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51.
We study the spatial structure and sub‐structure of regions rich in Hipparcos stars with blue BT – VT colours. These regions, which comprise large stellar complexes, OB associations, and young open clusters, are tracers of on‐going star formation in the Galaxy. The DBSCAN (Density‐Based Spatial Clustering of Applications with Noise) data clustering algorithm is used to look for spatial overdensities of early‐type stars. Once an overdensity, “agglomerate”, is identified, we carry out a data and bibliographic compilation of their star member candidates. The actual membership in agglomerate of each early‐type star is studied based on its heliocentric distance, proper motion, and previous spectro‐photometric information. We identify 35 agglomerates of early‐type Hipparcos stars. Most of them are associated to previously known clusters and OB associations. The previously unknown P Puppis agglomerate is subject of a dedicated study with Virtual Observatory tools. It is actually a new, nearby, young open cluster (d ∼ 470 pc, age ∼ 20 Ma) with a clear radial density gradient.We list P Puppis and other six agglomerates (including NGC 2451 A, vdBH 23, and Trumpler 10) as new sites for substellar searches because of their youth, closeness, and spatial density. We investigate in detail the sub‐structure in the Orion, CMa‐Pup and Pup‐Vel OB complexes (“super‐agglomerates”). We confirm or discover some stellar overdensities in the Orion complex, like the 25 Ori group, the Horsehead region (including the σ Orionis cluster), and the η Orionis agglomerate. Finally, we derive accurate parallactic distances to the Pleiades, NGC 2451 A, and IC 2391, describe several field early‐type stars at d < 200 pc, and discuss the incompleteness of our search. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
52.
José A. Caballero 《Monthly notices of the Royal Astronomical Society》2008,383(1):375-382
I have re-visited the spatial distribution of stars and high-mass brown dwarfs in the σ Orionis (σ Ori) cluster (∼3 Ma, ∼360 pc). The input was a catalogue of 340 cluster members and candidates at separations less than 30 arcmin to σ Ori AB. Of them, 70 per cent have features of extreme youth. I fitted the normalized cumulative number of objects counting from the cluster centre to several power-law, exponential and King radial distributions. The cluster seems to have two components: a dense core that extends from the centre to r ≈ 20 arcmin and a rarified halo at larger separations. The radial distribution in the core follows a power law proportional to r 1 , which corresponds to a volume density proportional to r −2 . This is consistent with the collapse of an isothermal spherical molecular cloud. The stars more massive than 3.7 M⊙ concentrate, however, towards the cluster centre, where there is also an apparent deficit of very low mass objects ( M < 0.16 M⊙ ) . Last, I demonstrated through Monte Carlo simulations that the cluster is azimuthally asymmetric, with a filamentary overdensity of objects that runs from the cluster centre to the Horsehead Nebula. 相似文献
53.
Concepción Jiménez-López Christopher S Romanek F.Javier Huertas Emilia Caballero 《Geochimica et cosmochimica acta》2004,68(16):3367-3377
Mg-bearing calcite was precipitated at 25°C in closed system free-drift experiments from solutions containing NaHCO3, CaCl2 and MgCl2. The chemical and isotope composition of the solution and precipitate were investigated during time course experiments of 24-h duration. Monohydrocalcite and calcite precipitated early in the experiments (<8 h), while Mg-calcite was the predominant precipitate (>95%) thereafter. Solid collected at the end of the experiments displayed compositional zoning from pure calcite in crystal cores to up to 23 mol% MgCO3 in the rims. Smaller excursions in Mg were superimposed on this chemical record, which is characteristic of oscillatory zoning observed in synthetic and natural solid-solution carbonates of differing solubility. Magnesium also altered the predominant morphology of crystals over time from the {104} to {100} and {110} growth forms.The oxygen isotope fractionation factor for the magnesian-calcite-water system (as 103lnαMg-cl-H2O) displayed a strong dependence on the mol% MgCO3 in the solid phase, but quantification of the relationship was difficult due to the heterogeneous nature of the precipitate. Considering only the Mg-content and δ18O values for the bulk solid, 103lnαMg-cl-H2O increased at a rate of 0.17 ± 0.02 per mol% MgCO3; this value is a factor of three higher than the single previous estimate (Tarutani T., Clayton R.N., and Mayeda T. K. (1969) The effect of polymorphims and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim. Cosmochim. Acta 33, 987-996). Nevertheless, extrapolation of our relationship to the pure calcite end member yielded a value of 27.9 ± 0.02, which is similar in magnitude to published values for the calcite-water system. Although no kinetic effect was observed on 103lnαMg-cl-H2O for precipitation rates that ranged from 103.21 to 104.60 μmol · m−2 · h−1, it was impossible to disentangle the potential effect(s) of precipitation rate and Mg-content on 103lnαMg-cl-H2O due to the heterogeneous nature of the solid.The results of this study suggest that paleotemperatures inferred from the δ18O values of high magnesian calcite (>10 mol% MgCO3) may be significantly underestimated. Also, the results underscore the need for additional experiments to accurately characterize the effect of Mg coprecipitation on the isotope systematics of calcite from a chemically homogeneous precipitate or a heterogeneous material that is analyzed at the scale of chemical and isotopic zonation. 相似文献
54.
