The OAE1a in Cuchia (Early Aptian, Spain): C and O geochemistry and global correlation

C-isotopes, TOC and O geochemical data from the lower Aptian Cuchia section in the western Basque-Cantabrian basin (BCB) allow an accurate delimitation of the OAE1a-equivalent and its geochemical Menegatti's segments, a detailed delta C-13(carb) correlation with regional and interregional sections, and a high-resolution construction of TOC and bulk-rock delta O-18(carb) curves and their interpretation. The delta C-13(carb) values range from -2 parts per thousand and +4 parts per thousand (VPDB). They agree with previous data from the eastern BCB sections (Aralar) confirming the ammonite age of the OAE1a in the Basque-Cantabrian basin: Deshayesites forbesi, Deshayesites deshayesi, and Deshayesites deshayesi-Dufrenoyia furcata transition Zones. Interregional delta C-13(carb) correlation with pelagic (Cismon, Italy, and Mid-Pacific Mountains, DSDP Site 463) and neritic (Roquefort-La Bedoule, France) core sections, reveals a common profile of a wide negative excursion characteristic of the OAE1a. It consists of a double trough separated by a flat relative maximum, with two negative spikes in the upper trough of neritic sections. TOC absolute values range from 0.12% to 1.37%. Segments of the TOC curve with persistent low values closely correspond with descending segments of the delta C-13(carb) curve, and are attributed to lesser organic productivity in the BCB. Detailed bulk-rock delta O-18(carb) data (-5.71 parts per thousand to -1.05 parts per thousand PDB) and variation curve show two main positive O-isotope shifts and three minor positive inflections, within a general negative trend characteristic of the OAE1a. The two major positive shifts correspond to both shallowing upwards sequences and the lowermost can be related to a eustatic sea level fall. Independent interregional correlation of the O-isotope shifts with C-isotopes supports their interpretation as punctuating colder events within a general warming trend.