N2-fixing cyanobacteria supplied nutrient N for Cretaceous oceanic anoxic events

The abundance of specific membrane lipids, 2-methylhopanoids, indicates that cyanobacteria played a key role in the seemingly global deposition of black shales during the early Aptian (ca. 120.5 Ma) and late Cenomanian (ca. 93.5 Ma) oceanic anoxic events. Organic matter–rich sediments deposited during these events are characterized by a ¹⁵N content typical of newly fixed N₂, indicating that cyanobacterial N₂ fixation was the main source for nutrient N. We propose that denitrification and anaerobic ammonium oxidation effectively cut off the return of nutrient N from the anoxic deep waters to the photic zone in the oceans, giving N₂-fixing cyanobacteria a competitive advantage over algae during these oceanic anoxic events.