Marine sub-sediment burial

This is a Carbon Storage Module and covers Marine sub-sediment burial. This module is part of the Rainbow BiCRS methodology, which allows Project Developers to choose the relevant modules for their project, and shall be used with the necessary accompanying modules.

See more details on how modules are organized in the BiCRS home page.

How to use this module

BiCRS Methodology

Eligible technologies

Project type

This module covers marine sub-sediment burial projects that inject waste and residual biomass feedstock inputs directly into the anoxic layer of marine sub-sediments. Projects shall meet all of the following criteria:

• Demonstrate capability to perform MRV as agreed upon in the validated project documentation

• Demonstrate a net-negative project carbon footprint based on initial LCA estimates of induced emissions and initial CDR estimates based on modeling

• Projects that sink biomass to the seafloor but do not bury and embed it into marine sub-sediments are not eligible.

• The entity eligible for receiving carbon finance is the operator performing storage at the sub-sediment burial site. Biomass producers and sub-sediment burial machinery manufacturers are not eligible Project Developers.

Project scope

A project is defined as all burial activities that take place from one port over the project lifetime (by default a maximum of 5 years, renewable), and all removal that occurs as a result of that burial, plus the upstream/downstream activities associated with that burial (e.g. GHG emissions from feedstock sourcing, transport...).

See the Storage batch section for more details on how a project is organized into different burial areas and burial events.

Eligible sites

Storage must be done in anoxic conditions.

Storage must be done in existing accessible marine sub-sediment. Projects that excavate, dredge or build wells for the sole purpose of accessing sub-sediments or creating sub-sediment conditions are not eligible, due to the associated environmental risks.

See the Site characterization section for more specific requirements.

Eligible feedstock

Only particulate terrestrial biomass feedstock that also meets the requirements of the BiCRS Biomass Feedstock module is eligible in this module. Injection of liquefied or gaseous CO$_2$ into sediments is outside the scope of this module.

See the BiCRS Biomass Feedstock module for more specific feedstock requirements.

Crediting timeline and process

Storage batches

Measurements and reporting are performed for storage batches. Verification and credit issuance is done at the reporting period scale (by default, annually), and groups results for all storage batches concerned during that reporting period. The organization of a project into storage batches, sites and points is described below, and depicted in Figure 1.

Sedimentary conditions for storage points within one storage site must be within the following ranges (data requirements are outlined in the Data Sources section):

• Grain size: the majority grain size at the burial sediment depth must be either defined as either clay (0.002-0.05 mm) or sand (0.05-2 mm) for all storage points within a storage site, indicated by >50% of sediment grain size. Defined by recommendations of USGS.

• Water depth at storage point: At water depths 1-20 m, water depths must be within 0.5 m. At water depths 20-200 m, water depths must be within 5 m.

• Sub-sediment depth of storage: At sub-sediment depths 2-3 m, storage depths must be within 0.5 m. At sub-sediment depths >3 m, storage depths must be within 1 m.

Ongoing burial into the sub-sediment shall last no longer than 31 days per storage batch, to standardize sampling timescales. If burial continues after 31 days, it shall be considered a separate storage batch.

One project may work with different storage batches simultaneously. Each storage batch shall be monitored and reported separately within the same Monitoring Report. Storage batch information shall be monitored and reported at least once per calendar year.

Information about storage batches may be monitored and issued credits continuously by Project Developers by uploading claim information to the Rainbow MRV platform.

Feedstock mixture

A feedstock mixture is defined as one biomass feedstock or uniform mixtures of feedstocks. One feedstock mixture may be used across several storage batches, but any time the feedstock mixture of one storage batch changes, a new storage batch shall be started.

Any water used in the feedstock mixture must come from within the 24 km$^2$ storage batch area.

The feedstock mixture composition may vary by no more than 20% to be considered the same homogeneous feedstock mixture, where the composition is made of feedstocks of a specific type from a specific supplier.

See the Pre-burial sampling section for requirements on feedstock sampling.

Storage site and storage point requirements

Storage points must meet the criteria outlined in Table 1 to be eligible. The criteria are set to ensure storage points are suitable for permanent carbon storage, are anoxic, and have low reversal risks.

