The Arctic Ocean was once a major source of greenhouse gases in the atmosphere – and it could become so again, researchers warn.
Methane (CH4) East just behind carbon dioxide (CO2) by trapping heat in the Earth’s atmosphere. Since 2020, human-caused greenhouse gas emissions have increased atmospheric methane about 10 parts per billion per yearmore than twice as much as CO2. However, scientists don’t yet know how the methane cycle will respond as our planet continues to warm.
The team focused on a period of rapid warming and ocean acidification that occurred around 56 million years ago, known as the Paleocene-Eocene Thermal Maximum (PETM). The PETM is one of the best examples of major climate change caused by disruptions in the carbon cycle on Eartha bit like the global warming we experience today.
Scientists have already shown that PETM is accompanied by widespread release of CO2 and CH4 in the oceans and atmosphere, which left distinct geochemical fingerprints in the sedimentary rocks of this period. But despite 30 years of research, scientists still cannot determine where these gases come from.
To explore how the carbon cycle worked during the PETM, the researchers behind the new study examined a 15-meter core of marine sediment drilled in the central Arctic Ocean by the Integrated Ocean Drilling Program. Arctic coring expedition. The sediments date back 66 million years, preserving the PETM warming event and the subsequent “recovery” period, during which the climate eventually restabilized.
The team extracted organic molecules from the sediments and measured different forms of carbon they contained. They identified organic molecules, called biomarkers, to determine which microbes were living on the seafloor when the sediment was deposited. They used forms of carbon, called isotopes, to determine what these microbes ate.
Methane generally contains lighter carbon isotopes than CO2meaning that methane-eating microbes produce biomarkers with typically light carbon isotopes. The researchers tracked these biomarkers in the cores and found that the main methane eaters in the Arctic Ocean had changed during the PETM.
Before PETM, methane formed deep beneath the seafloor and was consumed by microbes that breathe sulfate instead of oxygen, via a process known as anaerobic methane oxidation (AOM). But during PETM, biomarkers of AOM microbes decreased.
Today, AOM consumes the majority of methane found in marine sediments because sulfate is abundant in modern oceans. However, scientists believe that the sulfate was considerably lower during the PETM, meaning the amount of methane they could eat was limited to these microbes. The researchers suggest that a massive methane burp during PETM could have “overwhelmed the AOM sediment biofilter,” releasing methane into the seawater, they wrote in the study.
Once the methane reached the water column, biomarkers indicated that another set of microbes took over. These microbes consumed methane while breathing oxygen, through a process known as aerobic methane oxidation (AeOM).
Researchers suggest this change could have turned the Arctic into a major source of CO2 after the start of PETM warming. They explained that AOM in sediment produces bicarbonate, an alkaline compound, which helps buffer the ocean and stabilize its pH. But AeOM in the water column releases CO2which contributes to the warming and acidification of the oceans. AeOM microbes also consume O2allowing other oxygen-intolerant organisms to spread and ingest sulfate, further starving AOM microbes.
Could a similar methane shift in the Arctic accelerate climate change today? “We think it’s possible and very likely,” said the study’s lead author. Bumsoo Kimorganic geochemist at NASA’s Johnson Space Center. The Arctic Ocean is becoming warmer and cooler, which would consume more oxygen, leading to similar changes in the methane cycle, Kim, who was a researcher at Texas A&M University at the time of the study, said in an email to Live Science.
However, other scientists are less sure. “The factors that led to the Arctic becoming a source of carbon in the past may not be directly analogous in the future: the Arctic Ocean was physically further from the global ocean and the chemistry of the ocean was significantly different,” said Sandra Kirtland Turnerassociate professor of paleoclimate and paleoceanography at the University of California, Riverside, who was not involved in the study.
Kirtland Turner also noted that the results are a reminder that carbon cycle feedbacks can amplify or prolong warming. “Today, carbon cycle feedbacks remain poorly constrained and are rarely considered beyond 2100,” limiting our understanding of all their impacts, she told Live Science.