Using high-precision dating of geologic regions, scientists have found evidence that one of the most significant warming spikes of the last 500 million years had a surprisingly short duration, about 300,000 years. And it coincides with a carbon event that was half a million years after an earlier igneous episode than had been thought to cause the hyperthermal spike. Their work has been published in the journal Geology.

The Toarcian oceanic anoxic event (T-OAE), also called the Jenkyns event, occurred about 183 million years ago, in the early part of the Jurassic era and just 18 million years after the Triassic–Jurassic extinction event, which was the fourth of Earth's five major extinction events. (We are currently in the sixth mass extinction.)

"Although we know quite a lot about the kinds of environmental changes that accompanied warming, the exact age and timescale of the Toarcian OAE have long been debated," said David Kemp of the China University of Geosciences in Wuhan and lead author of the paper.

"This knowledge is important for establishing the likely cause of the event and for determining rates of warming and associated environmental changes."

Such information is important not only to paleoclimatologists and geologists, but also important to climate scientists who study today's global warming and use any information they can get from the past to determine our climate's sensitivity—how much the planet's surface will ultimately warm for a doubling of atmospheric carbon dioxide. It's also important to other scientists who study the Earth's water cycle, changes in ice, ocean and sea level, and their impact on life.

"Previous estimates have suggested durations of anywhere between 300,000 years to over one million years," Kemp said. "Our work shows that the event was quite short-lived, probably lasting around 300,000 years."

The T-OAE was already known as a "hyperthermal" warming event—significant global warming of short duration. It's been known as one of the most significant hyperthermal events of the last 540 million years, a period known as the Phanerozoic Eon. It saw seawater warming of up to 5°C, widespread ocean deoxygenation ("anoxia"), ocean acidification, increased chemical weathering and a faster hydrological cycle. Extinctions took place during the T-OAE as well, mostly of marine animal life.

To better determine the timescale of the T-OAE and the source of its warming, Kemp and his group used high-precision dating of grains of the mineral zircon found in volcanic ash in layered sedimentary rocks in Japan.

"Previous work had already suggested that the event could have been linked to volcanism," Kemp said, "and our study provides compelling support for this."

As volcanoes erupt, they eject a wide variety of material: magma, aerosols such as sulfur dioxide that can reach into the stratosphere, and gases such as carbon dioxide through vents. Magma also off-gases carbon dioxide, which in the atmosphere traps some of the heat given off by the surface of the Earth that provides a greenhouse effect.

"It seems probable that greenhouse gas emissions associated with volcanism could have triggered global warming at the Toarcian OAE," Kemp said. (However, today's additional carbon dioxide definitely comes as a waste product of human burning of fossil fuels; humans emit about 60 times more carbon dioxide than do volcanoes.)

"One way in which this could have happened," Kemp continued, "is that rising magma associated with the volcanism could have intruded into rocks like coal and shale that contain a lot of organic matter," and its heating could have generated large amounts of carbon dioxide and methane, another powerful greenhouse gas.

"The rapid release of these gases to the atmosphere would have triggered warming."

Besides finding an unexpectedly short T-OAE duration of 300,000 years (with measurement bounds of 169,000 years to 407,000 years), the hyperthermal coincided with Ferrar large igneous province (LIP) magmatism, associated with the breakup of the supercontinent Gondwana.

A large igneous province (LIP) is a very large accumulation of igneous rocks, coming about when magma travels through the Earth's crust and to the surface, and they often occur at the same time as mass extinctions and rapid climate change.

The Karoo LIP began about 500,000 years before the beginning of the Ferrar LIP; Kemp and his colleague's dating methodology is precise enough to refute previous work that linked the Karoo LIP to the T-OAE warming. Rather, both events were driven by extremely rapid carbon out-gassing produced by heat during their respective LIPs, perhaps less than a millennium in duration.

"Our new timescale for the event will help us to better understand the pace and timing of these emissions," Kemp said, "as well as the pace and timing of the various environmental changes associated with global warming."

He notes in particular that the rate of the marked reduction in seawater oxygen levels during the T-OAE can now be determined, and the pace of species loss and ecosystem recovery can be quantified.

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