CONSTRAINING THE EARLY NEOPROTEROZOIC NITROGEN CYCLE (Invited Presentation)

Denitrification and nitrogen fixation impart distinct isotopic signatures on biologically available nitrogen that can be traced by the nitrogen isotopic composition of sediments. Water-column redox cycling of nutrient nitrogen occurring at the oxic-anoxic interface results in significant ¹⁵N-enrichments, and nitrogen fixation is relatively ¹⁵N-depleted reflecting the atmospheric N₂ source. To reach a basic, first-order understanding of the state of the Neoproterozoic nitrogen cycle here we present bulk sediment δ¹⁵N from over 250 shale and mudstone samples of low thermal maturity. Materials were collected from outcrops and cores from the early Neoproterozoic through the end-Cryogenian (Marinoan) glaciation from Svalbard, Australia, Tasmania, Northwestern Canada and the Grand Canyon. Relatively little is know about the Neoproterozoic nitrogen cycle. Given the redox sensitivity of the nitrogen cycle it may be useful for enhancing our understanding the transition from a sulfidic early Neoprterozoic ocean to a ferruginous mid-Neoproterozoic ocean prior to the Ediacaran rise in atmospheric O₂. We recognize the drawbacks of bulk δ¹⁵N analyses and the relatively local signature of nitrogen cycle processes. However, these data are useful for constraining the range of variability in the nitrogen cycle that may be further elucidated by kerogen or compound-specific isotope analyses, and broader spatial and temporal sampling. Despite the range of environments, the immense magnitude of climatic perturbations and long period of time encompassed by these samples, the range of values is surprisingly small, from +1‰ to +9‰. In fact, aside from the narrow time interval following the mid-Cryongenian glaciation (Sturtian) the data range only from +1‰ to +6‰. This range of values is easily placed in the context of a modern nitrogen cycle despite the low atmospheric O₂ and anoxic deep-water conditions indicated by a range of geochemical proxies. Like the modern ocean, water-column redox cycling in the Neoproterozoic exerted similar influence on the d¹⁵N of dissolved inorganic nitrogen. Additionally, processes that result in δ¹⁵N values that are below the nitrogen-fixation end member (-1‰) that are characteristic of Phanerozoic Oceanic Anoxic Events and upwelling of ammonium from a shallow chemocline are not observed.