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: Grain size must be predominantly (> 50%) mud (< 63 µm grain size in diameter)

• 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 site requirements that identify suitable sites for permanent burial

• 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 will be permanently stable.

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.

At verification, the organic carbon content in the buried feedstock mixture of each storage batch is measured via sampling, and observed via remote sensing, at 1-3 months (optional) and 12 months (mandatory) to ensure permanent storage and negligible risk of reversal.

If measured organic carbon loss at 3 or 12 months exceeds 2% of the initially buried carbon, degradation/reversal may be triggered. In this case, the project is considered compromised, and carbon credit issuance for the affected storage batches will be paused. The Rainbow Certification team will collaborate with Project Developers to determine the cause of the unexpected loss and decide on appropriate corrective actions, including canceling issued credits according to the Rainbow Procedures Manual and suspension of future credit issuance.

The amount of permanently stored carbon that is issued credits is conservatively modeled, as detailed in the GHG quantification section. Note that when default literature values for biomass are used, the modeled fraction of organic carbon that is still stored after 1000 years is 92%.

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 Rainbow Certification team shall evaluate the assessment and may recommend changes to the assigned scores.

The Project Developer, Rainbow 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 Rainbow 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

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, and

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

Any identified carbon removal reversals shall result in canceled credits according to the Rainbow Procedures Manual.

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.

Project Developers shall measure heavy metal content from biomass samples and demonstrate that it is below thresholds set by the relevant jurisdiction.

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

Additional optional environmental impacts to monitor are described in Appendix C.

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.

Assumptions

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

2. 12 months is an appropriate and sufficiently long timeframe to determine if carbon degradation will likely occur over 1000 years, given that organic carbon degradation is front-loaded and logarithmic.

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

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

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

6. 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. 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.

7. 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.

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 may be used to define default values for the parameters outlined in Table 4. If instead, 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

Carbon storage is calculated by multiplying the fraction of organic carbon still stored over 1000 years, by the amount of initially buried organic carbon (Eq 1). Each component is described in the following sections.

Carbon burial

The amount of carbon initially buried shall be calculating using using primary data, measured by Project Developers, for each storage batch, following Eq. 2 below.

Carbon degradation

A small fraction of the buried organic carbon may be decomposed by microbes in the sub-sediment. This is expected to be small because of:

1. the site requirements that ensure anoxic conditions, preventing degradation,

2. use of terrestrial biomass in marine settings, where microbial communities are not well adapted to degrade terrestrial biomass (see Appendix B), and

3. sediment conditions in the site requirements, ensuring that if degradation occurs, any evolved CO$_2$ would likely 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 using a multi-G kinetic model as shown in Eq. 3 (see justification in Appendix A and Appendix B).

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 Project Developer is the party responsible for adhering to the Monitoring Plan.

Monitoring Plans to issue credits for this module shall include, but are not limited to, tracking of the following information for each new Storage Batch:

Reversal Monitoring Plans to check for reversals shall include, but are not limited to, tracking of the following information for a representative sample of storage sites:

Appendix

Appendix A: Scientific Basis of Marine Sub-Sediment Burial

This appendix outlines the scientific foundation for marine 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 (HS), 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: Incubations, models, and field evidence support minimal carbon loss in buried wood

Written by Daniel Babin (Ph.D.), Head of Science, Sinkco Labs. See reference list at the end of the Appendix.

Marine sub-sediment burial is a carbon storage method that stores waste biomass products in anoxic marine sediment to prevent rot. This carbon storage method is supported by laboratory incubations, biogeochemical models, and numerous natural examples of well-preserved, million-old subfossil wood found in sediment around the world. The purpose of this annex is to:

1. Provide general evidence of the feasibility of permanent carbon removal from biomass burial in or below aquatic sediments,

2. Use model and literature data to determine a target preservation threshold for a 1 year monitoring period that justifies claims of 1000 year permanent removals.

3. Assess an appropriate and conservative estimate for how much carbon will remain stored after 1000 years based on biogeochemical models and chemical and physical data subfossil wood preservation from the literature.

A model for organic matter decay based on laboratory incubations indicates 92% of organic carbon will remain after 1000 years. This aligns with findings on the decay of subfossil wood in a wide variety of geologic settings that indicate preservation 88-97% preservation on the timescale of thousands of years.

Introduction Marine sediment has the potential to lock away carbon for millennia. Marine sediments are the final resting place for terrestrial and marine carbon with 150-200 billion tons stored in top meters of marine sediments globally (Hedges & Keil, 1995; Hedges et al., 1997; Atwood et al., 2020). However, the pathway to permanent storage in sediments is relatively inefficient — only 0.1% will ultimately be buried for millions of years due microbial activity in both the water column and the upper few centimeters of sediment where oxygen is present (Burdige, 2007; LaRowe et al., 2012).

At depth in marine sediments, organic matter is much better preserved. This environment is typically anoxic or hypoxic, cold, and saline. Dissolved oxygen is consumed rapidly near the surface, and below a few centimeters, decomposition relies on less efficient anaerobic processes . Additionally, cooler temperatures slow microbial enzyme activity, further decelerating decomposition (Bulesco et al. 2019). The combined effect of low oxygen, high salinity, and low temperatures means that marine sediments preserve OM far longer than soils or freshwater sediments.

Marine sub-sediment burial (MSSB) as a carbon removal protocol leverages the geochemical stability of carbon at depth in anoxic marine sediments by placing biomass even deeper beneath layers of sediment where microbial access and oxygen infiltration are virtually nonexistent. To store carbon, low-value agricultural and forestry residues (normally burned or landfilled) are mixed into a slurry and injected more than 15 feet down into sediments.

