Global carbon cycling linked to eustatic changes in the aftermath of the early Aptian Oceanic Anoxic Event (OAE1a)

The Aptian experienced significant carbon-cycle disruptions, marked by notable paleoclimatic and paleoceanographic shifts. While the mechanisms behind the Aptian oceanic anoxic events (OAE) are relatively well-understood, the interplay between global carbon-cycle dynamics and sea-level changes immediately following the early Aptian OAE1a remains enigmatic. By analyzing a carbonate succession from the western Tethys (eastern Spain), dated using ammonite biostratigraphy as Deshayesites deshayesi and Dufrenoyia furcata zones, we identify a temporal correlation between a sea-level highstand and a subsequent fall, aligning respectively with a global carbon-isotope excursion exceeding +4‰, unprecedented in the Cretaceous, and a sharp negative isotopic shift from +4.3‰ to −0.7‰. The concurrence of these sea-level fluctuations and carbon isotope variations across multiple regions underscores the eustatic origin of sea-level change and its profound impact on global carbon cycling. We propose that during highstand and transgressive stages, sea expansion acted as a carbon sink, facilitating the increased burial of isotopically light carbon. Conversely, rapid sea-level falls likely triggered sediment reworking, reintroducing this light carbon into the dissolved inorganic carbon (DIC) of the global ocean. This study highlights the overlooked role of remobilized sediments as a source of carbon during the Aptian, commonly attributed to volcanic degassing, and emphasizes the immediate and significant response of the carbon cycle to eustatic changes.