All criteria shall be outlined in the Site Characterization Report, prepared before any burial events occur and submitted with the PDD for the validation audit. In addition, the Site Characterization Report shall provide GPS coordinates of each planned storage point, and a GIS-generated map showing each storage point and the delineation of the associated storage site.

Data sources characterizing storage points must be, in the following order of preference

1. primary data from a pilot survey e.g. site surveys, in situ measurements and measurements on samples collected at the project site, delivered by the Project Developer, or

2. secondary data from the specific area concerned (e.g. published peer-reviewed literature or database measurements) or

3. secondary data from an area that is proven to be sufficiently representative and similar to the project area in the appropriate factors that relate to permanent storage.

Table 1 The required measurements and information for a storage site that must be presented in the Site Characterization Report, before any burial occurs, to justify that the storage site is appropriate for permanent CDR via marine sub-sediment burial.

Sampling requirements

Sampling occurs at two stages of the project: sampling of the feedstock mixture before burial to establish organic carbon buried, and sampling the feedstock mixture after burial to check for any reversals (i.e. carbon degradation or diffusion. At both stages of sampling, laboratory testing shall provide the following measurements of the feedstock mixture:

• % organic carbon content of the solid biomass

• % moisture content of the feedstock mixture

• density of the feedstock mixture

Pre-burial sampling

Two representative samples of the feedstock mixture shall be prepared and sent for laboratory testing per storage batch: one at the beginning (day one) and one at the end of the storage batch (day 31 or an earlier date when the storage batch is complete).

Post-burial monitoring and sampling

Post-burial monitoring and sampling shall occur:

• at least 12 months after the burial event, and

• optionally, may also be performed within 1-3 months after the burial event if the Project Developer chooses the 50/50 credit issuance approach. See the Crediting timeline and process section for more details.

Post-burial monitoring and sampling should be completed using sediment coring, to access the buried biomass, extract samples, and send them to a laboratory to measure the organic content of the solid biomass. Alternative approaches may be considered on a case by case basis, and approved by the VVB, the Rainbow Certification team and, if deemed necessary by the Rainbow Certification team, an expert peer reviewer.

Sampling and laboratory testing shall be done separately for each storage point. At least three sub-samples shall be taken from each storage point and mixed together to obtain one composite sample for the storage point. Samples can not be mixed from all storage points in one storage site to perform laboratory tests on a composite sample.

Sampling plan

Project Developers shall prepare an ex-ante Sampling Plan before any burial events occur, and submit it with the PDD for the validation audit. The Sampling Plan shall describe:

• how representative samples will be taken of the feedstock mixture in pre-burial sampling

• how to preserve moisture content of feedstock mixture while sending it to the lab

• number of samples used for post-burial sampling

• strategy for ensuring random/representative/unbiased sampling locations for post-burial sampling

Sampling procedure

The Sampling Plan described above is developed ex-ante during validation and outlines the intended sampling approach. During monitoring and ex-post verification, Project Developers must provide a Sampling Procedure, described in the Monitoring Report, which documents the actual sampling approach that was implemented.

Ideally, the Sampling Procedure should align exactly with the Sampling Plan. However, given real-world challenges that may arise during monitoring, deviations are expected. The purpose of documenting the Sampling Procedure ex-post is to ensure transparency by capturing any adjustments made to the original plan.

The Sampling Procedure shall include all elements listed in the Sampling Plan components section.

Eligibility criteria

The eligibility criteria requirements specific to this module are detailed in the sections below. Other eligibility criteria requirements shall be taken from the accompanying modules and methodologies:

BiCRS methodology

• Additionality

• No double counting

• Targets alignment

• ESDNH

Other modules

• Substitution

• Co-benefits

• No double counting

• ESDNH

• Leakage

Rainbow Standard Rules

• Measurability

• Real

• TRL

• Minimum impact

Permanence

Removal Rainbow Carbon Credits (RCCs) issued from marine sub-sediment burial have a permanence horizon of 1000 years.

Permanence is assessed at two points during project certification:

• at ex-ante validation it is estimated using literature data and models

• during verification it is demonstrated using direct measurements.

Requirements for each stage are detailed below.

Estimating permanence at validation

To demonstrate that carbon in sub-sediment burial will remain permanently stable, indicators from the Storage site and storage point requirements section must be provided at validation, in the Site Characterization Report, demonstrating compliance with the requirements.

