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The Ocean Twilight Zone’s crucial carbon pump

When CO₂ enters the ocean, where does this heat-trapping gas go?

By Madeline Drexler | January 9, 2020

Ken Buesseler
WHOI marine chemist Ken Buesseler (left) and research specialist Steven Pike prepare to deploy a sediment trap to gain insight into the role of the upper ocean in Earth's carbon cycle and climate system. (Photo by Jennifer Kenyon, Woods Hole Oceanographic Institution)

When heat-trapping carbon dioxide from the atmosphere sinks into the ocean and turns to organic carbon, how much of it sinks to the deep ocean and how quickly?

Ken Buesseler, a geochemist at Woods Hole Oceanographic Institution (WHOI), has been working to answer these questions. He and other scientists have shown that one key to the ocean’s capacity to remove or sequester carbon is the ocean’s “twilight zone,” a mysterious stratum of ocean beneath the sunlit surface layer, which ranges from 30-200 meters (about 100-650 feet), to about 1,000 meters (3,300 feet) down.

Also called the mesopelagic zone, this enigmatic mid-layer of water is home to otherworldly creatures with names to match—bristlemouth, hatchetfish, devil squid, stoplight loosejaw. Were it not for these deep, inky-black waters, the climate on our planet would be unrecognizably warmer.

The removal of CO₂, its conversion to organic carbon, and the carbon’s dispatch to the ocean’s depths is known as the ocean’s biological pump.

“If you turned off this biological carbon pump—let’s say, you somehow made the ocean sterile and sucked out all of the twilight zone’s organisms—you would more than double the amount of CO2 in the atmosphere that humans have added,” said Buesseler. “That would take us to temperatures we haven’t seen in 50 million years, when the Earth was 20 degrees Fahrenheit warmer than today, there was no ice on either Pole, the seas were much higher, and many parts of the world where people now live were uninhabitable.”

The pump operates through different mechanisms. In one, a flurry of carbon-laden particles from the surface—dead phytoplankton, feces produced by zooplankton, and other material collectively known as “marine snow”—sink, with about 90 percent serving as the food supply for twilight zone animals and bacteria.

In addition, multitudes of fish, squid, zooplankton, and other mid-ocean denizens migrate nightly close to the ocean’s surface to feed, returning to the twilight zone by daybreak, carrying the carbon in their food with them. Theirs is the largest animal migration on the planet, and a vertical one at that. These migrating species transport massive amounts of carbon from surface waters to the deep ocean.

At the same time that the biological pump moves carbon, physical processes such as the global ocean’s overturning circulation—a system of surface and deep currents around the globe—transport suspended and dissolved carbon to the deep ocean, but on longer time scales.

Buesseler wants to know exactly what fraction of surface carbon is being shuttled to the twilight zone and on to the deep ocean, where it may stay for hundreds to thousands of years. That calculation would profoundly shape climate predictions.

“The twilight zone is the gateway to the deep ocean for carbon sequestration,” Buesseler explained. “But there’s a huge range of estimates of how much is entering the twilight zone—from four to 12 gigatons of carbon annually.” (For comparison, one gigaton weighs about the same as six million blue whales.) “That eight-gigaton difference in the estimates is huge—about the same magnitude as the amount of carbon humans put into the atmosphere every year. If we reduced that uncertainty, we could predict climate changes better and more wisely invest the billions of dollars that go to climate adaptation or mitigation. We could better predict sea level rise and help planners determine where to build roads and bridges and how to adapt our coastal cities to rising sea levels.”

Today, Buesseler sees a new potential danger on the horizon. Located largely in waters outside of national boundaries, the twilight zone is starting to pique the interest of commercial fishing operations harvesting organisms to be converted into feed for chickens, farmed fish, or the nutraceuticals market. Unless it is done sustainably, such extraction could disrupt the ocean’s vital carbon pump.

“This threat has motivated WHOI’s Ocean Twilight Zone program to some extent,” he said. “We need to understand how much carbon is getting through to the twilight zone, and this takes knowing not only how many organisms are there but much more about their life histories and diversity. We need to get ahead of the game before it becomes commercialized.”

This article was originally published as part of the in-depth feature “Oceans of Change” in the fall 2019 issue of Oceanus, Woods Hole Oceanographic Institution’s member magazine. Join and subscribe here.