Search results for " Boundary"
showing 10 items of 686 documents
Stage boundaries, global stratigraphy, and the time scale: towards a simplification
2004
International audience; This paper examines four facets of stratigraphic terminology and usage considered faulty and proposes corrective measures. The four perfectible areas are: (1) The system of dual nomenclature requiring discrete terminologies for the superpositional and temporal aspects of rock units. (2) The premise that a GSSP establishes the base of a stage as being coincident with the top of the preceding stage rather than simply defining it as the boundary between stages. (3) The rejection of supplementary (auxiliary) sections that would broaden the knowledge of a GSSP and enlarge the area in which it is easily usable. (4) The current dual system of nomenclature for Precambrian an…
Chronostratigraphy and geochronology: A proposed realignment.
2013
We propose a realignment of the terms geochronology and chronostratigraphy that brings them broadly into line with current use, while simultaneously resolving the debate over whether the Geological Time Scale should have a “single” or “dual” hierarchy of units: Both parallel sets of units are retained, although there remains the option to adopt either a single (i.e., geochronological) or a dual hierarchy in particular studies, as considered appropriate. Thus, geochronology expresses the timing or age of events (depositional, diagenetic, biotic, climatic, tectonic, magmatic) in Earth’s history (e.g., Hirnantian glaciation, Famennian-Frasnian mass extinction). Geochronology can also qualify r…
Challenges in defining the base of Cambrian Series 2 and Stage 3
2017
Formal subdivision of the Cambrian System into four series and ten stages is in progress. The base of Cambrian Stage 3 (provisional), which is conterminous with the base of Cambrian Series 2 (provisional), is expected to be placed at a horizon close to the first appearance of trilobites, which marks the onset of the largest phase of the Cambrian explosion. Conceptually, an ideal boundary position would be marked by a significant and globally recognizable bioevent that divides the lower part of the Cambrian System into a sub-trilobitic Terreneuvian Series and a trilobite-dominated Series 2. If the level is to be identified principally through biostratigraphic means, its position also needs t…
The end-Triassic mass extinction: A new correlation between extinction events and δ13C fluctuations from a Triassic-Jurassic peritidal succession in …
2018
Abstract A new δ13Ccarb curve was obtained from an expanded peritidal succession in western Sicily and was used to investigate the relationships between isotopic signatures and biological events on carbonate platforms across the Triassic-Jurassic boundary (TJB). The resulting curve shows two main negative carbon isotopic excursions (CIEs) that fit well with the “Initial” and “Main” CIEs that are recognized worldwide and linked to the End-Triassic Extinction (ETE). In the studied section, the first negative CIE marks the disappearance of the large megalodontids, which were replaced by small and thin-shelled specimens, while the “Main” CIE corresponds to the last occurrence (LO) of the megalo…
Planktonic foraminifera biozones and isotope stratigraphy of the Cenomanian-Turonian boundary in the western part of the Kopet-Dagh Basin, NE Iran
2018
Abstract A biostratigraphic study based on the planktonic foraminifera and isotope stratigraphy of the Cenomanian-Turonian interval has been undertaken in the western part of the Kopet-Dagh Basin (NE Iran). In this research, two stratigraphic sections including: Maraveh Tappeh and Qareh Galdy have been sampled and studied. Based on planktonic foraminifera, three biozones were assigned to this interval. These are the Rotalipora cushmani Zone, Whiteinella archaeocretacea Zone and Helvetoglobotruncana helvetica Zone, These biostratigraphic data confirm a gradual faunal turnover of planktonic foraminifera, at the Cenomanian-Turonian boundary in Whiteinella archaeocretacea Zone characterized by:…
Diffusion through thin membranes: Modeling across scales
2016
From macroscopic to microscopic scales it is demonstrated that diffusion through membranes can be modeled using specific boundary conditions across them. The membranes are here considered thin in comparison to the overall size of the system. In a macroscopic scale the membrane is introduced as a transmission boundary condition, which enables an effective modeling of systems that involve multiple scales. In a mesoscopic scale, a numerical lattice-Boltzmann scheme with a partial-bounceback condition at the membrane is proposed and analyzed. It is shown that this mesoscopic approach provides a consistent approximation of the transmission boundary condition. Furthermore, analysis of the mesosco…
The moral work of becoming a professional
2021
Abstract In contemporary working life, art-based initiatives are increasingly used in organizational training and development. For artists, this has created new employment opportunities as creative entrepreneurs who provide specialist services for workplaces. In this article, we study the dynamics of such encounters through the narrated accounts of training professionals. Our data come from a professional mentoring program where the working pairs of artists and consultants shared stories about their customer projects. By using conversation analysis as a method, we analyze the way stories are interactionally accomplished in peer group sessions of the program. In particular, we analyze how pa…
Characterizing the initial conditions of heavy-ion collisions at the LHC with mean transverse momentum and anisotropic flow correlations
2022
Physics letters / B 834, 137393 (2022). doi:10.1016/j.physletb.2022.137393
Accretionary orogens through Earth history
2009
Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in th…
A NEW SYMMETRIC AND POSITIVE DEFINITE BOUNDARY ELEMENT FORMULATION FOR LATERAL VIBRATIONS OF PLATES
1997
Abstract A new symmetric and positive definite boundary element method in the time domain is presented for the dynamic analysis of thin elastic plates. The governing equations of the problem are obtained from a variational principle in which a hybrid modified functional is employed. The functional is expressed in terms of the domain and boundary basic variables in plate bending, assumed to be independent of each other. In the discretized model the boundary variables are expressed by nodal values, whereas the internal displacement field is modelled by a superposition of static fundamental solutions. The equations of motion are deduced from the functional stationarity conditions and they cons…