High-resolution chemo- and biostratigraphic records of the Early Aptian oceanic anoxic event in Cantabria (N Spain): Palaeoceanographic and palaeoclimatic implications

Abstract

During the Early Aptian, major palaeoenvironmental changes occurred leading to an oceanic anoxic event (OAE 1a) and a perturbation of the global carbon cycle. New detailed litho-, bio-, and chemostratigraphic (TOC, δ13Ccarb, δ13Corg) records of two superbly exposed and expanded Lower Aptian sections in Cantabria (La Florida and Cuchía) allow to recognize the expression of the OAE 1a in shallow shelf environments of northern Spain. The succession consists of shallow platform limestones that include a marly unit (Patrocinio Formation), the deposition of which occurred mostly at the onset of the OAE 1a (~120.5Ma). This study presents a new integrated biostratigraphy based on ammonites, planktonic foraminifera, calcareous nannofossils and palynomorphs that allows an accurate age resolution of the succession. The marly unit records an abrupt negative δ13C excursion in both bulk organic matter (up to 5‰) and carbonate (up to 6‰, mean 3‰), as has been already observed at the onset of the OAE 1a in other Lower Aptian deposits worldwide. In detail, however, the negative excursion presents two minima in the studied sections. This negative spike is confidently attributed to the upper half of the Hayesites irregularis nannofossil Zone, to the upper part of the Blowiella blowi foraminiferal Zone, and to the middle–upper part of the Deshayesites weissi ammonite Zone. A third negative excursion occurs at the base of the Rhagodiscus angustus nannofossil Zone, which may be correlatable with the Dufrenoyia furcata ammonite Zone. This data set refines the age of the OAE 1a and reveals the existence of a stratigraphic gap in the westernmost margin of the Basque Cantabrian Basin that covers at least a portion of the upper part of the Early Aptian. Sedimentary facies and quantitative analysis of palynomorphs and nannofossils document significant environmental changes associated with the OAE 1a: compositional changes of neritic carbonates and calcareous nannofossils data indicate the occurrence of a biocalcification crisis inferred to have been related to CO2-induced changes in seawater chemistry, and palynomorphs identify a thermal maximum followed by a cooling phase. The latter show a Classopollis maximum during the OAE 1a, which is followed by a decrease in Classopollis and an increase of bisaccate pollen after the event.