These indicators are suitable proof that a substantial fraction of the buried carbon is permanently stable. The amount of permanently stored carbon is determined using the models and equations detailed in the GHG reduction quantification section.

Demonstrating permanence at verification

At verification, it is assumed that 92% of organic carbon still remaining in the feedstock mixture 12 months after burial will remain permanently stored over 1000 years. This is based on modeled results for oxic marine sediments, and likely overestimate the non-permanent fraction of organic carbon in anoxic marine sediments, as required under this module.

For each storage batch, the organic carbon content in the buried feedstock mixture is measured via sampling, and observed via remote sensing, at 1-3 months (optional) and 12 months (mandatory) to determine the carbon permanently stored.

The actual amount of permanently stored carbon is measured as described in the GHG quantification section, replacing the modeled amounts used during validation to issue ex-post Rainbow Carbon Credits.

Risk of reversal

Project Developers shall fill in the Rainbow Marine sub-sediment burial risk evaluation to evaluate the risk of carbon storage reversal, based on social, economic, natural, and delivery risks.

Project Developers shall assign a likelihood and severity score to each risk, and provide an explanation of their choices. The Riverse Certification team shall evaluate the assessment and may recommend changes to the assigned scores.

The Project Developer, Riverse Certification team, or the third-party auditor may suggest additional risks to be considered for a specific project.

Each reversal risk with a high or very risk score is subject to:

• risk mitigation plan, developed by the Project Developer, that details the long-term strategies and investments for preventing, monitoring, reporting and compensating carbon removal reversal, or

• additional contributions to the buffer pool, at a rate of 3% of verified removal Riverse Carbon Credits for each high or very high risk

All projects under this module are estimated to have a material reversal risk, due to

• risk of degradation from improper burial in oxic conditions

• risk of physical leakage from burial sites

• the novelty of the technology, meaning the abovementioned points have not been proven as consistent and reliable.

Therefore, the risk mitigation plan includes adhering to all site characteristics, plus a reversal monitoring requirement. At least 5 years after burial, Project Developers shall

• at a subset of storage sites, measure remaining organic carbon in feedstock samples

• at all storage sites, confirm the presence and extent of buried feedstock using radar

Co-benefits

Project Developers shall prove that their project provides at least 2 co-benefits from the UN Sustainable Development Goals (SDGs) framework (and no more than 4).

Common co-benefits of Marine sub-sediment burial projects, and their sources of proof, are detailed in Table 2. Project Developers may suggest and prove other co-benefits not mentioned here.

Table 2 Summary of common co-benefits provided by Marine sub-sediment burial projects. Co-benefits are organized under the United Nation Sustainable Development Goals (UN SDGs) framework.

Environmental and social do no harm

Project Developers shall prove that the project does not contribute to substantial environmental and social harms.

Projects must follow all national, local, and European (if located in Europe) environmental regulations related to the project activities.

Feedstock sustainability risks shall be taken from the Biomass feedstock module.

ESDNH risk evaluation

Project Developers shall fill in the Rainbow Marine sub-sediment burial risk evaluation to evaluate the identified environmental and social risks of Marine sub-sediment burial projects. The identified risks include:

• Release of biomass via improper embedding

• Release of aqueous CO$_2$ or methane at sediment-water interface

• Release of hydrogen sulfide at oxic-anoxic transition zone

• Project activities impacting benthic life

• Transfer of harmful pollutants in biomass feedstock

• Marine pollution due to ship time spent over storage site

Project Developers shall assign a likelihood and severity score of each risk, and provide an explanation of their choices. The VVB and Rainbow’s Certification team shall evaluate the assessment and may recommend changes to the assigned scores.

All risks with a high or very high risk score are subject to a Risk Mitigation Plan, which outlines how Project Developers will mitigate, monitor, report, and if necessary, compensate for any environmental and/or social harms.

Additional proof may be required for certain high risk environmental and social problems.

The Project Developer, the Rainbow Certification Team, or the VVB may suggest additional risks to be considered for a specific project.

Permitting

Project Developers must follow all relevant laws and legal requirements for reporting operations to local, federal and international governing bodies. Project Developers must follow the requirements outlined in their permit relating to the amount of tonnes injected if specified, and geographic area permitted for operations.

