Hidden carbon layer may have sparked ancient bout of global warming

There is no perfect parallel in Earth’s past for present-day climate change—human-driven warming is simply happening too fast and furiously. The closest analog came 56 million years ago, when over the course of 3000 to 5000 years, greenhouse gases soared in the atmosphere, causing at least 5°C of warming and pushing tropical species to the poles.

The cause of the Paleocene-Eocene Thermal Maximum (PETM) has long been debated, with researchers invoking exotic mechanisms such as catastrophic releases of methane from the sea floor or even asteroid strikes. But over the past few years, evidence has mounted for a more prosaic culprit: carbon-spewing volcanoes that emerged underneath Greenland as it tore away from Europe. Now, researchers have found signs of an effect that would have supercharged the warming effect of the volcanoes, making them a stronger suspect. The underside of Greenland is thought to be encrusted with carbon-rich rocks, like barnacles on the keel of a ship. During the rifting, they might have liberated a gusher of carbon dioxide (CO₂), says Thomas Gernon, a geologist at the University of Southampton and leader of the new study. “It’s a perfect storm of conditions.”

The PETM has long fascinated paleoclimatologists. “Since dinosaurs kicked the bucket, this is the biggest global warming event we have,” says Pincelli Hull, a paleoclimate scientist at Yale University. It can yield clues to how quickly Earth warms as greenhouse gas levels rise and how climate extremes alter ecosystems. But the comparison to today isn’t exact. Although the total release of carbon during the PETM exceeded the total of today’s known oil and gas reserves, it was slower than today’s surge of greenhouse gases and drove more gradual warming. Life had more time to adapt than it does today: Fossil records show trees migrated uphill and to higher latitudes, with animals following in their wake, even as tropical corals disappeared and ecosystems wholly changed.

Past explanations for the PETM centered on methane, a greenhouse gas even more powerful than CO₂ although shorter lived. Samples of ancient plankton shells seemed to show the atmosphere during the brief hothouse was enriched in light carbon, the isotope favored by life. That suggested the carbon responsible for the warming surge originated in living things, as most methane does, rather than in the gases spewed by volcanoes, which rise from deep Earth.

At first, researchers thought a small amount of warming might have destabilized methane hydrates—seafloor deposits of methane trapped in cages of ice crystals—triggering a massive release of carbon. But the 2010 Deepwater Horizon oil spill in the Gulf of Mexico put a dent in that theory. Microbes simply chewed up the methane the broken well released into the ocean, suggesting seeps of seabed methane would rarely get all the way into the air. “Most modeling studies suggest you can’t release enough greenhouse gases just through hydrates,” says Sev Kender, a palaeoceanographer at the University of Exeter.

Mudrocks on the sea floor also contain carbon that originated in living things, and magma from submarine eruptions could have heated the rocks and liberated the carbon. But in 2017, researchers analyzed plankton fossils from an ocean core and found the carbon released during the PETM was heavier than previously thought. For some, that indicated the carbon wasn’t from living sources. “Given the current state of knowledge, it seems likely to be volcanism,” says Marcus Gutjahr, a geochemist at GEOMAR Helmholtz Centre for Ocean Research Kiel, who led the 2017 study.

Greenland was rifting away from Europe at the time of the PETM as a mantle plume traveled under the island, priming the 180-kilometer-thick crust above to be pulled apart. Like all volcanism, the process would have released CO₂. Gernon calculated, however, that the eruptions during the rifting would have only provided one-fifth of the more than 10,000 gigatons of carbon needed to explain the PETM warming. But he knew that over the eons, CO₂ and other gases can bubble out of tectonic plates as they dive into the mantle, percolating up into the underside of thick crusts like Greenland’s, and forming carbonate formations that can be stable for millions or even billions of years.

If the crust is ever pulled apart by rifting, however, the trapped carbon can spill upward and erupt as rare carbonatite lava, which contains far more CO₂ than standard lava. Indeed, such a process appears to be underway in East Africa right now, where a rift has begun to tear the horn of Africa away from the rest of the continent, says James Muirhead, a structural geologist at the University of Auckland. “At the very edge of the craton we get these carbonatite lavas,” he says. “And adjacent to the craton we get high CO₂ fluxes.”

Similarly, the hot spot that burned through Greenland starting 60 million years ago could have mobilized any carbonate under its crust, Gernon says. When the rifting began to open up what today is the northeastern Atlantic Ocean, “you’ll have a huge amount of carbon venting.”

Evidence of the carbon-rich melt is abundant on either side of the North Atlantic rift, the tectonic division that marks the old boundary between Greenland and Europe, Gernon and his co-authors report in a study published today in Nature Geoscience. In an ocean core collected in 1981, they found volcanic tuffs indicating a sharp increase in volcanism during the PETM. They also combed the literature for studies of other rocks matching the core, and found reports in East Greenland and the Faroe Islands of anomalous lavas rich in magnesium, titanium oxide, and rare earth elements—signatures of melting of carbonate rock from deep in the crust. The lavas date roughly to 56.1 million years ago, and the investigators calculate that the rifting would have produced enough of them to explain nearly all of the needed carbon emissions.

Kender says Gernon makes a compelling case, but adds the timing is key. The PETM happened in a geological instant, lasting only several thousand years. Meanwhile, the volcanism has not been precisely dated. “Whether it was at the onset, in the middle, or later, we can’t say yet,” Kender says. Gernon’s team says more precise geochemical dating from the ocean core, still unpublished, supports the idea that the lavas they’re studying could be from the onset of the PETM. “I’m quietly confident the story works,” Gernon says.