Overview
How this pathway works
Seawater carbonate chemistry naturally buffers atmospheric CO₂ concentrations — the ocean already holds roughly fifty times more carbon than the atmosphere, primarily as dissolved bicarbonate ion. Ocean Alkalinity Enhancement (OAE) accelerates this natural buffering by deliberately adding alkalinity to seawater, shifting the carbonate equilibrium so that more atmospheric CO₂ dissolves and converts to stable bicarbonate, effectively increasing the ocean's CO₂ storage capacity without a corresponding increase in seawater acidity (pCO₂).
Under the Teravent Technology-Based Carbon Standard (TTS v1.0) Annex K, OAE projects earn Teravent Technology Credits of the Removal type (TTC-R) for verified net tonnes of atmospheric CO₂ drawn into stable ocean bicarbonate storage, net of full lifecycle project emissions. Because seawater re-equilibration with the atmosphere occurs over months to a few years following alkalinity addition — rather than instantaneously — and because open-ocean monitoring carries inherent measurement challenges relative to enclosed or terrestrial systems, this pathway defaults to Class II durability, with a pathway to Class III for projects that demonstrate exceptionally robust monitoring and modelling confidence.
Three methodology variants are approved under Annex K, spanning mineral alkalinity addition, electrochemical alkalinity generation, and managed coastal weathering. Each carries distinct measurement approaches and ecological safeguard requirements given the marine environment.
📌
Class II–III, by monitoring confidence. Credits issued under TTS Annex K typically carry Class II permanence (100–1,000 years, buffer 7–25%) given open-ocean measurement uncertainty, though the underlying bicarbonate storage mechanism itself persists on multi-thousand-year timescales once atmospheric equilibration is confirmed. Projects with exceptional dMRV coverage and validated ocean carbon modelling may achieve Class III designation subject to independent Science Advisory Board review.
Governing Standard
TTS v1.0 — Annex K
This pathway is governed exclusively by the Teravent Technology-Based Carbon Standard (TTS v1.0). No external registry, standard, or methodology is referenced or incorporated. All requirements — additionality, quantification, durability, safeguards, and credit issuance — are defined within TTS v1.0 and Annex K specifically.
TCR›
TTS v1.0›
Annex K — Ocean Alkalinity Enhancement›
OAE-M01 through OAE-M03
M02
Three-test additionality with TRL-based screening; OAE remains a pre-commercial marine CDR pathway
M03
Quantification and MRV — ocean carbon uptake modelling, alkalinity dosing metering, dMRV buoy/sensor network
M04
Class II default, Class III pathway with validated modelling · Buffer 7–25% · reversal risk minimal once equilibrated
M05
Environmental and social safeguards — marine ecosystem DNSH, trace metal impurity limits, fisheries consultation
M06
Ten-stage registration process; 17-element PDD including marine environmental impact assessment
M07
TTC serial: TCR–TTS–R–OAE–[Country]–[ProjectID]–[Vintage]–[Unit]
Teravent Technology Credit — Serial Number Format (TTS Annex K · Removal)
TCR
–
TTS
–
R
–
OAE
–
GB
–
00021
–
2025
–
000001
Methodologies Accepted
Three approved methodology variants
TTS v1.0 Annex K approves three discrete methodology types for the Ocean Alkalinity Enhancement pathway, differentiated by alkalinity source and delivery mechanism. All three require ocean carbon uptake modelling validated against a dedicated distributed monitoring, reporting, and verification (dMRV) sensor network.
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Air-sea equilibration takes time: Unlike direct capture pathways, OAE credits cannot be issued for alkalinity dosing alone — the atmospheric CO₂ uptake occurs progressively as surface seawater re-equilibrates with the atmosphere over months to a few years. Crediting follows a validated ocean carbon uptake model calibrated against dMRV sensor data, not the alkalinity dosed at the point of release.
Quicklime (calcium oxide) or finely crushed olivine is dispersed into seawater, either from dedicated dosing vessels operating along defined transects or from coastal outfall points, where the material dissolves and reacts to raise local seawater alkalinity. Quicklime dissolves rapidly, providing near-immediate alkalinity increase; crushed olivine dissolves more slowly, offering a lower-cost feedstock at the expense of a longer, less certain dissolution timeline requiring more conservative crediting assumptions.