Permits are typically required for accessing coastal marine sediments and performing sub-sediment burial. The Project Developer must provide written authorization by either 1) the permit granting regulatory authority or 2) by the partner providing the permit demonstrating freedom to operate and perform sub-sediment burial in the geographic area defined in the PDD.

Environmental Impacts Assessment (EIA)

Typically, the EIA should be completed in advance of obtaining permitting for credit generation, and will be completed over the course of operations and reported to Rainbow.

EIA may not be required for all permits for storage. When EIA is not required for permitting (e.g. for a research permit or permit exemption), the Project Developer shall demonstrate that a baseline environmental survey has been completed, assessing the elements listed below, and that the potential impacts have been considered to be within regulatory guidelines. This justification shall be evaluated by both the VVB and the Rainbow Certification Team. Project Developers shall provide the same information as they would in a full EIA to Rainbow for project validation, and cover aspects including:

• Marine protected areas

• Benthic habitat

• Fishing grounds

• Shipping lanes

• Subsea infrastructure

• Materials of historical significance

Baseline environmental survey and/or EIA must address how the project adheres to regulatory requirements such as limitations on sediment resuspension and habitat destruction due to seabed intervention.

GHG quantification

The system boundary of this quantification section starts after burial of feedstock mixture and covers carbon storage through end of life after 1000 years, and accounts for potential re-emission and decay modeled for 1000+ years. Sources of GHG emissions covered in this module include only permanent carbon storage modeling. Other GHG emissions shall be taken from the accompanying modules.

Data sources

The required primary data for GHG reduction calculations from projects are presented in Table 3. These data shall be included in the project’s PDD and made publicly available.

Table 3 Summary of primary data needed from projects and their source for project validation and verification. See the Monitoring Plan section for more details on monitoring and verification requirements. Asterisks (*) indicate which data shall be updated for each storage batch.

Secondary data taken from the literature are used to define default values for the parameters outlined in Table 4. These values are only used for ex-ante validation models, and will be replaced by project measurements during verification.

If project incubation experiments or in situ experiments are used to provide values for $G$ and $k$parameters, these experiments must either 1) be scientifically peer reviewed and published in academic journals, or 2) undergo independent external peer review for the specific project.

Table 4 Values from scientific literature that may be used instead of primary data, for validation stage ex-ante carbon degradation modeling.

Carbon storage modeling

Carbon storage is calculated by subtracting the amount of organic carbon degraded over 1000 years from the amount of initially buried organic carbon.

• Carbon burial is measured using the amount of feedstock mixture buried, and its measured organic carbon content.

• Carbon degradation is subtracted from this carbon burial. It is modeled at validation and measured at verification.

Carbon degradation

The greatest risk to carbon removal reversal is degradation of buried feedstock mixture by microbes in the sub-sediment. This is limited by the site requirements that ensure anoxic conditions preventing degradation in the first place, and by sediment conditions ensuring that if degradation occurs, any evolved CO$_2$ would stay trapped in the sub-sediment. Nevertheless, the calculations conservatively assume that any CO$_2$ degraded is diffused out of the sub-sediment.

Carbon degradation is conservatively modeled during validation using a multi-G kinetic model (see Appendix A and Example 1), and measured during verification.

Example 1: GHG quantification validation modeling

Assumptions

1. The entirety of buried biomass will be securely located in the seabed, allowing point-source monitoring of organic carbon degradation via measurement of organic carbon content.

2. Organic carbon degradation is front-loaded and logarithmic. 12 months is an appropriate and sufficiently long timeframe to determine how much carbon degradation (if any) will occur over 1000 years.

   1. Biomass degradation will either begin after embedding in the sub-sediment following a logarithmic relationship (Keil et al., 2010), or it will not occur at all.

3. The rate of organic carbon degradation under oxic conditions is greater than the rate under anoxic conditions.

4. Biomass degradation can be measured by tracking organic carbon content of samples of the buried feedstock mixture over time.

5. Storage points will not experience re-suspension or re-working such that burial biomass is exposed to the water column over 1000 years.

6. The site characteristics and requirements detailed in Table 1 are suitable to identify sub-sediment areas that are anoxic.

