# Blue carbon

Why carbon stored in tidal marsh, mangrove and seagrass sediment is more stable than forest carbon: the physics of anoxia, the sulfate switch that shuts down methanogenesis, sediment accretion, and what permanence and credit accounting have to answer.

Coastal wetlands bury carbon thousands of years deep in the accounting sense: anoxia slows decomposition, sulfate chemistry suppresses methane, and rising sediment carries the carbon out of oxygen's reach for centuries.

Source: https://en.bioecon.ru/docs/blue-bioeconomy/blue-carbon-restoration/blue-carbon/
Updated: 2026-09-07



Blue carbon is not a label for coastal nature in general; it names a geochemical accident: photosynthesis followed by burial under water, where oxygen cannot follow. Forests fix far more carbon in total, but almost all of it is respired back within years to decades. Tidal habitats fix less and lose far less of it, and the difference is entirely about what happens below the surface.

## Why the sediment keeps the carbon

Oxygen diffuses through water roughly ten thousand times more slowly than through air. In a waterlogged, tidally flooded soil the oxic zone is therefore a skin of millimetres to centimetres; below it, aerobic decomposers — and with them the fast carbon cycle — simply stop. What replaces them is set by chemistry. In freshwater wetlands the terminal pathway is methanogenesis, which leaks methane, a far more potent molecule per unit mass than carbon dioxide, enough to erase much of the storage benefit. In seawater the switch is sulfate: sulfate is abundant in the sea, sulfate-reducing bacteria outcompete methanogens for the same substrates, and the end product — sulfide, eventually pyrite and sulfate salts — does not warm the planet. Saline wetlands thus bury carbon without a methane bill; brackish and freshwater sites do not, and their methane accounting is part of any honest inventory.

The second mechanism is accretion. Marsh grasses, mangrove roots and seagrass canopies slow the water, and suspended particles settle. The soil surface builds upward year by year, carrying organic matter below the oxic skin before it decomposes. A forest soil carbon pool is close to steady state — decomposition balances inputs — while an accreting tidal sediment is cumulative: the stock grows with every decade the surface keeps rising, which is exactly what these wetlands do as sea level rises and they trap more mineral sediment to keep pace.

## What a carbon credit has to answer

The accounting inherits the chemistry. Additionality asks what the site was doing before: a drained or pond-aquaculture-converted wetland that is rewetted restarts burial, and the counterfactual is measurable in cores. Permanence is not a property but a rate race — burial continues only while sedimentation keeps pace with sea-level rise and the anoxic layer stays anoxic; if the site drowns or is drained again, decades of stored carbon oxidise back in a few years. Release is fast, burial is slow, and that asymmetry is the real risk profile. A further subtlety is provenance: sediment traps carbon that was fixed upstream as well as carbon fixed on site, and whether a restored habitat adds new sequestration or merely re-buries carbon that would have settled elsewhere must be established by measurement, not assumed. Cultivated seaweed sits at the contested edge of all this: carbon exported to the open deep sea has left the measurable system, and the defensible form of blue carbon remains burial in coastal sediment.

## The limiting quantity

The stock a coastal habitat builds scales with time only while two conditions hold — oxygen excluded below a rising surface, and surface rising with the sea. Everything upstream of that is ecology: the sequestration engine runs only where the habitat itself is alive, which is the subject of [blue-carbon habitat restoration](../blue-carbon-habitat-restoration/). The general rules of why degraded systems do and do not rebuild themselves are common to land and coast and are treated in the ecosystem-restoration cluster.