Permanence
Class II (default); Class III with validated modelling
Buffer Pool
10–25% (by monitoring confidence)
Dosing Metering
Vessel/outfall alkaline material mass, ±5%
dMRV Requirement
Buoy/sensor network measuring pH, alkalinity, pCO₂
Trace Metal Screening
Nickel/chromium content limits for olivine feedstock
Dissolution Time
Days (quicklime) to years (olivine)
Key Monitoring Indicators
- Alkaline material mass dosed per dispersal event, metered at ±5% accuracy
- dMRV sensor network measuring seawater pH, total alkalinity, and pCO₂ across the plume footprint and reference control sites
- Ocean carbon uptake model validated against dMRV data at each verification, quantifying net atmospheric CO₂ drawdown
- Trace metal impurity assay of the alkaline feedstock (particularly nickel and chromium for olivine sources), verified against DNSH thresholds
- Marine ecosystem monitoring at dispersal sites — phytoplankton community composition, local pH excursion extent
Seawater is electrolysed to split it into an alkaline (hydroxide-rich) stream and an acidic stream. The alkaline stream is returned to the ocean, directly raising local alkalinity without requiring mined mineral feedstock, while the acidic byproduct stream is either neutralised, used industrially, or reacted with silicate minerals to permanently sequester the acid — avoiding simple ocean acidification at the discharge point. This methodology avoids feedstock mining and transport emissions at the cost of higher electricity intensity relative to mineral addition.
Permanence
Class II (default); Class III with validated modelling
Buffer Pool
10–20% (by monitoring confidence)
Alkalinity Output Metering
Electrolysis cell output flow and concentration, ±3%
Acid Stream Management
Neutralisation or mineral reaction required, not direct discharge
Energy Intensity
Higher than mineral addition; grid factor deducted
TRL
4–6, early pilot stage
Key Monitoring Indicators
- Alkaline stream output flow rate and hydroxide concentration, metered at ±3% accuracy
- Acid byproduct stream fate documented and verified — neutralisation, industrial use, or silicate mineral reaction, never direct ocean discharge
- dMRV sensor network measuring seawater pH, alkalinity, and pCO₂ near the discharge point and reference control sites
- Electrolysis cell energy consumption per tonne alkalinity generated, logged for lifecycle deduction
- Ocean carbon uptake model validated against dMRV data at each verification
Rather than dispersing alkaline material from vessels into open water, this methodology spreads crushed reactive silicate minerals — typically olivine — directly on beaches or shallow coastal zones, where continuous wave action accelerates dissolution and mixing into the surrounding seawater. This approach lowers logistics costs relative to vessel-based dispersal but concentrates dissolution and any associated ecological effects within a defined, more easily monitored coastal footprint.
Permanence
Class II (default); Class III with validated modelling
Buffer Pool
10–22% (by monitoring confidence)
Application Metering
Mineral mass spread per site, ±5%
dMRV Requirement
Fixed coastal sensor array plus offshore reference sites
Trace Metal Screening
Nickel/chromium content limits, coastal sediment monitoring
Dissolution Time
Months to years, wave-energy dependent
Key Monitoring Indicators
- Mineral mass applied per coastal site, metered at ±5% accuracy
- Fixed coastal dMRV sensor array measuring pH, alkalinity, and pCO₂, paired with offshore reference sites for comparison
- Coastal sediment and beach ecosystem monitoring for trace metal accumulation and benthic community impacts
- Ocean carbon uptake model validated against dMRV data at each verification, accounting for site-specific wave energy and dissolution kinetics
- Recreational and fisheries use consultation documented for publicly accessible coastal sites
Project Boundary & GHG Accounting
Which emission sources must be counted
TTS v1.0 Module 3 requires a full lifecycle GHG emissions inventory within the project boundary, deducted from the modelled net atmospheric CO₂ uptake to arrive at the Net TTC figure.
Required
Modelled Atmospheric CO₂ Uptake (Gross)
Primary benefit quantity. Ocean carbon uptake model output, validated against dMRV sensor network data measuring pH, alkalinity, and pCO₂ changes attributable to the alkalinity addition.
Required
Feedstock Mining, Processing & Transport
Emissions from mining and crushing mineral feedstock (OAE-M01, OAE-M03) or manufacturing electrolysis cell components (OAE-M02), and transport to the dispersal site.