7. Any organic carbon degradation from the buried biomass leads to CO$_2$ released to the water column, and eventually back to the atmosphere, via diffusive transport (see Eq. 4, $F_{diffuse\, 1000}$ = 1). This is a conservative assumption, because degraded carbon may remain trapped permanently in the sediment matrix as CO$_2$. Indeed, the site requirements are set to ensure that CO$_2$ diffusion out of the sediment matrix is minimized.

8. Methane diffusion can be measured using oxygen penetration depth as a proxy. If O$_2$ is measurable in the surface layer of marine sediments, methane is unable to diffuse out of the sediment-water interface.

Uncertainty assessment

See general instructions for uncertainty assessment in the Rainbow Standard Rules. The outcome of the assessment shall be used to determine the percent of RCCs to eliminate with the discount factor.

The uncertainty in this module is assessed below for each component.

The uncertainty at the module level is estimated to be low. This translates to an expected discount factor of at least 3% for projects under this module.

Risk evaluation template

👉 Download the template here

Monitoring plan

The following information shall be provided for verification of each storage batch:

Appendix

Appendix A: Scientific Basis of Sub-Sediment Biomass Storage

This appendix outlines the scientific foundation for sub-sediment biomass storage, summarizing key research on organic carbon degradation and preservation in marine sediments. While no studies directly replicate the conditions described in this module, relevant literature on similar processes is compiled.

Introduction

Marine sediments serve as the final carbon sink, storing 150–200 billion tons of organic carbon in their upper layers (Hedges & Keil, 1995; Hedges, Keil & Benner, 1997; Atwood et al., 2020). The biological pump transfers oceanic carbon to sediments via microbial fixation, food chain dynamics, and sinking particulate matter. Despite its inefficiency—only ~1% of sinking carbon reaches sediments, and just 0.1% is buried long-term (Burdige, 2007; LaRowe et al., 2012)—this process significantly influences atmospheric CO₂ levels.

Biomass degrades rapidly in oxygenated sediments, but in anoxic environments, it can persist for millennia. Oxygen exposure time (OET) controls degradation: prolonged exposure breaks macromolecules into labile forms, accelerating conversion to CO₂. Reducing OET preserves biomass, as seen in bog bodies and historic wooden structures preserved in compacted, oxygen-deprived sediments (Ceccato et al., 2014; Macchioni et al., 2016).

Organic Carbon Preservation in Marine Sediments

Decades of research (Hedges & Keil, 1995; Arndt et al., 2013; LaRowe et al., 2020) indicate that organic carbon degrades slowly in anoxic sediments due to low substrate availability, microbial competition, mineral protection, and biochemical inaccessibility (Kristensen & Holmer, 2001; LaRowe et al., 2022).

Biomass preservation for over 1,000 years is common in coastal zones with high sedimentation rates and low OET. For example, rapid burial in the Bay of Bengal (30 cm/yr sedimentation) protects wood from microbial degradation, preserving organic material for millions of years (Lee et al., 2019). Similarly, wood fragments up to 11,900 years old have been recovered from the Gulf of Mexico (Schwab et al., 1996), and entire ancient forests remain buried off the Alabama coast (Delong et al., 2021; Moran et al., 2024).

Studies show organic carbon degradation slows exponentially over time, with rates up to 1,000× lower in anoxic sediments than in oxic environments (Kristensen & Holmer, 2001; Keil et al., 2010; LaRowe et al., 2012; Arndt et al., 2013). This supports the assumption that degradation rates in oxic conditions (Keil et al., 2010) represent a worst-case scenario for anoxic sub-sediment burial.

Biomass Degradation in Marine Sediments

Biomass degradation begins with extracellular enzymatic hydrolysis, where aerobic microbes break down macromolecules into small organic compounds. These are further processed via anaerobic fermentation into substrates for redox reactions. However, without sufficient OET, enzymatic hydrolysis cannot begin, preventing degradation (Hartnett et al., 1998; Hedges et al., 1999). Ligno-cellulosic biomass requires longer OET than algal biomass to initiate breakdown.

