Possible source of ancient methane eruption identified – Ars Technica

Fifty-six million years ago, Trillions of tons Carbon found its way into the atmosphere, acidification oceans and causing the already warm global climate to warm by another amount 5°C (9°F) – An episode known as the Paleocene-Eocene Thermal Maximum or PETM.

As it stands today, a warming climate has affected the environment on land and at sea Heavy rain And Heat stress Plankton are at the base of the food web. Wild animals had a high percentage of extinction And replacing by Smaller typesThere was a mass extinction of the small creatures that make shells and live at the bottom of the sea. Support hotter climate Crocodiles And Cypress swamp forestssuch as those found in the southeastern United States today, in ice-covered arctic latitudes and tundra today.

Where did all this carbon come from?

Its source has been debated for years, with some scientists blaming the destabilization of methane ice on the seafloor, and others pointing to widespread volcanic activity in the North Atlantic at the time. Modeling the carbon isotope transition indicates the carbon it originates from Both organic and volcanic sourcesBut the relative proportions have not been settled.

a New study Writing in the journal Nature Geoscience, Professor Christian Berndt of the GEOMAR Helmholtz Center for Ocean Research in Germany puts the blame on underground magma pushing out methane and carbon dioxide.₂ From marine sediments to the atmosphere via gas explosions called hydrothermal venting. Berndt worked with an international group of 35 co-authors on the paper.

Waiting 17 years for an appointment

The idea that hydrothermal venting played a major role in the PETM dates back to 2013 2004. Seismic images collected for oil and gas exploration showed that marine sediments off Norway were filled with thousands of PETM-aged craters, and other studies have found similar craters. Near Greenland. But seismic images couldn’t pinpoint the time when the craters formed with enough precision to determine whether they played a role in triggering the PETM: “This was basically conjecture,” Berndt said.

To find out if the vents actually caused the PETM, they had to obtain samples of them to date them, which required drilling deep into the seafloor 5,600 feet (1.7 km) below the Atlantic Ocean.

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

Berndt and his colleagues were aboard the scientific drillship JOIDESsolution as it drilled five wells in Modgunn Vent, about 200 miles off the Norwegian coast. The crater at the top of the opening is about 1.3 kilometers (4,300 ft) wide and about 80 meters (260 ft) deep. Below the crater, seismic images show a chimney-like feeding zone 400 meters (1,300 feet) deep, connecting the crater to a layer of now-frozen magma called the “sill.”

Right time?

“This was an interesting thing at the beginning of the PETM period,” Berndt told Ars.

Samples recovered from the wells provide “conclusive evidence of hydrothermal venting immediately before the onset of the PETM period,” supporting the “major role” of vents in the warming of the PETM period, Berndt and colleagues say in their paper. They base this on two lines of evidence found in the hole: the globally recognized shift in carbon isotopes that characterizes the PETM, and the presence of a type of plankton that was only present during the PETM.

“What is crucial and more precise is the carbon isotope excursion,” Berndt said.

But these two lines of evidence only appear in the sediments that filled the crater after Formed in the beginning; They were found 10-15 meters above the floor of the crater. This distance leaves room for maneuver in associating the hole with the beginning of the PETM period. “This means that the hole formed very shortly before the PETM, and during the PETM, it filled up,” Berndt said.

Professor Abe Slogs from Utrecht University, who was not involved in Berndt’s study, agrees that “the crater is older than the PETM period.” But Slugs points out that the plankton species found in these sediments were present throughout the PETM. “The species therefore cannot distinguish between the beginning or the body of an event,” Slogs said. In other words, the presence of this species cannot narrow the time in which the crater formed to less than a fairly large window.

So, how long before a hole forms in a PETM period?

If this had been thousands of years ago, the gas from its eruption would have reached the atmosphere much earlier to trigger a PETM. But if the volcano, and other volcanoes, had erupted a few centuries or a few thousand years ago, it could have raised temperatures.

Berndt argues that the 10-15 m of sediment that filled the crater before the isotopic shift and the appearance of PETM plankton represents only a short period of time. “It could take 200 years, maybe up to 3,000 years, or something like that,” he added.

He points to an example Blast drilling It happened in the North Sea in 1964 [a] He said that a 50-meter-deep hole in the North Sea is now almost filled. Furthermore, some of the Modgunn Vent deposits contain annual layers Seasonal plankton blooms It appears that it was filling up quickly indeed.

Correct depth

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

Evidence for a shallow eruption comes from the fact that the crater fill contains a lot of Earth-derived material and fossils of plankton that lived in shallow water. But there was no sign of wave movement, so it must have been deep enough not to be affected by the waves. These facts place limits on the depth of water at the eruption of the crater; “Maybe 30 to 150 meters is a good estimate,” Berndt said.

Furthermore, seismic images show that soon after the crater filled with sediment, the sea floor became shallow enough to be eroded by waves, so it could not have been much deeper when the crater erupted.

“Why was this climate relevant?” Berndt asked.

The depth of an eruption makes a big difference in its effect on climate. This is because methane is a powerful greenhouse gas in the atmosphere, even more so 25 times stronger of carbon dioxide₂But it must enter the atmosphere to warm it. Most of the methane rising into deep waters today is converted into carbon dioxide₂ before it can escape into the atmosphere. For Modgunn Vent, “this shallow water depth will allow the methane to reach directly into the atmosphere…and that’s really the importance,” Berndt explained.

If you were to travel back in time to watch one of these explosions from a vent 100 meters below sea level, “you would probably see a lot of muddy water on the surface and probably a lot of methane bubbles,” Berndt said. But if the vent was only 30 meters deep, the eruption would “really be shot into the air!” He said.

Based on the number of vents that appear in seismic data on both sides of the North Atlantic, Berndt estimates that there were thousands of them that exploded at the beginning of the PETM, so their cumulative impact on climate would be enormous. Some of them were huge – right there An opening 11 kilometers wide Off Greenland, it’s the size of Buffalo, New York, or Savannah, Georgia.

Modeling How heat spreads from underground magma “sills” to release gas from sediments shows that many vents may have continued to release methane for a long time, prolonging their thermal effect — for “maybe 10,000 years or so,” Berndt said.

It may take decades to settle the debate

Despite the “conclusive” evidence presented by Berndt and his colleagues, Gernon and Sluis are not convinced. “In terms of the potential major role of hydrothermal vents in increasing global warming, I think the jury is still out on that one,” said Gernon, who participated in the study. Recently published a paper Blame the company₂ Emitted by a sudden eruption of volcanic activity as a cause. “The vents were not the cause of the onset of the PETM period, but they contributed to the abnormally long duration of the PETM period,” Slugs told Ars.

Sampling more vents in the North Atlantic would help settle the debate. But interested scientists had to wait 17 years for these wells, and the National Science Foundation The ship has since been cancelled Who dug them up and has no replacement planned. So further drilling could take decades. “The IODP has been the most important and successful geoscience program in the world for the past 50 years, so it would be crazy to completely abandon it and not replace it,” Berndt said.

Meanwhile, other scientists are currently examining rocks recovered by boreholes for materials suitable for high-resolution radiometric dating, which may better constrain the timing of the hole.

Natural Earth Sciences, 2023. DOI: 10.1038/s41561-023-01246-8