Other Factors Influencing Isotopic Fractionations

Productivity and C-isotope records—Attributing trends in bulk S Corg only to the trade-off between dominance of respiration-influenced local carbon reservoirs versus dominance of production-influenced global carbon reservoirs is clearly a gross oversimphfication, as a number of other factors may influence C-isotope fractionation in the biogenic and aqueous reservoirs of the carbon cycle. However, this hypothesis finds support in the observations of Cretaceous and Devonian basins summarized herein. In each of these case studies, relative depletion of S Corg values are observed in association with units interpreted to represent examples of the anoxia-nutrient feedback mechanism, and recycling of... 

C02 concentrations (Rau et al. 1992 Pancost et al. 1997). Under conditions of abundant C02, mass transport into cells is unlikely to limit the rate of photosynthesis, so the large isotopic fractionation of rubisco should dominate the overall fractionation. There are many other factors that can influence Ephotosynthesis... 

Other than the suppression of isotope fractionation at low temperatures for mesophilic sulfate reducers, as discussed above, temperature has played a prominent role in the discussion of the factors influencing fractionation. Thus, for Desulfovibrio desulfuricans, specific rates of sulfate reduction increase with increasing temperature, and reduced fractionations down to 16-17 %o have been observed between 40 to 45°C (Kaplan and Rittenberg 1964). These results have been extrapolated to suggest higher specific rates should accompany fractionations reduced to near 0 %o at even higher... 

Combined effects. Because temperature, pressure and solution composition all affect the physical properties of water, these three variables can act in concert to influence fractionation factors (Horita et al., in press Hu and Clayton, in press). Increases in pressure and NaCl concentration both work to decrease the fluid s affinity for deuterium. The isotopic effect is most pronounced at low pressures and NaCl concentrations, which relates to the fact that the largest changes in the density of the fluid occurs over this region of pressure-composition space. It appears that the fractionation factor and the density of aqueous NaCl solutions are closely related to each other. With additional systematic experiments, empirical equations can be designed that relate the isotope salt effects and the density of aqueous solutions. 

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