Receding ice, energetic volcanoes and the Southern Ocean iron experiment: be careful what you wish for

One Sunday late last month, I jounced up 15 kilometres of torturous four-wheel-drive road to a trailhead high on the flank of Oregon’s 3,425-meter Mt. Hood volcano. My goal was to see if my soon to be 69 year-old body could still hike to a 2,600m viewpoint on the moraine of the mountain’s Eliot Glacier, which a friend and I had visited 34 years before.

As I hiked, however, I was quickly struck by more than the thin air and unrelenting climb. Where did the glacier go?

The answer, of course, was that it had retreated, dramatically, over a little less than half my lifetime. Where once its snout was only a slightly higher than the trailhead, now there was nothing but rock. Its lowest remaining part was hundreds of meters higher and perhaps a kilometre farther away.

The retreat of mountain glaciers, of course, is well known. But a paper in Nature, only a few days after my hike, revealed their demise in a new light. At least once in Earth’s history, the scientists found, rapidly melting glaciers had not only denuded many volcanoes, but triggered them to erupt.

It happened, says study author Professor Alan Mix, an oceanographer and paleoclimatologist at Oregon State University, US, by “uncorking” them by reducing the weight of ice pressing down on them. “That releases the stress, which then can trigger volcanoes,” he says.

At least once in Earth’s history, the scientists found, rapidly melting glaciers had not only denuded many volcanoes, but triggered them to erupt.

Not that this means the demise of the Eliot Glacier is likely to cause Mount Hood to erupt in a titanic blast. “We’re not saying volcanoes are going to start popping off and we’re going to be wiped out,” Mix says, “though I imagine some accounts will misquote me on that.”

That’s because there’s a difference between what he calls “big ice,” and “little ice,” and on this scale, the Eliot Glacier is little ice.

In fact, the big ice he’s talking about doesn’t currently exist in many parts of the globe. Yes, it’s in Greenland, but Greenland doesn’t have volcanoes.

The ice his study examined was in the Cordilleran ice sheet, which blanketed much of Western North America, from Alaska to Washington, at the peak of the last Ice Age.

On this scale, the Eliot Glacier is little ice.

“Most of the Cordilleran ice sheet disappeared about 10,000 years ago,” says the study’s lead author Dr Jianghui Du, now a postdoctoral researcher at ETH Zurich, Switzerland. “A remnant of the ice sheet is left in southern Alaska, mainly as mountain glaciers [which] are melting fast. Some studies have suggested that glacial retreat may be linked to increasing earthquakes in this region, [but] the remaining ice is much smaller than what existed in the Ice Age.”

Other volcanic regions with significant remaining ice are Iceland, the Antarctic Peninsula, and perhaps Patagonia. But even if volcanism rises in these places as glaciers retreat, the result won’t be a disaster-movie scenario because the uncorking of volcanoes is a slow process occurring over hundreds or thousands of years.

What really matters, Mix and Du say, is what the new find teaches about oceans, global warming, and the potential unintended consequences of possible efforts to protect the planet.

That story begins more than two decades ago in the North Pacific Ocean, when Mix wrangled funds for a cruise to collect sediment cores from the region. The goal was to learn how the North Pacific plays into the global climate system, but in the process, Mix and other scientists discovered that it had seen prolonged episodes of “humungous” low-oxygen “dead zones” that persisted for millennia.

Understanding why this happened is important, Du says, because global warming is already threatening oxygen levels in the ocean. As water warms, its ability to hold dissolved oxygen drops. That means that oxygen-depleted dead zones are expected to expand, with potentially dire consequences for marine ecosystems and fisheries dependent on them.

“This threat is particularly palpable in parts of the ocean where the natural oxygen background is already very low, mainly in the North and East Pacific,” Du says. “Slight decreases of oxygen in these regions may have disproportionally large impacts.”

Even if volcanism rises in these places as glaciers retreat, the result won’t be a disaster-movie scenario.

That’s where volcanoes and glaciers enter the picture, because one of the things Mix’s cores reveal is an extended low-oxygen event at the end of the last Ice Age, roughly 17,000 to 10,000 years ago.

For years, scientists struggled to understand this, but now, Du and Mix’s team has discovered that sediments from the hypoxic areas were sprinkled with unusually high levels of volcanic ash, indicative not of a single big volcanic blast, but of a steady drumbeat of smaller eruptions—occurring at precisely the time the giant ice sheets were melting.

The most likely sources, Du says, are volcanoes in the Aleutian Islands and the Wrangel Mountains of Alaska, all of which, at the peak of the last ice age, were deeply covered in ice (in places 2,000 meters thick).

The relevance of this to oceanic dead zones is that volcanic ash is rich in iron, an extremely good fertiliser for explosive blooms in marine phytoplankton. When these organisms gobble up all the available iron, they die, sink, and decompose—in the process now absorbing much of the available oxygen.  

Unlike the surface waters, the waters in which this happens about 200 meters to 1000 meters below the surface aren’t in contact with the atmosphere. That means they can’t easily replenish their oxygen, say Dr Weiqi Yao and Associate Professor Ulrich Wortmann, of Southern University of Science and Technology, China, and University of Toronto, Canada.

All of this, of course, occurred thousands of years ago. But there are proposals to offset climate change via iron fertilisation, in which powdered iron is sprinkled into iron-poor ocean waters in a deliberate effort to induce phytoplankton blooms whose growth might capture vast amounts of carbon dioxide from the air, and whose deaths would cause most of it to fall to the depths and be buried in sediment.

As far back as 2002, this was tested in an experiment called SOFeX—for Southern Ocean Iron (Fe) Experiment—and proposals have been floated to do it on a larger scale not only there, but in the North Pacific. The idea is that for each atom of iron deposited in these waters, a whopping 100,000 molecules of carbon dioxide might be removed from the air.

When organisms gobble up all the available iron, they die, sink, and decompose – absorbing much of the available oxygen.

But is it a good idea?

The new research says maybe not. “The Earth has done the experiment for us,” Mix says. “One lesson is that large-scale iron fertilisation does indeed sequester carbon dioxide in the deep sea. But there are dangerous consequences, because it creates dead zones. That would be a pretty catastrophic thing, so we should be careful about it.”

Other scientists agree “Du and colleagues’ work implies that short periods of iron fertilisation can lead to long-lasting oxygen deficiencies in marine ecosystems,” Yao and Wortmann wrote in their own paper in Nature. “The resulting marine dead zones could affect fisheries for millennia.”

“There has long been interest in the possibility that iron fertilisation could be used to draw down carbon dioxide,” adds Professor Richard Alley, a Nobel-laureate climate-change researcher at Pennsylvania State University. “I am not an expert in this area, but it cannot be a major solution for human carbon dioxide.”