Stable carbon-isotope stratigraphy and ammonite biochronology at Madotz, Navarra, northern Spain: implications for the timing and duration of oxygen depletion during OAE-1a

► OAE-1a is identified for the first time at Madotz and is equivalent to subunit 2b. ► Bulk Sedimentary rate ranges from 1.8 to 2.09 cm/ky for OAE-1a Stages C4-C6. ► During OAE-1a episodic dysoxic conditions occurred only between δ13C stages C4 and C5. ► The Longest Period of oxygen deficiency lasted between 95.23 and 120 ky. ► The Deshayesites forbesi Zone is identified in subunits 2a and 2b at Madotz.