434 research outputs found
Group Frames and Partially Ranked Data
We give an overview of finite group frames and their applications to calculating summary statistics from partially ranked data, drawing upon the work of Rachel Cranfill (2009). We also provide a summary of the representation theory of compact Lie groups. We introduce both of these concepts as possible avenues beyond finite group representations, and also to suggest exploration into calculating summary statistics on Hilbert spaces using representations of Lie groups acting upon those spaces
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<p>Confuciusornis, leveled 16bit data, resampled as cubic voxels, resliced in YZ plane</p>
<p>265 slices; TIF format; 2.520 Mb each</p>
<p>Voxel dimension X = 0.2148 mm</p>
<p>Voxel dimension Y = 0.2148 mm</p>
<p>Voxel dimension Z = 0.2148 mm</p>
<p>These data are 16bit leveled TIF files that are viewable in most viewers (see Usage Notes)</p
Chemical abrasion: the mechanics of zircon dissolution
Chemical abrasion is a technique that combines thermal annealing and partial
dissolution in hydrofluoric acid (HF) to selectively remove
radiation-damaged portions of zircon crystals prior to U–Pb isotopic
analysis, and it is applied ubiquitously to zircon prior to U–Pb isotope
dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of
zircon dissolution in HF and the impact of different leaching conditions on
the zircon structure, however, are poorly resolved. We present a
microstructural investigation that integrates microscale X-ray computed
tomography (µCT), scanning electron microscopy, and Raman
spectroscopy to evaluate zircon dissolution in HF. We show that µCT
is an effective tool for imaging metamictization and complex dissolution
networks in three dimensions. Acid frequently reaches crystal interiors via
fractures spatially associated with radiation damage zoning and inclusions
to dissolve soluble high-U zones, some inclusions, and material around
fractures, leaving behind a more crystalline zircon residue. Other acid paths
to crystal cores include the dissolution of surface-reaching inclusions and
the percolation of acid across zones with high defect densities. In highly
crystalline samples dissolution is crystallographically controlled with
dissolution proceeding almost exclusively along the c axis. Increasing the
leaching temperature from 180 to 210 ∘C results in
deeper etching textures, wider acid paths, more complex internal dissolution
networks, and greater volume losses. How a grain dissolves strongly depends
on its initial radiation damage content and defect distribution as well as
the size and position of inclusions. As such, the effectiveness of any
chemical abrasion protocol for ID-TIMS U–Pb geochronology is likely
sample-dependent. We also briefly discuss the implications of our findings
for deep-time (U-Th)/He thermochronology.</p
Geometric, kinematic, and erosional history of the central Andean Plateau, Bolivia (15–17°S)
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95425/1/tect1910.pd
Thermochronometer record of central Andean Plateau growth, Bolivia (19.5°S)
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94850/1/tect1973.pd
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Sediment structure and physicochemical changes following tidal inundation at a large open coast managed realignment site
Managed realignment (MR) schemes are being implemented to compensate for the loss of intertidal saltmarsh habitats by breaching flood defences and inundating the formerly defended coastal hinterland. However, studies have shown that MR sites have lower biodiversity than anticipated, which has been linked with anoxia and poor drainage resulting from compaction and the collapse of sediment pore space caused by the site's former terrestrial land use. Despite this proposed link between biodiversity and soil structure, the evolution of the sediment sub-surface following site inundation has rarely been examined, particularly over the early stages of the terrestrial to marine or estuarine transition. This paper presents a novel combination of broad- and intensive-scale analysis of the sub-surface evolution of the Medmerry Managed Realignment Site (West Sussex, UK) in the three years following site inundation. Repeated broad-scale sediment physiochemical datasets are analysed to assess the early changes in the sediment subsurface and the preservation of the former terrestrial surface, comparing four locations of different former land uses. Additionally, for two of these locations, high-intensity 3D-computed X-ray microtomography and Itrax micro-X-ray fluorescence spectrometry analyses are presented. Results provide new data on differences in sediment properties and structure related to the former land use, indicating that increased agricultural activity leads to increased compaction and reduced porosity. The presence of anoxic conditions, indicative of poor hydrological connectivity between the terrestrial and post-inundation intertidal sediment facies, was only detected at one site. This site has experienced the highest rate of accretion over the terrestrial surface (ca. 7 cm over 36 months), suggesting that poor drainage is caused by the interaction (or lack of) between sediment facies rather than the former land use. This has significant implications for the design of future MR sites in terms of preparing sites, their anticipated evolution, and the delivery of ecosystem services
Statistics for Fission-Track Thermochronology
This chapter introduces statistical tools to extract geologically meaningful information from fission-track (FT) data using both the external detector and LA-ICP-MS methods. The spontaneous fission of 238U is a Poisson process resulting in large single-grain age uncertainties. To overcome this imprecision, it is nearly always necessary to analyse multiple grains per sample. The degree to which the analytical uncertainties can explain the observed scatter of the single-grain data can be visually assessed on a radial plot and objectively quantified by a chi-square test. For sufficiently low values of the chi-square statistic (or sufficiently high p values), the pooled age of all the grains gives a suitable description of the underlying ‘true’ age population. Samples may fail the chi-square test for several reasons. A first possibility is that the true age population does not consist of a single discrete age component, but is characterised by a continuous range of ages. In this case, a ‘random effects’ model can constrain the true age distribution using two parameters: the ‘central age’ and the ‘(over)dispersion’. A second reason why FT data sets might fail the chi-square test is if they are underlain by multimodal age distributions. Such distributions may consist of discrete age components, continuous age distributions, or a combination of the two. Formalised statistical tests such as chi-square can be useful in preventing overfitting of relatively small data sets. However, they should be used with caution when applied to large data sets (including length measurements) which generate sufficient statistical ‘power’ to reject any simple yet geologically plausible hypothesis
Impact Factor: outdated artefact or stepping-stone to journal certification?
A review of Garfield's journal impact factor and its specific implementation
as the Thomson Reuters Impact Factor reveals several weaknesses in this
commonly-used indicator of journal standing. Key limitations include the
mismatch between citing and cited documents, the deceptive display of three
decimals that belies the real precision, and the absence of confidence
intervals. These are minor issues that are easily amended and should be
corrected, but more substantive improvements are needed. There are indications
that the scientific community seeks and needs better certification of journal
procedures to improve the quality of published science. Comprehensive
certification of editorial and review procedures could help ensure adequate
procedures to detect duplicate and fraudulent submissions.Comment: 25 pages, 12 figures, 6 table
Factors underlying age-related changes in discrete aiming
Age has a clear impact on one’s ability to make accurate goal-directed aiming movements. Older adults seem to plan slower and shorter-ranged initial pulses towards the target, and rely more on sensory feedback to ensure endpoint accuracy. Despite the fact that these age-related changes in manual aiming have been observed consistently, the underlying mechanism remains speculative. In an attempt to isolate four commonly suggested underlying factors, young and older adults were instructed to make discrete aiming movements under varying speed and accuracy constraints. Results showed that older adults were physically able to produce fast primary submovements and that they demonstrated similar movement-programming capacities as young adults. On the other hand, considerable evidence was found supporting a decreased visual feedback-processing efficiency and the implementation of a play-it-safe strategy in older age. In conclusion, a combination of the latter two factors seems to underlie the age-related changes in manual aiming behaviour
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