Potential source of ancient methane eruption identified

Fifty-six million years ago, trillions of tons of carbon found its way into the atmosphere, acidifying oceans and causing the already-warm global climate to heat up by another 5º C (9º F)—an episode known as the “Paleocene-Eocene Thermal Maximum” or “PETM.”

Like today, the warming climate affected the environment on land and in the sea, with extreme downpours and heat-stressed plankton at the base of the food web. Land animals had a high rate of extinction and replacement by smaller species, and there was a mass extinction of tiny shell-making creatures that lived on the sea bed. The hotter climate supported alligators and swamp-cypress forests, like those in today’s southeastern United States, in Arctic latitudes that are covered by ice and tundra today.

Where did all that carbon come from?

Its source has been debated for years, with some scientists blaming the destabilization of methane ice in the seabed and others pointing to the widespread volcanic activity in the North Atlantic at the time. Modeling of the carbon isotope shift points to carbon originating from organic and volcanic sources, but the relative proportions aren’t settled.

A new study published in Nature Geoscience by Professor Christian Berndt of the GEOMAR Helmholtz Centre for Ocean Research in Germany blames underground magma that drove methane and CO2 from marine sediments into the atmosphere via gassy eruptions dubbed “hydrothermal venting.” Berndt worked with an international group of 35 coauthors on the paper.

Waiting 17 years for a date

The idea that hydrothermal venting played a major role in the PETM dates back to 2004. Seismic images gathered for oil and gas exploration showed that the marine sediments off Norway were pockmarked with thousands of craters that are about PETM age, and other studies have found similar craters near Greenland. But the seismic images couldn’t constrain the time when the craters formed precisely enough to determine if they played a role in triggering the PETM: “That was conjecture, basically,” said Berndt.

To see if the vents could have triggered the PETM, they needed to retrieve samples from them to date them—and that required drilling deep into the seabed that lies below 5,600 feet (1.7 km) of the Atlantic Ocean.

So in 2004, Berndt and several coauthors formally proposed a project to drill and sample a hydrothermal vent, but they had to wait 17 years before drilling finally started in 2021 as part of the International Ocean Discovery Program (IODP). “You have to be patient,” said Berndt.

Berndt and colleagues were aboard the scientific drilling ship “JOIDES Resolution” as it drilled five boreholes into the “Modgunn Vent,” some 200 miles off the Norwegian coast. The crater at the top of the vent is about 1.3 km (4,300 feet) wide and approximately 80 meters (260 feet) deep. Beneath the crater, seismic images show a 400-meter-deep (1,300 feet), chimney-like feeder zone that connects the crater to a sheet of now-frozen magma called a “sill.”

The right time?

“This was bang on at the beginning of the PETM,” Berndt told Ars.

The samples recovered from the boreholes provide “conclusive evidence for hydrothermal venting immediately before the PETM onset,” supporting the “major role” of the vents in the PETM warming, Berndt and colleagues say in their paper. They base this on two lines of evidence in the crater: a globally recognized shift in carbon isotopes that marks the PETM and the presence of a species of plankton that only existed during the PETM.

“The crucial one and the most precise one… is the carbon isotope excursion,” said Berndt.

But these lines of evidence only show up in the sediments that filled the crater after it initially formed; they’re found 10–15 meters up from the crater floor. That distance leaves some wiggle room in tying the crater to the start of the PETM. “That means the vent was formed very shortly before the PETM, and during the PETM, it filled in,” said Berndt.

“The crater is older than the PETM,” agreed Professor Appy Sluijs of Utrecht University, who was not involved in Berndt’s study. But Sluijs points out that the plankton species in these deposits existed throughout the PETM. “The species can therefore not distinguish between the onset or the body of the event,” said Sluijs. In other words, the presence of this species can’t narrow down the time the crater formed to less than a fairly large window.

So how long before the PETM did the vent form?

If it was many millennia before, the gas released by its eruption would have made it to the atmosphere too early to trigger the PETM. But if it, and many like it, erupted just a few centuries or a couple of millennia before, then it could have triggered the warming.

Berndt argues that the 10–15 meters of sediment that filled the crater before the isotope shift and PETM-specific plankton appear represents just a short period. “It could be as quick as 200 years and up to maybe 3,000 years, something like that,” he said.

He points to the example of a drilling blowout that happened in the North Sea in 1964. “That generated [a] 50-meter-deep hole in the North Sea that is almost filled up now,” he said. Moreover, some of the Modgunn Vent sediments have annual layers with seasonal plankton blooms showing it was indeed filling quickly.

The right depth

“The main advance in this study is that the team convincingly show that the vents formed in a fairly shallow water column at roughly the time of the PETM,” Professor Tom Gernon of the University of Southampton, who was not involved in Berndt’s study, told Ars.

Evidence for a shallow eruption comes from the fact that the crater fill has a lot of land-derived material and fossils of plankton that lived in shallow water. But there was no indication of wave action, so it must have been deep enough to be unaffected by waves. These facts set limits on the water depth when the vent erupted; “between maybe 30 and 150 meters would be a good estimate,” said Berndt.

Moreover, seismic images show that shortly after the crater filled with sediments, the seabed became shallow enough to be eroded by waves, so it can’t have been much deeper when the vent erupted.

“Why was this climate-relevant?” asked Berndt.

The depth of the eruption makes a big difference to its climate impact. That’s because methane is a powerful greenhouse gas in the atmosphere, more than 25 times more potent than CO2, but it has to get into the atmosphere to warm it. Most methane bubbling up in deep water today is converted to CO2 before it can escape into the atmosphere. For the Modgunn Vent, “this shallow water depth would allow methane to get directly into the atmosphere… that's really the significance,” explained Berndt.

If you were to travel back in time to watch one of these eruptions from a vent 100 meters below the sea surface, “you would probably see a lot of muddy water at the surface and probably a lot of bubbling methane,” said Berndt. But if the vent was only 30 meters deep, the eruption would “really jet into the air!” he said.

Based on the number of vents that show up in seismic data on both sides of the North Atlantic, Berndt estimates that there were thousands of them erupting at the start of the PETM, so their cumulative effect on the climate would have been enormous. And some were massive—there’s an 11-kilometer-wide vent off Greenland, about the size of the cities of Buffalo in New York or Savannah in Georgia.

Modeling of how heat from magma “sills” spread underground to liberate gas from sediments shows that many vents probably continued emitting methane for a long time, prolonging their warming effect—for “maybe 10,000 years or so,” said Berndt.

It could be decades to settle the debate

Despite the “conclusive” evidence presented by Berndt and colleagues, both Gernon and Sluijs are unconvinced. “Regarding the potential 'major role' of hydrothermal vents in driving warming, I think the jury is still out on this,” said Gernon, who recently published a paper blaming CO2 emitted by a sudden flare-up of volcanic activity as the cause. “The vents were not the cause of the onset of the PETM, but they contributed to the anomalously long duration of the PETM,” Sluijs told Ars.

Sampling more vents in the North Atlantic would help to settle the debate. But the interested scientists had to wait 17 years for these boreholes, and the National Science Foundation has since scrapped the vessel that drilled them and has no replacement planned. So further drilling could be decades away. “IODP was the most important and successful geoscience program in the world for the last 50 years, so it would be crazy to completely give it up and not replace it,” said Berndt.

Meanwhile, other scientists are currently checking the rock recovered by the boreholes for material suitable for high-precision radiometric dating, which may better constrain the vent’s timing.

Nature Geoscience, 2023.  DOI: 10.1038/s41561-023-01246-8