The Mesozoic is marked by several widespread occurrences of intense organic matter burial. Sediments from the largest of these events, the Cenomanian-Turonian Oceanic Anoxic Event (OAE 2) are characterized by lower nitrogen isotope ratios than are seen in modern marine settings. It has remained a challenge to describe a nitrogen cycle that could achieve such isotopic depletion. Here we use nitrogen-isotope ratios of porphyrins to show that eukaryotes contributed the quantitative majority of export production throughout OAE 2, whereas cyanobacteria contributed on average approximately 20%. Such data require that any explanation for the OAE nitrogen cycle and its isotopic values be consistent with a eukaryote-dominated ecosystem. Our results agree with models that suggest the OAEs were high-productivity events, supported by vigorous upwelling. Upwelling of anoxic deep waters would have supplied reduced N species (i.e., NH 4 þ ) to primary producers. We propose that new production during OAE 2 primarily was driven by direct NH 4 þ -assimilation supplemented by diazotrophy, whereas chemocline denitrification and anammox quantitatively consumed NO 3 − and NO 2 − . A marine nitrogen reservoir dominated by NH 4 þ , in combination with known kinetic isotope effects, could lead to eukaryotic biomass depleted in 15 N.biomarkers | nitrogen fixation | stable isotopes | paleoceanography

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