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81.
Suzanne Mahlburg Kay Beatriz Coira Gerhard Wörner Robert W. Kay Bradley S. Singer 《Bulletin of Volcanology》2011,73(10):1487-1511
The giant ignimbrites that erupted from the Cerro Galán caldera complex in the southern Puna of the high Andean plateau are
considered to be linked to crustal and mantle melting as a consequence of delamination of gravitationally unstable thickened
crust and mantle lithosphere over a steepening subduction zone. Major and trace element analyses of Cerro Galán ignimbrites
(68–71% SiO2) that include 75 new analyses can be interpreted as reflecting evolution at three crustal levels. AFC modeling and new fractionation
corrected δ18O values from quartz (+7.63–8.85‰) are consistent with the ignimbrite magmas being near 50:50 mixtures of enriched mantle
(87Sr/86Sr ~ 0.7055) and crustal melts (87Sr/86Sr near 0.715–0.735). Processes at lower crustal levels are predicated on steep heavy REE patterns (Sm/Yb = 4–7), high Sr
contents (>250 ppm) and very low Nb/Ta (9-5) ratios, which are attributed to amphibolite partial melts mixing with fractionating
mantle basalts to produce hybrid melts that rise leaving a gravitationally unstable garnet-bearing residue. Processes at mid
crustal levels create large negative Eu anomalies (Eu/Eu* = 0.45–0.70) and variable trace element enrichment in a crystallizing
mush zone with a temperature near 800–850°C. The mush zone is repeatedly recharged from depth and partially evacuated into
upper crustal magma chambers at times of regional contraction. Crystallinity differences in the ignimbrites are attributed
to biotite, zoned plagioclase and other antecrysts entering higher level chambers where variable amounts of near-eutectic
crystallization occurs at temperatures as low as 680°C just preceding eruption. 40Ar/39Ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the Galán
ignimbrite erupted in more than one batch including a ~ 2.13 Ma intracaldera flow and outflows to the west and north at near
2.09 and 2.06 Ma. Episodic delamination of gravitationally unstable lower crust and mantle lithosphere and injection of basaltic
magmas, whose changing chemistry reflects their evolution over a steepening subduction zone, could trigger the eruptions of
the Cerro Galán ignimbrites. 相似文献
82.
Beatriz Peña-Molino Terrence M. Joyce 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(10):1019-1030
Water properties measured by the central mooring in the Line W mooring array southeast of Cape Cod document a large character shift during the period of November 2001 to April 2008. The observed temperature, salinity and planetary potential vorticity (PPV) anomalies manifest changes in the formation region of the water masses present at Station W, specifically upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and Overflow Water (OW). During the observation period, the minimum in the PPV anomaly field relative to the record mean PPV profile migrated from 1500 m, where it was originally found, to 700 m. Temporal changes in the vertical distribution of temperature and salinity were correlated with the PPV changes. This suggests a dLSW-dominated first half of the record versus an uLSW-dominated second half. The structure of these anomalies is consistent with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the western boundary. In that context, the observed water properties at Line W in the early 2000s reflected the intense deep convection in the Labrador Sea in the mid-1990s, with less intense convection subsequently affecting lighter isopycnals. The observed velocity field is dominated by high-frequency (periods of days to months) fluctuations, however, a fraction of the velocity variability is correlated with changes in water mass properties, and indicate a gradual acceleration of the southwestward flow, with a corresponding increase in Deep Western Boundary Current transport. 相似文献
83.
María Paula Iglesia Llanos Claudia Beatriz Prezzi 《International Journal of Earth Sciences》2013,102(3):745-759
From the Late Carboniferous until the Middle Jurassic, continents were assembled in a quasi-rigid supercontinent called Pangea. The first palaeomagnetic data of South America indicated that the continent remained stationary in similar present-day latitudes during most of the Mesozoic and even the Palaeozoic. However, new palaeomagnetic data suggest that such a scenario is not likely, at least for the Jurassic. In order to test the stationary versus the dynamic-continent model, we studied the Jurassic apparent polar wander paths of the major continents, that is, Eurasia, Africa and North America that all in all show the same shape and chronology of the tracks with respect to those from South America. We thus present a master path that could be useful for the Jurassic Pangea. One of the most remarkable features observed in the path is the change in pole positions at ~197 Ma (Early Jurassic), which denotes the cessation of the counter-clockwise rotation of Pangea and commencement of a clockwise rotation that brought about changes in palaeolatitude and orientation until the end of the Early Jurassic (185 Ma). Here, we analyse a number of phenomena that could have triggered the polar shift between 197 and 185 Ma and conclude that true polar wander is the most likely. In order to do this, we used Morgan’s (Tectonophysics 94:123–139, 1983) grid of hotspots and performed “absolute” palaeogeographical reconstructions of Pangea for the Late Triassic and Jurassic. The palaeolatitudes changes that we observe from our palaeomagnetic data are very well sustained by diverse palaeoclimatic proxies derived from geological and palaeoecological data at this time of both the southern and northern hemispheres. 相似文献
84.
Gypsum is one of the most universally distributed salts in weathered materials but little is known about the influence of environmental conditions on the damage generated by gypsum in stones. To quantify the damage induced by gypsum crystallization acoustic emission techniques are employed to record the elastic energy released during salt crystallization cycles in a limestone. Different environmental conditions have been established during the cooling and drying periods in traditional salt crystallization cycles. During drying two different temperatures (50 and 25 °C) and relative humidity (low, 25 % at 50 °C and 65 % at 25 °C, and high, 99 %) have been applied. The acoustic emission signals are filtered by a frequency analysis in order to eliminate signals corresponding to external noise or artifacts. Our experimental results show that acoustic emission activity is higher under high relative humidity conditions than under low relative humidity conditions, and also higher when drying at 50 °C than at room temperature. Microscopic observations on the weathered samples indicated that under high relative humidity conditions and at room temperature, gypsum crystallizes not on the sample surface, like in the other samples, but deeper in the inner part of the sample, in good agreement with previously published data. We show that using acoustic emissions as usually done in rock mechanics is also very useful in the study of stone decay and weathering processes in the laboratory. 相似文献
85.
