A new NASA-led geochemical study reveals that the Arctic Ocean floor, once a sink for methane, may again turn into a powerful source of carbon dioxide if current warming continues. Researchers analysing ancient molecules from sediments beneath the Lomonosov Ridge found that around 56 million years ago, during the Paleocene Eocene Thermal Maximum (PETM), microbial activity switched gears transforming methane removal processes and amplifying a global heatwave. Could history now be repeating itself as modern Arctic waters warm and stratify?

Clues from the Ancient Seafloor

The research, guided by organic geochemist Bumsoo Kim at NASA’s Johnson Space Center, examined biomarkers molecular remnants of ancient microbes buried deep within Arctic sediment cores. By comparing carbon isotopes and molecular traces, the team reconstructed how microbes processed methane during one of Earth’s most intense warming events. Their findings revealed a remarkable transition: methane oxidation that once occurred without oxygen in sediments gave way to oxygen-driven reactions in open water. This aerobic process converts methane into carbon dioxide and consumes oxygen, subtly acidifying the surface ocean and altering its chemistry. Before the PETM, the Arctic’s methane output was largely contained within the sediments, thanks to anaerobic microbes that used sulfate to convert methane safely underground, creating alkalinity that buffered the ocean. But as sulfate availability dropped during the ancient warming, this natural brake weakened. Methane leaked upward into the water, where oxygen-breathing microbes completed its oxidation—producing CO₂, drawing down oxygen, and creating hypoxic conditions hostile to marine life.

From Methane Sink to Carbon Source

This transition effectively turned the Arctic Ocean from a stabilizing methane sponge into a net source of carbon dioxide. The long tail of the PETM shows that such feedbacks can outlast their triggers by thousands of years, with lasting effects on climate balance. The researchers warn that similar microbial shifts could unfold again in today’s Arctic, where freshening surface waters and rapid warming phenomena collectively known as polar amplification are changing how gases circulate between the seafloor and atmosphere. Methane is a potent greenhouse gas, second only to carbon dioxide in its radiative impact, and a critical player in feedback loops that accelerate planetary heating. A modern resurgence of methane oxidation in Arctic waters could, therefore, multiply the long-term effects of today’s emissions, extending warming far beyond human control.

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Modern Parallels: Methane Bubbling to the Surface

Contemporary field data already show methane seeping from the seafloor in multiple Arctic regions, including the Western Svalbard margin and the Eastern Siberian Arctic Shelf. These emissions stem from melting permafrost and the breakdown of methane hydrates, frozen gas structures destabilized by rising ocean temperatures. Though current releases are relatively small compared to fossil fuel emissions, the pace is quickening in certain hotspots, hinting at the start of a feedback cycle similar to that seen in the ancient record. Scientists now use a suite of technologies satellite spectroscopy, autonomous submersibles, and aerial drones to trace methane concentrations from the seabed to the atmosphere. These systems detect how much gas escapes oxidation and directly contributes to greenhouse forcing. The emerging picture is that the Arctic methane network is dynamic, sensitive, and increasingly active in response to subtle environmental shifts.

Reconstructing the Past: A Window into Earth’s Climate Memory

The study draws upon deep-sea cores recovered through the Integrated Ocean Drilling Program’s Arctic Coring Expedition, a multinational project that probes ancient sediments to decode Earth’s climatic evolution. The cores from the Lomonosov Ridge captured the transition into and out of the PETM, offering a rare glimpse of how Arctic biogeochemistry responded to extreme warming. Scientists focused on a specific hopanoid compound, an organic molecule produced by methane-consuming bacteria whose unusually light carbon isotopic signature confirmed widespread methane oxidation. By comparing layers of sediment, they identified a clear “handoff” between anaerobic sediment microbes and aerobic water-column microbes. This shift reflected a stratified ocean, with reduced vertical mixing and declining oxygen levels, mirroring trends now observed in modern Arctic waters.

Lessons for Today’s Arctic Future

Current Arctic conditions echo many features of that ancient transformation. Warming, ice melt, and declining salinity are creating more pronounced stratification, preventing oxygen from reaching deeper layers and altering microbial ecosystems. If sediment-based methane consumption declines again, more methane could ascend into the water column, where it is converted into carbon dioxide rather than neutralized. The result would be greater ocean acidification and prolonged atmospheric heating, a slow-burning feedback that could outlast initial human-driven warming. Researchers emphasize that today’s Arctic differs from the PETM in many ways: the ancient ocean was more isolated and chemically distinct. Yet the parallel mechanisms raise sobering questions about how Earth’s carbon systems behave under sustained stress. With climate models still uncertain about long-term carbon feedbacks beyond the year 2100, understanding these ancient events becomes crucial to predicting our own trajectory.

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A Warning Written in Sediment

The Arctic’s geological record has preserved a cautionary tale of how subtle microbial shifts can transform the planet’s climate balance. As methane seeps intensify and ocean layers grow more stratified, the risk of a large-scale methane-to-CO₂ flip grows increasingly real. The new findings suggest that feedbacks operating on millennial timescales could amplify warming in ways that current models underestimate. In essence, the study’s message is clear: Earth’s ancient climate archive isn’t just a window into the past, it may be a preview of what lies ahead if Arctic change continues unchecked.

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