1,539 research outputs found
High resolution carbon isotope stratigraphy of the basal Silurian stratotype (Dob's Linn, Scotland) and its global correlation
Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dob's Linn, Southern Scotland, has received criticism due to the difficulties of relating its well constrained graptolite biostratigraphy to shallow water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dob's Linn have yielded carbon stable isotope signatures consistent with those recorded elsewhere, in particular showing a clear positive ÎŽ13C excursion in the terminal Ordovician. The architecture of the ÎŽ13C curve from Dob's Linn enables very high resolution stratigraphic subdivision and direct correlation between the deep water Dob's Linn section and time equivalent carbonate shelf deposits. An integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites, conodonts and shelly faunas has been constructed. This direct correlation shows that the shallow water successions, including the former stratotype candidate at Anticosti Island, are generally incomplete, with hiatuses related to the rapid sea level changes during the Hirnantian stage. This confirms and greatly increases the global utility of Dob's Linn as a boundary stratotype
Ultrasound by emergency physicians to detect abdominal aortic aneurysms: a UK case series
Early identification of abdominal aortic aneurysms in some
patients can be difficult and the diagnosis is missed in up to
30% of patients. Ultrasound cannot be used to identify a leak,
but the presence of an aneurysm in an unstable patient is
conclusive. With minimal training emergency physicians can
easily identify the aorta and thus in the early phase of
resuscitation an aneurysm can be confidently excluded. The
purpose of the examination is not to delineate the extent of
the aneurysm, but to identify those patients that will need
emergency surgery. A series of patients presented to the
department in an unstable condition with equivocal abdominal
signs. An ultrasound scan in the resuscitation room by
members of the emergency department revealed an aneurysm,
which was enough to convince the vascular surgeons to
take the patient straight to theatre with good results. In
patients who are stable, computed tomography will continue
to be used to evaluate the extent of the aneurysm and identify a leak
Statoviruses, a novel taxon of RNA viruses present in the gastrointestinal tracts of diverse mammals
Formation of misfit dislocations in strained-layer GaAs/In<sub>x</sub>Ga<sub>1âx</sub>As/GaAs heterostructures during postfabrication thermal processing
It is demonstrated that relaxation of GaAs/InxGa1âxAs/GaAs strained-layer heterostructures can be brought about by postfabrication thermal processing. Misfit dislocations are introduced into the structure during thermal processing, even though the thickness of the strained layer is well below the critical value predicted by the MatthewsâBlakeslee model. The misfit dislocations are observed to be of both 60° mixed type and 90° pure edge type. As no relaxation occurs at the lower temperatures encountered during fabrication by molecular-beam epitaxy, it can be inferred that the critical condition for the formation of misfit dislocations is not only a function of strained-layer thickness and composition, but also of temperature. This observation cannot be accounted for by differential thermal expansion or diffusion across the strained-layer interfaces, but the temperature-dependent Peierls force may offer an explanation. The high temperature required to produce relaxation of these structures suggests that they are sufficiently thermally stable for most practical applications
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