TECTONIC AND CLIMATIC CONTROLS ON THE PHANEROZOIC MARINE NITROGEN CYCLE

Nitrogen (N) plays a key role in marine productivity and organic carbon fluxes and, thus, is potentially a major influence on the global climate system. Although variations in the N-isotopic composition (δ¹⁵N) of late Neogene sediments are well known, long-term (multimillion-year) changes in the marine N cycle have received little attention to date. We analyzed secular δ¹⁵N variation since 660 Ma based on a survey of 153 marine units and examined its relationship to climate change. δ¹⁵N_sed has varied systematically from higher values (~+4 to +6‰) during icehouse climate modes to lower values (~-2 to +2‰) during greenhouse climate modes. Given that the marine N cycle must be in an approximate homeostatic steady state at geologic timescales (i.e., loss of fixed N to denitrification must be balanced by new N fixation), this pattern of secular isotopic variation is likely due to coupled changes in the rates of water-column denitrification and cyanobacterial N fixation. Icehouse climate modes are characterized by high rates of denitrification and N fixation, related to enhanced marine productivity and expansion of oxygen-minimum zones under conditions of more vigorous thermohaline circulation. In contrast, greenhouse climate modes are characterized by low globally integrated rates of denitrification and N fixation, an inference at odds with the existing paradigm of enhanced N fixation during oceanic anoxic events (OAEs). At geologic timescales, tectonic-plate configurations and ocean circulation patterns appear to have been the dominant control on δ¹⁵N_sed. Large (~5‰) positive shifts in δ¹⁵N_sed were associated with the Early Mississippian climate transition, which resulted from closure of an equatorial seaway and formation of the Tethys Ocean, and the late Cretaceous climate transition, which resulted from opening of the South Atlantic. During both events, rapid climatic cooling may have been due to changes in the locus of deepwater formation, with consequent effects on the marine N cycle.