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
55.
We present the applicability of differential system (DS) method for identification of hydraulic conductivity and effective porosity in a phreatic aquifer. In the original setting, the first step of the DS system is to solve an overdetermined algebraic system in the least squares sense. A natural extension of the method is to pose a least squares problem in an appropriate functional space. We show an improvement of the identification by considering the least square problem in the space of square integrable functions in the time variable for a finite interval. 相似文献
56.
Ana Carolina Ruiz-Fernández Claude Hillaire-Marcel Federico Páez-Osuna Bassam Ghaleb Margarita Caballero 《Quaternary Research》2007,67(2):181-192
Lago Verde is a fresh-water maar found on the lower slopes of San Martin volcano, at the Sierra de Los Tuxtlas, Mexico, currently the northernmost remnant of the tropical rain forest in America. 210Pb and 137Cs analyzed in a sediment core were used to reconstruct the historical fluxes of Ag, Cd, Cu, Pb, Hg and Zn to the site during the last ∼ 150 yr. The 210Pbxs-derived sediment accumulation rates, the magnetic susceptibility, C/N ratios and δ13C data evidenced background conditions at the lake until 1960s, when enhanced erosion related to the clearing of large forested areas at Los Tuxtlas promoted higher accumulation rates of a heavier and more magnetic sedimentary material. Recent sediments from Lago Verde were found enriched by Pb (26-fold natural concentration level [NCLs]) and moderately enriched by Cd > Cu > Zn and Hg (6-, 5-, 4- and 4-fold corresponding NCLs, respectively). The fluxes of Cu, Hg, Pb and Zn have significantly increased since 1940s, with peak ratios of total modern to pre-industrial fluxes of 11, 11, 19 and 49, respectively. The lake occupies a relatively pristine, non-industrialized basin, and therefore the increased metal fluxes might be related to long-distance aeolian transport of trace metals. 相似文献
57.
This paper presents the first detailed multi-element geochemical data from the late Quaternary sediments of the Tecocomulco lake basin (central Mexico) and rocks exposed in the basin catchments to understand the extents of chemical weathering and provenance of the siliciclastic fractions. Ternary diagrams of A-CN-K, A-C-N and A-CNK-FM and elemental ratios suggest that most of the lacustrine sediments were derived from mafic volcanic deposits comprising the Chichicuatla and the Apan-Peñon andesites and the Apan-Tezontepec basaltic-andesites. The felsic tephra layers have chemical compositions comparable to the Acoculco volcanic sequences. The calculated indices of chemical weathering such as chemical index of alteration (CIA), plagioclase index of alteration (PIA) and chemical index of weathering (CIW) indicate low to extreme chemical weathering for the lacustrine sediments and low chemical weathering for tephra layers. The varying degree of chemical weathering in lacustrine sediments is related to the fluctuating average annual precipitation during the late Quaternary. However, the low weathering of tephra layers are due to their higher rate of deposition. The dacite-rhyolitic tephra layers of ca. 31,000 14C yr BP are relatively more weathered compared to the unweathered rhyolitic tephra of ca. 50,000 14C yr BP. This could be due to the rapid deposition of ca. 200 cm of tephra layers during the ca. 50,000 14C yr BP volcanic eruption that might have prevented the interaction between tephra layers and weathering agents. 相似文献
58.
