Driver of eustatic change during the early Aptian Oceanic Anoxic Event 1a (-120 Ma)

Sea-level changes exert an important control on oceanic circulation and climate evolution. Researchers have proposed that sea-level rise favored accumulation of sediments enriched in organic carbon during oceanic anoxic events (OAEs), although high-frequency sea-level changes and their controlling mechanism have remained poorly constrained. Here we present a detailed sedimentological and geochemical study on Aptian (Lower Cretaceous) shallow-water carbonates of the Dariyan Formation exposed in the Zagros fold belt of southern Iran. Microfacies analysis reveals that a regression occurred during carbon isotopic stages C3-C4, followed by a transgression during stages C6-C7. Frequency analysis of carbonate allochems indicates a transition from rudists and other skeletal grains to microbial grains with reduced bioturbation during regression at late C3-C5 stages, whereas the opposite occurred during transgression at stages C6-C7. In the transition interval (C3-C5) nutrients (P, Ba) and redox indicators (Ce anomaly and U concentration) document low nutrient levels and oxygen-rich sea waters, suggesting sea-level changes, rather than by seawater chemistry control. Worldwide correlation of relative sea-level changes across Tethyan, Northern Atlantic, Pacific and Arctic areas is consistent with regression followed by transgression during OAE1a. The regression and transgression occurred at a period of climatic warming and cooling respectively, suggesting that aquifer-eustasy rather than thermo-eustasy or glacio-eustasy may have represented the main cause of eustatic changes.