Category
Ocean-based Removal
Ocean-based removal is a family of methods that accelerate oceanic CO2 uptake by raising alkalinity or by helping marine life carry carbon into deep water. Measurement is the honest core of the story: in the first verified credits the removal was never directly observed and quantification was entirely model-based.
One of eight durable carbon removal pathways presented by CDR Türkiye, all intended to store carbon for centuries to millennia.
Overview
Open a heading to read the detail.
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How it works The ocean is already the largest mobile store of atmospheric carbon, and these methods try to speed up that exchange.
The most common route is alkalinity enhancement: adding alkaline material lets seawater hold more CO2, and the carbon is bound as bicarbonate. Other routes strengthen the biological pump, the way marine life carries carbon into deep water, or take seawater into a plant and strip its carbon out directly.
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Deployment today Deployment today is very small.
The world’s first verified ocean alkalinity enhancement credits were issued in June 2025 and totalled 625.6 tonnes of CO2. Across the sector, more than 90% of contracted marine credits remain undelivered.
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The measurement problem The measurement problem defines this family and should not be softened.
In that first issuance the CO2 removal was never directly observed: quantification was entirely model-based, a total of 14.35% was deducted from the credits to allow for uncertainty, and the risks of the added material turning back into solid carbonate in the water or building up on the seafloor were explicitly left unquantified. A 2025 review concluded that the current knowledge base is not sufficient to predict the outcome of applications at sea, and that no data exists from enclosed sea-tank experiments or large field pilots. For balance: small deployments have been tracked with high precision, and the dispute is whether that tractability extrapolates to climate-relevant scale.
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Governance and Türkiye Governance is a separate constraint: the London Convention and Protocol restrict commercially driven ocean research.
Türkiye’s coastline and maritime area are theoretically of interest, but no ocean-based trial has been run here and no regional proposition can be built while the global measurement problem is unresolved.
CDR.fyi Carbon to Sea, independent review of measurement and verification
Technologies and applications
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Technology types
The technical methods that capture, convert or store the carbon.
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Technology type
Ocean alkalinity enhancement Ocean alkalinity enhancement (OAE)
Alkaline mineral or solution added to seawater to raise its capacity to hold CO2.
- How it works
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- Alkaline material
Crushed alkaline mineral is prepared.
- Distribution
It is distributed into coastal or open seawater.
- Alkalinity
The alkalinity of the seawater rises.
- CO2 uptake
The ocean draws more CO2 from the atmosphere.
- Bicarbonate
The CO2 is held as bicarbonate.
- Alkaline material
- Durability
- Not measured
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Bicarbonate in the ocean
Bicarbonate stays in the ocean for a long time; the removal itself has never been directly observed.
- What it rests on
- In the first verified issuance the removal was never directly observed: quantification was entirely model-based, a total of 14.35% was deducted from the credits to allow for uncertainty, and the risks of the added material turning back into solid carbonate in the water or building up on the seafloor were left unquantified. Total issuance was 625.6 tonnes of CO2.
- Carbon to Sea, independent review of measurement and verification
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Technology type
Electrochemical acid removal
Seawater passed through an electrochemical cell that splits it into acid and base streams, with the acid removed and the base returned to the sea.
- How it works
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- Intake
Seawater is taken into the plant.
- Electrochemical split
The cell separates the water into acid and base streams.
- Acid removal
The acid stream is neutralised or disposed of separately.
- Base return
The alkaline stream is returned to the sea and CO2 uptake rises.
- Monitoring
Water chemistry around the discharge is monitored.
- Intake
- Durability
- Early stage
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Bicarbonate in the ocean
Carbon is held as bicarbonate; the removal has not been demonstrated in the field.
- What it rests on
- The method has a measurability advantage: input and output streams can be metered at the plant, so uncertainty is narrower than with open-sea dispersal. Against that, electricity consumption is high and acid disposal is a separate problem; there is no commercially demonstrated deployment.
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Technology type
Direct ocean capture Direct ocean capture (DOC)
Seawater taken into a plant and acidified electrochemically so that the carbon dissolved in it comes out as CO2 gas, with the decarbonised water returned to the sea.
- How it works
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- Intake
Seawater is taken into the plant and filtered.
- Acidification
An electrochemical cell makes the water more acidic and the carbon dissolved in it converts to CO2 gas.
