Duration of the Early Bajocian and the associated δ13C positive excursion based on cyclostratigraphy
The Early Bajocian, about 172 Ma ago, was a period of tectonic changes, a global carbon-isotope positive excursion, and biological diversification of marine invertebrates (e.g. ammonites, radiolarians and coccolithophores). Unfortunately, both the duration of the Early Bajocian and the associated palaeoenvironmental changes are still poorly constrained. We propose here an estimate of the duration of this sub-stage based on a cyclostratigraphic analysis of the carbonate content from the Chaudon–Norante section, French Subalpine Basin, France. The Chaudon–Norante succession has been correlated with Les Dourbes section using greyscale variations to detect possible local hiatuses. The duration estimated here for the entire Early Bajocian is 4.082 Ma. Two intervals were identified in the positive δ13C excursion: an increase of carbon isotope values lasting 1.36 Ma and climax isotope values lasting for 2.72 Ma. A cooling at high latitudes and warming at low latitudes may have enhanced the Earth’s temperature gradient leading to an increase in humidity, which in turn triggered ocean eutrophication at the origin of the δ13C positive excursion.