IMPLICATION OF DIAGENETIC ALTERATIONS IN PALEONTOLOGICAL INVESTIGATIONS: AN EXAMPLE USING ADÉLIE PENGUIN EGGSHELLS FROM ANTARCTICA

Carbon from fossil avian eggshell is commonly used for radiocarbon and/or stable isotope analyses to answer stratigraphic or paleoecological questions. A major consideration in this use is a fossil's state of preservation, which is often assumed based on the conditions of the local environment.

This study examines data from scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis of late Holocene Adélie penguin (Pygoscelis adeliae) eggshell fragments to investigate the role of diagenesis in an arid Antarctic environment. The results of this study demonstrate that clear and rapid diagenetic alterations occur in eggshell carbonate following burial in ornithogenic, or bird formed, soils. Diagenesis progresses from blocky rhombs of calcium carbonate (CaCO₃) in the modern eggshells to botryoidal fluorapatite crystals (Ca₅(PO₄)₃F), as indicated by the increased weight percentages of phosphorus and fluorine with sampling depth. Carbon weight percent was found to decrease, with statistical significance, in relation to phosphorus, indicating that carbon is preferentially removed from the eggshell through time.

Diagenesis in the fossils is predictable, with subtle, early stage alterations occurring first along the outer margins of the eggshells, and slowly progressing inward towards the eggshell's center. These data indicate either redeposition of eggshell carbonates, or a relatively rapid replacement of calcium carbonate with diagenetic minerals, typically within a few thousand years. Although the impact of this diagenesis on radiocarbon and stable isotope interpretations remains unknown, our data emphasize the importance of understanding the preservation of a sample before it is considered for use in paleontological reconstructions.