266-5 Investigating Nitrogen Cycling Dynamics Preserved in Neoproterozoic (Glacio)marine Sediments: Insights from the Southwestern Unites States

Session: The Neoproterozoic Earth and Life Co-evolution (Posters)

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During the Neoproterozoic, shallow marine settings experienced global glaciations, evolving redox structures, and eukaryotic expansion. Discerning the coevolution of habitability and eukaryotes within these settings depends on understanding nutrient availability. One key limiting nutrient is nitrogen (N), a bioessential element whose speciation varies relative to biologic activity and water column redox. Marine N cycling is reconstructed using sedimentary N isotope values (δ¹⁵N); however, the δ¹⁵N record remains spatially and temporally limited for the Neoproterozoic. To address this, we present N isotopic values for whole rock (δ¹⁵N_WR) and kerogen fractions (δ¹⁵N_ker), organic δ¹³C values, trace element concentrations, and mineralogical abundances from late Tonian, Cryogenian, and Ediacaran sediments in the southwestern Unites States.

Our results show aerobic N cycling persisted for much of the Neoproterozoic. Most Ediacaran, Cryogenian, and Tonian sediments have δ¹⁵N_WR values around 5- 7‰, similar to average nitrate δ¹⁵N in modern seawater. Within the same samples, most δ¹⁵N_ker values are ~2- 4‰ lower than δ¹⁵N_WR. During the late Tonian, δ¹⁵N_WR decreases 2- 4‰, then increases during the Cryogenian. This 2- 4‰ difference is also observed in the δ¹⁵N_ker fraction and occurs between Tonian- Cryogenian sections in the Death Valley area (Horse Thief Springs and Kingston Peak formations) and the Wasatch Mountains (Big Cottonwood and Mineral Fork formations). Enrichments of redox-sensitive trace metals mostly align with sections of low δ¹⁵N. Chemical Index of Alteration values are generally higher during the Tonian and Ediacaran (CIA=65-80) and lower during the Cryogenian (~55-70). C/N ratios of whole rock and kerogen, as well as mineralogy, reflect heterogenous inputs of allochthonous sediment. However, these inputs are shown to have low N concentrations and do not obscure the authigenic δ¹⁵N signal. The combined δ¹⁵N results and other geochemical proxies describe N cycle conditions where nitrate would have been available in local, partially oxygenated seawater during the Tonian, Cryogenian and Ediacaran. A more anaerobic N cycle developed near the Tonian-Cryogenian transition, perhaps driven by the onset of Sturtian glaciation.

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

doi: 10.1130/abs/2025AM-8493

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