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21.
The δ18O of mammalian bone-phosphate varies linearly with δ18O of environmental water, but is not in isotopic equilibrium with that water. This situation is explained by a model of δ18O in body water in which the important fluxes of exchangeable oxygen through the body are taken into account. Fractionation of oxygen isotopes between body and environmental drinking water is dependent on the rates of drinking and respiration. Isotopic fractionation can be estimated from physiological data and the estimates correlate very well with observed fractionation. Species whose water consumption is large relatively to its energy expenditure is sensitive to isotopic ratio changes in environmental water. 相似文献
22.
Phosphorites from sedimentary sequences ranging in age from Archaean to Recent were analysed for δ18O in both the PO4 (δ18Op) and CO3 (δ18Oc) in the apatite lattice. The oxygen isotope record is considerably better preserved in phosphates than in either carbonates or cherts. The use of the Longinelli and Nuti [8] temperature equation yields temperatures for Recent phosphorites that are in good agreement with those measured in the field. The δ18Op values of ancient phosphorites decrease with increasing age. These changes with time are not likely to be due to post-depositional exchange. Changes in δ18O values of seawater and variations of temperature with time can account for the δ18Op time trend, but the latter explanation is preferred. In Ancient phosphorites δ18Oc in structurally bound carbonate in apatite is not a reliable geochemical indicator. 相似文献