1,720,971 research outputs found
The role of water-rock ratio and temperature in the isotopic alteration of flysch rock in the Tolfa Mountain mining district (Latium, Central Italy)
No full textCarbon and oxygen isotope analyses have been performed on calcium carbonate samples from flysch rocks collected in the south of Tolfa and Allumiere villages (Central Italy). Large variation it) delta(18)O occurs from values around +28 per mil (SMOW), typical of marine carbonates, down to values of about +9 per mil (SMOW). The lower values are measured on samples collected close to phanerocrystalline carbonate deposits outcropping in the area and interpreted as part of a hydrothermal vein system. delta(13)C does not show large variation, and, aside from a few outliers, falls within a range of 0 and +2 per mil (PDB). Similar carbon and oxygen isotope spatial relationships between the flysch country rocks and the phanerocrystalline carbonate deposits were determined in a previous study by FERRINI & MASI (1987), who interpreted the O-18 depletion as due to interaction of the host rock with an aqueous fluid at hydrothermal temperatures below 250degreesC and with a water to rock ratio progressively increasing toward the carbonate vein. The combined results indicate that hydrothermal fluid temperature largely regulates oxygen isotope variation (and possibly the carbon and oxygen deltas co-variation) with respect to the variation of the water to rock ratio
Major constraints on the use of radiocarbon dating for tephrochronology
No full textIt is well recognized that radiocarbon dating is a powerful tool capable of providing a detailed chronostratigraphy for areas that have been subjected to recurrent volcanic activity during Late Pleistocene and Holocene time. In such geological contexts carbon-bearing materials suitable for 14C dating are, for the most part, refractory componetns of pedosphere (e.g. palaesol humic matter) and organic detritus (e.g. charred/uncharred remains of higher plants) which occur within the tephra suites. Since such materials derive initially from atmospheric carbon dioxide, as well as record significant lapses in volcanic activity, it is then often assumed that these conditions comply ideallly with the fundamental requirements of the radiocarbon dating theory. However, two recent 14C dating programmes based on materials from the volcanic districts of the Phlegrean Fields and the island of Procida (Campania, southern Italy) yielded evidence to the contrary and, in turn, gave rise to concern over the validity of the 14C dating method when applied in volcanic regions. In particular, it has been found that the conventional radiocarbon ages recorded by modern tree-leaves from the caldera of Solfatra volcano range from 'modern' to ca. 5000 yr BP. Furthermore, the magnitude of the age anomaly is determined by the pattern of localised dilution of the mean atmospheric 14C concentration by juvenile (14C-free and 13C-enriched) carbon dioxide issuing from nearby volcanic vents. In the case of paleosols, a series of 14C measurements on different chemical fractions isolated from the humic matter in a suite of four paleosol horizons entrained in tephra deposits at Procida island yielded, for each paleosol horizon, a systematic ca. 2000 yr scatter of 14C ages. It is shown that such intrasoil age differences, which reflect the origin and history of humic constituents, can be used to afford a better definition of the true paleosol age
Parte II Capitolo III: La determinazione della provenienza dei marmi usati in architettura a Ostia.
No full textNel Cap. III si è effettuata la detereminazione della provenienza dei marmi appartenenti ad elementi architettonici, prendendo in esame gli edifici di Ostia Antica. L'indagine archeometrica è stata condotta utilizzando l'analisi petrografica unitamente allo studio della composizione isotopica di ossigeno e carbonio tramite spettrometria di massa. Irisultati sono riportati nelle tabell
Geochemical evidences for biased radiocarbon ages yielded by multisource organic matter scattered in sediments.
No full textHistoric-artistic and archaeometric studies of the sculptures of the museum of Ostia
No full textAn archaeometric study was conducted on 21 marble sculptures belonging to the collection of the Museum of Ostia. The sculptures consist of statues, reliefs, portraits, and architectural freizes from public buildings, private residences and funerary monuments. The study, aimed at determining the provenance of the marbles used, is based upon the measurement of the oxygen and carbon isotopic composition and the evaluation of the macroscopic features of the marble of the various items, corroborated by the historic-artistic information available. The results obtained indicate that the majority of sculptures are of Luni marble and were very likely crafted by local workshops. Parian and Thasian marbles, two of the finest Greek marbles, were also used; most artefacts made of Parian marble were imported, but the two sculptures of Thasian marble so far studied were probably carved by urban workshops. Relatively limited seem the use of marbles from Anatolia. Our data suggest that at Ostia the marbles from Luni (Carrara) and, to a lesser extent, Paros were the most widely used varieties; only few artefacts carved in marbles from other Classical sources have been found so far in the collection of the Museum. However, more data are needed to confirm this indication
Capitolo II: Lidentificazione dei marmi di Cava di Porto, della Fossa Traiana e del deposito del tempio dei fabri navales a ostia.
No full textCapitolo IV: La determinazione della provenienza dei marmi usati nella statuaria e nei rilievi.
No full text
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
- …
