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61.
Mid to late Holocene constraints for continental shelf mud deposition in association with river input: the Guadiana Mud Patch (SW Iberia) 总被引:1,自引:1,他引:0
This study correlates Holocene sedimentation with regional environmental changes for the Guadiana middle shelf, SW Iberia,
based on a detailed sedimentological analysis combined with radiocarbon and amino acid racemization dating. The sedimentary
record of vibrocore CRIDA16 (307 cm) from the mud patch off the Guadiana River is characterized by a transgressive signature.
Terrigenous sand–gravel dominates the core base up to ca. 5,000 cal. years b.p., enriched in schist/greywackes and non-reworked quartz. The shelf area was under the direct influence of Guadiana bed load
discharges, as sea level was lower than today but rising, favouring the entrapment of fines during the Guadiana estuary infilling
in the early Holocene. Core upwards, fines dominate the record. Sea-level stabilization at ca. 5,700–3,700 cal. years b.p. led to estuary infilling by coarser sediments and export of suspended sediment to the middle shelf. After ca. 900–700 cal.
years b.p., the formation of the Guadiana Mud Patch signifies deposition in a marine environment dominated by suspended material from
the Guadiana River. Comparison with earlier publications showed that a similar evolutionary mechanism of middle shelf mud
deposition occurred in other parts of the Iberian Margin, pointing to a generalized establishment of this type of sedimentary
body in Iberia since the mid to late Holocene. This suggests that the formation of mud belts on the north and south Iberian
Margin was to a large extent synchronous. It is likely that the evolution of the Guadiana Mud Patch on the shelf was controlled
foremost by the overall climate trends identified for several parts of Iberia during the Holocene (i.e. increased aridity
punctuated by significant humidity stages), and less by human occupation of the region. Enhanced humidity stages led to higher
rainfall and intensified floods, with a consequent increase of discharges to the shelf which ultimately were responsible for
the Guadiana Mud Patch formation. It is expected that this pattern of sedimentary dynamics has been strongly disturbed since
2002, when the closing of the Alqueva Dam (the biggest on the Iberian Peninsula) was completed in the Guadiana River basin. 相似文献
62.
63.
P. Amram C. Mendes de Oliveira H. Plana C. Balkowski M. Bolte 《Astrophysics and Space Science》2002,281(1-2):397-400
We present kinematics and photometric evidence for the presence of seven candidate tidal dwarf galaxies in Stephan's Quintet.
The central regions of the two most probable parent galaxies, NGC 7319 and NGC 7318B, contain little or no gas whereas the
intragroup medium and, in particular, the optical tails that seem to be associated with NGC 7318B are rich in cold and ionized
gas. Two tidal dwarf candidates may be located at the edge of a tidal tail, another located within a tail, and for the four
others there is no obvious stellar/gaseous bridge between them and the parent galaxy. Two of the candidates are associated
with H I clouds, one of which is, in addition, associated with a CO cloud. All seven regions have low continuum fluxes and
high Hα luminosity densities [F(Hα) = (1-60) × 10-14 ergs s-1 cm-2]. Their magnitudes (MB = –16.1 to –12.6), sizes (∼ 3.5 h75
-1 kpc), colors (typically B – R = 0.7), and gas velocity gradients (∼ 8 –26 h75 km s-1 kpc-1) are typical for tidal dwarf galaxies. In addition, the ratios between their star formation rates determined from Hα and
from the B-band luminosity are typical of other tidal dwarf galaxies. The masses of the tidal dwarf galaxies in Stephan's
Quintet range from ∼ 2 × 108 to 1010 M⊙, and the median value for their inferred mass-to-light ratios is 7 (M/L)⊙. At least two of the systems may survive possible ‘fallbacks’ or disruption by the parent galaxies and may already be, or
turn into, self-gravitating dwarf galaxies, new members of the group.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
64.
We present results from a large scale R-band survey of the nearby Leo I group to measure the luminosity function down to dwarf spheroidal luminosities and surface
brightnesses. This program has utilized>7 square degrees of imaging with the Mosaic camera at the KPNO 0.9mand extensive,
on-going spectroscopic follow-up. We estimate our completeness from simulations to be 90% at μR(0) ≃ 24.5, M
R ≃ –11.Leo I is a nearby, low-density group for which a steep faint-end slope has been suggested (Ferguson and Sandage, 1991).We
find the luminosity function to be more similar to that of the Local Group at the faint-end (flat).We also find an unusual
gap in the luminosity function at intermediate luminosities, –19.5 < M
R < –16. This gap cannot be explained by pure Poisson fluctuations around a typical Schechter function, nor is it likely to
result from photometric incompleteness.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
65.
