Seabed carbon

The carbon cycle diagrams most teachers know show five main stores: atmosphere, ocean, vegetation, soil, and fossil fuels. What they rarely show is the carbon locked in the sediments on the seafloor.

Seabed carbon is not a classic blue carbon habitat like a mangrove or seagrass meadow. There is no visible vegetation doing the work. The carbon is simply there, buried in mud and sand, built up gradually over geological time.

Where is it, and how much?

Continental shelf sediments could be one of the largest long-term organic carbon reservoirs on the planet. Not all sediment stores carbon equally. Recent research shows that muddy, soft sediments hold significantly more organic carbon than sandier or coarser substrates.

Mapping this carbon globally is still difficult. Unlike a forest, which can be surveyed from above, shelf-sediment carbon has to be measured directly, sediment core by sediment core, which makes the full picture hard to establish and currently incomplete.

What could disturb it?

Bottom trawling, a form of fishing where nets are dragged across the seafloor, puts more than 20 gigatonnes of sediment annually back into the water column. This makes it the most widespread human physical disturbance to surface sediments on Earth. When that happens, carbon that was locked away from the atmosphere is resuspended, where it can be broken down and potentially re-enter the atmospheric carbon system.

Trawling is not the only disturbance. Recent research has begun estimating how much seabed carbon has historically been disturbed by port dredging and aggregate extraction across Northwest Europe, industries that physically remove or rework seabed sediment and can cause stored carbon to be released in much the same way.

Current research

This is the central focus of the Convex Seascape Survey, a five-year global research programme run by the University of Exeter, Blue Marine Foundation, and Convex Group. Fieldwork has taken place across the Irish Sea, North Sea, English Channel, Jersey, Scotland, South Africa, New Zealand, Canada, and Antarctica, with sites chosen to allow findings to be broadened to regional and global scales.

One study compared seabed carbon and biodiversity inside the South Arran Marine Protected Area, a rare, highly protected, mud-dominated site, against less-protected areas nearby. Soft, muddy habitats like Arran's have historically been overlooked for protection because they were seen as low in biodiversity value, but their potential role in long-term carbon storage is increasingly part of the case for protecting them.

A decade ago, seabed carbon barely featured in climate models. It's now recognised as one of the largest carbon reservoirs on the planet, and findings on where it sits, how stable it is, and what disturbs it are starting to come in.