Oxygen, determination fixation

Figure 3. Estimates of atmospheric CO2 levels (PCO2) based on observed isotopic fractionation (Sp) as determined by the 5 C content of individnal alkenone componnds from DSDP site 588. Estimates are based on a phosphate concentration of 0.3 pM and fractionation associated with enzymatic fixation of 27%o. Carbon burial events inferred from the inorganic carbon record are noted with CM, and Mi represents glacial events inferred from the oxygen isotopic record. Data for Sr isotope ratios are from sites 588 (open squares) and 289 (open triangle). Cli. Optimum is climate optimum, EAIS represents the time of the expansion of the East Antarctic Ice Sheet. Error estimates described in the original text represent 15% uncertainty of the calculated PCO2 values. [Used by permission of the editor of Paleoceanography, from Pagani et al. (1999), Fig. 14].

Reaction (3.4) is fixation of an oxygen molecule onto an alkyl radical. It is generally a very fast reaction if the concentration of oxygen in the polymer is moderate-to-high this rapidly transforms alkyl radicals into peroxy radicals. It is essentially reaction (3.5) that will determine the rate of oxidation of the polymer, the rate being a function of the bond strength of the C-H bond broken and the stability of the alkyl... 

For high-precision determinations of in situ oxygen concentrations the results have to be converted into yumol/kg because the amount of oxygen defined by a volume concentration depends on the sample temperature, which changes from in situ to subsampling conditions. The conversion of /rmol/L concentrations into in situ /rmol/kg requires the application of the equations for seawater density (UNESCO, 1983 see Appendix Table 4). The sample volume at subsampling temperature (fixation) is corrected to the in situ volume and mass. Temperature or volume change after fixation do not affect the analysis. 

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