1,721,041 research outputs found
Magnetostratigraphic evidence that "tiny wiggles" in the oceanic magnetic anomaly record represent geomagnetic paleointensity variations
No full textMagnetostratigraphy of Eocene-Oligocene boundary sections in Umbria Italy: no evidence for short subchrons within chrons 12R and 13R
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Magnetic properties of Pleistocene sediments from Mediterranean sea: a correlation with paleoclimatic record
No full textThe magnetic properties of the late Quaternary hemipelagic sediment from piston core BS77-15 taken in the Tyrrhenian Sea were investigated and the results compared with palaeoclimatic records. The magnetic mineralogy of the sediment was studied and the magnetic carriers were identified by their spectra of coercivity and unblocking temperature. Investigations of rock magnetic parameters such as the Natural Remanent Magnetization (NRM), the low-field magnetic susceptibility x, the frequency-dependent susceptibility xm, the acquisition of Anhysteretic Magnetization (ARM) and Isothermal Magnetization (IRM) and the Median Destructive Field (MDF) have established the downcore variations in terms of magnetic mineralogy, grain size and coercivity in the sediments. The rock magnetic parameters were compared with an alternative climatic record from the same core based on the faunal association of planktonic foraminifera. A significant correlation was found between faunal climatic record and xm. Together with the magnetic properties of the sediment this suggests that a palaeoclimatic signal is recorded by a fine-grained low-coercivity fraction of the magnetic minerals
Stratigrafia magnetica ad alta risoluzione del limite Eocene/Oligocene nella successione Umbro-Marchigiana
No full textHigh-resolution magnetostratigraphy of the Eocene-Oligocene boundary in the Umbro-Mar- chean Appenine. High-resolution magnetostratigraphy across the Eocene-Oligocene boundary has been employed in a detailed investigation of the nature of low-amplitude, short-wavelength oceanic magnetic anomalies. A core, 39.4 m long and 10 cm in diameter, was drilled through the Eocene-Oligocene boundary near to the Massignano Quarry stratotype section near Ancona, Italy. The stratigraphy of the core, which traverses the Scaglia Variegata and Scaglia Cinerea formations, was correlated precisely to the quarry section by linear regression of the depths of identifiable biotite-rich layers. The good recovery of intact material allowed an average sampling interval of about 12 cm, which is closer than in preceding magnetostratigraphic studies of Umbrian-Marche sequences. The characteristic remanent magnetization was obtained by both progressive alternating field and thermal demagnetizations. The stable component of the natural remanent magnetization could be isolated by thermal demagnetization at temperatures of 300-540 ◦ C or by alternating field demagnetization in fields higher than 20 mT. It is probably carried by magnetite in the Scaglia Cinerea marls, while some amount of hematite is present in the underlying Scaglia Variegata. A stratigraphic plot of the ChRM directions shows well-defined magnetozones and the resulting polarity sequence correlates well with polarity chrons C12r to C16n-2. A few single-sample normal magnetozones that do not correspond to the geomagnetic polarity timescale are found within chron 16n.1-r. The magnetozones corresponding to chrons C12r or C13r do not exhibit short subchrons that might account for the low-amplitude and short-wavelength magnetic anomalies reported in this part of the marine magnetic record. In investigation of relative paleointensity fluctuations has been carried out in this part of the core, which embraces the Scaglia Cinerea formation. Anhysteretic remanent magnetization (ARM) has been used to normalize the natural remanent magnetization (NRM), compensating variations in sedimentary input. The ensuing NRM/ARM ratio is taken to be a proxy for relative variation of paleomagnetic field intensity. The paleointensity fluctuates systematically and has minimum values close to the reported positions of low-amplitude, short-wavelength magnetic anomalies in the marine record
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Magnetostratigraphy and Biostratigraphy of the Carnian/Norian Boundary Interval from the Pizzo Mondello Section (Sicily)
Full text linkWe present new magnetostratigraphic and biostratigraphic data from an Upper Triassic limestone section named Pizzo Mondello. This section is 141m-thick and crops out in the Sicani Mountains of western Sicily (Italy). The Pizzo Mondello section is one of the Tethyan best in virtue of its high quality of exposure, accessibility, stratigraphic continuity and good magnetostratigraphic data. We recognize a sequence of six magnetozones, from M1 to M6, each subdivided into a lower predominantly normal and an upper predominantly reversed magnetozone. This sequence of magnetozones spans the Carnian/Norian boundary according to conodont biostratigraphy. The Carnian/Norian boundary should fall in the upper part of magnetozone M3 which should correspond to chron E14 in the Newark reference sequence of polarity reversals. The comparison of data from the Newark basin, Pizzo Mondello and the Late Triassic Bolcektasi Tepe section from the literature seems to suggest the existence of a disconformity in the Tethyan marine domain at around the Carnian/Norian boundary which we tentatively relate to intraPangea tectonic mobility. A consistent correlation of Newark/Pizzo Mondello with published data from the Norian Tethyan marine Kavur Tepe and Scheiblkogel sections is still elusive, suggesting that the validity of the original polarity interpretation of the Kavur Tepe section could usefully be critically reviewed
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