News•May 4, 2015

The Surprising Link Between Fjords and Carbon

The magnificent, steep-sided fjords that slice deeply into the coastlines of New Zealand, Norway and Alaska are hugely popular attractions for tourists. But they may be surprisingly important to the Earth’s climate system as well.

While fjords make up just one tenth of one percent of the oceans’ surface area they account for about 11 percent of the carbon locked away in marine sediments each year — carbon that can’t leak out into the atmosphere to add to global warming, says a new reportin Nature Geoscience.

A fjord in Norway.
Credit: Dag Endre Opedal/flickr

The sediments come from rapid erosion of soil and plant debris into fast-running rivers that flow into the fjords from surrounding mountains. “We suspected fjords were important to the global carbon cycle, but when we really analyzed it, we realized, ‘wow!’ ” said Richard Smith, of the consulting firm Global Aquatic Research, in Sodus, N.Y., lead author of the study.

Climate scientists already know that about half of the carbon that humans pump into the atmosphere, mostly through the burning of fossil fuels, is reabsorbed by the land and oceans in various ways. But the precise breakdown isn’t all that well understood.

Nailing down the role fjords play won’t directly aid in the fight to reduce the amount of CO2 in the atmosphere, since the erosion of sediments into fjords is a purely natural process that can’t really be sped up. Human activity such as overdevelopment of the land surrounding fjords could, however, slow the erosion process. “Fjords are pretty pristine areas,” Smith said. “They’re helping us by burying carbon. We should let them do their thing.”

Their “thing” basically is to channel far more organic matter into the ocean than ordinary rivers do.

“Classically, when you look at rivers like the Mississippi, they do transmit lots of material,” Smith said. That material is mostly soil filled with plant debris and carbon-rich organic matter, washed into the river from surrounding farmlands, grasslands and forests. Eventually, it washes downstream to form the silty Mississippi Delta, where that carbon will sit more or less indefinitely.

The Drygalski Fjord extends into the southern end of South Georgia Island.
Credit: David Stanley/flickr

“But about 10 years ago, people realized that small, fast-moving mountainous rivers are more efficient at transporting organic matter,” Smith said. The reason is that sediments in a slow-moving stream like the Mississippi are constantly churned up to the top of the river, where some the organic matter can break down and send CO2 into the atmosphere. A mountain stream racing down the steep side of a fjord, by contrast, rushes the sediments down to the sea before much of the carbon can break down.

Because fjords are basically narrow arms of the ocean that reach far inland, they dramatically increase the amount of shoreline that mountain streams can erode. “You’ve got an interface between mountainous land and the ocean that reaches hundreds of miles inland,” Smith said. “It’s a very quick means of transfer for sediments from land to sea.”

To calculate how big a role fjords play in the planet’s carbon cycle, Smith and his colleagues looked at 573 sediment samples from the bottoms of fjords around the world, and 124 additional samples drilled from beneath the bottoms of fjords, where centuries upon centuries of older sediments are buried.

At just 11 percent of the total carbon locked up in ocean sediments, which are themselves only a fraction of the carbon removed from the atmosphere by seawater, forests and other natural carbon absorbers, fjords are “a minor component of the overall annual sink,” according to Richard Keil, of the University of Washington, who was not involved in this research. “But if we’ve overlooked fjord carbon storage for so long, what else don't we know about?

“It’s amazing to me how much we still have to learn about how the Earth functions,” Keil said.