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An isotopic study of modem ostrich eggshell (OES) is presented as a calibration for terrestrial paleoenvironmental applications. The stable carbon and nitrogen isotope fractionations of OES were determined for various organic fractions of eggshell by measuring the isotopic ratios of modern OES samples collected from controlled settings (i.e., toes and farms) and corresponding ostrich diet. These fractionations were used to evaluate the relationship between the isotope composition of OES laid by free-range birds Living in South Africa and their environment. The carbon isotope composition of the total organic and inorganic fractions of OES were enriched by 2 and 16 parts per thousand, respectively, relative to the diet. In natural settings, the delta(13)C values of both the organic and inorganic fractions of OES reflected that of ambient vegetation, with a noted dietary preference for C-3 plants. The nitrogen isotope composition of the total organic fraction of OES was 3 parts per thousand enriched relative to the diet, and varied inversely with mean annual precipitation (MAP) in natural settings. A decrease in MAP of 100 mm was accompanied by an increase in delta(15)N values of approximately 1 parts per thousand. The oxygen isotope composition of the inorganic fraction of the OES varied linearly with that of the drinking water in controlled settings. However, in natural settings, the delta(18)O of OES values were highly variable and are thought to be controlled primarily by the delta(18)O of ingested plant leaf-water. The stability of the isotopic signal in the organic fraction of OES through geologic time was evaluated through a series of heating experiments. The delta(13)C and delta(15)N values of the total organic fraction of heated OES increased by less than 0.6 and 0.2 parts per thousand for carbon and nitrogen, respectively, in spite of extensive diagenetic alteration and changes in the amino acid composition of the samples. The results of this study indicate that the stable carbon and nitrogen isotope composition of OES is relatively stable under the experimental conditions used and may be used to derive a plethora of paleoenvironmental information, including changes in C-3 and C-4 vegetation and paleorainfall estimates.

Additionally, carbon isotopic analysis of individual amino acids (IAA) in the OES and corresponding diet were determined to elucidate information on isotopic fractionation during OES protein synthesis. The delta(13)C values of IAA in OES range over 12 parts per thousand and provide valuable information for future studies of (1) diagenesis in fossil OES samples and (2) comparative animal physiology, including the determination of digestive and feeding strategies of extant and extinct animals. Copyright (C) 1998 Elsevier Science Ltd.