How did volcanism trigger climate change before the eruptions started?

By: ARS Techica - Science Posted On: September 08, 2024 View: 5

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Image of a person in a stream-filled gap between two tall rock faces. — Enlarge / Loads of lava: Kasbohm with a few solidified lava flows of the Columbia River Basalts.
Joshua Murray

As our climate warms beyond its historical range, scientists increasingly need to study climates deeper in the planet’s past to get information about our future. One object of study is a warming event known as the Miocene Climate Optimum (MCO) from about 17 to 15 million years ago. It coincided with floods of basalt lava that covered a large area of the Northwestern US, creating what are called the “Columbia River Basalts.” This timing suggests that volcanic CO₂ was the cause of the warming.

Those eruptions were the most recent example of a “Large Igneous Province,” a phenomenon that has repeatedly triggered climate upheavals and mass extinctions throughout Earth’s past. The Miocene version was relatively benign; it saw CO₂ levels and global temperatures rise, causing ecosystem changes and significant melting of Antarctic ice, but didn’t trigger a mass extinction.

A paper just published in Geology, led by Jennifer Kasbohm of the Carnegie Science’s Earth and Planets Laboratory, upends the idea that the eruptions triggered the warming while still blaming them for the peak climate warmth.

The study is the result of the world’s first successful application of high-precision radiometric dating on climate records obtained by drilling into ocean sediments, opening the door to improved measurements of past climate changes. As a bonus, it confirms the validity of mathematical models of our orbits around the Solar System over deep time.

A past climate with today’s CO₂ levels

“Today, with 420 parts per million [of CO₂], we are basically entering the Miocene Climate Optimum,” said Thomas Westerhold of the University of Bremen, who peer-reviewed Kasbohm’s study. While our CO₂ levels match, global temperatures have not yet reached the MCO temperatures of up to 8° C above the preindustrial era. “We are moving the Earth System from what we call the Ice House world… in the complete opposite direction,” said Westerhold.

When Kasbohm began looking into the link between the basalts and the MCO’s warming in 2015, she found that the correlation had huge uncertainties. So she applied high-precision radiometric dating, using the radioactive decay of uranium trapped within zircon crystals to determine the age of the basalts. She found that her new ages no longer spanned the MCO warming. “All of these eruptions [are] crammed into just a small part of the Miocene Climate Optimum,” said Kasbohm.

But there were also huge uncertainties in the dates for the MCO, so it was possible that the mismatch was an artifact of those uncertainties. Kasbohm set out to apply the same high-precision dating to the marine sediments that record the MCO.

A new approach to an old problem

“What's really exciting… is that this is the first time anyone's applied this technique to sediments in these ocean drill cores,” said Kasbohm.

Normally, dates for ocean sediments drilled from the seabed are determined using a combination of fossil changes, magnetic field reversals, and aligning patterns of sediment layers with orbital wobbles calculated by astronomers. Each of those methods has uncertainties that are compounded by gaps in the sediment caused by the drilling process and by natural pauses in the deposition of material. Those make it tricky to match different records with the precision needed to determine cause and effect.

The uncertainties made the timing of the MCO unclear.

Enlarge / Tiny clocks: Zircon crystals from volcanic ash that fell into the Caribbean Sea during the Miocene.
Jennifer Kasbohm

Radiometric dating would circumvent those uncertainties. But until about 15 years ago, its dates had such large errors that they were useless for addressing questions like the timing of the MCO. The technique also typically needs kilograms of material to find enough uranium-containing zircon crystals, whereas ocean drill cores yield just grams.

But scientists have significantly reduced those limitations: “Across the board, people have been working to track and quantify and minimize every aspect of uncertainty that goes into the measurements we make. And that's what allows me to report these ages with such great precision,” Kasbohm said.

The right stuff

Kasbohm needed a drill core with both Miocene sediments and volcanic ash layers. It’s the volcanic ash that contains the zircon crystals used for uranium-lead dates, so the ash layers act like bookmarks inserted between sediment layers, dating specific points in the sequence.

She located a core that had been drilled in 1996 from the bed of the Caribbean Sea between Nicaragua and Jamaica. “The location was, I think, the biggest contributor to success,” said Kasbohm. That’s because it is downwind from volcanoes that produce a lot of volcanic ash that contains plenty of zircons.

The zircon crystals are tiny—just a 10^th of a millimeter or less—and they must be separated from everything else in the sample, a process that requires extreme care to avoid contamination. “I end up with half a picogram of uranium and lead to measure, and so even small amounts of trace metals… are actually the biggest source of my uncertainty,” said Kasbohm. (A picogram is a trillionth of a gram.)

She used a series of methods to separate the zircons, including the classic gold-panning technique: “I literally use these gold pans as part of the process,” said Kasbohm. She then bathed the zircons in acid to eat away damaged parts of each crystal before using a mass spectrometer to measure the amount of uranium that had decayed to lead since the ash erupted, giving an age.

