STABLE ISOTOPES IN MODERN OSTRICH EGGSHELL - A CALIBRATION FOR PALEOENVIRONMENTAL APPLICATIONS IN SEMIARID REGIONS OF SOUTHERN AFRICA

An isotopic study of modem ostrich eggshell (OES) is presented as a ca libration 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 u sed to evaluate the relationship between the isotope composition of OE S laid by free-range birds Living in South Africa and their environmen t. The carbon isotope composition of the total organic and inorganic f ractions of OES were enriched by 2 and 16 parts per thousand, respecti vely, 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. Th e nitrogen isotope composition of the total organic fraction of OES wa s 3 parts per thousand enriched relative to the diet, and varied inver sely with mean annual precipitation (MAP) in natural settings. A decre ase in MAP of 100 mm was accompanied by an increase in delta(15)N valu es of approximately 1 parts per thousand. The oxygen isotope compositi on of the inorganic fraction of the OES varied linearly with that of t he 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-wate r. The stability of the isotopic signal in the organic fraction of OES through geologic time was evaluated through a series of heating exper iments. The delta(13)C and delta(15)N values of the total organic frac tion of heated OES increased by less than 0.6 and 0.2 parts per thousa nd for carbon and nitrogen, respectively, in spite of extensive diagen etic alteration and changes in the amino acid composition of the sampl es. The results of this study indicate that the stable carbon and nitr ogen isotope composition of OES is relatively stable under the experim ental conditions used and may be used to derive a plethora of paleoenv ironmental information, including changes in C-3 and C-4 vegetation an d paleorainfall estimates. Additionally, carbon isotopic analysis of i ndividual amino acids (IAA) in the OES and corresponding diet were det ermined to elucidate information on isotopic fractionation during OES protein synthesis. The delta(13)C values of IAA in OES range over 12 p arts per thousand and provide valuable information for future studies of (1) diagenesis in fossil OES samples and (2) comparative animal phy siology, including the determination of digestive and feeding strategi es of extant and extinct animals. Copyright (C) 1998 Elsevier Science Ltd.