No abstract

The extent and persistence of anoxia in the South Atlantic Ocean during its early opening phase in the Early Cretaceous is not well constrained, hindering a holistic understanding of the processes and mechanisms that drive past changes in water column redox conditions, as well as the impacts of such changes on marine ecosystems. Here we provide high‐resolution geochemical records from Deep Sea Drilling Project Site 364 that document variations in redox conditions, chemocline depth, marine productivity, and marine ecosystem dynamics in the northern South Atlantic during the Aptian. We show that many of these parameters varied across discrete sedimentary cycles expressed in the Deep Sea Drilling Project 364 succession. Our data indicate that during the initial stages of basin development, anoxic and euxinic conditions were prevalent and occasionally extended into the upper water column. However, strong cyclicity in sedimentological and geochemical parameters imply that the anoxia/euxinia was not a persistent state. We argue that the water column redox conditions during the Aptian were driven by changes in the hydrological cycle, induced by variations in astronomical forcing. We suggest that the episodically amplified hydrological cycle not only enhanced nutrient availability and marine productivity, but might also have caused density‐driven upper ocean stratification. The presence of black shales of similar age in other ocean basins suggests that this mechanism is broadly important for the formation of Early Cretaceous organic‐rich successions.