Mud is what matters for the ocean’s natural ability to store massive amounts of carbon and buffer global warming.

Oceans absorb an enormous amount of human-caused emissions, sinking as much as 48 per cent of the carbon produced by burning fossil fuels. And sea floor sediments, in particular, are estimated to hold more of these “blue carbon” stores than all the soil on land.

“Muddy sediment is where you often have really high carbon,” said Graham Epstein, a member of Canada’s Blue Carbon project, a collective scientific effort to assess the country’s coastal carbon reservoirs.

“It’s often a hard sell to tell people that thick, gloopy mud — which often doesn't look that pretty — is the really good stuff that we need to think about protecting.”

Boasting the longest coast in the world, Canada potentially has numerous seabed reserves, but needs a better understanding of where carbon hot spots are to preserve them, said Epstein, a researcher at University of Victoria.

Fortunately, he and other university researchers and ocean conservation groups involved in the blue carbon initiative have produced the first national map predicting the location of Canada’s richest ocean carbon deposits.

Canada is estimated to hold 10.9 billion tonnes (10 gigatons) of carbon within the top 30 cm of its ocean sediment, according to the study.

Across the Pacific, Atlantic and Arctic oceans, the highest carbon density was determined in small areas near shore, particularly in B.C.'s deep, winding inlets and fiords and sections of the Salish Sea, along with sheltered near-shore areas of the Atlantic east coast.

Relatively high carbon densities were also predicted to occur further offshore in deeper channels or troughs within the Gulf of St Lawrence, the central Scotian Shelf and at the edge of the continental slope west of Vancouver Island.

Carbon is best stored in deeper sites where there isn’t typically a lot of wave action or disturbance from strong currents, Epstein noted.

“Fjords are a really good example of this because you have these really steep, steep topographies going to quite deep depths at the bottom,” he said.

“You can have a lot of delivery of carbon to the sea floor in sheltered, enclosed environments where the sediment and organic matter can accumulate.”

Atmospheric carbon is stored in the ocean in two ways, either through a “physical” or “biological” pump. In the first instance, carbon dioxide diffuses or mixes into seawater at the ocean’s surface as part of a chemical process before being transferred to the deep ocean in regions where cold, denser surface water sinks and remains trapped for long periods of time.

However, organic carbon finds its way to the sea floor when phytoplankton absorb carbon dioxide during photosynthesis to generate energy when they also release oxygen. The carbon stored in the small organisms may be “captured” again as larger marine animals eat them.

However, dead phytoplankton or other dead marine animals or waste can sink to the sea floor as “marine snow,” where they form thick layers of carbon-rich matter in sediment that can remain trapped for millennia, as long as they remain undisturbed.

These blue carbon stores are often overlooked as potential nature-based solutions in the fight against climate change. More attention is paid to land-based efforts to protect forests, tundra or soil, Epstein said.

However, estimates suggest Canada’s ocean sediments potentially store 100 times more carbon than all coastal seagrass beds and salt marshes combined and about half of all the carbon in forests.

Blue carbon storage is vulnerable to human activities, including bottom trawl fishing, shipping, energy generation, telecommunication construction, deep-sea mining and pollution from land-based activity, Epstein added.

But knowing where the best carbon storage sites are can help inform fisheries plans, or sites for marine-protected areas, or help prioritize locations for further research on the ground, he added.

The new ocean carbon map, which relied on limited existing data about ocean carbon storage in Canada and employs a predictive model to determine the estimates around carbon storage, is a good tool but needs to be followed up with more effort and scientific data, he suggested.

“The map is still a guide and the best guess based on the data we have,” Epstein said.

“It’s a first step to start directing research or management to start thinking about which areas are most important and what sort of activities or conservation or management actions we might want to consider.”