USING CARBONATE STABLE ISOTOPES TO IDENTIFY SEASONAL TRENDS IN LATE CRETACEOUS AMMONITES

Extinct ammonites of the genus Baculites may constrain valuable seasonal information that can aid in the reconstruction of paleoenvironmental conditions. Here we present oxygen isotope data suggesting that well-preserved aragonitic Baculites shells can record sinusoidal δ¹⁸O signals. This signal can be explained by seasonal seawater temperature variation. The intensity of seasonality can be derived from the amplitude of the signal. Growth rates of Baculites may also be determined from the length of shell material recording a single seasonal cycle. As very little is known about these organisms, growth rates may provide important details about their life strategies.

In this study, carbonate powder samples were taken from late Cretaceous Baculites shells in sequence of ontogenetic succession. Specimens were obtained from three Late Cretaceous locales that represent deposition from the Western Interior Seaway. Mass spectrometry was then employed to quantify the δ¹⁸O and δ¹³C of each sample. In addition, potential artifacts in isotope data due to the use of floor waxes and other consolidants during museum display preparation of these specimens were investigated. The sealants tested so far have not shown any measurable alteration of isotopic signals.

Preliminary trials from two Baculites specimens have produced promising data. In a large Baculites, we observed one complete seasonal cycle manifested as a sinusoidal curve. The single sinusoidal period spans approximately 35 cm of shell material, which provides evidence that Baculites grew at extremely fast rates. Additionally, we observed a 0.6‰ δ¹⁸O amplitude that suggests very small seasonal intensity of only around a 2.5 °C. In another much shorter specimen from a higher latitude, we have identified a unidirectional δ¹⁸O signal in a short Baculites with a 0.9‰ decrease across 12 cm of shell material. This trend may represent a partial seasonal cycle or part of a previously unrecognized ontogenetic effect. As further confirmation of these observations is necessary, at least two further large Baculites specimens are being analyzed, and their results will also be presented at the conference. Ultimately, fossilized ammonite specimens from the Western Interior Seaway may prove to be useful tools for reconstructing climatic conditions during the Upper Cretaceous.