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321.
322.
Torsten Henkel Thomas Stephan Elmar K. Jessberger Peter Hoppe Roger Strebel Sachiko Amari Roy S. Lewis 《Meteoritics & planetary science》2007,42(7-8):1121-1134
Abstract— Thirteen presolar silicon carbide grains—three of supernova (SN) origin and ten of asymptotic giant branch (AGB) star origin—were examined with time‐of‐flight‐secondary ion mass spectrometry (TOF‐SIMS). The grains had been extracted from two different meteorites—Murchison and Tieschitz—using different acid residue methods. At high lateral resolution of ~300 nm, isotopic and elemental heterogeneities within the micrometer‐sized grains were detected. The trace elemental abundances, when displayed in two‐element correlation plots, of Li, Mg, K, and Ca show a clear distinction between the two different meteoritic sources. The different concentrations might be attributed to differences of the host meteorites and/or of extraction methods whereas the stellar source seems to be less decisive. In one SN grain with 26Mg‐enrichment from extinct 26Al, the acid treatment, as part of the grain separation procedure, affected the Mg/Al ratio in the outer rim and therefore the inferred initial 26Al/27Al ratio. A second SN grain exhibits a lateral heterogeneity in 26Al/27Al, which either is due to residual Al‐rich contamination on the grain surface or to the condensation chemistry in the SN ejecta. 相似文献
323.
Kanan K. Datta Somnath Bharadwaj T. Roy Choudhury 《Monthly notices of the Royal Astronomical Society》2007,382(2):809-818
Linear transient phenomena induced by flow non-normality in thin self-gravitating astrophysical discs are studied using the shearing sheet approximation. The considered system includes two modes of perturbations: vortex and (spiral density) wave. It is shown that self-gravity considerably alters the vortex mode dynamics; its transient (swing) growth may be several orders of magnitude stronger than in the non-self-gravitating case and two to three times larger than the transient growth of the wave mode. Based on this finding, we comment on the role of vortex mode perturbations in a gravitoturbulent state. We also describe the linear coupling of the perturbation modes, caused by the differential character of disc rotation. The coupling is asymmetric: vortex mode perturbations are able to excite wave mode perturbations, but not vice versa. This asymmetric coupling lends additional significance to the vortex mode as a participant in spiral density waves and shock manifestations in astrophysical discs. 相似文献
324.
Six new radiocarbon ages and a pollen sequence are provided for Laguna Stibnite, a small lake on the Taitao Peninsula, Chile (latitude 46°25'S, longitude 74°24'W). The sediments record a late-glacial to present sequence, with a basal age of 14 335 ± 145 yr BP (Q-2840). The radiocarbon ages provide the basis for a sound chronology in a region of Chile where few palynological studies have been made and where previous radiocarbon ages are ambiguous. The chronology from Laguna Stibnite supports the acceptance of a radiocarbon chronology based on wood fragments (rather than bulk analysis) at a site near Puerto Edén further south in the Chilean Channels (49°08'S). These data from Laguna Stibnite provide evidence for an early deglaciation (before 14 000 yr BP) in this region of southern Chile. The sequence provides no evidence for a climatic reversal between 11 000 yr BP and 10 000 yr BP, the so-called Younger Dryas chron. 相似文献
325.
The Magellan mound province in the Porcupine Basin 总被引:5,自引:0,他引:5
V. A. I. Huvenne W. R. Bailey P. M. Shannon J. Naeth R. di Primio J. P. Henriet B. Horsfield H. de Haas A. Wheeler K. Olu-Le Roy 《International Journal of Earth Sciences》2007,96(1):85-101
The Magellan mound province is one of the three known provinces of carbonate mounds or cold-water coral banks in the Porcupine
Seabight, west of Ireland. It has been studied in detail using a large and varied data set: 2D and 3D seismic data, sidescan
sonar imagery and video data collected during ROV deployment have been used to describe the mounds in terms of origin, growth
processes and burial. The aim of this paper is to present the Magellan mounds and their setting in an integrated, holistic
way.
More than 1,000 densely spaced and mainly buried mounds have been identified in the area. They all seem to be rooted on one
seismic reflection, suggesting a sudden mound start-up. Their size and spatial distribution characteristics are presented,
together with the present-day appearance of the few mounds that reach the seabed. The underlying geology has been studied
by means of fault analysis and numerical basin modelling in an attempt to identify possible hydrocarbon migration pathways
below or in the surroundings of the Magellan mounds.
Although conclusive evidence concerning the processes of mound initiation proves to be elusive, the results of both fault
analysis and 2D numerical modelling failed to identify, with confidence, any direct pathways for focused hydrocarbon flow
to the Magellan province. Diffuse seepage however may have taken place, as drainage area modelling suggests a possible link
between mound position and structural features in the Hovland-Magellan area. During mound development and growth, the interplay
of currents and sedimentation seems to have been the most important control. Mounds which could not keep pace with the sedimentation
rates were buried, and on the few mounds which maintained growth, only a few corals survive at present. 相似文献
326.
Roy W. Spencer 《GeoJournal》2007,70(1):11-14
Al Gore’s movie An Inconvenient Truth gives a variety of unusually biased interpretations of the state of climate science and global warming theory. These cover
a wide range of natural events and processes which could potentially be impacted by global warming, but which the movie misrepresents
as clear examples of the human influence on climate. A few examples include the mixing up of cause and effect in his graphical
portrayal of temperature and carbon dioxide variations over hundreds of thousands of years; the repeated depiction of ice
calving from glaciers as a sign of global warming; the implication that Hurricane Katrina was the fault of humans; and the
particularly extreme view that the Greenland ice sheet will melt, flooding coastal cities worldwide. Ultimately, all of these
are related to the widespread perception that scientists have uniquely tied global warming to anthropogenic greenhouse gas
emissions. The real inconvenient truth is that science has no idea how much of recent warming is natural versus the result
of human activities. 相似文献
327.
