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

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doi: 10.1073/pnas.1104313109
Author(s): Higgins, Meytal B.; Robinson, Rebecca S.; Husson, Jonathan M.; Carter, Susan J.; Pearson, Ann
Author Affiliation(s): Primary:
Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA, United States
Other:
University of Rhode Island, United States
Volume Title: Proceedings of the National Academy of Sciences of the United States of America
Source: Proceedings of the National Academy of Sciences of the United States of America, 109(7), p.2269-2274. Publisher: National Academy of Sciences, Washington, DC, United States. ISSN: 0027-8424 CODEN: PNASA6
Note: In English. 63 refs.; illus.
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., 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 15 N.
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Ammonium ion; Atlantic Ocean; Biochemistry; Biomarkers; C-13/C-12; Carbon; Cenomanian; Cretaceous; Cyanobacteria; Cyclic processes; Demerara Rise; Depletion; Equatorial Atlantic; Eukaryotes; Fixation; Geochemistry; Hydrochemistry; Isotope ratios; Isotopes; Kerogen; Leg 207; Marine environment; Marine sediments; Mesozoic; N-15; N-15/N-14; Nitrogen; North Atlantic; Northwest Atlantic; ODP Site 1258; Ocean Drilling Program; Oceanic anoxic events; Organic compounds; Paleo-oceanography; Paleoecology; Pigments; Porphyrins; Recycling; Sediments; Stable isotopes; Stratigraphic boundary; Turonian; Upper Cretaceous; West Atlantic
Coordinates: N092600 N092600 W0544400 W0544400
Record ID: 2014038001
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