Sequential Isotopic Analysis of Ammonite Aptychi

Abstract

Stable oxygen and carbon isotope data from ammonite carbonate (CaCO3) shell material and/or aptychi can be interpreted to reveal information about paleoenvironmental conditions because oxygen isotope ratios can vary based on temperature and oxygen isotope composition of ancient seawater and carbon isotope ratios varies depending on dissolved inorganic carbon (DIC) of the local environment and biological factors. Previous stable isotopic studies on ammonite aptychi were based on bulk sampling, and thus cannot reveal information about ammonite life history. This thesis reports the first time series δ18O and δ13C values measured in eight ammonite aptychi and assesses their preservation via scanning electron microscopy and electron backscatter diffraction techniques. The samples were collected from the Late Cretaceous Mooreville Chalk in Greene County and Harrel Station in Dallas County, Alabama, USA. Results show that these ontogenetic time series data provided insight into possible migration patterns, potentially both short-term and long-term movements between cooler and warmer water. Additionally, calculated paleotemperature values based on δ18O data vary between 15oC and 26.4oC for Dallas County and 18.6oC and 27.6oC for Greene County in the Upper Cretaceous eastern Gulf Coastal Plain, similar to other estimates from this region and time. These analyses suggest time series δ18O and δ13C data from aptychi may be a useful method to assess aspects of ammonite behavior and environmental conditions.