Light isotopes

Flesh 6 N values decrease systematically with age (Fig. 12.3). The mean difference between flesh and diet is 3.0 0.79%oon day 91, 2.8 0.44%o on day 131 and 2.2 0.33%o on day 171. If muscle has a relatively high turnover rate, then this result is extremely coimterintuitive given the assumptions of Minagawa and Wada (1984) that selective catabolism of isotopically light molecules should occur and should cause tissue enrichment rather than depletion. 

The evolution of the profiles of the isotope ratio is shown in Figure 8-12, which plots the profiles at various times in the calculation. Early in the calculation, isotope ratios at shallow depths have been driven more negative by the release of isotopically light respiration carbon, but little change has occurred at greater depths. As the evolution proceeds, the ratios at shallow depths become more positive as the result of the dissolution and diffusion of heavier carbon from both above and below. In the final steady state, after some 15,000 years, the isotope ratio is nearly constant at about -0.6 per mil at depths below 100 centimeters, rising rapidly to the seawater value, +2 per mil in the top 100 centimeters. The final values reflect a balance between the release of isotopically light carbon by respiration and the release of isotopically heavy carbon by dissolution, with the additional influence of the diffusion of isotopically heavy seawater carbon. 

In our example, we test the consequences of reacting an isotopically light (i.e., nonmarine) limestone at 60 °C with an isotopically heavier groundwater that is relatively rich in magnesium. We start by defining the composition of a hypothetical groundwater that is of known CO2 fugacity (we initially set /co2 to 1) and in equilibrium with dolomite ... 

Hehre and co-workers (DeFrees et al., 1977, 1979a) have published both experimental and theoretical evidence in support of negative ion (anionic) hyperconjugation. These workers determined the free energies for the gas-phase hydron3 transfer equilibria (31), (32) and (33) by pulsed ion cyclotron resonance spectroscopy (Wolf et al., 1976). These equilibria, which involve the gas-phase formation of a methylamino, a methoxy and a thiomethoxy anion, all lie to the right, i.e. the formation of the isotopically light anion is favoured. These results were rationalized in terms of the MO... 

When Equations 11.19 and 11.20 are rewritten for isotopic light (L) and heavy (H) species, the isotope effects are obtained by taking ratios, recognizing [Eol] = [Eqh] ... 

The isotope fractionation factor for isotopes (light) and (heavy) between two substances X4 sm XB is usually expressed in terms of a ... 

James et al. (2003) performed laboratory experiments in which aqueous fluids were reacted with natural samples of unaltered MORE (5 Li = +4.6) and a quartzofeldspathic mud/turbidite mixture (5 Li = +1.3). The starting fluids had elemental compositions generally intermediate between seawater and hydrothermal fluids, and were isotopically light (5 Li = -5.5). The initial fluid/solid was 4. Fluid samples were extracted as the experiments were raised to 350°C over 400-500 h, and then during cooling (c. 740 h total). In both experiments Li was removed from solution at low temperatures, but the isotopic composition of the fluids... 

Kieffer (1982) proposed detailed calculation of partition function ratio / in crystalline solids through direct evaluation of the Helmholtz free energies of isotopically light and heavy compounds ... 

Figure 11,10 Schematic representation of vibrational spectra for an isotopically light compound (heavy bottom line) and an isotopically heavy compound (light upper line) of the same bulk stoichiometry and structure. Reprinted with permission from Kieffer (1982), Review of Geophysics and Space Physics, 20, 827-849, copyright 1982 by the American Geophysical Union.

As shown schematically in figure 11.10, the frequency distribution for an isotopically light compound g °(v) [and the corresponding g °(co)] can be expected to differ from the frequency distribution of the isotopically heavy counterpart g(v) [or... 

Variations in content of some important carbon compounds are schematically demonstrated in Fig. 2.11 The two most important carbon reservoirs on Earth, marine carbonates and the biogenic organic matter, are characterized by very different isotopic compositions the carbonates being isotopically heavy with a mean 5 C-valne around 0%c and organic matter being isotopically light with a mean 5 C-value aronnd -25%c. For these two sedimentary carbon reservoirs an isotope mass balance mnst exist such that ... 

Jeffrey et al. (1983) interpreted this trend as the loss of labile, C-enriched amino acids and sugars through biological reworking which leaves behind the more refractory, isotopically light lipid components. 

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