252-1 Reconstructing Atmospheric pN2O Across Phanerozoic Climate Transitions

Session: Climate Transitions in the Paleozoic

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Abstract:

Despite its low abundances today, the trace gas nitrous oxide (N₂O) modulates Earth’s atmospheric chemistry and climate. Since N₂O acts as a potent greenhouse gas and drives the catalytic destruction of stratospheric ozone (O₃), its abundance over deep time may have influenced surface temperatures and UV irradiation, with manifold effects on plant evolution and climate change over deep time. However, no proxy record exists for atmospheric pN₂O over the Paleozoic, leaving its role in ancient climate and atmospheric chemistry enigmatic.

To investigate the temporal variability in N₂O emissions and steady-state atmospheric pN₂O over the Phanerozoic, we apply a reconstructive modeling approach. Here we present an Earth-system box model of the nitrogen (N) and phosphorus (P) cycles over geologic time, which accounts for biogeochemical N-cycling in marine and terrestrial ecosystems, N₂O sources and sinks, and the photochemical lifetime of N₂O. The model relies extensively on independently derived boundary conditions to better account for the varying effects of GMST, atmospheric pO₂, marine pH, and tectonic evolution on biogeochemical dynamics over the past 550 Ma.

Our results represent the first detailed reconstructions of the N₂O cycle over the Phanerozoic. We find that global productivity and N-cycle fluxes covary strongly with GMST and pO₂, as enhanced crustal weathering, faster metabolic rates, and more efficient nutrient recycling amplify marine eutrophication and N-fixation. Consequently, N₂O emissions vary by two orders of magnitude across the Phanerozoic. The strongest variability is observed in the Paleozoic, with pN₂O ranging from <100 ppb to >6 ppm between the late Ordovician “icehouse” and the Silurian-Devonian “hothouse”.

Though this study does not resolve the climatic effects of N₂O, the strong covariance between N₂O emissions and GMST suggests a potential positive feedback coupling between climate and N₂O emissions. This feedback, driven by a “hothouse N-cycle” coupled to redox stratification in hyper-productive and eutrophic oceans, may help to explain the extremely high temperatures and stability of Phanerozoic hothouse climates. Additionally, intense N₂O emissions may have disrupted the stratospheric O₃ layer and increased exposure of surface life to harmful UV radiation, especially during the later Paleozoic. In future work, we intend to explore the role of N₂O in amplifying climate extrema and attenuating the stratospheric O₃ layer, particularly during key transitions in Phanerozoic climate and biological evolution.

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

doi: 10.1130/abs/2025AM-7145

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