Deep beneath the cold waters off Herschel Island in Canada's Arctic, something surprising is happening: the ocean is acting like a giant storage locker for carbon that could otherwise warm the planet.
Scientists have long worried that as Arctic permafrost thaws due to rising temperatures, enormous amounts of stored carbon would pour into the ocean. There, tiny microorganisms would break it down and release it as greenhouse gases like CO₂, making climate change worse. But new research suggests this picture may be incomplete.
A team from the Alfred Wegener Institute and the University of Bremen analyzed sediment samples from the seabed near Herschel Island, also known by its Inuit name Qikiqtaruk. What they found was unexpected. Although huge quantities of carbon wash into the sea from thawing coasts and rivers each year, most of it does not return to the atmosphere. Instead, it stays locked in the ocean floor.
The Arctic stores a tremendous amount of carbon in its frozen ground — roughly 1,300 gigatonnes, comparable to several decades of global human emissions. Another 400 gigatonnes sits in ocean and river sediments. Scientists estimate that about 0.02 gigatonnes of carbon enter the Arctic Ocean from land every year, and that number could climb 70 to 150 percent higher by the year 2100.
To understand what happens to this carbon, researchers examined layers of mud deposited over roughly 50 years. They found that microorganisms digest only about 10 percent of the organic carbon buried in seabed sediments and convert it into gases that escape into the water and eventually the air. The remaining 90 percent stays in the seabed, effectively removed from the climate equation.
The team used carbon isotopes — natural atomic markers — to trace where the microorganisms' food comes from. Their discovery? The tiny bacteria are surprisingly picky eaters.
"The sediment is home to 'gourmet' bacteria that apparently prefer fresh carbon stemming from, for example, more recent algal remains over the 'old' carbon from permafrost deposits," explained Gesine Mollenhauer, a geochemist at the Alfred Wegener Institute.
This suggests that fears about permafrost carbon accelerating warming may be somewhat overstated, at least for carbon that reaches the seabed. However, researchers caution that some carbon may break down before it ever gets there.
The findings, published in the journal Nature Geoscience, offer a more nuanced understanding of how Arctic change affects the global climate system.