56 Million Years Ago, the Arctic Ocean Supercharged Earth’s Climate
- MM24 News Desk
- Oct 17
- 3 min read
A groundbreaking study led by Chinese scientists has unveiled how tiny shifts in ocean chemistry helped amplify global warming 56 million years ago — offering new insight into how the Arctic Ocean once turned from a carbon sponge into a carbon source.
The research, led by Zhang Yige from the Guangzhou Institute of Geochemistry under the Chinese Academy of Sciences, The international team discovered that subtle changes in the concentration of oceanic sulfate acted as a “chemical switch,” altering how methane — one of the most powerful greenhouse gases — was consumed in the ocean. This switch, they found, played a critical role in intensifying the Paleocene-Eocene Thermal Maximum (PETM), a period of extreme global warming and ocean acidification roughly 56 million years ago.
A Chemical Trigger for Ancient Climate Change
During the PETM, Earth’s temperatures soared, oceans became more acidic, and massive amounts of carbon were released into the atmosphere. For decades, scientists have puzzled over what fueled this ancient climate catastrophe, which bears striking similarities to modern global warming.
According to Zhang, the answer lies beneath the waves — in the chemistry of the ancient Arctic Ocean. “Due to a severe shortage of sulfate, just like a lack of fuel in a power plant, methane could not be efficiently processed,” Zhang explained. “Instead, oxygen-loving bacteria began to rapidly oxidize methane, directly consuming oxygen and releasing carbon dioxide — much like a high-temperature combustion that produces exhaust gases.”
The Slow Burn vs. the Fast Flame
In today’s oceans, about 90 percent of methane is consumed by microorganisms living in oxygen-free sediments. These microbes use sulfate as “fuel,” converting methane slowly and efficiently while releasing alkaline by products that help counteract ocean acidification. Zhang likens this process to a “slow-burning power plant.”
However, during the PETM, the sulfate concentration in Arctic seawater was less than one-third of what it is today. This scarcity forced methane-consuming processes into overdrive. Without enough sulfate, the “slow burners” could not function, and the “fast burners” — oxygen-dependent bacteria — took over. This led to a rapid surge in carbon dioxide emissions, dramatically accelerating global warming.
Tracing Ancient Clues
To uncover this ancient process, Zhang’s team reconstructed the methane oxidation pathways using molecular tracers — chemical fossils left behind by bacteria millions of years ago. They analyzed the compound hop-17(21)-ene and its carbon isotope composition, which serve as “identity cards” for ancient microorganisms.
“By reading these ‘identity cards,’ we can tell which microorganisms were active — the slow-burning generators or the fast burners — and how intense their activity was,” explained Kim Bum-soo, the study’s first author from South Korea.
The findings revealed that during the later stages of the PETM, activity from the “fast burners” surged dramatically, coinciding with a spike in carbon dioxide levels. CO₂ concentrations reconstructed from marine phytoplankton tracers showed that the Arctic Ocean’s CO₂ levels were 200–700 ppm higher than the global average, suggesting that the region had transformed from a carbon “sponge” into a “chimney” venting greenhouse gases into the atmosphere.
Lessons for a Warming Future
Researcher Shen Jiaheng, a co-author of the study, noted that as the Arctic Ocean freshened and sulfate levels dropped, methane decomposition could only proceed through the rapid oxidation route — directly generating large amounts of CO₂. “This fundamentally changed the Arctic’s role in the global carbon cycle, turning it into a source of greenhouse gas emissions,” she said.
The study also highlights how geological processes — such as volcanic activity, continental weathering, and tectonic shifts — influence sulfate levels in the ocean, thereby shaping the global carbon cycle over geological timescales.
Zhang emphasized that these ancient mechanisms could have modern parallels. As today’s Arctic Ocean warms and freshens due to melting ice and changing salinity, similar methane oxidation patterns could emerge once again. “When the Arctic’s chemical environment shifts,” Zhang warned, “the scenario from 56 million years ago could repeat itself — with methane moving from slow, efficient use to rapid, climate-boosting release. This is a signal we must pay close attention to.”
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