286 research outputs found
Testing fluvial erosion models using the transient response of bedrock rivers to tectonic forcing in the Apennines, Italy
The transient response of bedrock rivers to a drop in base level can be used to
discriminate between competing fluvial erosion models. However, some recent studies of
bedrock erosion conclude that transient river long profiles can be approximately
characterized by a transportâlimited erosion model, while other authors suggest that a
detachmentâlimited model best explains their field data. The difference is thought to be
due to the relative volume of sediment being fluxed through the fluvial system. Using a
pragmatic approach, we address this debate by testing the ability of endâmember fluvial
erosion models to reproduce the wellâdocumented evolution of three catchments in the
central Apennines (Italy) which have been perturbed to various extents by an
independently constrained increase in relative uplift rate. The transportâlimited model is
unable to account for the catchmentsâresponse to the increase in uplift rate, consistent with
the observed low rates of sediment supply to the channels. Instead, a detachmentâlimited
model with a threshold corresponding to the fieldâderived median grain size of the
sediment plus a slopeâdependent channel width satisfactorily reproduces the overall
convex long profiles along the studied rivers. Importantly, we find that the prefactor in the
hydraulic scaling relationship is uplift dependent, leading to landscapes responding faster
the higher the uplift rate, consistent with field observations. We conclude that a slopeâ
dependent channel width and an entrainment/erosion threshold are necessary ingredients
when modeling landscape evolution or mapping the distribution of fluvial erosion rates in
areas where the rate of sediment supply to channels is low
Isolation of a Campylobacter lanienae-like Bacterium from Laboratory Chinchillas (Chinchilla laniger)
Routine necropsies of 27 asymptomatic juvenile chinchillas revealed a high prevalence of gastric ulcers with microscopic lymphoplasmacytic gastroenteritis and typhlocolitis. Polymerase chain reaction (PCR) analysis using Campylobacter genus-specific partial 16S rRNA primers revealed the presence of Campylobacter spp. DNA in the faeces of 12 of 27 animals (44.4%). Species-specific partial 16S rRNA PCR and sequencing confirmed that these animals were colonized with Campylobacter lanienae, a gram-negative, microaerophilic bacterium that was first identified on routine faecal screening of slaughterhouse employees and subsequently isolated from faeces of livestock. Campylobacter lanienae was isolated from the faeces of six PCR-positive animals and identified with species-specific PCR and full 16S rRNA sequencing. Phylogenetic analysis showed that these isolates clustered with C. lanienae strain NCTC 13004. PCR analysis of DNA extracted from gastrointestinal tissues revealed the presence of C. lanienae DNA in the caecum and colon of these chinchillas. Gastrointestinal lesions were scored and compared between C. lanienae-positive and C. lanienae-negative animals. There was no correlation between colonization status and lesion severity in the stomach, liver, duodenum, or colon. Possible routes of C. lanienae infection in chinchillas could include waterborne transmission and faecalâoral transmission from wild mice and rats or livestock. Based on these findings, the authors conclude that C. lanienae colonizes the lower bowel of chinchillas in the absence of clinical disease. This is the first report of C. lanienae in any rodent species. Campylobacter lanienae isolates from different mammalian species demonstrate heterogeneity by 16S rRNA sequence comparison. Analysis using rpoB suggests that isolates and clones currently identified as C. lanienae may represent multiple species or subspecies.Ruth L. Kirschstein National Research Service AwardNational Institutes of Health (U.S.) (Grant R01-OD011141)National Institutes of Health (U.S.) (Grant T32-OD007036)National Institutes of Health (U.S.) (Grant P30-ES02109
Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath
Landslide hazard motivates the need for a deeper understanding of the events that occur before, during, and after catastrophic slope failures. Due to the destructive nature of such events, in situ observation is often difficult or impossible. Here, we use data from a network of 58 seismic stations to characterise a large landslide at the Askja caldera, Iceland, on 21 July 2014. High data quality and extensive network coverage allow us to analyse both long- and short-period signals associated with the landslide, and thereby obtain information about its triggering, initiation, timing, and propagation. At long periods, a landslide force history inversion shows that the Askja landslide was a single, large event starting at the SE corner of the caldera lake at 23:24:05âŻUTC and propagating to the NW in the following 2âŻmin. The bulk sliding mass was 