Required
Vessel, Dosing & Electrolysis Energy
Fuel or electricity consumed by dosing vessels, coastal outfall pumping, or electrolysis cell operation, applying the applicable emissions factor per TLP v1.0.
Required where material
Lime Calcination Emissions
Where quicklime feedstock (OAE-M01) is produced by calcining limestone, the process CO₂ released during calcination must be deducted from the net atmospheric uptake claimed, unless already captured and separately accounted.
Excluded
Background Ocean Carbon Cycling
Natural background variability in ocean carbon uptake unrelated to the alkalinity addition is excluded from the crediting boundary, isolated through paired reference control site comparison in the ocean carbon uptake model.
MRV Confidence
Measurement, reporting
& verification
Teravent's Science Advisory Board assesses each pathway against four MRV dimensions. Ocean Alkalinity Enhancement carries the most significant open-ocean measurement challenge of any Teravent pathway, driving comparatively lower permanence and quantification confidence relative to terrestrial or industrial pathways, offset by very high additionality clarity given the pathway's early commercial stage.
Dosing QuantificationHigh
Atmospheric Uptake ConfirmationMedium
Permanence ConfidenceMedium–High
Additionality ClarityVery High
🔬 Measurement Requirements — TTS Module 3
Alkaline material dosing is metered at the vessel, outfall, or coastal application site to ±5% accuracy (±3% for electrolysis output). The defining MRV challenge for this pathway is confirming the resulting atmospheric CO₂ uptake, since the ocean's air-sea equilibration process operates over months to years and across a diffuse, moving water mass rather than a fixed containment boundary. Teravent requires a distributed monitoring, reporting, and verification (dMRV) sensor network — moored buoys, autonomous underwater gliders, or satellite-linked sensors — measuring seawater pH, total alkalinity, and pCO₂ across the affected plume and paired reference control sites, feeding a validated ocean carbon uptake model reviewed by an independent oceanographic expert at each verification.
Additionality
Demonstrating additionality
TTS v1.0 Module 2 requires all projects to pass a three-test additionality framework, with a Technology Readiness Level (TRL) screen applied ahead of the common practice test. Ocean Alkalinity Enhancement remains an early-stage marine CDR approach globally, so virtually all current projects qualify for the streamlined TRL-based screen.
1
TRL-Based Common Practice Screen
Projects deploying OAE technology at Technology Readiness Level 6 or below automatically satisfy the common practice test — deliberate alkalinity enhancement for carbon removal purposes remains uncommon outside a small number of pilot and demonstration projects globally across all three methodology variants.
2
Regulatory Surplus Test
The alkalinity addition activity must not be mandated by any legally binding coastal water quality, ocean fertilisation, or marine discharge regulation. Where a jurisdiction's environmental permitting requires baseline monitoring as a condition of any marine activity, that requirement does not itself disqualify the project's carbon removal additionality.
3
Financial Additionality Test
Carbon revenue must be necessary for project viability. Developers must submit a discounted cash flow analysis, net of any government grants or research funding (disclosed under Module 8), demonstrating that vessel operations, feedstock, dMRV instrumentation, and monitoring costs exceed available revenue absent carbon credit income.
ℹ️
Government incentive disclosure: Where a project receives direct government grants, research funding, or per-tonne marine CDR tax credits, this must be disclosed to the TSA at registration under Module 8. Double-claiming the same carbon benefit under both a government incentive programme and Teravent credits is prohibited.
Leakage Assessment
Leakage types & deductions
TTS v1.0 Module 3 defines three leakage types applicable across all engineered pathways. Ocean Alkalinity Enhancement carries distinctive marine leakage considerations related to feedstock sourcing and localised ecosystem effects.
Feedstock Extraction Leakage
Mining Emissions & Land Disturbance
Crushing and mining olivine or limestone feedstock (OAE-M01, OAE-M03) carries embodied mining emissions and potential land disturbance at the quarry site, which must be included in the project's lifecycle inventory.
Included as required boundary item, not separately deducted
Energy-Source Leakage
Vessel Fuel & Grid Electricity Displacement
Where dosing vessels use marine fuel or electrolysis equipment draws grid electricity, associated combustion or displaced generation emissions must apply the applicable emissions factor absent verified low-carbon alternatives.
Deduction: applicable marine fuel or grid factor, TLP v1.0
Local Ecosystem Displacement Leakage
Fisheries & Recreational Use Impact
Where alkalinity dosing or coastal mineral application measurably displaces local fishing or recreational activity from the affected zone, this must be documented and considered as part of the project's community consultation and safeguard requirements, though it is not deducted from the carbon credit calculation itself.
