Articles | Volume 11, issue 4
Biogeosciences, 11, 1273–1295, 2014
Biogeosciences, 11, 1273–1295, 2014

Research article 27 Feb 2014

Research article | 27 Feb 2014

Icehouse–greenhouse variations in marine denitrification

T. J. Algeo1, P. A. Meyers2, R. S. Robinson3, H. Rowe4, and G. Q. Jiang5 T. J. Algeo et al.
  • 1Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA
  • 2Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA
  • 3Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA
  • 4Department of Earth and Environmental Sciences, University of Texas at Arlington, Arlington, TX 76019, USA
  • 5Department of Geoscience, University of Nevada Las Vegas, Las Vegas, Nevada, USA

Abstract. Long-term secular variation in the isotopic composition of seawater fixed nitrogen (N) is poorly known. Here, we document variation in the N-isotopic composition of marine sediments (δ15Nsed) since 660 Ma (million years ago) in order to understand major changes in the marine N cycle through time and their relationship to first-order climate variation. During the Phanerozoic, greenhouse climate modes were characterized by low δ15Nsed (∼−2 to +2‰) and icehouse climate modes by high δ15Nsed (∼+4 to +8‰). Shifts toward higher δ15Nsed occurred rapidly during the early stages of icehouse modes, prior to the development of major continental glaciation, suggesting a potentially important role for the marine N cycle in long-term climate change. Reservoir box modeling of the marine N cycle demonstrates that secular variation in δ15Nsed was likely due to changes in the dominant locus of denitrification, with a shift in favor of sedimentary denitrification during greenhouse modes owing to higher eustatic (global sea-level) elevations and greater on-shelf burial of organic matter, and a shift in favor of water-column denitrification during icehouse modes owing to lower eustatic elevations, enhanced organic carbon sinking fluxes, and expanded oceanic oxygen-minimum zones. The results of this study provide new insights into operation of the marine N cycle, its relationship to the global carbon cycle, and its potential role in modulating climate change at multimillion-year timescales.

Final-revised paper