Oceans Lock Away Carbon Slower Than Previously Thought

Research expeditions conducted at sea using a rotating gravity machine and microscope found that the Earth's oceans may not be absorbing as much carbon as researchers have long thought.

Oceans are believed to absorb roughly 26 percent of global carbon dioxide emissions by drawing down CO₂ from the atmosphere and locking it away. In this system, CO₂ enters the ocean, where phytoplankton and other organisms consume about 70 percent of it. When these organisms eventually die, their soft, small structures sink to the bottom of the ocean in what looks like an underwater snowfall.

This marine snow" pulls carbon away from the surface of the ocean and sequesters it in the depths for millennia, which enables the surface waters to draw down more CO₂ from the air. It's one of Earth's best natural carbon-removal systems. It's so effective at keeping atmospheric CO₂ levels in check that many research groups are trying to enhance the process with geoengineering techniques.

But the new study, published on 11 October in Science, found that the sinking particles don't fall to the ocean floor as quickly as researchers thought. Using a custom gravity machine that simulated marine snow's native environment, the study's authors observed that the particles produce mucus tails that act like parachutes, putting the brakes on their descent-sometimes even bringing them to a standstill.

The physical drag leaves carbon lingering in the upper hydrosphere, rather than being safely sequestered in deeper waters. Living organisms can then consume the marine snow particles and respire their carbon back into the sea. Ultimately, this impedes the rate at which the ocean draws down and sequesters additional CO₂ from the air.

The implications are grim: Scientists' best estimates of how much CO₂ the Earth's oceans sequester could be way off. We're talking roughly hundreds of gigatonnes of discrepancy if you don't include these marine snow tails," says Manu Prakash, a bioengineer at Stanford University and one of the paper's authors. The work was conducted by researchers at Stanford, Rutgers University in New Jersey, and Woods Hole Oceanographic Institution in Massachusetts.

Oceans Absorb Less CO₂ Than Expected

Researchers for years have been developing numerical models to estimate marine carbon sequestration. Those models will need to be adjusted for the slower sinking speed of marine snow, Prakash says.

The findings also have implications for startups in the fledgling marine carbon geoengineering field. These companies use techniques such as ocean alkalinity enhancement to augment the ocean's ability to sequester carbon. Their success depends, in part, on using numerical models to prove to investors and the public that their techniques work. But their estimates are only as good as the models they use, and the scientific community's confidence in them.

We're talking roughly hundreds of gigatonnes of discrepancy if you don't include these marine snow tails." -Manu Prakash, Stanford University

The Stanford researchers made the discovery on an expedition off the coast of Maine. There, they collected marine samples by hanging traps from their boat 80 meters deep. After pulling up a sample, the researchers quickly analyzed the contents while still on board the ship using their wheel-shaped machine and microscope.

The researchers built a microscope with a spinning wheel that simulates marine snow falling through sea water over longer distances than would otherwise be practical.Prakash Lab/Stanford

The device simulates the organisms' vertical travel over long distances. Samples go into a wheel about the size of a vintage film reel. The wheel spins constantly, allowing suspended marine-snow particles to sink while a camera captures their every move.

The apparatus adjusts for temperature, light, and pressure to emulate marine conditions. Computational tools assess flow around the sinking particles and custom software removes noise in the data from the ship's vibrations. To accommodate for the tilt and roll of the ship, the researchers mounted the device on a two-axis gimbal.

Slower Marine Snow Reduces Carbon Sequestration

With this setup, the team observed that sinking marine snow generates an invisible halo-shaped comet tail made of viscoelastic transparent exopolymer-a mucus-like parachute. They discovered the invisible tail by adding small beads to the seawater sample in the wheel, and analyzing the way they flowed around the marine snow. We found that the beads were stuck in something invisible trailing behind the sinking particles," says Rahul Chajwa, a bioengineering postdoctoral fellow at Stanford.

The tail introduces drag and buoyancy, doubling the amount of time marine snow spends in the upper 100 meters of the ocean, the researchers concluded. This is the sedimentation law we should be following," says Prakash, who hopes to get the results into climate models.

The study will likely help models project carbon export-the process of transporting CO₂ from the atmosphere to the deep ocean, says Lennart Bach, a marine biochemist at the University of Tasmania in Australia, who was not involved with the research. The methodology they developed is very exciting and it's great to see new methods coming into this research field," he says.

But Bach cautions against extrapolating the results too far. I don't think the study will change the numbers on carbon export as we know them right now," because these numbers are derived from empirical methods that would have unknowingly included the effects of the mucus tail, he says.

Marine snow may be slowed by parachutes" of mucus while sinking, potentially lowering the rate at which the global ocean can sequester carbon in the depths.Prakash Lab/Stanford

Prakash and his team came up with the idea for the microscope while conducting research on a human parasite that can travel dozens of meters. We would make 5- to 10-meter-tall microscopes, and one day, while packing for a trip to Madagascar, I had this aha' moment," says Prakash. I was like: Why are we packing all these tubes? What if the two ends of these tubes were connected?"

The group turned their linear tube into a closed circular channel-a hamster wheel approach to observing microscopic particles. Over five expeditions at sea, the team further refined the microscope's design and fluid mechanics to accommodate marine samples, often tackling the engineering while on the boat and adjusting for flooding and high seas.

In addition to the sedimentation physics of marine snow, the team also studies other plankton that may affect climate and carbon-cycle models. On a recent expedition off the coast of Northern California, the group discovered a cell with silica ballast that makes marine snow sink like a rock, Prakash says.

The crafty gravity machine is one of Prakash's many frugal inventions, which include an origami-inspired paper microscope, or foldscope," that can be attached to a smartphone, and a paper-and-string biomedical centrifuge dubbed a paperfuge."