Nitrogen isotopes atmosphere

Figure 2.6 Carbon- and nitrogen-isotopic compositions of presolar SiC grains. Predictions from stellar models are shown for comparison. Solar metallicity AGB star models Nollett et al. (2003), Type II SN Rauscher et al. (2002), novae Jose et al. (2004). For data sources see Lodders Amari (2005) Zinner (2007). Note that for the solar 14N/15N ratio the value inferred for Jupiter s atmosphere is shown.

The atmosphere of Mars has a high value of = (-1-620 160)%c (Nier and McElroy, 1977) that may be due to fractionating losses (see section 7 in Chapter 4.12), and (4.7 1.2) X 10 g N, which is equivalent to 0.7 ppb when divided into the mass of the entire planet (Owen et al, 1977). Therefore, Mars appears to have 10 " times the nitrogen on the Earth. Mathew et al (1998) reported evidence for a component in a martian meteorite with < —22%c, suggesting that, like the Earth, the solid planet may contain nitrogen that is isotopically lighter than the atmosphere, and consistent with the modification of atmospheric nitrogen isotopes by losses. 

Junk G. and Svec H. (1958) The absolute abundance of the nitrogen isotopes in the atmosphere and compressed gas from various sources. Geochim. Cosmochim. Acta 14, 234-243. 

Beaumont V. and Robert F. (1999) Nitrogen isotope ratios of kerogens in Precambrian cherts a record of the evolution of atmosphere chemistry Precamb. Res. 96(1-2), 63-82. 

Sulfur isotope studies have also provided insights into the transition from Archean low Po to higher values in the Proterozoic. In the same studies that revealed extremely low Archean ocean sulfate concentrations, it was found that by —2.2 Ga, isotopic compositions of sedimentary sulfates and sulfides indicate bacterial sulfate reduction under more elevated seawater sulfate concentrations compared with the sulfate-poor Archean (Habicht et al., 2002 Canfield et al., 2000). As described above, nitrogen isotope ratios in sedimentary kerogens show a large and permanent shift at —2.0 Ga, consistent with denitrification, significant seawater nitrate concentrations, and thus available atmospheric O2. 

Stable nitrogen isotope compositions are useful tracers of both the source of N to the biomass and the history of the organic nitrogen. The of dissolved N2 in surface waters (c.+ 0.6%o) is slightly enriched compared with atmospheric N2 (0%o) because the heavier gas molecules have a lower vapor pressure and are more soluble in water. In contrast, dissolved NO3 (by far the most abundant form of oxidized inorganic nitrogen in the sea) exhibits values that... 

Schematic diagram of the stable nitrogen isotope ratios of different nitrogen reservoirs in the sea. The general range of stable isotope ratios (with respect to the atmosphere) found in nature is given in the boxes and the difference fractionation factors e (in %o) accompany arrows between the boxes. Many of the values are approximations because of the wide variations of observations. See Table 5.3 for more details of some of the reactions and the text for explanation. Values are based on data presented by Altabet and Small (1990), Altabet and Francois (1994) and Sigman and Casciotti (2001).

Another type of radioactivity, known as nuclear transmutation, results from the bombardment of nuclei by neutrons, protons, or other nuclei An example of a nuclear transmutation is the conversion of atmospheric to gC and H, which results when the nitrogen isotope captures a neutron (from the sun). In some cases, heavier elements are synthesized from hghter elements. This type of transmutation occurs naturally in outer space, but it can also be achieved artificially, as we will see in Section 23.4. 

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