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Study of ancient rocks helps predict potential for future marine anoxia
Chinese Academy of Sciences Headquarters
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Paleozoic marine biodiversity, atmospheric composition, and seafloor oxygenation history.
view moreCredit: Image by Prof. CHEN Jitao's team
Earth's current climate is considered an "icehouse climate" due to the existence of polar ice caps. This is important because previous icehouse climates can better predict how atmospheric oxygen and carbon dioxide (CO2) levels today may affect the risk of marine anoxia and subsequent marine biodiversity loss in the future.
To understand the interplay among atmospheric oxygen and CO2 levels and oxygenation conditions in the ocean during an earlier icehouse climate, an international team led by Prof. CHEN Jitao from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences studied ancient sedimentary rocks in Naqing, South China, to analyze their chemical compositions.
Specifically, the researchers derived high temporal-resolution records of carbonate uranium isotopes from a marine carbonate slope succession dating from the late Carboniferous to early Permian (310–290 million years ago). This geologic epoch is part of the Late Paleozoic Ice Age (LPIA) (360–260 million years ago), which is recognized as the longest icehouse climate since advanced plants and terrestrial ecosystems appeared.
By combining these records with previously published carbonate carbon isotopes, paleo-CO2 data, and records of volcanic activity and plant evolution, the researchers quantitatively explored, through biogeochemical modeling, the global carbon cycle and marine oxygen conditions for this geological period. This work was published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).
The study revealed rapid drops in levels of carbonate uranium isotopes, which occurred alongside rapid increases in atmospheric CO2 levels. This suggests that seafloor anoxia expanded even during the Phanerozoic maximum of atmospheric oxygen and the glacial peak of the LPIA.
Using a carbon–phosphorus–uranium (C-P-U) biogeochemical model coupled with Bayesian inversion, the researchers quantitatively examined the interactions among marine anoxia, carbon cycling, and climate evolution during this paleo-glacial period. Model results indicated that enhanced burial of marine organic carbon likely drove the overall decline in atmospheric CO2 and the rise in oxygen levels in the atmosphere–ocean system throughout this interval. However, despite these high oxygen levels, episodic massive carbon emissions could have triggered recurrent global warming and seafloor deoxygenation.
Furthermore, the team's model showed an increase of 4–12% in the extent of the anoxic seafloor, which could have led to a pause or decline in marine biodiversity. This study emphasizes that under current icehouse conditions, which mirror the high-oxygen state of the LPIA, ongoing warming may still provoke widespread ocean anoxia.
This study advances our understanding of the processes and feedback mechanisms within the Earth system during icehouse conditions, enabling more accurate projections of the future trajectory of current global warming and marine deoxygenation.
Journal
Proceedings of the National Academy of Sciences
DOI
Geochemical, geologic, and biotic records for the late Carboniferous to early Permian, showing repeated occurrences of anoxic events (AE1–5).
Credit
Image by Prof. CHEN Jitao's team
Massive burps of carbon dioxide led to oxygen-less ocean environments in the deep past
University of California - Davis
New research from the University of California, Davis, the Chinese Academy of Sciences and Texas A&M University reveals that massive emissions, or burps, of carbon dioxide from natural earth systems led to significant decreases in ocean oxygen concentrations some 300 million years ago.
Combining geochemical analyses of sediment cores and advanced climate modeling, the study, published June 23 in Proceedings of the National Academy of Sciences, highlights five periods when significant decreases in ocean oxygen levels (by 4% to 12%) coincided with significant increases in levels of carbon dioxide in the atmosphere. Such oxygen-less, or anoxic, events are known for their detrimental effects on marine life and biodiversity.
Despite their roots in the deep past, the findings are relevant to the current global climate and its future. If events of a similar scale were to happen today, they would likely affect coastal areas that are important for fisheries and marine biodiversity.
“This is our only analog for big changes in carbon dioxide at levels comparable to what we’re living in today, where we see doublings and triplings of the levels,” said senior author Isabel P. Montañez, a Distinguished Professor in the Department of Earth and Planetary Sciences at UC Davis.
What’s different, though, is the source of the carbon dioxide. While carbon dioxide levels of long-past climates were influenced by natural systems like volcanic eruptions, human-produced and human-related carbon dioxide emissions strongly influence today’s levels.
“We’re creating a burp now and at a rate two, maybe three, orders of magnitude faster than in the past,” Montañez said.
Sediment cores and deep climate modeling
In the study, the team used sediment cores sourced from a geological formation in South China called the Naqing succession. By analyzing the geochemical makeup of these deep-water cores, specifically carbonate uranium isotopes, the team chronicled Earth’s environmental conditions from 310 to 290 million years ago.
“Through that analysis, we see these ‘burps’ not just in carbon dioxide but in the ocean’s uranium isotope signature too,” Montañez said. “They’re totally aligned, and the size of those uranium spikes tell us about the magnitude of the ocean anoxia.”
The team then used that information to inform leading-edge climate models, developed by the authors of this study, that are used to better understand ancient climates.
“It’s a mathematical framework in which we put in all our proxy information and we run it hundreds of thousands of times on a supercomputer,” Montañez said. “It basically best models what is most realistic given all the uncertainties, all the knowns, all the information that it’s given.”
Based on the modeling, the team found five instances of decreased oxygen in the global ocean by 4% to 12% from 290 to 310 million years ago. Each period lasted for roughly 100,000 to 200,000 years.
While the decrease in ocean oxygen doesn’t appear to correlate to any known mass extinctions, it does align with pauses in biodiversity that can be seen in the geological record.
“We do see these pauses in biodiversity each time these burps happen,” Montañez said. “It had an impact, most likely coastal regions were impacted the most.”
Records of the past, lessons for the future
The Earth of 300 million years ago was vastly different than the Earth of today. For one, oxygen in the atmosphere was 40% to 50% higher than it is today. Despite the differences between past and present, the magnitude of the rises in carbon dioxide levels are similar.
That could be interpreted as a warning, according to Montañez.
“This is a huge discovery because how do you take an ocean sitting under an atmosphere with much more oxygen than today and permit this?” Montañez said. “The message for us is, ‘Don’t be so sure that we can’t do this again with our current human-driven release of carbon dioxide.’”
Additional authors are: Jitao Chen, Chinese Academy of Sciences; Shihan Li and Shuang Zhang, Texas A&M University; Terry Isson, University of Waikato, New Zealand; Tais Dahl, University of Copenhagen, Denmark; Noah Planavsky, Yale University; Feifei Zhang, Xiang-dong Wang and Shu-Zhong Shen, Nanjing University, China.
The research was supported in part by grants from the National Natural Science Foundation of China and the U.S. National Science Foundation.
Journal
Proceedings of the National Academy of Sciences
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Repeated occurrences of marine anoxia under high atmospheric O2 and icehouse conditions
Article Publication Date
23-Jun-2025
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