Search results for "ISOTOPES"
showing 10 items of 1216 documents
Subsecond fluorine-19 MRI of the lung
2006
Minimal scan times in rapid fluorine-19 MRI using sulfur hexafluoride (SF6) have been on the order of 10 s. Because of the very short T1 relaxation time of SF6 (T1 = 1.65 ms), high receiver bandwidths are necessary to allow for a high number of excitations. Since high bandwidths cause high levels of electronic noise, SNR per acquisition has been too low to further reduce scan time. The purpose of this study was to investigate whether scan times could be reduced using hexafluoroethane (C2F6), a gas with a longer T1 (T1 = 7.9 ms) at a relatively low bandwidth of 488 Hz/pixel. Gradient-echo images were acquired during and after completion of the wash-in of a 70% C2F6- 30% O2 mixture. Peak SNR …
A low-level spectrometer with a planar low-energy HPGe: shielding arrangement tests and system performance for 210Pb determination in air filter samp…
2001
A system for low-energy photon spectrometry using a planar germanium detector with appropriate specifications is presented. A spectrometric background investigation has been carried out with various detector shielding arrangements. The characteristics of the system for measurements of 210Pb in air-particulate matter on filters have been determined.
Planktonic foraminifera as proxies of the Holocene climatic variability (Tyrrhenian, Mediterranean Sea)
2017
Introduction. The focus of this study is the paleoclimatic reconstruction of the southern Tyrrhenian between ~9.2 and 2.9 ka, through the study of planktonic foraminiferal assemblages and stable isotopes, and comparing data with other coeval intervals. Several authors have studied the climatic sensitivity of Holocene planktonic foraminifera in different parts of the Mediterranean. Planktonic foraminifera produce good records of Holocene climatic variability, especially as regards the suborbital events such as Bond events (Bond et al., 1997) and other cooling/warming oscillations. Therefore, the obtained eco-biostratigraphy has allowed us to analyze how climatic forcing influenced sea surfac…
The late Messinian "Lago-Mare" event and the Zanclean Reflooding in the Mediterranean Sea: New insights from the Cuevas del Almanzora section (Vera B…
2020
Abstract The return to normal marine conditions in the Mediterranean Sea after the end of the Messinian Salinity Crisis (MSC), that led to the deposition of thick evaporitic succession followed by settlement of brackish to freshwater conditions of the “Lago-Mare”, is still subject to extensive debate between two opposite scenarios. One scenario implies an abrupt reflooding through the Gibraltar gateway of the Mediterranean Sea previously disconnected from the world ocean and partly desiccated. The second scenario postulates that the Mediterranean Sea kept a high-water level throughout the Messinian Salinity Crisis and was connected continuously to the Atlantic Ocean and to the Paratethys. T…
Turonian-Maastrichtian biostratigraphy and isotope stratigraphy of the Kopet-Dagh Basin deposits, northeastern Neo-Tethys, Iran
2021
Abstract Planktonic foraminiferal biostratigraphy and isotope stratigraphy analyses of Turonian-Maastrichtian deposits have been carried out in the western part of the Kopet-Dagh Basin. The identification of the planktonic foraminiferal assemblages has led to the recognition of twelve Upper Cretaceous zones (Helvetoglobotruncana helvetica Zone to the Contusotruncana contusa-Abathomphalus mayaroensis zone) from the Tethyan realm. The obtained stable carbon isotope profile correlates with other reference curves, revealing many Late Cretaceous events including the Hitchwood, Navigation, Light Point, East Cliff, White Fall, Late Campanian Event (LCE), Campanian-Maastrichtian Boundary Event (CMB…
An evaluation of inoceramid single-prism sclerochronology
2020
Abstract Inoceramid prisms are among the most common microfossils that can be found in Late Cretaceous strata. Since these biomineral units were formed by sequential accretion in the outer shell layer of the bivalves, they potentially serve as archives of short-term (circa-annual) paleowater temperature fluctuations. In the present study, we tested to what extent intra-prismatic stable isotope variations of individual inoceramid prisms reflect sub-annual δ13C and the δ18O patterns. We obtained multiple carbonate samples from prisms recovered from the sediment and from a cross-sectioned inoceramid shell fragment and compared data from these samples to data from classical sclerochronological …
Environmental impact of early Basque mining and smelting recorded in a high ash minerogenic peat deposit.
2004
International audience; More than four metres of core, covering almost 5000 years of deposition, were collected in a high ash minerogenic peat deposit located in the High Aldudes valley (Basque country), an area well known for its mineral abundance, exploited from Roman Times at least.Although minerogenic peatlands are not generally considered as the best archives to reconstruct past atmospheric metal deposition history, lead isotopic geochemistry demonstrates the integrity of the Pb record at least within the three upper meters; that is to say over the last four millennia.Zn, Cd and Cumay have been widely redistributed either by biological cycling, advective groundwater movements, or diffu…
Observations of molecular hydrogen mixing ratio and stable isotopic composition at the Cabauw tall tower in the Netherlands
2016
AbstractMeasurements of the stable isotopic composition (δD(H2) or δD) of atmospheric molecular hydrogen (H2) are a useful addition to mixing ratio (χ(H2)) measurements for understanding the atmospheric H2 cycle. δD datasets published so far consist mostly of observations at background locations. We complement these with observations from the Cabauw tall tower at the CESAR site, situated in a densely populated region of the Netherlands. Our measurements show a large anthropogenic influence on the local H2 cycle, with frequently occurring pollution events that are characterized by χ(H2) values that reach up to ≈1 ppm and low δD values. An isotopic source signature analysis yields an apparent…
Geochemical records of limestone façades exposed to urban atmospheric contamination as monitoring tools?
2008
International audience; Magnetic susceptibility, surface rock soiling, elemental composition and lead isotope ratios were measured in surface stone samples collected at different heights of a late 19th century building in Dijon, France. We targeted four limestone facades that differ in orientation and proximity to car traffic. It seems that zinc, copper, sulphur and cadmium are present as diffuse pollutants in urban atmosphere, at least at the scale of the building studied. In contrast, lead and arsenic exhibit point sources: automotive traffic and past coal-burning fly-ash emissions; both coherent with lead isotopic composition measurements. Parameter variations primarily result from expos…
Assessing anthropogenic pressures on coastal marine ecosystems using stable CNS isotopes: State of the art, knowledge gaps, and community-scale persp…
2015
Abstract In recent decades, the analysis of carbon, nitrogen and sulfur stable isotopes (SIA) has emerged as a powerful, viable methodology for examining food web structure and dynamics, as well as addressing a number of applied issues. Here, we provide a state-of-the-art review of the use of SIA for assessing anthropogenic pressures on natural ecosystems, in order to establish current knowledge gaps and identify promising applications for evaluating the ecological status of marine coastal waters. Specifically, the potential of SIA to provide food web-scale indicators for estimating cumulative anthropogenic pressures is addressed. The review indicates that the methodology has been used for …