Geology & Geophysics News

Iron Readings Hint That Ocean Depth Influences Seabed Volcanism

Water pressure on mid-ocean ridges may affect magma production kilometers beneath the ocean floor.

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Scientists long doubted that changes in sea level could affect volcanoes erupting deep in the sea along mid-ocean ridges. Recently, however, measurements of iron that bubbled up as magma, released long ago from hydrothermal vents along those ridges, suggest a possible connection.

As Earth’s climate cycled in and out of periods of cooling and glaciation for the past 2.6 million years, sea levels fell as ice sheets that locked up water on land formed and then rose with glacial melting that returned water to the oceans again. The new findings indicate that the activity of magma at mid-ocean ridges, which lie an average of 2.5 kilometers underwater, may fluctuate in response to those sea level ups and downs and the accompanying relatively small changes in water pressure, said Jennifer Middleton, a geochemist who was a Harvard Ph.D. student at the time of the studies.

Previous studies in Iceland have suggested that thickening glaciers on the summits of volcanoes exert massive increases in pressure on these volcanoes that suppress eruptions and vice versa. Taking cues from those effects, researchers have moved this sort of research offshore to explore the potentially more nuanced effects of decreases and increases in glaciation, and the associated changes in sea level and water pressure, on underwater volcanoes.

Forging an Iron Link

Iron arises in plumes from hydrothermal vents on mid-ocean ridges and is often used by scientists as a measure of the activity of magma at these locations. In preliminary findings from new investigations conducted at the Mid-Atlantic Ridge, East Pacific Rise, and Juan de Fuca Ridge, “we see that when sea levels are changing, you see changes in hydrothermal iron deposition at all of the sites that have been explored,” said Middleton. “This suggests there is a global change in magmatic activity associated with changing sea level.”

Measurements of copper, which has likewise served as a measure for the activity of magma, during those same investigations yielded a different pattern than iron, Middleton noted. However, there are reasons to think that the iron correlation remains valid, she said.

The new research gives a snapshot of the Juan de Fuca Ridge between 400,000 and 600,000 years ago, Middleton pointed out this past December as she reported their findings in a poster presentation at the American Geophysical Union’s 2017 Fall Meeting in New Orleans, La.

Unexpected Results

Middleton and her fellow researchers took a 2-week cruise in the Pacific Ocean to gather samples at the Juan de Fuca Ridge, just a few hundred kilometers west of Seattle. They were accompanied by scientists across many disciplines, each aiming to provide different sides of the same story.

To get the “hydrothermal story,” Middleton gathered sediment cores from the ocean floor near the ridge. “It’s pretty low-tech,” she said. “We essentially attach a rope to a length of PVC pipe and drive it into the ocean floor with a rock on top.” The sediment cores keep a historical record in their layers. The researchers examined variations in iron depositions in the samples and looked for correlations with variations in sea level. They did the same for copper, which was thought to behave much like iron.

Their findings from the cruise were not quite what they expected: The researchers had anticipated that iron and copper would fluctuate with sea level and in tandem with each other. However, copper levels occasionally spiked in a way that iron levels didn’t. Middleton told Eos that copper goes through chemical reactions upon settling on the seafloor. These reactions complicate the measurements, which may make copper levels a poor estimate of hydrothermal activity.

Although iron readings seemed tied to sea level variations, those brought their share of surprises, too. Initially, the researchers thought magma production and iron deposition would increase in times of low sea level and water pressure. They observed the opposite, they reported in their poster, finding that iron quantities instead increased as sea levels did.

Lagging or Cracking?

Middleton offered two possible ways to account for this relationship. Magma is produced several kilometers beneath the ocean floor, and scientists don’t know how long it takes to reach the surface. “It may take 1,000 years, it may take 10,000,” she said. Therefore, there may be a lag between changes in water pressure and shifts in hydrothermal activity.

Alternatively, there may be no lag, but higher water pressure may lead to cracks in Earth’s crust. This explanation draws upon a theory that glacial cycles affect crust thickness under the ocean, leaving it more vulnerable to fracture at certain times. In this case, plumes of hydrothermal metals would be released when ocean levels are high. Bridgit Boulahanis, a marine geophysicist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y., investigated this cracking hypothesis on the research cruise to Juan de Fuca.

Newfound Inconsistency

Middleton said that the output of hydrothermal metals from mid-ocean ridges was thought to be steady over large timescales, but the iron findings suggest otherwise. The findings tie the fluctuation of iron release to global climate variations. She said she finds it fascinating “how the climate cycle and the solid Earth interact with each other on timescales of tens of thousands of years.”

David Lund, a professor of marine sciences at the University of Connecticut–Avery Point in Groton who was not involved in the research, said the new findings highlight a growing understanding of links between liquid and rocky components of Earth. “The research suggests a connection between the fluid Earth and the solid Earth that we didn’t know about 10 years ago,” he said. “It’s very exciting.”

—Nicoletta Lanese (email: [email protected]; @NicolettaML), Science Communication Program Graduate Student, University of California, Santa Cruz

Citation: Lanese, N. (2018), Iron readings hint that ocean depth influences seabed volcanism, Eos, 99, https://doi.org/10.1029/2018EO090821. Published on 18 January 2018.
Text © 2018. The authors. CC BY-NC-ND 3.0
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