It is well known that volcanic eruptions on the earth's surface can directly influence the atmosphere and thus the global climate. However, around 55 million years ago, undersea volcanism was apparently also able to do this, as a study shows: When the Atlantic was born, so much methane apparently bubbled into the atmosphere from hydrothermal vents in the shallow water that strong global warming began. This is based on seismic data and analysis of drill cores from one of the cooled volcanic vents at the bottom of the North Atlantic.
Today, humans are responsible for global warming through the massive release of greenhouse gases. However, it is clear that natural processes have repeatedly led to major climate changes throughout the history of the earth. Insights into the factors of the climate-historical development of the earth can serve the general understanding of this complex system. In this context, the international research team led by Christian Berndt from the GEOMAR Helmholtz Center for Ocean Research in Kiel is now focusing on the last strong climate warming before our time: The so-called Paleocene-Eocene temperature maximum around 55 million years ago culminated in global warming five degrees. This also confused nature - many living beings died out during this time, according to previous studies.
On the trail of a geological climate change
The well-known strong volcanism at this time seems to be the cause of global warming, but geologists have not been able to explain the connection exactly. As part of their study, Berndt and his colleagues have now investigated a suspicion that volcanism in the course of the birth of the Atlantic could have contributed significantly to the Paleocene-Eocene temperature maximum through greenhouse gas emissions. Because the sea was formed around 55 million years ago by dividing a once-connected landmass. Intense volcanism occurred in the fracture zone. Its traces are still clearly visible today on the kilometer-deep seabed between Greenland and Europe.
The extent to which this undersea rift volcanism could actually have brought relevant amounts of the strong greenhouse gas methane into the atmosphere seemed questionable. In order to test the hypothesis, the scientists have now evaluated information obtained during an expedition with the drill ship "JOIDES Resolution" to the Vøring Plateau off the coast of Norway. There they conducted seismic surveys of the seafloor and extracted drill cores from one of the thousands of hydrothermal vents that have long since cooled down in the area that was once characterized by volcanism. Through their analytical methods, they were able to gain structural clues and information about the age of the formations.
Methane release in shallow water
As they report, their results confirmed that the volcanic vent had been active shortly before the Paleocene-Eocene temperature maximum - i.e. released gases from the Earth's interior. Once global warming had fully unfolded, activity died down and the resulting crater was filled in, the data show. This timing was thus already consistent with the role of volcanic activity in global warming. But another finding was even more important: the characteristics of the filling material showed that the vent must have been in a relatively shallow water depth of less than a hundred meters.
This is an important aspect, explains Berndt: “Most of the methane that enters the water column from active hydrothermal sources in the deep sea today is quickly converted by oxidation into carbon dioxide, which has a far less climactic effect. Since the vent examined is in the middle of the rift valley, where the water depth should be greatest, it can be assumed that other vents were even closer to the water surface or even protruded above the water. This allowed much larger amounts of methane to enter the atmosphere directly than if the vents had been active in deep water," says Berndt.
Finally, the geophysicist emphasizes another finding from the study that is related to today's climate change: "It is becoming apparent that it took many millennia for the climate to cool down naturally. So the Earth system was able to regulate itself back then – but not on a time scale that would be relevant to today’s climate crisis,” says Berndt.
Source: GEOMAR Helmholtz Center for Ocean Research, specialist article: Nature Geoscience, doi: 10.1038/s41561-023-01246-8