Impact of Early Aptian carbon cycle perturbations on evolution of a marine shelf system in the Basque-Cantabrian Basin (Aralar, N Spain)

The Aptian Oceanic Anoxic Event 1a (OAE1a, ca.120 Ma ago) is one of the most prominent of a series of geologically brief intervals in the Cretaceous reflecting a major perturbation in the global carbon cycle. This carbon cycle perturbation is recorded in the C-isotope stratigraphy. In this study we present a new carbonate and organic carbon isotope record across OAE1a from expanded shallow marine sections (Igaratza and Iribas) in the Basque-Cantabrian Basin (N Spain). The sediments studied were accumulated on a mixed carbonate–siliciclastic and carbonate shelf. The δ13Ccarb and δ13Corg curves obtained show a pronounced negative spike in the Deshayesites weissi Zone and a subsequent positive excursion within the newly described D. deshayesi–D. furcata transition Zone, approximately equivalent to the upper D. deshayesi Zone of other sections. Specific changes in facies patterns and a reduction in neritic carbonate production predate the negative C-isotope perturbation. These changes coincide with global carbonate crises in neritic and pelagic environments. Corresponding with the beginning of OAE1a (Selli level) a condensed interval rich in ammonites is observed. This horizon is linked to a major transgression with high biodiversity. The “ammonite bloom” suggests enhanced primary productivity in the surface waters of the Basque-Cantabrian Basin at improving environmental conditions for selected biocalcifiers. The ultimate recovery of shelf carbonate deposition occurred after OAE1a and it coincided in time with the most positive values of the carbon-isotope excursion. Based on carbon-isotope stratigraphy it has been deduced that black shales corresponding to Selli Level are missing in Aralar, so that anoxia is not recorded in the studied sections of the marine shelf environment. An organic-rich interval (Aparein level) postdates the Early Aptian major positive excursion. The Aparein event should be considered in the future research of OAEs.