1,754 research outputs found
North American Jurassic Apparent Polar Wander: Implications for Plate Motion, Paleogeography, and Cordilleran Tectonics
Eight paleomagnetic poles are considered to be reliable Jurassic reference poles for cratonic North America. These poles form a consistent chronological progression defining two arcuate tracks of apparent polar wander (APW) from Sinemurian through Tithonian time (203-145 Ma). Combined with reliable Triassic and Cretaceous reference poles, the resulting path is well modeled by paleomagnetic Euler pole (PEP) analysis and is significantly different from previous APW compilations. These differences reflect differences in original data sets, modes of analysis, and geologic time scales and translate into substantial and important differences in paleolatitude estimates for cratonic North America. PEP analysis reveals two cusps, or changes in the direction of APW: one in the Late Triassic to Early Jurassic (Jl) and one in the Late Jurassic (J2). The J1 cusp represents the change in North American absolute plate motion associated with rifting of the central Atlantic and Gulf of Mexico, while the J2 cusp correlates temporally with the marine magnetic anomaly M21 plate reorganization and to various North American intraplate tectonomagmatic events (e.g., Nevadan Orogeny). Analysis of pole progression along the J1 to J2 and J2 to Cretaceous APW tracks indicates constant angular plate velocity of 0.6°-0.7°/m.y. from 203 to 150 Ma followed by significantly higher velocity from 150 to 130? Ma. Late Triassic-Jurassic reference poles indicate more southerly paleolatitudes for cratonic North America than have previous compilations requiring modification of displacement scenarios for suspect terranes along the western Cordillera
Magnetic Polarity Stratigraphy and Stratigraphic Completeness
A fundamental limiting factor in the precision of magnetostratigraphic correlation is stratigraphic completeness. Sadler [1981] has suggested a method by which the expected completeness of any given stratigraphic section can be calculated given the thickness, duration, and depositional environment. The technique is probabilistic and requires the investigator to specify a meaningful short-term time scale at which completeness is to be estimated. For magnetostratigraphy, the short-term time scale is defined by the duration of the polarity chron or other polarity feature of interest. Sadler\u27s method allows the probability of observing such features to be quantified. It is a useful tool for evaluating the reliability of magnetostratigraphic correlations and for judging between alternative correlations
North American Apparent Polar Wander, Plate Motion and Left-oblique Convergence: Late Jurassic - Early Cretaceous Orogenic Consequences
The North American apparent polar wander (APW) path indicates an episode of unusually rapid absolute northward motion of western North America between 150 and 135 Ma. During this time the northward component of absolute motion of points along the Washington-Oregon-California coast was in excess of 150 km/m.y. and perhaps as high as 230 km/m.y. We believe that such high absolute northward velocity for North America probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time.
The inception of rapid northward motion and left-oblique convergence was abrupt and should be recorded in the geology of the western Cordillera. It is tempting to correlate this period of unusual Pacific basin-North American interaction with the “Nevadan orogeny” in the Klamath Mountains as well as with left-lateral strike-slip structures such as the Pine Nut fault and Bear Mountains fault zone. Significant differences exist between North American plate motion recorded by the Late Jurassic-Cretaceous APW path and that predicted by a fixed hotspot model. We believe that this discrepancy reflects uncertainty associated with pre-Late Cretaceous hotspot tracks and poorly constrained relative plate motions during the Cretaceous normal polarity superchron
Impact of Processing Conditions on Inter-tablet Coating Thickness Variations Measured by Terahertz In-Line Sensing.
