1,721,121 research outputs found
A new application of 238U series in carbonates dating: 226Ra chronology of a Holocenic freshwater (uranium poor) travertine
No full textGeocronologia radiometrica dei depositi travertinosi posti al tetto dell’unità di Roccarespampani col metodo del Th-230/U-234. Procedura L/L.
No full textRadiogenic isotope, cosmogenic radionuclides and radiation damage methods applied to the dating of continental carbonates
No full text""Speleothems, fresh-water tufa and travertines are generally dated using methods based on uranium-series disequilibria, but also on cosmogenic radionuclides and radiation damage. The basic theory of these methods is introduced and related applications in the study of continental carbonates are shown with examples derived from recent research.. U-Th is the most popular method and is based on the measurement of 230Th formed in situ by radioactive decay of uranium (234U and 238U) co-precipitated with CaCO3. In the absence of detrital minerals (see for example pure speleothems) the extent to which the 230Th\\\/234U activity ratio has returned towards unity is a function of time, taking into account also the state of disequilibrium between 234U and 238U. Tufa and travertines, however, are impure mixtures of calcium carbonate and incorporated detrital minerals and deserve the recourse to specific correction schemes to determine the authigenic radionuclide component and thus a fitting age. The method extends back to at least 350,000 years. The disequilibria between other parent-daughter pairs in 238U and 235U decay chains (namely, 234U\\\/238U, 231Pa\\\/235U, 226Ra\\\/230Th) are also used in carbonate dating. 234U\\\/238U method can be applied to dating carbonates as old as 1.25 Ma, assuming that the uranium activity ratio in the parent fluids has remained constant over time. 226Ra\\\/230Th method is used to date continental carbonates younger than 8 ka, presupposing that 226Ra\\\/230Th initial activity ratio can be determined on present-day deposition. Recently, the use of the classic U-Pb method has been extended to speleothems dating, opening new perspectives on the study of older speleothems, not datable using standard U-series methods.. The use of radiocarbon dating in carbonate chronology is considered problematic, principally because of potential variability in the contribution of “dead carbon” from the host limestone. Actually, precipitating waters may contain a proportion of carbon inherited from the dissolution of a 14C-free carbonate component in the bedrock. Another problem could arise from the contamination of the material with younger carbon from soil organic matter that might be deposited in the original structure of porous carbonate. In addition to that, bomb-14C may affect modern precipitation, leading to an inconsistently too young age. Because of these problems, relatively few speleothem chronologies are based on 14C ages. . Radiation damage dating methods (electron spin resonance, thermoluminescence, optical stimulated luminescence) are based on the time-dependent accumulation of electrons in the crystal lattice of certain common minerals, acting as natural radiation dosimeters. Assuming a constant radiation dose, the extent of trapped electrons is proportional to the accumulated radiation and to the time elapsed since a “zeroing” event. Unfortunately, not all the samples are suited for radiation damage dating: indeed, the presence of cationic impurities such as Mn2+, Fe2+, or Fe3+, humic acids (organic matter), can mask the signal of interest, or interfere with it. These methods can be applied to date materials ranging in age from a few hundreds years to about a million year.. . "
Palaeoclimate and Palaeoenvironmental reconstructions from speleothems in artificial caves (Lazio, Italy)
No full textPalaeoclimatic and tectonic evolution of Acque Albule basin (Tivoli, Roma, Italy): geochronological and structural constraints on travertine deposition.
No full text
Variazioni delle concentrazioni di radon indoor in un edificio a tre piani costruito sui depositi alluvionali del Tevere (Roma). Risultati preliminari
No full textCorrecting for CO2 interference in soil radon flux measurements
No full textIn the present paper the interference of high CO2 concentration in soil radon flux measurement using a solid state silicon detector is
discussed. A possible correction, making use of a chemical trap consisting of soda lime pellets, is successfully experimented at Cava dei Selci
site (Roma, Italy). The interference is likely due to neutralization processes of 218Po ions in the CO2 dominant atmosphere occurring in the
chamber of radon monitor and resulting in a reduced efficiency in electrostatic collection on the surface of the solid state detecto
226Ra chronology of Holocene travertine from Esanatoglia (Central Italy)
No full textHere we investigate the feasibility of using alpha-spectrometric 226Ra methodology to date low U freshwater carbonate deposits
(<0.1 μg g-1) by analysing Holocene freshwater travertine deposits from Esanatoglia, Umbro-Marchean Apennines, Central Italy.
Previously, such methods have been tested on hydrothermal deposits, which are characterised by high Ra and U. We calculate a 226Ra
age estimate of 2.9 ± 0.4 ka (1σ) for Holocene travertine that is in reasonable agreement with archaeological evidence by assuming that
the 226Ra initial activity of the Holocene travertine is comparable with that of present-day carbonate deposits in the same area. We also
investigate the use of Ba to normalise 226Ra activities. Comparison of age estimates based on Ra methods with ages derived from alphaspectrometric
230Th/234U isochron techniques for the same co-eval sub-samples indicates that, where only alpha-spectrometric methods
are available, the former are likely to be much more useful for Holocene travertine material, especially if there is a significant detrital
component
- …
