Carbon and oxygen stable isotopes across the Coniacian/Santonian boundary at Olazagutia, northern Spain

Carbon and oxygen stable isotopes have been analysed from 45 bulk sediment samples taken through a 40-m section across the Coniacian/Santonian boundary at Olazagutia, Navarra, Spain, a candidate Global Boundary Stratotype section for this stage boundary, and related to key bioevents. Carbon isotope values lie between +2.68 and +3.05‰; oxygen isotope values between −3.12 and −2.26‰, VPDB. Neither carbon nor oxygen isotope curves include significant excursions. Nevertheless, the carbon isotope curve can be correlated precisely with a published carbon isotope curve from southeast England. Within the limits of our sampling resolution (approximately one sample per 22,000 years), correlations based on carbon isotopes and on first and last occurrences of the inoceramid bivalve Platyceramus undulatoplicatus (the primary biomarker for the Coniacian/Santonian boundary) are equally accurate. Minor fluctuations in the oxygen isotope curve may reflect periodic incursions of cooler Atlantic water into the area around Olazagutia.