Carbonate S13C values

As we have already seen, D-enriched macromolecules have a normal 513C value, but it is conceivable that the D-enriched component might come from gas-phase interstellar molecules with a normal S13C value and an abnormal 8D value. On the other hand, some carbon carrier phases could contain both polycyclic aromatic molecules (quasigraphite ) and elemental carbon with all this carbon being abnormal because it forms part of the grains. 

Fig. 7 Stable carbon isotope ratios of 1-chloronaphthalene, dichloronaphtalenes and 1-chloronaphthoic acid methylester as detected in the groundwater samples B and C. In addition the S13C-values of corresponding reference compounds determined prior and after a recovery experiment are presented.

Additionally, significant variations of the standard deviation values were observed with respect to the individual compounds. The precision of S13C-values of n-alkanes, 2,2,4,4,6,8,8-heptamethylnonane, fatty acid methyl esters, phthalates and musk fragrances was generally acceptable down to amounts of approx. 5 ng with standard deviation values below 1.0 %o. On the contrary, already at the 25ng- and lOng-levels the stable carbon isotope ratios of the chlorinated compounds (CI5- and Cf,-benzene, Clfi-butadiene) as well as of tetrabutyl tin were measured with high standard deviations between 0.8 %o and 3.2 %o. 

With respect to the different shifts of S13C-values in the recovery procedure it has to be noted, that a satisfactory determination of carbon isotope ratios depends not only on the analytical methods applied but also on the individual substances analysed. Therefore, in order to evaluate the quality of compound specific isotope analyses of riverine contaminants measurements of field samples have to be accompanied by recovery experiments of individual substances. 

For the interpretation of the distribution of the carbon isotope ratios within the riverine system investigated two different variations have to be considered. If the mean values summarized from all data of all sampling locations vaiy with a higher deviation than their individual standard deviations as well as the variations obtained from the recovery experiments a significant alteration of the isotopic composition of the contaminants can be inferred. These assumptions were supported by the S13C-values of d34-hexadecane. As expected for the internal standard the variation of the individual data points (standard deviation between 0.5 and 1.4 %o) was in the same range as compared to the standard deviation derived from the data of all sample extracts analysed (standard deviation 0.5 %o). 

For most of the anthropogenic contaminants investigated carbon isotope analyses were performed with an acceptable standard deviation down to amounts of approx. 5 ng absolutely applied to the gas chromatograph. These amounts correspond to concentrations in water samples at a natural abundance level of low to medium contaminated river systems. At lower amounts standard deviations of S13C-values of individual compounds increase significantly. 

We then set the isotopic compositions of each oxygen and carbon-bearing component in the reactant, the Fountain brine, to SI8O and S13C values of zero, as might be expected in a sedimentary brine. Finally, we segregate each mineral in the calculation from isotopic exchange, as discussed in Chapter 15. The procedure for small water/rock ratios is... 

To set up the calculation, we specify initial isotopic compositions for the fluid and calcite. We choose a value of —13 %c for r) 18 Os mow of the Lyons fluid, reflecting Tertiary rainfall in the region, and set the calcite composition to +11 %o, the mean of the measured values (Fig. 25.4). We further set S13C pi)B for the fluid to — 12 %o. We do not specify an initial carbon composition for the calcite, so the model sets this value to — 11 %c, in isotopic equilibrium with the fluid. Again, this value is near the mean of the measurements. 

Carboxylic acids are structurally diverse, with equal concentrations of straight-chain and branched compounds. The abundances of carboxylic acids decrease as the number of carbon atoms in the structure increase. Dicarboxylic acids, containing two carboxyl groups (Fig. 10.2), also occur. These acids are abundant in Murchison. The measured S13C and 8D values are higher for branched than straight-chain acids. This isotopic difference points to synthesis of the branched and linear molecules under different conditions. 

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