Beverly J. Johnson

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

An isotopic study of modern 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., zoos 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‰, respectively, relative to the diet. In natural settings, the δ13C values of both the organic and inorganic fractions of OES reflected that of ambient vegetation, with a noted dietary preference for C3 plants. The nitrogen isotope composition of the total organic fraction of OES was 3‰ 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 δ15N values of approximately 1‰. 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 δ18O of OES values were highly variable and are thought to be controlled primarily by the δ18O 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 δ13C and δ15N values of the total organic fraction of heated OES increased by less than 0.6 and 0.2‰ 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 C3 and C4 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 δ13C values of IAA in OES range over 12‰ 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.

The determination of late Quaternary paleoenvironments at Equus Cave, South Africa, using stable isotopes and amino acid racemization in ostrich eggshell

Abstract The stable carbon and nitrogen isotopes from organic compounds, and stable oxygen and carbon isotopes from inorganic carbonate in modern and fossil ostrich eggshell (OES) at Equus Cave, South Africa, were used to determine relative changes in C 3 and C 4 vegetation, rainfall, and temperature throughout the last 17 ka. Amino acid racemization and three new radiocarbon dates were used to assign ages to the individual OES fragments analyzed for stable isotopes, and to evaluate the stratigraphic integrity of the samples. The amino acid racemization and radiocarbon data indicate that most of the OES fragments are not in correct stratigraphic context, and may have been upwardly and downwardly mixed; thus, each fragment was placed in chronological sequence using radiocarbon-calibrated amino acid racemization. The stable carbon isotope data from the fossil OES do not change systematically through the last 17 ka. The ostriches primarily consumed C 3 plants (between 70% and 90%), and smaller quantities of C 4 plants (between 30% and 10%), indicating that a mixture of C 3 and C 4 plants has been in the vicinity of Equus Cave for the last 17 ka. Stable nitrogen isotope data from OES indicate that at 17 ka, mean annual precipitation (MAP) was at a minimum (190±50 mm/yr), increased steadily to modern values by 6 ka (600±150 mm/yr), and remained relatively unchanged until present. Stable oxygen isotope data coupled with nitrogen isotope data indicate that paleotemperatures were at a minimum between 14 and 17 ka, and reached their maximum in the latest Holocene. This study demonstrates the plethora of paleoenvironmental information that can be derived from analysis of three different stable isotopes (carbon, nitrogen, and oxygen) in the same sample-type (OES), and presents a new tool with wide applicability for reconstructing paleoenvironments in semi-arid and arid regions of Africa.