The Early Callovian Positive Carbon Isotopic Excursion as a Marker for the Base of the Callovian and Associated Environmental Trends

D'Orbigny named the Callovian stage after Kellaways in Wiltshire, UK, in the 1850s. However, agreement on its boundaries and, more recently, on the position and location of the Global Boundary Stratotype Section and Point (GSSP) for the base of the Callovian has proven difficult. This is mainly due to a lack of agreement on the appropriate index fossil, its regional and global correlations, and the location of the stratotype section. Stable carbon-isotope chemostratigraphy and event stratigraphy have become important tools for refining boundary definitions and supporting GSSP selection. In this study, we analyze stable carbon-isotope records from two Bathonian–Callovian stratigraphic sections in Portugal and Greenland in order to identify and correlate isotopic events previously recognized in other European basins. We propose that the early Callovian positive δ13CTOC excursion represents a robust secondary marker for the base of the Callovian. In addition to improving superregional correlation between candidate GSSPs and Standard Auxiliary Boundary Stratotypes, this excursion offers a framework for ordering and age-calibrating global oceanographic isotopic events across the Bathonian–Callovian transition. Ongoing work at the BETY Lab is further exploring the oceanographic and environmental processes responsible for the succession of δ13C excursions around the Bathonian–Callovian boundary.