Influence of local factors on the expression of the early Aptian OAE 1a – insights from a pelagic basin in the Western Tethys

The early Aptian Oceanic Anoxic Event (OAE 1a) was characterized by a major perturbation in the global carbon cycle, linked to the rapid input of 13C-depleted carbon into the Earth's system from volcanogenic and/or methanogenic sources. Although the related environmental and climatic changes are relatively well understood on a global scale, local and regional impacts are less well constrained. Here we provide new insights into the local expression of the biotic and environmental changes related to OAE 1a in pelagic domains, by analysing a pelagic succession from the Western Tethys, La Frontera section (S. Spain), using biomarkers, TOC content, and elemental composition data. These data revealed a complex variability before, during, and after OAE 1a (defined as isotope segments C2 to C7), but with a clear episode of biotic and environmental perturbation within OAE 1a at the upper C4 segment. Comparison with the nearby pelagic Carbonero section within the same basin reveals a heterogeneous expression of this event, recording three episodes of accelerated environmental change (including C4 segment) that correlate with C-isotope shifts. These episodes also correlate with cooling and warming periods based on sea-surface temperature data and can be linked to variable Ontong-Java Plateau activity, as indicated by osmium isotopes. Local palaeogeographic conditions likely controlled the heterogeneous expression of OAE 1a, with the Carbonero section situated in a more subsident and more restricted water circulation area. Our results highlight the importance of understanding the local and regional context when evaluating the global expression of OAE 1a in pelagic domains.