A novel in-line technique utilising pulsed terahertz radiation for direct measurement of the film coating thickness of individual tablets during the coating process was previously developed and demonstrated on a production-scale coater. Here, we use this technique to monitor the evolution of tablet film coating thickness and its inter-tablet variability during the coating process under a number of different process conditions that have been purposefully induced in the production-scale coating process. The changes that were introduced to the coating process include removing the baffles from the coater, adding uncoated tablets to the running process, halting the drum, blockage of spray guns and changes to the spray rate. The terahertz sensor was able to pick up the resulting changes in average coating thickness in the coating drum and we report the impact of these process changes on the resulting coating quality.This work was conducted with financial support from the UK Technology Strategy Board (AB293H). H.L. And J.A.Z. would like to acknowledge the Engineering and Physical Sciences Research Council (EP/L019922/1) and the Newton Trust Cambridge for research funding. J.A.Z. would like to thank Gonville & Caius College, Cambridge for a research fellowship. The authors acknowledge Colorcon Ltd. and Meggle AG for providing the excipients used in this study, Provel Ltd. (Bolton, UK) for the kind loan of the mixing equipment to disperse the coating polymer and Staffan Folestad (AstraZeneca) for useful discussions. Additional data related to this publication is available as a supplementary data to this publication as well as at the University of Cambridge data repository (https://www.repository.cam.ac.uk).This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/jps.2450
Warming experiments elucidate the drivers of observed directional changes in tundra vegetation
Few studies have clearly linked long-term monitoring with insitu experiments to clarify potential drivers of observed change at a given site. This is especially necessary when findings from a site are applied to a much broader geographic area. Here, we document vegetation change at Barrow and Atqasuk, Alaska, occurring naturally and due to experimental warming over nearly two decades. An examination of plant cover, canopy height, and community indices showed more significant differences between years than due to experimental warming. However, changes with warming were more consistent than changes between years and were cumulative in many cases. Most cases of directional change observed in the control plots over time corresponded with a directional change in response to experimental warming. These included increases in canopy height and decreases in lichen cover. Experimental warming resulted in additional increases in evergreen shrub cover and decreases in diversity and bryophyte cover. This study suggests that the directional changes occurring at the sites are primarily due to warming and indicates that further changes are likely in the next two decades if the regional warming trend continues. These findings provide an example of the utility of coupling insitu experiments with long-term monitoring to accurately document vegetation change in response to global change and to identify the underlying mechanisms driving observed changes
Comparative Analysis of Tandem Repeats from Hundreds of Species Reveals Unique Insights into Centromere Evolution
Centromeres are essential for chromosome segregation, yet their DNA sequences
evolve rapidly. In most animals and plants that have been studied, centromeres
contain megabase-scale arrays of tandem repeats. Despite their importance, very
little is known about the degree to which centromere tandem repeats share
common properties between different species across different phyla. We used
bioinformatic methods to identify high-copy tandem repeats from 282 species
using publicly available genomic sequence and our own data. The assumption that
the most abundant tandem repeat is the centromere DNA was true for most species
whose centromeres have been previously characterized, suggesting this is a
general property of genomes. Our methods are compatible with all current
sequencing technologies. Long Pacific Biosciences sequence reads allowed us to
find tandem repeat monomers up to 1,419 bp. High-copy centromere tandem repeats
were found in almost all animal and plant genomes, but repeat monomers were
highly variable in sequence composition and in length. Furthermore,
phylogenetic analysis of sequence homology showed little evidence of sequence
conservation beyond ~50 million years of divergence. We find that despite an
overall lack of sequence conservation, centromere tandem repeats from diverse
species showed similar modes of evolution, including the appearance of higher
order repeat structures in which several polymorphic monomers make up a larger
repeating unit. While centromere position in most eukaryotes is epigenetically
determined, our results indicate that tandem repeats are highly prevalent at
centromeres of both animals and plants. This suggests a functional role for
such repeats, perhaps in promoting concerted evolution of centromere DNA across
chromosomes
Efficient energy transfer in light-harvesting systems, I: optimal temperature, reorganization energy, and spatial-temporal correlations
Understanding the mechanisms of efficient and robust energy transfer in
light-harvesting systems provides new insights for the optimal design of
artificial systems. In this paper, we use the Fenna-Matthews-Olson (FMO)
protein complex and phycocyanin 645 (PC 645) to explore the general dependence
on physical parameters that help maximize the efficiency and maintain its
stability. With the Haken-Strobl model, the maximal energy transfer efficiency
(ETE) is achieved under an intermediate optimal value of dephasing rate. To
avoid the infinite temperature assumption in the Haken-Strobl model and the
failure of the Redfield equation in predicting the Forster rate behavior, we
use the generalized Bloch-Redfield (GBR) equation approach to correctly
describe dissipative exciton dynamics and find that maximal ETE can be achieved
under various physical conditions, including temperature, reorganization
energy, and spatial-temporal correlations in noise. We also identify regimes of
reorganization energy where the ETE changes monotonically with temperature or
spatial correlation and therefore cannot be optimized with respect to these two
variables
Severe Acute Pharyngitis Caused by Group C Streptococcus
INTRODUCTION: Adult group C beta-hemolytic streptococcal pharyngitis has a prevalence of approximately 5%. It can present with a broad spectrum of severity. CASE REPORT: We report a 30-year-old woman who presented with severe Group C streptococcal pharyngitis. DISCUSSION: She presented with a 9-day history of progressive symptoms, including fever, sore throat, neck swelling, and recent onset of hoarseness. In the 9Â days before the emergency room (ER) presentation, the patient had visited the ER twice complaining of a sore throat. At both visits, the physicians performed rapid antigen strep testing. Each time her test was negative and the physicians recommended symptomatic therapy. Her symptoms continued to worsen leading to her repeat presentation. At this time she had severe pharyngitis with markedly enlarged tonsils. Neck CT excluded peritonsillar abscess. Rapid strep testing was again negative, but her throat culture grew group C beta-hemolytic streptococcus. CONCLUSION: This presentation illustrates the importance of a systematic approach to evaluating patients with negative rapid strep tests and worsening pharyngitis
- …