Dominant eukaryotic export production during ocean anoxic events reflects the importance of recycled NH₄⁺

Authors

Meytal B. Higgins
Rebecca S. Robinson, University of Rhode IslandFollow
Jonathan M. Husson
Susan J. Carter
Ann Pearson

Document Type

Article

Date of Original Version

2012

Department

Oceanography

Abstract

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., NH₄⁺) to primary producers. We propose that new production during OAE 2 primarily was driven by direct NH₄⁺-assimilation supplemented by diazotrophy, whereas chemocline denitrification and anammox quantitatively consumed NO₃^- and NO₂^-. A marine nitrogen reservoir dominated by NH₄⁺, in combination with known kinetic isotope effects, could lead to eukaryotic biomass depleted in ¹⁵N.

Citation/Publisher Attribution

Higgins, M. B., Robinson, R. S., Husson, J. M., Carter, S. J., & Pearson, A. (2012). Dominant eukaryotic export production during ocean anoxic events reflects the importance of recycled NH₄⁺. Proc Natl Acad Sci USA, 109(7), 2269-2274. doi: 10.1073/pnas.1104313109

Available at: http;//dx.doi.org/10.1073/pnas.1104313109