates in coarse and fine litter bags (k_c/k_f) was lower in disturbed Tancada lagoon (0.82) than in Cesine lagoon (2.11), a similar Mediterranean coastal water body with almost pristine conditions. These results indicate that, in addition to data on macroinvertebrate community structure, decomposition rates could also be used to assess water quality in coastal lagoons.  相似文献   

Rock magnetic and geochemical proxies for iron mineral diagenesis in a tropical lake: Lago Verde, Los Tuxtlas, East–Central Mexico     

Beatriz Ortega  Margarita Caballero  Socorro Lozano  Gloria Vilaclara  Alejandro Rodríguez 《Earth and Planetary Science Letters》2006,250(3-4):444-458

Magnetic and non-magnetic mineral analyses were conducted on a lacustrine sequence from Lago Verde in the tropical coast along the Gulf of Mexico that covers the last 2000 years. The site witnessed the transformation of the environment since the early Olmec societies until forest clearance in the last century. Through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes, and compare this model with the archeological record and inferred climatic changes in the northern hemisphere of tropical America. Volcanic activity has played a major influence on sediment magnetic properties, as a purveyor of Ti-magnetites/Ti-maghemites, and as a factor of instability in the environment. Anoxic reductive conditions are evident in most of Lago Verde's sedimentary record.

Direct observations of magnetic minerals and ratios of geochemical (Fe, Ti), and ferrimagnetic (χf) and paramagnetic (χp) susceptibility (χ) data, are used as parameters for magnetite dissolution (χp/χ, Fe/χf), and precipitation (χf/Ti) of magnetic minerals. Intense volcanic activity and anoxia are recorded before A.D. 20, leading to the formation of framboidal pyrite. Increased erosion, higher evaporation rates, lower lake levels, anoxia and reductive diagenesis in non-sulphidic conditions are inferred for laminated sediments between A.D. 20–850. This deposit matches the period of historical crisis and multiyear droughts that contributed to the collapse of the Maya civilization. Dissolution of magnetite, a high organic content and framboidal pyrite point to anoxic, sulphidic conditions and higher lake levels after A.D. 850. Higher lake levels in Lago Verde broadly coincide with the increased precipitation documented during the Medieval Warm Period (A.D. 950–1350) in the northern tropical and subtropical regions of the American continent. For the Little Ice Age (A.D. 1400–1800), the relatively moist conditions inferred are in concordance with the glacial advances recorded in central Mexico. Higher erosion rates reflect destruction of the rainforest over the last 40 years.  相似文献   

International Study on Artemia LVIII. Morphologic differentiation and reproductive isolation of Artemia populations from South America     

Gonzalo Gajardo  Nelson Colihueque  Margarita Parraguez  Patrick Sorgeloos 《International Journal of Salt Lake Research》1998,7(2):133-151

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