Oxygen/carbon ratios, variation with

FIGURE 9.4 Variation of equilibrium composition and enthalpy change with oxygen-steam ratio for the carbon-oxygen-steam system at atmospheric pressure and 900 K. From Parent and Katz (1948). 

Isotope-selective photodissociation of gaseous carbon monoxide is a well-known process in molecular clouds. This process may have been important in the early solar system, with the nascent Sun as the source of ultraviolet radiation. The consequent self-shielding gives rise to the non-mass-dependent oxygen isotope variations observed in primitive meteorites. This model implies that the solar oxygen isotope ratios, and are about 5% smaller than those ratios in... 

Because of their expected small size, atmospheric variations of the 0/" 0 in O, have not been observed up to now. Seasonal variations and mean annual gradients are expected to be on the order of a few penneg (Seibt, 1997). Just as the combination of CO, and O, provides a very powerful constraint on the carbon cycle, a combination with measurements of the oxygen isotope ratios in both CO, and O, would constitute two further powerful constraints. 

Of special geological interest is the isotopic analyses of coeval carbonate-phosphate pairs (Wenzel et al. 2000), which helps to distinguish primary marine signals from secondary alteration effects and sheds light on the causes for 5 0 variations of fossil ocean water. Wenzel et al. (2000) compared Silurian cal-citic brachiopods with phosphatic brachiopods and conodonts from identical stratigraphic horizons. They showed that primary marine oxygen isotope compositions are better preserved in conodonts than in brachiopod shell apatite and suggested that conodonts record paleotemperature and ratios of Silurian sea water. 

Knauth LP, Lowe DR (1978) Oxygen isotope geochemistry of cherts from the Onverwacht group (3,4 billion years), Transvaal, South Africa, with implications for secular variations in the isotopic composition of chert. Earth Planet Sci Lett 41 209-222 Knoll AH, Hayes JM, Kaufman AJ, Swett K, Lambert IB (1986) Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East Greenland. Nature 321 832-838... 

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

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