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)

Author:

Saitoh Masafumi1234ORCID,Nishizawa Manabu15ORCID,Ozaki Kazumi26,Ikeda Masayuki7,Ueno Yuichiro256,Takai Ken5,Isozaki Yukio8

Affiliation:

1. Laboratory of Ocean‐Earth Life Evolution Research (OELE) Japan Agency for Marine‐Earth Science and Technology (JAMSTEC) Yokosuka Japan

2. Earth‐Life Science Institute Tokyo Institute of Technology Tokyo Japan

3. School of Geosciences and Civil Engineering Kanazawa University Kanazawa Japan

4. The University Museum The University of Tokyo Tokyo Japan

5. Institute for Extra‐cutting‐edge Science and Technology Avant‐garde Research (X‐star) Super‐cutting‐edge Grand and Advanced Research (SUGAR) Program Japan Agency for Marine‐Earth Science and Technology (JAMSTEC) Yokosuka Japan

6. Department of Earth and Planetary Sciences Tokyo Institute of Technology Tokyo Japan

7. Department of Earth and Planetary Environmental Science The University of Tokyo Tokyo Japan

8. Department of Earth Science and Astronomy The University of Tokyo Tokyo Japan

Abstract

AbstractThe 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 (δ15N) 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 δ15N value of limestone after acid treatment is substantially high for ∼5‐Myr up to +28‰, the highest through the Phanerozoic oceans, suggesting that the nitrogen source (nitrate) was substantially enriched in 15N via denitrification within subsurface oxygen‐deficient zones (ODZs; O2 < 5 µM). Numerical modeling of nitrogen isotope dynamics in the upwelling system along the seamount suggests that the possible minimum δ15N value of a global deep‐oceanic nitrate reservoir was ca. +9‰ in the Capitanian (∼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 (O2 ≤ 20 µM) prevailed at intermediate water depths (100–1,000 m), in association with expanded ODZs with anoxic/euxinic cores along continental margins (≥ ∼0.4% of global ocean volume), while the deep‐ocean remained to be more oxidizing (O2 ≤ 60 µM). 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 (∼5‐Myr) development of marine dysoxia and the extinction.

Publisher

American Geophysical Union (AGU)

Subject

Paleontology,Atmospheric Science,Oceanography

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