Demise of Organic Matter–Rich Facies and Changing Paleoenvironmental Conditions Associated with the End of Carbon Isotope Segment C5 of Oceanic Anoxic Event 1a in the North and Northeastern Iberian Peninsula

During the Cretaceous, the concurrence of changing paleoceanographic and paleoclimatic conditions, coupled with variations in eustatic sea level, contributed to episodes of globally widespread deposition of organic matter (OM)–rich marine sediments collectively termed oceanic anoxic events (OAEs). Here, we aim to investigate the response of a lower Aptian hemipelagic sequence from the northeastern Iberian margin in the context of OAE1a. Stable–carbon isotope (δ13Corg) data are consistent with the pattern reported for the end of carbon isotope segment C5 within OAE1a. Moreover, high sediment accumulation rates (bulk: ∼37.13 cm/ky, wet: ∼63.29 cm/ky) permit the establishment of refined details suitable for precise chemostratigraphic correlations. We recognized three distinct lithologic units. Within unit 1, variable pulses of fluvial fluxes explain the alternating lithology, with limestones depicting the least terrestrially influenced end member and marlstones representing episodes of highest terrigenous input. In the ensuing marlstone-dominated unit 2 interval, results show relatively higher OM, redox-sensitive trace elements (RSTEs), P, Fe, Al, Si, and Ti values than before, thus suggesting an increase in runoff with quasi-permanent eutrophic surface waters and continuous oxygen-deprived conditions, but without a fully anoxic phase, as benthic fauna, while relatively reduced, are present throughout. Unit 3 registers limestones impoverished in OM, Al, Si, Ti, P, and Fe, with a lower relative proportion of autochthonous to allochthonous OM, indicating a reduction in runoff and surface water fertility linked to drier climate conditions. Simultaneous changes in microfacies, with coarser packed biosparites, reduced planktonic foraminifera, and an increase in benthic taxa, imply shallowing of the basin, similar to that reported for sequences in the neighboring Basque-Cantabrian Basin synchronous with the negative δ13Corg shift heralding the end of segment C5.