Nitrogen Isotope Record From a Mid-oceanic Paleo-Atoll Limestone to Constrain the Redox State of the Panthalassa Ocean in the Capitanian (Late Guadalupian, Permian)

The Capitanian stage is characterized by marine anoxia possibly related to the extinction, although the global redox structure of the Capitanian ocean has not been constrained. We newly report a nitrogen isotope (d15N) record from a paleo-atoll limestone at the top of a mid-Panthalassan seamount to constrain the spatial extent and duration of the Capitanian marine anoxia. The d 15N value of limestone after acid treatment is substantially high for similar to 5-Myr up to +28%, the highest through the Phanerozoic oceans, suggesting that the nitrogen source (nitrate) was substantially enriched in delta N-15 via denitrification within subsurface oxygen-deficient zones (ODZs; O-2 < 5 mu M). Numerical modeling of nitrogen isotope dynamics in the upwelling system along the seamount suggests that the possible minimum delta N-15 value of a global deep-oceanic nitrate reservoir was ca. +9% in the Capitanian (similar to 4% higher than at the present). Furthermore, a redox-dependent nitrogen isotope mass balance model constrained the global redox structure of the Capitanian superocean. Substantially reducing conditions (O-2 <= 20 mu M) prevailed at intermediate water depths (100-1,000 m), in association with expanded ODZs with anoxic/euxinic cores along continental margins (similar to 0.4% of global ocean volume), while the deep-ocean remained to be more oxidizing (O-2 <= 60 mu M). The enhanced open-ocean productivity associated with the low sea-level and high nutrient flux to the ocean resulted in the global ocean deoxygenation during the cooling stage. Our model is consistent with previous geologic observations and with a possible link between the long-term (similar to 5-Myr) development of marine dysoxia and the extinction. The Capitanian stage is a peculiar stage characterized by several environmental changes including marine anoxia, possibly related to the large extinction. Previous studies reported the evidence for marine anoxia developed at least locally in the Capitanian, although the spatial extent and duration of anoxia in the superocean Panthalassa in the stage have been poorly constrained. Bioavailable nitrogen is essential for life and the nitrogen isotopic composition of sedimentary record is useful to reconstruct the global nitrogen cycle in association with oceanic redox condition in the past. We report a similar to 5-Myr-long nitrogen isotope record from a shallow-water limestone in the lost mid-superocean during the Capitanian. This unique record reveals major changes in global nitrogen cycle in the stage, that is, the appearance of a substantially N-15-enriched oceanic nitrate reservoir indicating enhanced anaerobic respiration under reducing conditions. Numerical modeling further constrains that the reducing water masses (O-2 < 20 mu M) prevailed at intermediate water depths (100-1,000 m) in the superocean, in association with the expanded oxygen-deficient zones with anoxic/euxinic cores developed along the continental margins (at least twice as much as in the modern oceans). Our new model is consistent with the scenario that the long-term subsurface anoxia/euxinia contributed to the shelf extinction.

Automated summary

The Capitanian stage is a peculiar stage characterized by marine anoxia and a large extinction. Previous studies have confirmed the occurrence of marine anoxia at least locally in the Capitanian, but the spatial extent and duration of anoxia in the superocean Panthalassa during this stage have been poorly understood. This study provides new insights into the global nitrogen cycle and redox conditions, showing that the nitrogen source was substantially enriched in δ15N via denitrification within subsurface oxygen-deficient zones. Numerical modeling further reveals that reducing water masses prevailed at intermediate water depths in the superocean, contributing to the shelf extinction.