Klaus Keil Maria E. Zucolotto Alexander N. Krot Patricia M. Doyle Myriam Telus Tatiana V. Krot Richard C. Greenwood Ian A. Franchi John T. Wasson Kees C. Welten Marc W. Caffee Derek W. G. Sears My Riebe Rainer Wieler Edivaldo dos Santos Rosa B. Scorzelli Jerome Gattacceca France Lagroix Matthias Laubenstein Julio C. Mendes Philippe Schmitt‐Kopplin Mourad Harir Andre L. R. Moutinho 《Meteoritics & planetary science》2015,50(6):1089-1111
The Vicência meteorite, a stone of 1.547 kg, fell on September 21, 2013, at the village Borracha, near the city of Vicência, Pernambuco, Brazil. It was recovered immediately after the fall, and our consortium study showed it to be an unshocked (S1) LL3.2 ordinary chondrite. The LL group classification is based on the bulk density (3.13 g cm?3); the chondrule mean apparent diameter (0.9 mm); the bulk oxygen isotopic composition (δ17O = 3.768 ± 0.042‰, δ18O = 5.359 ± 0.042‰, Δ17O = 0.981 ± 0.020‰); the content of metallic Fe,Ni (1.8 vol%); the Co content of kamacite (1.73 wt%); the bulk contents of the siderophile elements Ir and Co versus Au; and the ratios of metallic Fe0/total iron (0.105) versus total Fe/Mg (1.164), and of Ni/Mg (0.057) versus total Fe/Mg. The petrologic type 3.2 classification is indicated by the beautifully developed chondritic texture, the standard deviation (~0.09) versus mean Cr2O3 content (~0.14 wt%) of ferroan olivine, the TL sensitivity and the peak temperature and peak width at half maximum, the cathodoluminescence properties of chondrules, the content of trapped 132Xetr (0.317 × 10?8cm3STP g?1), and the Raman spectra for organic material in the matrix. The cosmic ray exposure age is ~72 Ma, which is at the upper end of the age distribution of LL group chondrites. The meteorite is unusual in that it contains relatively large, up to nearly 100 μm in size, secondary fayalite grains, defined as olivine with Fa>75, large enough to allow in situ measurement of oxygen and Mn‐Cr isotope systematics with SIMS. Its oxygen isotopes plot along a mass‐dependent fractionation line with a slope of ~0.5 and Δ17O of 4.0 ± 0.3‰, and are similar to those of secondary fayalite and magnetite in the unequilibrated chondrites EET 90161, MET 96503, and Ngawi. These data suggest that secondary fayalite in Vicência was in equilibrium with a fluid with a Δ17O of ~4‰, consistent with the composition of the fluid in equilibrium with secondary magnetite and fayalite in other unequilibrated ordinary chondrites. Secondary fayalite and the chondrule olivine phenocrysts in Vicência are not in isotopic equilibrium, consistent with low‐temperature formation of fayalite during aqueous alteration on the LL parent body. That alteration, as dated by the 53Mn‐53Cr chronology age of secondary fayalite, took place Ma after formation of CV CAIs when anchored to the quenched angrite D'Orbigny. 相似文献
66.
Alfred Hirn Luisa Senos Martine Sapin Luis Mendes Victor 《Geophysical Journal International》1982,70(1):1-10
Summary. Seismic probing of the upper crust in a catazonal massif in Tras os Montes, Hercynian Iberia, demonstrates the absence of a root towards the lower crust and favours a mechanism of thrust and nappe emplacement. Similar high to low velocity successions in the upper crust are evidenced in particular segments of the Hercynian domain of France. All these occurrences are related to a type region to which a particular meaning has been given in a recent plate tectonics interpretation of the Hercynian orogenesis. This model, implying large-scale nappe displacement and intracrustal thrusts furnishes in turn a general mechanism for the tectonic formation of upper crustal high and low velocity layers of limited extent in orogenic cycles. 相似文献
67.
