251-6 Nitrogen isotope evidence for rapid depletion of bioavailable nitrate in the Illinois Basin across the Frasnian-Famennian boundary

Session: Phanerozoic Earth System Shifts in the Marine Sedimentary Record

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The expansion of marine anoxia is often implicated as a driver for the Late Devonian mass extinctions, but the role of other aspects of ocean chemistry such as nutrient availability is less well understood. In particular, nitrate availability plays a key role in maintaining ocean habitability for eukaryotes that lack the ability to fix their own nitrogen. Here, we present nitrogen isotope (d¹⁵N) data for three cores that span the Frasnian-Famennian transition in the New Albany Shale, Illinois Basin, North America. Our previous paleo-redox and paleosalinity analysis of these same cores revealed a fundamental hydrographic reorganization across the epeiric sea at the Frasnian-Famennian boundary (FFB). In the Frasnian, the Illinois Basin was relatively isolated and persistent anoxia only developed along the shallow Cumberland Sill under marine salinities, with the deep interior of the basin exhibiting intermittently oxygenated conditions. In the Famennian, however, the basin potentially became better connected to the broader North American epeiric seaway and an increasing influx of freshwater led to the development of strong density stratification, driving a major expansion of anoxia and euxinia in the deep basin interior. In the two cores representing intermediate and deep-water sites, d¹⁵N values are mostly positive in the Frasnian, indicating an aerobic nitrogen cycle with nitrification and partial denitrification, and thus replete bioavailable nitrate. Across the FFB, however, there is a rapid shift to d¹⁵N values of ~0‰ in both cores, indicating the onset of a completely anaerobic nitrogen cycle dominated by nitrogen fixation, and thus a lack of bioavailable nitrate. In both cores, this rapid decline in d¹⁵N is accompanied by a positive excursion in d¹³C_org which may correspond to the Upper Kellwasser Event. A completely different d¹⁵N signal is recorded in a third, shallower core deposited along the Cumberland Sill where d¹⁵N values are ~0‰ across the FFB, but then rise to positive values later in the Famennian. Together, our data indicate that hydrographic and redox changes across the FFB drove the rapid depletion of bioavailable nitrate and the onset of an anaerobic nitrogen cycle in the Illinois Basin, suggesting that bioavailable nitrate depletion in epeiric seas may represent an overlooked contributing factor to the Late Devonian mass extinctions.       

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9091

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