A continental-weathering control on orbitally driven redox-nutrient cycling during Cretaceous Oceanic Anoxic Event 2

The Cretaceous period (~145–65 m.y. ago) was characterized by intervals of enhanced organiccarbon burial associated with increased primary production under greenhouse conditions.The global consequences of these perturbations, oceanic anoxic events (OAEs), lasted upto 1 m.y., but short-term nutrient and climatic controls on widespread anoxia are poorly understood.Here, we present a high-resolution reconstruction of oceanic redox and nutrient cyclingas recorded in subtropical shelf sediments from Tarfaya, Morocco, spanning the initiation ofOAE2. Iron-sulfur systematics and biomarker evidence demonstrate previously undescribedredox cyclicity on orbital time scales, from sulfidic to anoxic ferruginous (Fe-rich) water-columnconditions. Bulk geochemical data and sulfur isotope modeling suggest that ferruginousconditions were not a consequence of nutrient or sulfate limitation, despite overall low sulfateconcentrations in the proto–North Atlantic. Instead, fluctuations in the weathering influxes ofsulfur and reactive iron, linked to a dynamic hydrological cycle, likely drove the redox cyclicity.Despite the potential for elevated phosphorus burial in association with Fe oxides underferruginous conditions on the Tarfaya shelf, porewater sulfide generation drove extensive phosphorusrecycling back to the water column, thus maintaining widespread open-ocean anoxia