Still, few thought that igneous activity could act fast enough. Geologists thought the sills formed over a few million years, whereas fossilized sediments show it took just a few thousand years to start the PETM.
The Birmingham team has closed that gap. They found that with the Iceland plume, as Jones put it, “you can turn the tap on … in five to ten thousand years.”
Cooking Organic
In earlier work on V-shaped ridges of lava near Iceland, Jones had shown that pulses of hot mantle periodically ride up the Iceland plume, pushing up adjacent tectonic plates. Jones called the ridges “a smoking gun” proving that such pulses happen, but unfortunately, the ridges do not go back to the time of the PETM.
For more clues, Jones and his colleagues turned to the Forties Sandstone Member near Scotland. A target for oil drilling, the Forties Sandstone Member has been extensively studied, drilled and seismically scanned by the oil industry, allowing geologists to work out that it formed from the eroded remains of the land bridge between Scotland and Greenland that was uplifted from the Atlantic 56 million years ago, coincident with the PETM. “We can see marine sediments being uplifted and exposed above the sea level,” said study co-author Tom Dunkley Jones.
That uplift is a clear sign of the giant pulse of mantle arriving beneath the crust, and differences in the timing of the uplift at different locations told the Birmingham team how fast the subsurface magma “bruise” spread.
But to figure out how much organic matter the underground sills would have cooked up, the scientists first had to find and measure them. That task fell to Stephen Jones’ former graduate students Murray Hoggett and Karina Fernandez, who scrutinized tens of thousands of square kilometers of seismic scans to infer that there are between 11,000 and 18,000 sills in the region.
“Until we had that database of geometries and dimensions, we couldn’t even tell you how fast or how regular [the sills] needed to be to get to the right carbon release,” said Sarah Greene, a co-author.
The scientists then combined a standard oil-industry model for calculating the rate at which individual sills generated gas with a statistical technique called Monte Carlo simulation to calculate the rate at which the sills would emit gas collectively.
“Each sill is small and generates a small amount of carbon,” Greene explained. “You need a bunch to be active at the same time to sum up to the kinds of total release that we see.”
Igneous Trigger
Remarkably, the team’s calculated emissions agree with independent estimates of the carbon release during the PETM calculated from isotopes in 56-million-year-old sediments. “The fact that it overlaps quite nicely is … quite powerful,” said Greene.
The Birmingham group’s work has changed the minds of several previously pro-clathrate scientists. One such expert, the geoscientist Lee Kump of Pennsylvania State University, called the new study “compelling evidence” that the North Atlantic Igneous Province is “the trigger for, and main mechanism of, carbon emission during the PETM. Methane clathrate involvement is not needed.” Similarly, James Zachos of the University of California, Santa Cruz, who discovered some of the earliest evidence for the PETM and formerly attributed the event to methane clathrates, said he now sees igneous activity as “the trigger and main source of carbon.” Appy Sluijs of Utrecht University agreed: “Volcanism could certainly have triggered the event.”
Clathrates or permafrost may have amplified the warming, they say, but the new study strongly suggests that igneous activity dominated.
In contrast, Richard Zeebe of the University of Hawaii, Manoa stands by his previous opinion that the PETM and later warm episodes coincided with times when Earth’s orbit around the sun would have delivered extra solar warmth. “The PETM is part of a long series of hyperthermals,” he said, “and invoking a special trigger for one—say volcanism for the PETM—but not for all others seems illogical.”