86.
87.
Maider Armendáriz Idoia Rosales Beatriz Bádenas Laura Piñuela Marc Aurell José Carlos García‐Ramos 《地学学报》2013,25(6):439-445
Palaeotemperature estimates from the oxygen‐isotope compositions of belemnites have been hampered by not knowing ancient seawater isotope compositions well enough. We have tackled this problem using Mg/Ca as a proxy for temperature and here, we present a ~2 Ma record of paired Mg/Ca and δ18O measurements of Jurassic (Early Pliensbachian) belemnites from the Asturian basin as a palaeo‐proxy of seawater oxygen‐isotope composition. From the combined use of the two approaches, we suggest a δ18Ow composition of about ?0.1‰ for the Jamesoni–Ibex zones. This value may have been increased by about 0.6‰ during the Davoei Zone due to the effect of waters with a different δ18Ow composition. These findings illustrate the inaccuracy of using a globally homogeneous ice‐free value of δ18Ow = ?1‰ for δ18Ocarb‐based palaeotemperature reconstructions. Our data suggest that previous palaeotemperatures calculated in the region from δ18O values of belemnites may have been underestimated as the seawater oxygen isotopic composition could have been higher. 相似文献
88.
Building an environmental quality index for a big city: a spatial interpolation approach combined with a distance indicator 总被引:1,自引:1,他引:0
One of the main tasks of regional and environmental economics is to construct Environmental Quality Indexes for big cities.
A standard method is to generate a single measure as a linear combination of several contaminants by applying Principal Component
Analysis. Spatial interpolation is then carried out to determine pollution levels across the city. We innovate on this method
and propose an alternative approach. First, we combine a set of noise and air pollutants measured at a number of monitoring
stations with data available for each census tract. This yields a mixed environmental index that is socioeconomically more
complete. We then apply kriging to match the monitoring station records to the census data. Finally, we construct a composite
pollution index using the Pena Distance method (DP2), which proves more robust than traditional approaches. 相似文献
89.
Margarita Caballero Gabriela Vázquez Socorro Lozano-García Alejandro Rodríguez Susana Sosa-Nájera Ana Carolina Ruiz-Fernández Beatriz Ortega 《Journal of Paleolimnology》2006,35(1):83-97
The Sierra de Los Tuxtlas is a recently active volcanic field, with eruptions in 1664 and 1792. It holds one of the reserves
of tropical evergreen forest in Mexico, as well as several maar lakes. One of them, Lago Verde, was chosen for a three-fold
study (1) on its present limnological conditions, (2) on the algal community living in the water column and preserved in the
surface sediments; and (3) on its recent history (ca. 340 yr). The palaeolimnological study was based on multiproxy analyses
on core material dated by 210Pb, 137Cs and 14C. Lago Verde is a small, shallow lake with dilute, slightly alkaline water (CO32− + HCO3− > Cl− > SO42−, Na+ + K+ > Ca2+ > Mg2+). It is turbid, eutrophic, with high phosphorus levels. It is a warm polymictic lake, with thermal and oxygen stratification
establishing by midday during the warm months. The lake does not stratify in winter. Diatoms dominate the phytoplankton community
in the cold ‘nortes’ season, Cyanobacteria in summer, and Chlorophyta in autumn. Cyanobacteria (Chroococcales) are not well
preserved in the surface sediments while Chlorophyta are better preserved. Sedimentary diatoms are well preserved, dominated
by the three most abundant species in the water column: Achnanthidium minutissimum, Fragilaria capucina and Aulacoseira granulata. The base of the studied sequences is constrained by the historic eruption of 1664. The period from 1664 to 1963 is characterised
by a meso-eutrophic lake. Tropical Forest vegetation reaches maximum values between ca. 1800 and 1963. Between ca. 1785 and
1885 the lake was slightly shallower than in the rest of the 1664–1963 period, probably recording climatic variability. An
early pulse of anthropogenic disturbance was recorded by ca. 1921 and after ca. 1963 intense forest clearance and high erosion
rates led to a more turbid, more productive, nutrient-rich lake. The highest anthropogenic impact was reached by ca. 1988;
afterwards the lake and its basin reached a new balance, with the establishment of the present modern conditions. 相似文献
90.
Hugo Levato Beatriz García Carlos Loustó Nidia Morrell 《Astrophysics and Space Science》1988,146(2):361-373
We have determined radial velocities for 31 stars in the field of the open cluster IC 2391, and derived the percentage of probable radial-velocity variables. We improved the orbit of one previously known spectroscopic binary, and confirmed the existence of two double-line binaries. We discussed with such data the membership to the cluster, the average cluster radial velocity and compared the incidence of short-period binaries among the Main-Sequence members of IC 2391 with the incidence for other clusters with similar average axial rotation of its members.Member of the Carrera del Investigador Científico del Consejo Nacional de Investigaciones Científicas y Técnicas de la República ArgentinaVisiting Astronomer, Cerro Tololo Inter-American Observatory, operated by AURA, Inc. under contract with the National Science Foundation. 相似文献