Noble gases atmospheric mixing ratio

As partly mentioned before, natural and anthropogenic induced variations of the atmospheric C02 concentration and of the 14C/ 12C and 13C/12C ratios have been observed. For a quantitative discussion of these variations in relation to possible causes, models for the carbon cycle dynamics have been developed [21-25]. Compared to the noble gas radioisotopes 39Ar and 81Kr, for which we only have to consider a well mixed atmospheric reservoir, we have a much more complicated system for 14C. The C02 in the atmosphere exchanges with the carbon in the biosphere and with the... 

The observed noble-gas abundances and isotopic ratios on Venus are summarized in Tables 3 and 4. The helium mixing ratio is a model-dependent extrapolation of the value measured in Venus upper atmosphere, where diffusive separation of gases occurs. The main differences between Venus and Earth are that Venus is apparently richer in He, Ar, and Kr than the Earth, and the low " Ar/ Ar ratio of — 1.1 on Venus, which is —270 times smaller than on Earth. The low " Ar/ Ar ratio may reflect more efficient solar-wind implantation of Ar in solid grains accreted by Venus and/or efficient early outgassing that then stopped due to the lack of plate tectonics. Wieler (2002) discusses the noble-gas data. Volkov and Frenkel (1993) and Kaula (1999) describe implications of the " Ar/ Ar ratio for outgassing of Venus. 

The principal difficulty encountered by these mixing models in their present form is their inability to account for differences in nonradiogenic noble gas isotopic distributions between Earth and Mars, and between both of these and solar compositions (Swindle 2002 Wieler 2002), which imply processing of primordial atmospheres by isotopically fractionating mechanisms. As noted above, solar isotope ratios would be expected in the ambient nebular gases surrounding accreting cometary matter. Recent experiments... 

Figure 9. A variety of mantle models for noble gas distributions within the mantle, with He fluxes shown as arrows. Dark mantle regions provide He with high He/ He ratios. In the limited interaetion models (A), this region eomprises the lower mantle, convectively isolated by the 670-km boundary. Upper mantle eharaeteristies refleet progressive depletion to form the atmosphere. In the steady state models (B), there is also layered eonvection, but the upper mantle composition is due to mixing of lower mantle, subducted, and radiogenic components. Mantle convection across the 670-km discontinuity is accomodated in the remaining models, where the high He/ He ratios reside in heterogeneities or deeper layers (C) a lower boimdary layer of residual depleted mantle (RDM) of subducted oceanic lithosphere (D), or in the core (E). See text for references and detailed discussion.

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