古代岩石の研究が将来の海洋無酸素状態の予測に寄与（Study of Ancient Rocks Helps Predict Potential for Future Marine Anoxia）

2025-06-24 中国科学院（CAS）

Paleozoic marine biodiversity, atmospheric composition, and seafloor oxygenation history. (Image by Prof. CHEN Jitao’s team)

＜関連情報＞

• https://english.cas.cn/newsroom/research_news/earth/202506/t20250624_1046056.shtml
• https://www.pnas.org/doi/10.1073/pnas.2420505122

大気中の酸素濃度が高く、氷室のような状況下で、海洋無酸素状態が繰り返された Repeated occurrences of marine anoxia under high atmospheric O2 and icehouse conditions

Jitao Chen, Shihan Li, Shuang Zhang, +6 , and Isabel P. Montañez
Proceedings of the National Academy of Sciences  Published:June 23, 2025
DOI:https://doi.org/10.1073/pnas.2420505122

Significance

The overall well-oxygenated Phanerozoic ocean–atmosphere system experienced discrete periods of ocean anoxia that are closely associated with global carbon cycle perturbations under primarily greenhouse climate states. Here we document, through coupled U and C isotopic excursions and biogeochemical modeling, repeated occurrences of CO₂ -induced marine anoxia at the 10⁵ -y-scale during the highly oxygenated, but overall low CO₂, deep glacial (310 to 290 Ma) of the penultimate icehouse. Our joint proxy-model inversion approach indicates moderate-scale seafloor anoxia (4 to 12%) that may have led to a pause or decline in marine biodiversity and reveals the potential for the development of widespread marine anoxia under CO₂ concentrations not much different from today or projected for within this century.

Abstract

The Late Paleozoic Ice Age (~340 to 260 Ma) occurred under peak atmospheric O₂ (1.2 to 1.7 PIAL, pre-industrial atmospheric levels) for Earth history and CO₂ concentrations comparable to those of the preindustrial to that anticipated for our near future. The evolution of the marine redox landscape under these conditions remains largely unexplored, reflecting that oceanic anoxia has long been considered characteristic of carbon cycle perturbation during greenhouse times. Despite elevated O₂, a 10⁵-y period of CO₂-forced oceanic anoxia was recently identified, but whether this short-term interval of widespread oceanic anoxia was anomalous during this paleo-ice age is unexplored. Here, we investigate these issues by building a high-resolution record of carbonate uranium isotopes (δ²³⁸U_carb) from an open-marine succession in South China that permits us to reconstruct the global marine redox evolution through the deep glacial interval (310 to 290 Ma) of near peak O₂. Our data reveal repeated, short-term decreases in δ²³⁸U_carb coincident with negative C isotopic excursions and rises in paleo-CO₂, all superimposed on a longer-term rise in δ²³⁸U_carb. A carbon–phosphorus–uranium biogeochemical model coupled with Bayesian inversion is employed to quantitatively explore the interplay between marine anoxia, carbon cycling, and climate evolution during this paleo-glacial period. Although our results indicate that protracted, enhanced organic carbon burial can account for the long-term O₂ increase, seafloor oxygenation, and overall low CO₂, episodic pulses of C emissions had the potential to drive recurring short-term periods of marine anoxia (with 4 to 12% of seafloor anoxia) despite up to 1.7 times higher atmospheric O₂ than present day.