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

• Published in: Acta geologica polonica Vol. 65; no. 4; pp. 525 - 543
• Main Authors: García-Mondéjar, Joaquín, Fernández-Mendiola, Pedro A., Owen, Hugh G.
• Format: Journal Article
• Language: English
• Published: De Gruyter Open 01.12.2015
• Subjects: Aptian; Basque-Cantabrian Basin; C-O isotopes; Cretaceous; TOC
• ISSN: 2300-1887; 2300-1887
• DOI: 10.1515/agp-2015-0023

C-isotopes, TOC and O geochemical data from the lower Aptian Cuchía section in the western Basque-Cantabrian basin (BCB) allow an accurate delimitation of the OAE1a-equivalent and its geochemical Menegatti´s segments, a detailed δ correlation with regional and interregional sections, and a high-resolution construction of TOC and bulk-rock δ curves and their interpretation. The δ values range from -2‰ and +4‰ (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 δ correlation with pelagic (Cismon, Italy, and Mid-Pacific Mountains, DSDP Site 463) and neritic (Roquefort-La Bédoule, 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 δ curve, and are attributed to lesser organic productivity in the BCB. Detailed bulk-rock δ data (-5.71‰ to -1.05 ‰ 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.