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

Abstract

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