Kasbohm and colleagues also measured climate proxies from the sediments: oxygen isotopes to determine the temperature changes and carbon isotopes that reflect carbon cycle changes.

Enlarge / A deep-sea sediment core with volcanic ash (gray) that erupted 16.67 million years ago, in whitish limestone.
IODP

Surprisingly, the results showed that the warming of the MCO began about 200,000 years before the flood basalt eruptions. “That is a mismatch… first warming, then volcanism is not your usual causal chronology!” remarked Kasbohm.

In a Universe where time moves only in one direction, an effect before a cause is frowned upon.

Despite that mismatch with the start of warming, the study did find a match between the eruptions and the later peak warmth during the MCO. “You can definitely see that the interval with the greatest sustained warmth of the Miocene Climate Optimum is concurrent with the eruption of the Columbia River Basalts,” said Kasbohm.

Basalt stalled in the crust

Can a single factor explain warming before the eruptions and also peak warming with the eruptions?

Kasbohm and colleagues cite work by Xiaochuan Tian and Roger Buck of Columbia University, who modeled the physics of bringing basalt through Earth’s crust to erupt. Before a flood basalt happens, “You have to bring that basalt through the crust,” said Kasbohm. “How can you actually get that really dense magma to the surface?”

Tian and Buck showed that basalt magma has a buoyancy problem, so it tends to stall out within the crust, collecting in horizontal-ish sheets called “sills.” But as more and more sills form, the crust becomes denser, making the magma buoyant in comparison. It can then rise all the way to the surface and erupt. Crucially, Tian and Buck calculated that this process takes several hundred thousand years, with each sill emitting CO₂ as it cools. These greenhouse gas emissions could plausibly explain why warming started before the basalts erupted.

The Sun is setting on US ocean drilling, as the drill ship J<em>OIDES Resolution</em> is being scrapped in 2024. — Enlarge / The Sun is setting on US ocean drilling, as the drill ship JOIDES Resolution is being scrapped in 2024.
Shuhao Xie/IODP

This could explain small mismatches in timing between basalt eruptions and climate effects noted for other Large Igneous Provinces, such as in the end-Permian and end-Triassic mass extinctions. Similarly, at the end of the Cretaceous, there’s a significant climate warming before the main volume of Deccan Traps lavas erupted in India.

Thomas Gernon, a professor at the University of Southampton, who was not involved in Kasbohm’s study, agrees, saying it’s “likely that the emplacement of these flood basalts… contributed to the sustained climate warmth,” though he questions the link between the underground magma and the MCO.

“The 200,000-year time interval between the onset of Miocene warming and volcanism doesn’t fit well with our understanding of Large Igneous Provinces like the Columbia River Basalts, which typically have a… rapid onset,” said Gernon. “The simplest explanation, therefore, is that the Columbia River Basalts did not trigger the Miocene Climate Optimum, which begs the question, what did?” Gernon pointed to a proposed uptick in plate tectonic production of ocean crust at the time, which would also have emitted CO₂ into the atmosphere.

Confirming astronomical models

The significance of this first successful application of high-precision radiometric dating to marine sediments in drill cores goes far beyond the Miocene Climate Optimum. It paves the way for an improved understanding of environmental changes through hundreds of millions of years of geological climate records.

“This is really outstanding,” said Westerhold about the paper. “I think when people see this potential, they will say, ‘Oh! We should try this also.’”

Westerhold’s specialty is using orbital wobbles calculated by astronomers to date sediments: “They look a little bit like a bar code, and this bar code can be reproduced by astronomers, and if you fit the bar codes, you know the time,” explained Westerhold.

Kasbohm and colleagues compared their results with sediments drilled from two sites in the Pacific Ocean that also recorded the MCO and had been dated using those orbital “bar codes,” whereas Kasbohm’s Caribbean core had not. The ages calculated for the Pacific cores using orbital wobbles were close to Kasbohm’s radiometric ages on her Caribbean core.

“That makes you jump in your seat,” said Westerhold, “because you have two different records dated with two different methods, nothing to do with each other, [and] they come up with the same solution.”

This shows that astronomers’ models of planetary orbits are valid into deep time. “It's basically a fundamental confirmation of the orbital theory,” said Westerhold.

New use for old core…

The workhorse ship for scientific ocean drilling, the JOIDES Resolution, is being decommissioned in 2024 with no sign of a replacement, but it produced nearly 500 kilometers of core that is stored in Texas, Germany, and Japan. Kasbohm’s study may unlock new results from those old cores: “There is a ton of work that can be done on these cores that are sitting in those three repositories,” said Kasbohm.

“It's a really outstanding paper,” said Westerhold. “She is proposing a way forward that we are missing.”

Geology, 2024. DOI: 10.1130/G52255.1

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