Ernst Zinner Sachiko Amari Cristine Jennings Aaron F. Mertz Ann N. Nguyen Roberto Gallino Maria Lugaro Roy S. Lewis 《Geochimica et cosmochimica acta》2007,71(19):4786-4813
We report isotopic ratio measurements of small SiC and Si3N4 grains, with special emphasis on presolar SiC grains of type Z, and new nucleosynthesis models for 26Al/27Al and the Ti isotopic ratios in asymptotic giant branch (AGB) stars. With the NanoSIMS we analyzed 310 SiC grains from Murchison (carbonaceous CM2 chondrite) separate KJB (diameters 0.25-0.45 μm) and 153 SiC grains from KJG (diameters 1.8-3.7 μm), 154 SiC and 23 Si3N4 grains from Indarch (enstatite EH4 chondrite) separate IH6 (diameters 0.25-0.65 μm) for their C and N isotopic compositions, 549 SiC and 142 Si3N4 grains from IH6 for their C and Si isotopic compositions, 13 SiC grains from Murchison and 66 from Indarch for their Al-Mg compositions, and eight SiC grains from Murchison and 10 from Indarch for their Ti isotopic compositions. One of the original objectives of this effort was to compare isotopic analyses with the NanoSIMS with analyses previously obtained with the Cameca IMS 3f ion microprobe. Many of the Si3N4 grains from Indarch have isotopic anomalies but most of these apparently originate from adjacent SiC grains. Only one Si3N4 grain, with 13C and 14N excesses, has a likely AGB origin. The C, N, and Si isotopic data show that the percentage of SiC grains of type Y and Z increase with decreasing grain size (from ∼1% for grains >2 μm to ∼5-7% for grains of 0.5 μm), providing an opportunity for isotopic analyses in these rare grains. Our measurements expand the number of Al-Mg analyses on SiC Z grains from 4 to 23 and the number of Ti analyses on Z grains from 2 to 11. Inferred26Al/27Al ratios of Z grains are in the range found in mainstream and Y grains and do not exceed those predicted by models of AGB nucleosynthesis. Cool bottom processing (CBP) has been invoked to explain the low 12C/13C ratios of Z grains, but this process apparently does not lead to increased 26Al production in the parent stars of these grains. This finding is in contrast to presolar oxide grains where CBP is needed to explain their high 26Al/27Al ratios. The low 46,47,49Ti/48Ti ratios found in Z grains and their correlation with low 29Si/28Si ratios extend the trend seen in mainstream grains and confirm an origin in low-metallicity AGB stars. The relatively large excesses in 30Si and 50Ti in Z grains are predicted by our models to be the result of increased production of these isotopes by neutron-capture nucleosynthesis in low-metallicity AGB stars. However, the predicted excesses in 50Ti (and 49Ti) are much larger than those found. Even lowering the strength of the 13C pocket cannot solve this discrepancy in a consistent way. 相似文献
328.
ZHANG Jingxiong David Roy Sadashiva Devadiga ZHENG Min 《地球空间信息科学学报》2007,10(1):44-50
With remote sensing information products becoming increasingly varied and arguably improved, scientific applications of such products rely on their quality assessment. In an operational context such as the NASA (National Aeronautics and Space Administration) information production based on the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument on board Earth Observing System (EOS) Terra and Aqua satellites, efficient ways of detecting product anomaly, i.e., to discriminate between product artifacts and real changes in Earth processes being monitored, are extremely important to assist and inform the user communities about potential unreliability in the products. A technique for anomaly detection, known as MAD (the median of absolute deviate from the median), in MODIS land products via time series analysis is described, which can handle intra- and in-ter-annual variation in the data by using MAD statistics of the original data and their first-order difference. This method is shown to be robust and work across major land products, including NDVI, active fire, snow cover, and surface reflectance, and its applicabil-ity to multi-disciplinary products is anticipated. 相似文献
329.
330.
Earthquakes in Iran and neighbouring regions are closely connected to their position within the geologically active Alpine-Himalayan
belt. Modern tectonic activity is forced by the convergent movements between two plates: The Arabian plate, including Saudi
Arabia, the Persian Gulf and the Zagros Ranges of Iran, and the Eurasian plate. The intensive seismic activity in this region
is recorded with shallow focal depth and magnitude rising as high as Mw = 7.8. The study region can be attributed to a highly complex geodynamic process and therefore is well suited for multifractal
seismicity analysis. Multifractal analysis of earthquakes (mb ≥ 3) occurring during 1973 – 2006 led to the detection of a clustering pattern in the narrow time span prior to all the large
earthquakes: Mw = 7.8 on 16.9.1978; Mw = 6.8 on 26.12.2003; Mw = 7.7 on 10.5.97. Based on the spatio-temporal clustering pattern of events, the potential for future large events can be
assessed. Spatio-temporal clustering of events apparently indicates a highly stressed region, an asperity or weak zone from
which the rupture propagation eventually nucleates, causing large earthquakes. This clustering pattern analysis done on a
well-constrained catalogue for most of the fault systems of known seismicity may eventually aid in the preparedness and earthquake
disaster mitigation. 相似文献