Documented under Module 5 safeguards, not credit-deducted
Permanence & Reversal Risk
Buffer pool & reversal risk
Once atmospheric CO₂ has re-equilibrated into stable ocean bicarbonate, reversal risk is exceptionally low — bicarbonate ion has a residence time in the ocean of tens of thousands of years. The principal source of uncertainty driving buffer pool rates is measurement confidence in confirming how much atmospheric uptake actually occurred, not durability of the storage mechanism itself once confirmed.
| Methodology |
Durability Class |
Buffer Pool Rate |
Primary Uncertainty Drivers |
| OAE-M01 Mineral Addition |
Class II (default) |
10–25% |
Dissolution timeline uncertainty (olivine); dMRV plume tracking accuracy |
| OAE-M02 Electrochemical |
Class II (default) |
10–20% |
Early-stage technology; acid stream fate verification |
| OAE-M03 Coastal Weathering |
Class II (default) |
10–22% |
Wave-energy-dependent dissolution rate variability |
⚠️
Pathway to Class III: Projects that demonstrate exceptionally comprehensive dMRV sensor coverage, multi-year validated ocean carbon uptake modelling, and independent oceanographic expert corroboration may apply for Class III designation at a subsequent verification, subject to Science Advisory Board review. Reversal notification requirements are limited given the diffuse, already-equilibrated nature of ocean bicarbonate storage; instead, proponents must notify the TSA within 30 days of any material downward revision to the ocean carbon uptake model estimate.
Eligibility Requirements
Key registration criteria
Projects must meet all of the following minimum requirements to qualify for registration under TTS Annex K. Additional methodology-specific requirements are detailed in the Annex K methodology tables for each OAE-M code.
✓
Distributed monitoring, reporting, and verification (dMRV) sensor network installed and operational prior to alkalinity dosing, covering the affected plume and reference control sites
✓
Validated ocean carbon uptake model submitted at registration, calibrated against dMRV sensor data and reviewed by an independent oceanographic expert
✓
Trace metal impurity assay of alkaline feedstock (nickel, chromium content) confirming compliance with DNSH thresholds
✓
Ten-stage registration process completed, from technology eligibility assessment through validation to ongoing verification, per TTS Module 6
✓
Seventeen-element Project Design Document submitted, including a marine environmental impact assessment
✓
Three-test additionality demonstrated with TRL-based screening applied first; TRL documentation updated at each verification
✓
Marine ecosystem monitoring plan covering phytoplankton community composition and local pH excursion extent at dispersal sites
✓
Fisheries and coastal community consultation documented for sites with recreational or commercial marine use overlap
✓
Applicable marine environmental permits obtained from relevant national or international maritime authorities prior to dosing
✓
Government research funding or grant disclosure submitted at registration and updated at each verification period
Co-Benefits & SDGs
Sustainable Development
Goal alignment
All Teravent registered Ocean Alkalinity Enhancement projects must complete an SDG impact assessment at registration and at each verification period. Three SDGs are systematically tracked for this pathway, reflecting its dual climate and marine ecosystem dimensions. Projects may apply for co-benefit quality labels where independently verified indicators are met.
SDG 13 · Climate Action
SDG 14 · Life Below Water
SDG 9 · Industry, Innovation & Infrastructure
Frontier Technology+
TSA TAP-designated first-of-kind OAE deployments with independently peer-reviewed methodology and validated dMRV coverage are eligible for the Frontier Technology label.
Ocean Health+
Projects demonstrating measurable localised ocean acidification mitigation alongside carbon removal, verified through marine ecosystem monitoring, are eligible for the Ocean Health co-benefit label.
Zero Fossil Input+
Facilities using verified renewable or nuclear electricity for electrolysis (OAE-M02) or low-emission vessel operation are eligible for the Zero Fossil Input co-benefit label.
Community Benefit+
Projects with verified coastal community consultation and demonstrated compatibility with local fisheries and recreational use are eligible for the Community Benefit+ label.
Deployment scope: Coastal regions with access to shipping infrastructure, mineral feedstock sourcing, or suitable coastal application sites — currently most active in the North Atlantic, Northern Europe, and select Pacific coastal regions with established marine research infrastructure.