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.

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