How dead stuff at the bottom of the ocean helps slow the pace of climate change
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In the deep ocean, thousands of feet below the surface, it looks like it's snowing.
At those depths, the water is filled with slowly drifting particles known as "marine snow," part of a never-ending deluge of debris from the world above. The tiny globs of "snow" are made up of leftovers from life at the surface: dead and dying plankton, their poop and mucus, and the bacteria that feed on them.
It turns out this process is vital to understanding climate change.
The particles are part of a conveyor belt carrying waste to the deep. That waste contains carbon dioxide absorbed from the atmosphere. As it sinks, it locks away vast amounts of carbon, including some of the heat-trapping gases emitted by cars and power plants. Without that process, the Earth would be much hotter than it is today.
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To predict how fast the planet is warming, climate scientists need to understand how much marine snow reaches the ocean floor. A new study finds these particles are sinking more slowly than previously thought – which may mean less carbon is ultimately stored away.
The culprit, the study finds, is invisible halos of mucus. That mucus acts like a parachute, slowing the particles as they sink. Understanding that dynamic could give scientists a much more accurate idea of what's happening to carbon dioxide on the planet.
"The small stuff controls the big stuff," says Manu Prakash, bioengineering professor at Stanford University, one of the authors of the study. "On our planet, that's a rule that we should think about. That the small stuff really matters."
Feast of the dead
Even when the surface of the ocean looks fairly empty to human eyes, it's teeming with tiny life.
"We tend to think the ocean is just this clear water," says Alice Alldrege, emeritus professor of marine biology at the University of California, Santa Barbara. "But in fact the ocean is full of little islands of particulate matter that get eaten by other organisms or decomposed by bacteria. It's like little islands of very hot spots."
Tiny algae grow at the surface, using sunlight and carbon dioxide to build their bodies. They're eaten by communities of other plankton. As all those plankton die, their bodies clump together. The dead and dying clusters become a feast for microbes and yet more plankton, creating tiny ecosystems.
Alldredge was one of the first to study marine snow in the 1970s, as scientists began to understand the vital role it plays in the ocean. She and others found that marine snow particles also contain mucus. It's produced by bacteria and algae as they grow, as well as creatures known as larvaceans, which feed on their prey by creating entire "snot palaces," as they're known.
"There's nothing yucky about it," Alldredge says, with a laugh. "For many of these organisms, it's just part of life and it's important."
The ocean's biological pump
Once it forms, particles of marine snow can take weeks to slowly drift to the ocean floor. There, it becomes a vital food source for marine life that inhabits the deep sea, where no sunlight reaches. Some of the marine snow is also deposited in the sediment on the seafloor, storing carbon dioxide.
This process is one of the key ways the ocean locks away carbon from the atmosphere. Because plankton use carbon dioxide to grow at the surface, it's stored in their bodies. After they die, that carbon comes with them to the seafloor as they sink. In this way, the ocean is like a sponge, soaking up the heat-trapping gas, including much of what humans generate from burning fossil fuels.
"Every year, between 30 to 40 percent of all carbon that we're emitting, the ocean absorbs it," Prakash says. "We have this remarkable technology that does it for free, and it's the living ocean."
This process is known as a "biological pump," through which carbon dioxide in the atmosphere eventually makes its way to the deep ocean, where it stays for thousands of years, preventing it from heating up the planet. As a result, climate scientists say the pace of climate change is inherently tied to how much marine snow is sinking to the deep – and how fast.
"If that sinking rate is slow, you absorb carbon slowly," Prakash says. "If that sinking rate is fast, you can actually absorb a lot of carbon. If that sinking rate is zero, you actually don't absorb any carbon."
Using a gravity machine
To understand this sinking process, Prakash and his colleagues developed a special microscope they named the "gravity machine." Tiny particles are observed while in a spinning wheel, keeping them perpetually falling. To study highly delicate marine snow, the team collected samples at sea in the Gulf of Maine and studied them onboard the ship (with occasional seasickness breaks).
They found marine snow particles don't fall quite as scientists expected. Instead, they're slowed down by invisible mucus around the particle, which creates drag in the water like a parachute.
That's a worrisome result, Prakash says, because the slower the particles sink, the less carbon actually makes it to the sea floor. Sinking slowly gives bacteria and plankton feeding on the detritus more time to break it down. That means more carbon stays in the upper ocean, instead of sinking to the deep where it's locked away.
Predicting future warming
Advancing our understanding of the dynamics of marine snow could help climate scientists better predict future warming. Researchers use that information in complex computer models to simulate how the planet will respond to increasing levels of carbon dioxide.
"We do not have a very accurate estimate of how much stuff is getting to the bottom of the ocean," says Colleen Durkin, oceanographer at the Monterey Bay Aquarium Research Institute, who was not involved in the research. "The estimates can vary by an order of 100 percent when it comes to models."
As climate change continues to impact the ocean, including its fundamental water chemistry, scientists say the dynamics of marine snow could also change. That would alter the balance of how the planet handles carbon dioxide.
"This is a really critical issue for understanding climate change – how it's going to change in the future and even how it's changing right now," Durkin says.
It's an example of why studying the ocean is crucial, Prakash says.
"The oceans truly are the biggest unknown on our planet," Prakash says. "It's so remarkable to think about the mysteries that exist. And literally our life depends on it."
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