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41.
42.
Nicole Chorney 《Icarus》2004,167(1):220-224
We present R- and V-band rotational light curves for classical Kuiper belt object 1997 CV29. The imaging data was obtained from the Canada-France-Hawaii Telescope (CFHT) archive located at the Canadian Astronomical Data Center (CADC). The data consist of one nights observations of a series of 42, 8 minute exposures in R band followed by 33, 8 minute exposures in V band acquired on the following night. Using relative photometry we determined a highly significant variation in the brightness of 1997 CV29. Using phase-dispersion-minimization we find 8.0, 11.2, and 15.8 hrs to be the most likely, periods of rotation and we argue that the ∼16 hr period is the most likely based on our limited observing window. The phased light curve has a peak-to-peak range of Δm?0.4±0.1 mag suggesting an axial ratio of a/b?1.45. 相似文献
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44.
Dynamics of magma mixing and degassing recorded in plagioclase at Stromboli (Aeolian Archipelago,Italy) 总被引:12,自引:0,他引:12
Patrizia?LandiEmail author Nicole?Métrich Antonella?Bertagnini Mauro?Rosi 《Contributions to Mineralogy and Petrology》2004,147(2):213-227
Crystal-rich materials (scoriae and lava flows) emitted during the 1985–2000 activity of Stromboli were taken into consideration for systematic study of bulk rock/matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase, the most abundant mineral phase. Over the considered time period, bulk rock composition remained fairly constant in both major (SiO2 49.2–50.9 wt% and K2O 1.96–2.18 wt%) and trace elements. The quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol%. Plagioclase ranges in composition between An62 and An88 and is characterized by alternating, <10–100 m thick, bytownitic and labradoritic concentric layers, although the dominant and representative plagioclase of scoriae is An68. The labradoritic layers (An62–70) show small-scale (1–5 µm), oscillatory zoning, are free of inclusions, and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass. In contrast the bytownitic layers (An70-An88) are patchy zoned, show sieve structure with abundant micrometric glass inclusions and voids, and are attributed to rapid crystal growth.A key to understanding the origin of bytownitic layers can be retrieved from the texture and composition of the coronas of plagioclase xenocrysts, inherited from crystal-rich magma, in nearly aphyric pumice which are erupted during more energetic events and represent a deep, volatile-rich, HK-basaltic magma. They show a continuum from fine-sieve to evident skeletal texture from the inner to the outer part of the corona associated with normal compositional zoning from An90 to An75. In the light of these observations, we propose that input of H2O-rich melt blobs, and their mixing with the residing magma, causes partial dissolution of the labradoritic layers followed by the growth of bytownitic composition whose sieve texture attests of rapid crystallization occurring under undercooling conditions mainly induced by degassing. As a whole, the zoning of plagioclase in the scoriae records successive and discrete intrusions of volatile-rich magma blobs, its degassing and mixing with the resident degassed magma at shallow level.Editorial responsibility: T.L. GroveAn erratum to this article can be found at 相似文献
45.
Keyan Fang Nicole Davi Xiaohua Gou Fahu Chen Edward Cook Jinbao Li Rosanne D’Arrigo 《Climate Dynamics》2010,35(6):941-951
Spatial reconstructions of drought for central High Asia based on a tree-ring network are presented. Drought patterns for
central High Asia are classified into western and eastern modes of variability. Tree-ring based reconstructions of the Palmer
drought severity index (PDSI) are presented for both the western central High Asia drought mode (1587–2005), and for the eastern
central High Asia mode (1660–2005). Both reconstructions, generated using a principal component regression method, show an
increased variability in recent decades. The wettest epoch for both reconstructions occurred from the 1940s to the 1950s.
The most extreme reconstructed drought for western central High Asia was from the 1640s to the 1650s, coinciding with the
collapse of the Chinese Ming Dynasty. The eastern central High Asia reconstruction has shown a distinct tendency towards drier
conditions since the 1980s. Our spatial reconstructions agree well with previous reconstructions that fall within each mode,
while there is no significant correlation between the two spatial reconstructions. 相似文献
46.
Larry W. Esposito Nicole Albers Bonnie K. Meinke Miodrag Sremčević Prasanna Madhusudhanan Joshua E. Colwell Richard G. Jerousek 《Icarus》2012,217(1):103-114
UVIS occultation data show clumping in Saturn’s F ring and at the B ring outer edge, indicating aggregation and disaggregation at these locations that are perturbed by Prometheus and by Mimas. The inferred timescales range from hours to months. Occultation profiles of the edge show wide variability, indicating perturbations by local mass aggregations. Structure near the B ring edge is seen in power spectral analysis at scales 200–2000 m. Similar structure is also seen at the strongest density waves, with significance increasing with resonance strength. For the B ring outer edge, the strongest structure is seen at longitudes 90° and 270° relative to Mimas. This indicates a direct relation between the moon and the ring clumping. We propose that the collective behavior of the ring particles resembles a predator–prey system: the mean aggregate size is the prey, which feeds the velocity dispersion; conversely, increasing dispersion breaks up the aggregates. Moons may trigger clumping by streamline crowding, which reduces the relative velocity, leading to more aggregation and more clumping. Disaggregation may follow from disruptive collisions or tidal shedding as the clumps stir the relative velocity. For realistic values of the parameters this yields a limit cycle behavior, as for the ecology of foxes and hares or the “boom-bust” economic cycle. Solving for the long-term behavior of this forced system gives a periodic response at the perturbing frequency, with a phase lag roughly consistent with the UVIS occultation measurements. We conclude that the agitation by the moons in the F ring and at the B ring outer edge drives aggregation and disaggregation in the forcing frame. This agitation of the ring material may also allow fortuitous formation of solid objects from the temporary clumps, via stochastic processes like compaction, adhesion, sintering or reorganization that drives the denser parts of the aggregate to the center or ejects the lighter elements. Any of these more persistent objects would then orbit at the Kepler rate. We would also expect the formation of clumps and some more permanent objects at the other perturbed regions in the rings… including satellite resonances, shepherded ring edges, and near embedded objects like Pan and Daphnis (where the aggregation/disaggregation cycles are forced similar to Prometheus forcing of the F ring). 相似文献
47.