Sedimentological and palaeohydrological characterization of Late Pleistocene and Holocene tufa mound palaeolakes using trenching methods in the Spanish Pyrenees 
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下载免费PDF全文 Xavier M. Pellicer Juan Pablo Corella Francisco Gutiérrez Carles Roqué Rogelio Linares Domingo Carbonel Mario Zarroca Jesús Guerrero Xavier Comas 《Sedimentology》2016,63(6):1786-1819
Lakes developed in the inner depressions of tufa mounds are rare geomorphic features and still poorly understood. Sedimentation in this unusual type of endorheic lake with a very restricted catchment area is highly sensitive to environmental and hydrological changes. The Isona tufa mound complex, north‐eastern Iberian Peninsula, is associated with the discharge zone of a confined artesian aquifer and comprises 11 tufa mounds consisting of an annular rimstone enclosing a central depression filled with lake deposits. Data gathered from trenches excavated in four palaeolakes located within three different morphostratigraphic units permitted a precise analysis of the geometrical characteristics and stratigraphic relationships of the deposits and provided a sedimentation model for the Late Quaternary infilling of the spring‐fed lakes. The work illustrates that trenches allow a precise characterization of the stratigraphic arrangements, lateral facies changes and deformation structures, which are not apparent in studies relying solely on borehole records, and facilitate sampling for dating and geochemical analyses. The five sedimentary facies described represent different evolutionary stages of the lakes, including: (i) carbonate‐rich palustrine deposits probably related to periods with strong hydrological seasonality; (ii) massive highly bioturbated organic ooze; (iii) banded organic carbonate‐rich facies associated with an increase in the regional effective moisture; (iv) fine‐grained quartz‐rich aeolian/slope‐wash sediments; and (v) colluvial facies deposited following the desiccation of the lakes located at higher altitudes. Geochemical and sedimentological analyses of the lacustrine sequences provided information on the palaeohydrological evolution of the Isona tufa mound complex and the palaeoenvironmental conditions of the area over the last 28 ka. Radiometric dating suggests that deposition occurred simultaneously at ca 22 ka in palaeolakes situated at different elevations. A drop in the piezometric level prompted by the opening of springs at lower altitudes probably caused the deactivation of the upper springs and the desiccation of the lakes. Arid conditions prevailed in the area during the Late Glacial and the early Holocene (28·0 to 8·5 ka bp ). More humid conditions recorded from 8·5 to 4·2 ka and again since 1·7 ka are in accordance with palaeoenvironmental reconstructions available in the Western Mediterranean since the Last Glacial Maximum. 相似文献
59.
Alcocer Javier Prado Blanca Mora Lucy Oseguera Luis A. Caballero Margarita 《Journal of Paleolimnology》2021,66(3):333-353
Journal of Paleolimnology - We analyzed multiple variables in recent sediments from 18 tropical, karst lakes in “Lagunas de Montebello” National Park, a Mexican Protected Natural Area... 相似文献
60.
Beatriz Ortega Gabriel Vázquez Margarita Caballero Isabel Israde Socorro Lozano-García Peter Schaaf Esperanza Torres 《Journal of Paleolimnology》2010,44(3):745-760
Geochemical data obtained from X-ray fluorescence, physical properties, total organic and inorganic carbon content (TOC/TIC),
and diatom analysis from a 6.61-m-long sedimentary sequence near the modern northern shore of Lake Zirahuen (101° 44′ W, 19°
26′ N, 2000 m asl) provide a reconstruction of lacustrine sedimentation during the last approximately 17 cal kyr BP. A time
scale is based on ten AMS 14C dates and by tephra layers from Jorullo (AD 1759-1764) and Paricutin (AD 1943-1952) volcanoes. The multiproxy analyses presented
in this study reveal abrupt changes in environmental and climatic conditions. The results are compared to the paleo-record
from nearby Lake Patzcuaro. Dry conditions and low lake level are inferred in the late Pleistocene until ca. 15 cal kyr BP,
followed by a slight but sustained increase in lake level, as well as a higher productivity, peaking at ca. 12.1 cal kyr BP.
This interpretation is consistent with several regional climatic reconstructions in central Mexico, but it is in opposition
to record from Lake Patzcuaro. A sediment hiatus bracketed between 12.1 and 7.2 cal kyr BP suggests a drop in lake level in
response to a dry early Holocene. A deeper, more eutrophic and turbid lake is recorded after 7.2 cal kyr BP. Lake level at
the coring site during the mid Holocene is considered the highest for the past 17 cal kyr BP. The emplacement of the La Magueyera
lava flows (LMLF), dated by thermoluminiscence at 6560 ± 950 year, may have reduced basin volume and contributed to the relative
deepening of the lake after 7.2 cal kyr BP. The late Holocene (after 3.9 cal kyr BP) climate is characterized by high instability.
Extensive erosion, lower lake levels, dry conditions and pulses of high sediment influx due to high rainfall are inferred
for this time. Further decrease in lake level and increased erosion are recorded after ca. AD 1050, at the peak of Purepechas
occupation (AD 1300–1521), and until the eighteenth century. Few lacustrine records extend back to the late Pleistocene—early
Holocene in central Mexico; this paper contributes to the understanding of late Pleistocene-Holocene paleoclimates in this
region. 相似文献