Complex and protracted environmental and ecological perturbations during OAE 1a: Evidence from an expanded pelagic section from south Spain (Western Tethys)

The early Aptian Oceanic Anoxic Event (OAE) 1a represents a major perturbation in the global carbon cycle associated with significant environmental, biotic and sedimentary changes. The signature of this event is a global negative followed by a positive stable carbon isotope excursion (CIE), associated with the input of light-carbon into the climate system and subsequent widespread deposition of organic-rich sediments. This study uses biostratigraphy, C-isotope stratigraphy, biomarkers, and elemental geochemistry to stratigraphically and geochemically characterize OAE 1a at an expanded pelagic marine section from the western Tethys. The high-resolution δ¹³C_org analysis of the section highlights several successive negative CIEs during the onset of OAE 1a (isotope segment C3), pointing to rapid changes in global carbon cycling. The biomarker data indicate that the organic matter is thermally mature and is mainly of marine origin. The biomarker assemblage together with records of redox-sensitive trace elements suggest that sedimentation took place under generally well oxygenated waters punctuated by three short episodes of sea floor anoxia/dysoxia. The two first episodes of anoxia/dysoxia correlate with enhanced organic productivity during the main negative C-isotope excursion that represents the onset of the OAE 1a. They occurred at the final part of C-isotope negative excursions, likely as a response to C-release, increased temperatures and associated hydrological change and weathering, which resulted in increased fertilization-driven oxygen consumption. This was followed by the positive carbon isotope excursion, due to the burial of OM outpacing carbon input. The third episode of anoxia-dysoxia, which occurred later during the major positive C-isotope excursion, is not associated with fertilization, and perhaps instead reflects a general progressive depletion of oxygen during OAE 1a, linked to stagnation of marine waters. Interestingly, although the environmental parameters return to pre-OAE values at the end of the event, a biotic (especially microbial) perturbation persisted after OAE 1a. Our results suggest that instability in environmental conditions was the main feature during OAE 1a in the western Tethys with notable changes extending from the onset to the post-OAE 1a interval.