ICEHOUSE-GREENHOUSE CLIMATE IMPACTS ON THE PHANEROZOIC MARINE NITROGEN CYCLE

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 >80 marine units of Late Ordovician through Pliocene age (450 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 Ordovician-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 versus long geologic timescales.