We use the flexibility of the concept of a fermion–boson star to explore different configurations, ranging from objects of
atomic size and masses of the order 1018 g, up to objects of galactic masses and gigantic halos around a smaller core, with possible interesting applications to astrophysics
and cosmology, particularly in the context of dark matter.
PACS codes: 04.40.−b, 95.35.+d 相似文献
68.
Luiz?Alberto?Ribeiro?Mendon?a Horst?FrischkornEmail author Maria?Marlúcia?Freitas?Santiago Josué?Mendes Filho 《Environmental Geology》2004,46(2):295-302
The Araripe Plateau in northeastern Brazil has an area of about 8,000 km2, confined by 39°05E and 40°55E, and 7°10S and 7°50S. Due to high permeability of soils, a surface drainage system is practically inexistent. Water is stored in excavations with clayey soil, the barreiros. Monthly samples were taken for 18O measurements, from September 1999 to August 2000, from four barreiros, three dug wells and five drilled wells. Results show that (1) groundwaters in the eastern part of the plateau are derived from present-day rainfall (18O–3.2), whereas groundwaters in the western portion are isotopically different (18O–5.0); (2) barreiros are strongly marked seasonally by elevated 18O during the dry period due to elevated evaporation; (3) a dug well at a distance of 30 m from a barreiro exhibits 18O similar to that of the reservoir, indicating a strong interaction between groundwater and surface water; and (4) a tubular well of 242-m depth, located in a fault, exhibits strong seasonal changes in 18O and electrical conductivity, revealing downward leakage between aquifers. 相似文献
69.
Julio Cezar Mendes Silvia Regina de Medeiros Ian McReath Cristina Maria Pinheiro de Campos 《Gondwana Research》2005,8(3):337-345
In the central-southern region of Espírito Santo State, southeastern Brazil, several granitoids with a variable composition intruded high-grade metamorphic rocks, in the northern segment of the Ribeira fold belt. A close relationship between hydrous and anhydrous facies is present in some of the plutons, including the Varzea Alegre Igneous Complex, which has an inner domain formed by gabbro, diorite and granite, and an irregular outer ring of charnockitic rocks. These green megaporphyritic charnockites have primary anhydrous mineral assemblage, I-type and metaluminous character, and high-K calc-alkaline signature. U-Pb zircon single crystal ages obtained by TIMS indicate crystallization at about 500 Ma, similar to other late tectonic plutons of this part of the Ribeira belt. Sr and Nd isotopic ratios ranging from 0.7078 to 0.7096 and 0.5114 to 0.5116 respectively, are interpreted to be indicative of a hybrid origin from crustal and mantle-derived magmas. A binary diagram using Sr isotope ratios also demonstrates that the genesis of the charnockites probably included both magma mixing and fractional crystallization processes. 相似文献
70.
Luiz Bruner de Miranda Alessandro Luvizon Bérgamo Belmiro Mendes de Castro 《Ocean Dynamics》2005,55(5-6):430-440
Observations of thermohaline properties and currents were undertaken in the Curimataú River estuary (6°18′S), Rio Grande do
Norte state (RN), Brazil, during consecutive neap–spring tidal cycles in the austral autumn rainy season. Highly asymmetric
neap tide along channel velocities (−0.4 to 0.9 m s−1) and highly stratified conditions were generated by an increase of the buoyancy energy from the freshwater input (R
iE≈5.6). During the spring-tidal cycle the river discharge decreased and the longitudinal velocity components were higher, less
asymmetrical (−0.8 to 1.1 m s−1) and semidiurnal, associated with moderately stratified conditions (R
iE≈0.1) due to the increase of the kinetic tidal energy forcing mechanism. The overall salinity variation from surface to bottom
during two tidal cycles was from 20.5 to 36.3 and 29 to 36.7 in the neap and spring tide experiments, respectively; in the
last experiment, the tropical water (TW) mass intrusion was enhanced. The net salt transport reversed from down to up estuary
during the neap and spring tide experiments, respectively, varied from 6.0 to –2.0 kg m−1 s−1, an indication of changes in the main forcing of the estuary dynamics. Evaluation of a classical steady analytical model,
in comparison with nearly steady experimental vertical profiles of velocity, shows an agreement classifiable as reasonably
fair. 相似文献