The Cretaceous/Tertiary boundary carbon and oxygen isotope stratigraphy, diagenesis, and paleoceanography at Zumaya, Spain

Abstract

Carbon and oxygen isotope analysis of 240 m of Upper Cretaceous and Lower Tertiary limestones and marlstones from Zumaya, Spain, has revealed three distinct, negative carbon isotope excursions of up to 2 per mil, occurring both before and during the Cretaceous/Tertiary (K/T) transition. Although diagenesis has affected these rocks to varying degrees during burial and lithification, we believe that these excursions may reflect paleoceanographic changes. Sr/Ca, electron microscopic, and petrographic analysis have been used to assess the type and magnitude of diagenesis, allowing us to distinguish samples with significantly altered isotopic values. Carbon isotopes of rocks at Zumaya exhibit absolute values and ranges similar to semiconsolidated sediments of the same age. Oxygen isotopes of these rocks, however, exhibit a 1–2 per mil or more depletion, defined here as a diagenetic overprint, in comparison with values from semi‐consolidated sediments of the same age. Ammonite and inoceramid macrofossils disappear in this section 12.5 and 120 m below the K/T boundary, coincident with two distinct episodes of negative δ13C values. These may reflect localized paleoceanographic or ecological changes. The last and largest of the isotope excursions corresponds with the K/T boundary marl and mass extinctions of calcareous plankton. The K/T boundary carbon isotope excursion is one of three that took place during the last 1 million years of the Cretaceous, each one possibly being associated with biotic changes in the oceanic realm. The K/T boundary excursion coincides with iridium concentration and spherules of unidentified origin, perhaps indicating that paleoceanographic change coincided with either extraterrestrial impacts or volcanic events that amplified their effects and accelerated biotic changes during times of stressed oceanic ecosystems.