- Gas stripping
The released CO2 is separated from the water, collected and purified.
- Water returned
The water, stripped of its carbon and with its acidity neutralised again, goes back to the sea, where it draws CO2 from the atmosphere again.
- Storage
The stripped CO2 is compressed and injected into a geological formation; without that step the removal is not complete.
- Intake
- Durability
- Early stage
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Depends on the storage step
Unless the stripped CO2 is stored, the removal is not complete.
- What it rests on
- The measurability advantage is real: because the water enters a plant, inbound and outbound streams can be metered and the captured carbon leaves as a weighable stream of gas. The claim does not end there: how much CO2 the returned water then draws from the atmosphere is still a modelled question. Electricity consumption is high, the effect of intake structures on marine life and the local chemistry of the discharge are separate issues, and deployment is at pilot scale: the first field validation ran at a plant rated at 1,000 tonnes per year.
- Captura, Kona pilot plant
Deployment routes
The different routes by which the carbon reaches a durable store.
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Deployment route
Macroalgae cultivation and sinking
Cultivated macroalgae lowered into deep water so that its carbon is separated from surface waters and the atmosphere.
- How it works
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- Cultivation
Macroalgae is grown on offshore lines.
- Harvest
Mature biomass is collected.
- Sinking
The biomass is lowered into deep water and removed from surface circulation.
- Monitoring
The fate of the sunk biomass and effects at the seabed are monitored as far as possible.
- Cultivation
- Durability
- Not measured
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Not measured in the field
The claimed durability rests on the slowness of deep-water circulation; how much of the sunk carbon stays has not been measured.
- What it rests on
- The cultivation side is understood and measurable; uncertainty begins after sinking. How much biomass decays and returns to surface waters, and how much remains at the seabed, has not been directly observed. Effects on seafloor ecosystems are also an open question.
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Deployment route
Ocean nutrient fertilisation
Iron, or another nutrient whose shortage limits plankton growth, added to surface waters so that microscopic marine plants (phytoplankton) multiply and carry more carbon into deep water.
- How it works
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- Nutrient addition
The nutrient is dispersed over a defined surface area.
- Bloom
Phytoplankton production rises and CO2 is consumed at the surface.
- Sinking
Part of the dying biomass descends into deep water.
- Monitoring
The sinking fraction and ecosystem effects are monitored as far as possible.
- Nutrient addition
- Durability
- Contested
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Depends on the sinking fraction
That fraction has not been measured reliably.
- What it rests on
- The method is contested on both scientific and governance grounds and should be described that way. The nutrient balance of an ecosystem is altered directly, effects do not stay within the treated area, and the sinking fraction cannot be measured. The London Convention and Protocol restrict commercially driven ocean research.
- Carbon to Sea, independent review of measurement and verification
Frequently asked questions
What is ocean alkalinity enhancement?
Ocean alkalinity enhancement is the addition of alkaline material to seawater so the water can hold more CO2. Raised alkalinity increases the CO2 the ocean draws from the atmosphere, and the carbon is bound as bicarbonate.
Can removal be measured in ocean methods?
So far it has not been measured directly. In the world’s first verified ocean alkalinity enhancement credits the removal was never directly observed: quantification was entirely model-based, a total of 14.35% was deducted from the credits to allow for uncertainty, and the risks of the added material turning back into solid carbonate in the water or building up on the seafloor were left unquantified.
How much ocean-based removal has been delivered?
The first verified credits were issued in June 2025 and totalled 625.6 tonnes of CO2. Across the sector, more than 90% of contracted marine credits remain undelivered.
Why is ocean fertilisation controversial?
Fertilisation alters the nutrient balance of an ecosystem directly and its effects do not stay local, while the share of biomass that actually sinks cannot be measured. The London Convention and Protocol restrict commercially driven ocean research. The method is contested on both scientific and governance grounds.
Why are plant-based seawater methods easier to verify than dispersal?
Where seawater is taken into a plant, the inbound and outbound streams can be metered and the carbon removed leaves as a weighable stream of gas. In open-sea dispersal what is measured is a diluting chemical signal, and the removal can only be estimated with a model. That does not make closed systems proven: how much CO2 the returned water then draws from the atmosphere remains a modelled question, and energy consumption and the local effect of the discharge are separate issues.