In deep sediments, sulfate reduction is the dominant degradation process, accounting for 50% of total biomass decomposition globally (Jorgensen et al., 2019). This slow, energy-limited process produces CO₂ and hydrogen sulfide (HS). CO₂ diffuses upward, where it may be fixed by microbes or released at the sediment-water interface. Worst-case CO₂ diffusion rates align with modern dissolved inorganic carbon (DIC) fluxes (Krumins et al., 2013). CO₂ accumulates due to compaction, it can form hydrates at depths >10 m in cold marine sediments (Eccles & Pratson, 2012; Velaga et al., 2011).

Hydrogen sulfide, though toxic, is rapidly oxidized in oxygenated environments, preventing marine toxicity. Additionally, 10–20% of HS reacts with iron hydrates to form pyrite (FeS), further stabilizing organic matter (Barber et al., 2017; Baumgartner et al., 2023).

Methanogenesis, consuming 15% of CO₂ from sulfate oxidation, contributes to organic carbon degradation (Regnier et al., 2011). Over time, sediment compaction reduces porosity, slowing diffusion and promoting FeS formation. This further limits CO₂ and methane movement, allowing microbial utilization.

Conclusion

Long-term biomass preservation in marine sediments is driven by low OET, rapid burial, and anoxic conditions. Anoxic degradation is significantly slower than oxic processes, enhancing the stability of buried carbon. Existing research supports the feasibility of sub-sediment biomass storage as a durable carbon sequestration strategy.

Appendix B Additional optional ESDNH indicators to monitor

ESDNH indicators may be measured by the Project Developer within the validation stage to reduce project risk, and suggested monitoring during the verification stage.

To monitor environmental risk, Project Developers should understand the biogeochemical zonation of sediment depths where biomass is stored. In anoxic marine sediments, organic molecules degrade via sulfate reduction, producing hydrogen sulfide (H$_2$S), which diffuses upward and oxidizes to sulfate in oxygen-rich layers. In the absence of sulfate, methanogenesis dominates, producing methane (CH$_4$). Both processes can generate H$_2$S or CH$_4$, posing environmental risks.

• Hydrogen sulfide is toxic to benthic life, and excessive production may exceed oxidation rates, increasing ecological risk.

• Methane, a potent greenhouse gas, can also impact benthic organisms if released.

To mitigate risks, H$_2$S and CH$_4$emissions at the sediment-water interface should remain below environmental thresholds. Project Developers are encouraged to measure dissolved sulfate, H$_2$S, and CH$_4$concentrations in target sediment layers before burial and include these gases in their monitoring plans to ensure environmental safety.

Suggested monitoring plan additions to monitor environmental harms

Appendix C Reasoning for 2 m burial depth

In addition to reaching below the maximum oxygen penetration depth at any season, there is a required sub-sediment depth of at least 2 m depth into the sediment is required due to risk of reversal, due to maximum 2m sediment scouring during tropical zones, and infilling of previously scoured areas due to resuspension due to storms (Morton, 1979; Sherwood et al., 1994), fluid-mud flows (Wheatcroft, 2000), and erosion (Morton, 1979, Harris & Wiberg, 2001).

On the continental shelf, seafloor sediments are eroded and reworked by bottom currents and wave action, a process known as “scouring” (Flood et al. 1983). This creates linear or lobate depressions shaped by dominant environmental forces. Channel-shaped scours, or furrows, range from 10–100 meters wide and 100–1000 meters long, with coarse sand or gravel floors. Larger lobate deposits (100–500 meters wide) are often filled with mega-rippled coarse sand, forming "Rippled Scour Depressions" (Davis et al. 2013). Major storms can also transport large amounts of sediment to the deep sea without leaving scours (Teague et al. 2006).

Scouring and sediment resuspension pose risks to carbon storage in shelf sediments, as buried biomass must remain covered to prevent oxygen exposure. To assess this risk, we reviewed 29 studies on sediment furrows and ripple scour depressions across various depths and oceanographic settings (Figure 4, Table 5). Reported scour depths, including those from extreme events (e.g., Hurricanes Katrina, Ivan, Sandy), inform our recommendation of a >2-meter burial depth for carbon storage. While regional variation is significant, findings from Ferinni et al (2005) suggest that wider continental shelves may offer greater protection from erosion.

Table 5: Summary of observation of furrowing and rippled scour depressions in literature

Version history

This page describes the changes in the Marine sub-sediment burial module.