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Paper No. 7
Presentation Time: 3:00 PM


ALGEO, Thomas J., Geology, University of Cincinnati, Cincinnati, OH 45221-0013, MEYERS, Philip A., Geological Sciences, The University of Michigan, 1100 North University Avenue, Ann Arbor, MI 48109-1005 and ROWE, Harry, Earth and Environmental Sciences, University of Texas at Arlington, Box 19049, 500 Yates Street, Arlington, TX 76019,

The modern marine fixed nitrogen (N) budget is influenced predominantly by processes of cyanobacterial N fixation and denitrification. Pre-Quaternary changes in the marine N cycle have received little attention to date, despite the potential of sediment N isotopes to provide insights regarding its long-term evolution. Compilation of N isotope data for >150 marine units of Cambrian through Pliocene age (540 to 2.4 million years ago) reveals that sediment δ15N has varied systematically from higher values (~+4 to +6‰) during icehouse intervals (e.g., the Carboniferous and Neogene) to lower values (~-2 to +2‰) during greenhouse intervals (e.g., the Cambrian-Devonian and Jurassic-Cretaceous). This pattern implies that systematic changes in the relative rates of nitrogen fixation and denitrification occurred in the global ocean at multimillion-year timescales. Although possibly a passive response to tectonic forcings, such variation is more likely to represent an important negative feedback on long-term climate change via modulation of nutrient cycling in marine ecosystems. This pattern of forcing is substantially different from that observed for Quaternary glacial-interglacial cycles, which has been linked to variations in oceanic denitrification rates acting as a positive feedback on global climate change. Thus, the nature of climate-N cycle feedbacks may be fundamentally different at short (sub-million-year) versus long (supra-million-year) timescales.
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