The Cassini Ultraviolet Imaging Spectrograph (UVIS) has detected 27 statistically significant features in 101 occultations by Saturn’s F ring since July 2004. This work nearly doubles the number of features reported by Esposito et al. (Esposito, L.W. et al. [2008]. Icarus 194, 278–289). As the number of statistically significant features has grown, it has become useful to classify them for the purposes of cataloging. We define three classes: Moonlet, Icicle, and Core, which visually classify the shapes of features seen to date in the occultation profiles of Saturn’s F ring. Two features fall into the Moonlet class. Each is opaque in its occultation, which makes them candidates for solid objects. A majority of features are classified as Icicles, which partially block stellar signal for 22 m to just over 3.7 km along the radial expanse of the occultation. The density enhancements responsible for such signal attenuations are likely due to transient clumping of material, evidence that aggregations of material are ubiquitous in the F ring. Finally, the variety of core region shapes displays how even the general shape of the F ring is ever-changing. The core region of the F ring (typically ~10 km wide) usually has a smooth U-shape to it, but the core region takes the shape of Ws and Vs in some occultation profiles. Our lengthy observing campaign reveals that Icicles are likely transient clumps, moonlets are possible solid objects, and cores show the variety of F ring morphology. We suggest that icicles may evolve into moonlets, which are an order of magnitude less abundant. 相似文献
48.
49.
Gang Yang Aaron Zimmerman Nicole C. Hurtig Svetoslav V. Georgiev Vineet Goswami Judith L. Hannah Holly J. Stein 《Geostandards and Geoanalytical Research》2020,44(2):231-242
Rhenium–osmium geochronometry for samples with low Re and complex matrices requires improved Re extraction methods. Here, we investigate plausible controls on efficiency and efficacy of Re extraction during our anion resin bead purification. Four different protocols are compared, each isolating a single variable to test. Rhenium concentrations for solutions at each step of each protocol document differences in chemical recovery/yield. The negative‐thermal ionisation mass spectrometry (N‐TIMS) signal intensity serves as a proxy for Re yield and purity. These data document correlations between the N‐TIMS signal intensity and (a) the duration of anion resin bead conditioning prior to loading with Re‐bearing solution, and (b) both duration and strength of nitric acid used during rinsing of the Re‐loaded anion resin bead. The optimal protocol improved Re signal intensity around fourteen times compared with our current Re extraction protocol, an aggregate of 2.4 times improvement in chemical recovery (yield) and 5.8 times improvement in emission efficiency (purity). Repeated N‐TIMS isotopic measurements on our in‐house Re standard solution (1407) verify that our optimal protocol‐3 does not fractionate Re isotopes. The improved anion resin bead method considerably lowers the Re detection limit and allows Re‐Os isotopic analysis of picogram‐level Re hosted in geological samples with complex matrices. 相似文献
50.
Holocene paleoenvironmental changes inferred from diatom assemblages in sediments of Kusawa Lake, Yukon Territory, Canada 总被引:1,自引:0,他引:1
The southwest Yukon Territory, Canada, is an important region for recovering sensitive records of Holocene paleoclimatic change. More information is needed, however, to constrain the timing of the major Holocene climatic transitions, and to understand associated impacts on different ecosystems. For example, paleolimnological studies have focused on small lakes and ponds, but the history of large lakes has received little study. We analyzed diatom assemblages, species richness, valve concentrations, and biogenic silica, in the sediments of Kusawa Lake (60°16.5'N; 136°10.9'W; 671 m a.s.l.) to reconstruct the responses of this large (surface area = 142 km2), deep (Zmax = 135 m) freshwater ecosystem to Holocene climatic transitions. Diatoms colonized the lake soon after ice retreat, around 11,000 cal yr BP; assemblages throughout the record were dominated by planktonic types. Diatom concentrations and biogenic silica were high during the Holocene Thermal Maximum between 10,700 and 7300 cal yr BP, then began to decrease in response to cooling associated with orbitally driven reductions in insolation. Diatom assemblages shifted towards taxa with lower surface water temperature optima after 8300 cal yr BP, perhaps in response to abrupt and progressive cooling. Our study confirms that diatom assemblages in large lakes are sensitive to regional-scale paleoclimatic changes. 相似文献