Noble gases occurrence

Silylenes are short-lived intermediates, and their detection requires fast methods such as ultraviolet (UV)24 or laser-induced fluorescence spectroscopy.25 The characteristic absorption maxima in the UV-visible spectra of these species, which are assigned to n - p transitions of electrons at the silicon atom, were used as a fingerprint to prove the occurrence of silylenes in matrices or solution. In addition, these transient species, which under normal conditions are too short lived to be observed by a slow detection method such as infrared (IR) spectroscopy, can be isolated in inert hydrocarbon or noble gas matrices, thus allowing the accurate measurement of their IR spectra. 

To obtain information on the noble gas state in the mantle, it is necessary to analyze mantle-derived materials that have trapped mantle noble gases. Accessible samples include volcanic rocks, volcanic gases, mantle xenoliths, and diamonds. Among various mantle-derived materials, submarine volcanic rocks are particularly useful because of their wide occurrence and their relatively large (for mantle samples) amounts of trapped noble gases. So far, information has been obtained mainly from... 

Noble-gas geochemistry is a data-rich field, and the discussion above does not do justice to all the data. There are additional occurrences of elemental and/or isotopic patterns which may be indicative of still other significant noble-gas components. Except for one particularly significant case, U-Xe, which is discussed in the following section, the scope of this chapter does not permit adequate description (but see Ott (2002), for a more detailed review). 

Based only on He and Ne data, the appropriate value of Lex cannot be determined. Additional information from other noble gas concentrations or isotope ratios is needed to distinguish between the different models and to determine the model parameters (Peeters et al. 2002). However, up to now only very few H- He studies of shallow ground waters provide such additional information (Beyerle et al. 1999a Holocher et al. 2001). In all previous studies, only He and Ne were measured and the assumption Lex = Lair was applied. Because Lair is actually only the upper limit of Lex, this approach tends to overestimate the atmospheric and thus to underestimate the non-atmospheric He components. A clear sign that this approach is not always appropriate is the common occurrence of negative values for " Heter or even Hetri. 

The occurrence of thermal diffusion in nature was first observed by Severinghaus et al. (1996) for soil gas in sand dunes. The fractionation of soil air by water vapor diffusion, gravitational settling, and thermal diffusion studied by these authors may also have a small effect on dissolved noble gases in ground water, and hence the calculation of noble gas paleotemperatures. However, these comparatively minor effects have not yet been further studied in the soil-air-ground-water system. 

There has been much interest in the injuries to plants from environmental S02 emissions. These injuries may be caused by aqueous S02 or by the entrance of S02 through the stomata (Taekemoto and Noble, 1986). The general symptom of S02 toxicity is water-soaked areas on the leaves which develop into well-defined dry white necrotic spots (Mansfield and Lucas, 1990). Although acute S02 damage is quite common, the occurrences are usually very localised near the source, and related to weather conditions favouring slow dispersion of the gas. 

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