7â16âŻĂâŻ1010âŻkg, equivalent to a collapsed volume of 35â80âŻĂâŻ106âŻm3. The sliding mass was displaced downslope by 1260âŻÂ±âŻ250âŻm. At short periods, a seismic tremor was observed for 30âŻmin before the landslide. The tremor is approximately harmonic with a fundamental frequency of 2.3âŻHz and shows time-dependent changes of its frequency content. We attribute the seismic tremor to stick-slip motion along the landslide failure plane. Accelerating motion leading up to the catastrophic slope failure culminated in an aseismic quiescent period for 2âŻmin before the landslide. We propose that precursory seismic signals may be useful in landslide early-warning systems. The 8âŻh after the main landslide failure are characterised by smaller slope failures originating from the destabilised caldera wall decaying in frequency and magnitude. We introduce the term "afterslides" for this subsequent, declining slope activity after a large landslide
Case report: First case of neuromelioidosis in Europe: CNS infection caused by Burkholderia pseudomallei
Neuromelioidosis is a rare CNS infection caused by Burkholderia pseudomallei and is characterized by high morbidity and mortality. Our report presents the diagnostic and therapeutic approach of the first case of neuromelioidosis confirmed in Europe. A 47-year-old man with a medical history of recurrent otitis with otorrhea and fever after tympanoplasty and radical cavity revision operation on the left ear was admitted with headache, decreased level of consciousness, dysarthria, left-sided hemiparesis, and urinary incontinence. After extensive investigations including MRI, microbiological, serological, and CSF analyses, and, ultimately, brain biopsy, a diagnosis of neuromelioidosis was established. Despite antibiotic treatment, the patient showed no clinical improvement and remained in a severely compromised neurological state under mandatory mechanical ventilation. Neuromelioidosis can pose a diagnostic challenge requiring an extensive diagnostic evaluation because of its uncommon clinical and radiological presentations
A discrete random model describing bedrock profile abrasion
We use a simple, collision-based, discrete, random abrasion model to compute
the profiles for the stoss faces in a bedrock abrasion process. The model is
the discrete equivalent of the generalized version of a classical, collision
based model of abrasion. Three control parameters (which describe the average
size of the colliding objects, the expected direction of the impacts and the
average volume removed from the body due to one collision) are sufficient for
realistic predictions. Our computations show the robust emergence of steady
state shapes, both the geometry and the time evolution of which shows good
quantitative agreement with laboratory experiments.Comment: 9 pages, 6 figure
Geotechnical controls on erodibility in fluvial impact erosion
Bedrock incision by rivers is commonly driven by the
impacts of moving bedload particles. The speed of incision is modulated by
rock properties, which is quantified within a parameter known as erodibility
that scales the erosion rate to the erosive action of the flow. Although
basic models for the geotechnical controls on rock erodibility have been
suggested, large scatter and trends in the remaining relationships indicate
that they are incompletely understood. Here, we conducted dedicated
laboratory experiments measuring erodibility using erosion mills. In
parallel, we measured uniaxial compressive strength, tensile strength,
Young's modulus, bulk density, and the Poisson's ratio for the tested
lithologies. We find that under the same flow conditions, erosion rates of
samples from the same lithology can vary by a factor of up to 60. This
indicates that rock properties that may vary over short distances within the
same rock can exert a strong control on its erosional properties. The
geotechnical properties of the tested lithologies are strongly
cross-correlated, preventing a purely empirical determination of their
controls on erodibility. The currently prevailing model predicts that
erosion rates should scale linearly with Young's modulus and inversely with
the square of the tensile strength. We extend this model using
first-principle physical arguments, taking into account the geotechnical
properties of the impactor. The extended model provides a better description
of the data than the existing model. Yet, the fit is far from satisfactory.
We suggest that the ratio of mineral grain size to the impactor diameter
presents a strong control on erodibility that has not been quantified so
far. We also discuss how our laboratory results upscale to real landscapes
and long timescales. For both a revised stream power incision model and a
sediment-flux-dependent incision model, we suggest that long-term erosion
rates scale linearly with erodibility and that, within this theoretical
framework, relative laboratory measurements of erodibility can be applied at
the landscape scale.</p
- âŠ