Isotope sclerochronology of ammonites (Baculites Compressus) from methane seep and non-seep sites in the Late Cretaceous Western Interior Seaway, USA: Implications for ammonite habitat and mode of life

Ammonites, as well as other fauna, were common in methane seeps of the Late Cretaceous Western Interior Seaway (WIS) of North America. Biogeochemical processes at the seeps, in particular the anaerobic oxidation of methane, produced a dissolved inorganic carbon reservoir with a low δ13C, manifested in the carbon isotope composition of the inorganic calcium carbonate concretions associated with the seeps and recorded in well-preserved shells of ammonites documented at the sites. Detailed sclerochronological sampling of six well-preserved specimens of Baculites compressus collected at seep sites in the Pierre Shale of South Dakota reveals three patterns that can be explained by reference to two specimens of the same species collected at age-equivalent non-seep sites. Three of the specimens exhibit uniformly low values of δ13C that are significantly different (unpaired t-test, p < .0001) from similarly sized specimens of the same species collected at age-equivalent non-seep sites, suggesting that these ammonites lived at the seeps during the time interval over which the shell was secreted (adult portion of the shell). Two of the specimens collected from a seep site exhibit values of δ13C consistent with early ontogeny at a non-seep site followed by later ontogeny at a seep site. The values of δ18O of all the specimens reveal water temperatures of 16 to 28 °C. One small juvenile (15 mm long) collected at a seep site exhibits higher values of δ13C consistent with a non-seep environment, but values of δ18O that indicate very warm or slightly brackish water, suggesting that this animal lived in surface waters during its early ontogeny and died soon after arriving at the seep. Our results demonstrate that seep fluids affected the geochemistry of the water column above the seeps and that seeps provided habitats for ammonites in the WIS. Thus, although ammonites were mobile animals, they probably exploited a low-energy life style, remaining at the same site for extended periods of time.