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 (super 15) N content typical of newly fixed N (sub 2) , indicating that cyanobacterial N (sub 2) 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 N (sub 2) -fixing cyanobacteria a competitive advantage over algae during these oceanic anoxic events.