A hidden layer of carbon may have triggered an ancient episode of global warming | Science
There is no perfect parallel in Earth’s past for current climate change – human-induced warming is simply happening too fast and furiously. The closest analog occurred 56 million years ago, when over 3,000 to 5,000 years greenhouse gases skyrocketed in the atmosphere, causing a warming of at least 5 °C and pushing tropical species towards the poles.
The cause of the Paleocene-Eocene Thermal Maximum (PETM) has long been debated, with researchers citing exotic mechanisms such as catastrophic releases of methane from the seabed or even asteroid strikes. But over the past few years, evidence has been mounting for a more prosaic culprit: carbon-spewing volcanoes that emerged beneath Greenland as it moved away from Europe. Now researchers have found signs of an effect that may have supercharged the volcanoes’ warming effect, making them a stronger suspect. Greenland’s underside is thought to be encrusted with carbon-rich rocks, like barnacles on a ship’s keel. During rifting, they may have released a gush 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 the dinosaurs kicked the bucket, this is the biggest global warming event we’ve had,” says Pincelli Hull, a paleoclimate scientist at Yale University. This can give clues to how quickly the Earth is warming as greenhouse gas levels rise and how climate extremes are altering ecosystems. But the comparison with today is not exact. Although the total release of carbon during the PETM exceeded the total oil and gas reserves known today, it was slower than the current increase in greenhouse gases and resulted in more gradual warming. Life had more time to adapt than it does today: the fossil record shows that trees migrated upriver and to higher latitudes, with animals in their wake, even as tropical corals have disappeared and the ecosystems have completely changed.
Past PETM explanations centered on methane, a greenhouse gas even more potent than CO2 albeit short-lived. Samples of ancient plankton shells appeared to show that the atmosphere during the brief greenhouse was enriched in light carbon, the isotope favored by life. This suggests that the carbon responsible for the warming push comes from living things, as most methane does, rather than gases spewed by volcanoes, which rise from deep within the Earth.
At first, researchers thought that a small amount of warming could have destabilized methane hydrates – deposits of methane on the sea floor trapped in cages of ice crystals –triggering a massive release of carbon. But the 2010 Deep water horizon oil spill in the Gulf of Mexico has put a dent in this theory. The microbes simply chewed up the methane the broken well released into the ocean, suggesting methane seeps from the seabed would rarely find their way into the air. “Most modeling studies suggest that you cannot release enough greenhouse gases from hydrates alone,” says Sev Kender, a paleo-oceanographer at the University of Exeter.
Mudrocks on the sea floor also contain carbon from living things and magma from underwater eruptions could have heated the rocks and released the carbon. But in 2017, researchers analyzed plankton fossils from an ocean core and discovered that carbon released during PETM was heavier than previously thought. For some, this indicated that the carbon did not come from living sources. “Given the current state of knowledge, it seems likely that it is volcanism,” says Marcus Gutjahr, a geochemist at the GEOMAR Helmholtz Center for Ocean Research Kiel, who led the 2017 study.
Greenland was separating from Europe at the time of the PETM as a mantle plume moved beneath the island, preparing the 180-kilometre-thick crust above to separate. Like any volcanism, the process would have released CO2. Gernon calculated, however, that eruptions during rifting would have provided only one-fifth of the more than 10,000 gigatonnes of carbon needed to explain PETM’s warming. But he knew that over the eternities, CO2 and other gases can bubble out of tectonic plates as they plunge into the mantle, seeping under thick crusts like that of Greenland, and formation of carbonate formations which can be stable for millions or even billions of years.
If the crust is torn apart by the rift, however, the trapped carbon can spill upwards and erupt as rare carbonatite lava, which contains much more CO2 than standard lava. Indeed, such a process seems to be going on in East Africa right now, where a fault 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 next to the craton, we get high CO2 flow.
Similarly, the hotspot that burned in Greenland 60 million years ago could have mobilized any carbonate beneath its crust, Gernon says. When rifting began to open up what is now the northeast Atlantic Ocean, “you’ll have an enormous amount of carbon being vented.”
Evidence for carbon-rich melting is abundant on either side of the North Atlantic Rift, the tectonic divide that marks the ancient boundary between Greenland and Europe, report Gernon and his co-authors in a study published today in nature geoscience. In an ocean core taken in 1981, they found volcanic tuffs indicating a sharp increase in volcanism during the PETM. They also combed through the literature to study other rocks matching the core and found reports in eastern Greenland and the Faroe Islands of anomalous lavas rich in magnesium, titanium oxide and earth elements. rare – signatures of melting carbonate rocks from deep within the crust. . The lava is around 56.1 million years old, and the researchers calculate that rifting would have produced enough of it to account for almost all of the carbon emissions needed.
Kender says Gernon presents a compelling case, but adds that timing is key. The PETM occurred in a geological instant, which only lasted a few thousand years. In the meantime, the volcanism has not been precisely dated. “Whether it’s early, middle or later, we can’t tell yet,” Kender says. Gernon’s team says more precise geochemical dating of the ocean core, as yet unpublished, supports the idea that the lavas they are studying may have originated from the early PETM. “I’m quietly confident the story works,” Gernon says.