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

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 ch...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2012-02, Vol.109 (7), p.2269-2274
Main Authors: Higgins, Meytal B, Robinson, Rebecca S, Husson, Jonathan M, Carter, Susan J, Pearson, Ann
Format: Article
Language:English
Subjects:
Bacteria
Biological Sciences
biomass
Cyanobacteria
denitrification
ecosystems
Eukaryotes
eukaryotic cells
Isotopes
Nitrogen
nitrogen cycle
nitrogen fixation
organic matter
Physical Sciences
porphyrins
Sediments
trophic relationships
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Summary: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., Formula ) to primary producers. We propose that new production during OAE 2 primarily was driven by direct Formula -assimilation supplemented by diazotrophy, whereas chemocline denitrification and anammox quantitatively consumed Formula and Formula . A marine nitrogen reservoir dominated by Formula , in combination with known kinetic isotope effects, could lead to eukaryotic biomass depleted in 15N.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1104313109