The durability of this carbon storage technique is further enhanced because marine sediment microbial communities are adapted primarily to marine-derived organic matter—not to the complex compounds typical of terrestrial plant biomass (like lignin and cellulose) often used in MSSB. These microbes often lack the necessary enzymatic pathways to efficiently degrade such materials . Recent studies show that when terrestrial-derived organic substrates (e.g., lignin, cellulose-rich material) enter marine sediments, only select microbial clades—usually rare or uncultured—respond, and even then, degradation is slow (Bulesco et al. 2019).

Predicting carbon loss in marine sediments To predict the amount of carbon degradation, the MSSB protocol draws on sediment biogeochemistry models. A particularly relevant framework is the multi-G kinetic model of organic matter degradation, originally developed for marine sediments. As reviewed by Arndt et al. (2013), the multi-G model assumes organic matter comprises multiple discrete pools (“G” classes), each with its own characteristic degradation rate (Arndt et al. 2013). Rapidly decaying compounds are exhausted early, leaving progressively more refractory fractions that break down extremely slowly. This multi-component kinetic formulation quantifies how overall reactivity declines with depth and time in sediment burial (Arndt et al. 2013). The model has been successful in simulating long-term carbon preservation in many sedimentary settings (Arndt et al. 2013), making it a promising tool to predict the fate of buried biomass carbon. The model is the closest fit to MSSB projects, although it assesses degradation in shallow layers of marine sediment, where more degradation is expected to occur than in MSSB projects, which bury organic matter deeper in sub-sediment layers. Because the model was calibrated in oxic conditions and MSSBs projects are required to bury biomass in anoxic conditions, the mullti-G model predictions are expected to be a maximum degradation rate.

Empirical evidence for long-term stability of buried organic carbon comes from deep-sea experiments, and corroborates results from the model described above. Keil et al. (2010) applied a multi-G type analysis to test burying crop residues in marine sediment. In a 700-day incubation using marine sediments, an initial brief pulse of decay oxidized <1% of the added plant material in the first week Keil et al. (2010). Thereafter, degradation virtually stalled: over the following two years only 3–8% of the terrestrial biomass (soy straw, corn stover, wood chips) was re-mineralized. This decay rate is much lower than the 19% lost from more labile, marine-native plankton material (Keil et al. 2010). The fitted kinetic parameters (e.g. approximately 0.004 yr^-1 for terrestrial plant carbon) were orders of magnitude lower than typical decay rates for fresh organic matter (Keil et al. 2010).

For this study, we use the decay rate fit for alder wood from the laboratory experiments of Kiel et al. (2010) (Figure A1). Plankton, maize, and soy were also tested in their experiments, but alder was selected as the best representation of woody biomass (Figure A1). In Kiel et al. (2010), decay of alder is modeled as:

$G(t)=G_{int,1,0}e^{-k_{int1}t}+G_{int,2,0}e^{-k_{int2}t}+G_{res,0}e^{-k_{res}t}$

Where G(t) is the fraction of carbon left at time t, and G_int2 = 0.088, G_res = 0.911, k_int2 = 0.003, and k_res = 0 (Kiel et al. 2010).

The purpose of this annex is to validate the multi-G model’s predictions against real-world data from long-buried wood on land. We compiled carbon-loss measurements from subfossil wood recovered in a wide variety of settings (terrestrial and marine burial, archaeological settings, anaerobic bogs, landfills) to see if observed decay over years to millennia aligns with the multi-G kinetic curve. By comparing these field and laboratory observations to model expectations, we test whether biomass buried in marine sub-sediments can achieve the ultra-long-term carbon stability that the multi-G framework – and prior marine experiments – suggest.

Table A1: Summary comparing the conditions and parameters of 1) the multi-G model with input parameters from Keil et al. 2010, 2) empirical studies measuring carbon loss in buried wood, and 3) the requirements for marine sub-sediment burial projects under the present methodology.

Results The results are illustrated in a comparative plot (grouped data vs. model, Figure A2) which shows observed carbon loss at each timescale alongside the multi-G model prediction. Each empirical data point represents the mean percent carbon loss for one of the four age bins (0-1, 1-100, 100-1000, 1000-10,000 years), with error bars indicating the variability among studies in that bin. Light grey dots represent individual samples from studies measuring decay. The decay curve predicted by the multi-G model is overlaid for reference. The trajectory of the decay curve falls within the range of observations of subfossil wood, indicating that the multi-G kinetic model reproduces the real-world decay of buried wood within the margin of experimental error.

Quantitatively, very little carbon loss is observed in the early stages of burial, consistent with model expectations of initial rapid stability. In the 0–1 year group, the carbon loss was 1.2% ± 1.3% (Figure A2). Several studies reported essentially no measurable mass loss during the first year of anoxic burial. The multi-G model supports this, predicting negligible decay in year one due to the rapid exhaustion of only the most labile components. These observations suggest that if a maximum of <2.5% (1.2% + 1.3%) of carbon loss is observed in MSSB batches after 1 year, decay is consistent with empirical evidence for carbon loss in buried wood, and long-term storage can be estimated using model results. This directly supports purpose #2 of this annex—determining a <2.5% annual loss is a reasonable proxy for predicting 1,000-year carbon permanence.

Even after centuries of burial (100–1000 years bin), the compiled data show that most of the wood’s carbon is still retained (with 5.2% ± 3.3% lost), aligning with the model’s slow exponential decay phase (8% lost). The close alignment of model results (92% remaining) and empirical data (minimum 91.5% remaining) supports purpose #3 of this annex. Available evidence suggests that permanent carbon removal using MSSB is estimated to be a maximum of 91.5%.

Discussion These findings have positive implications for the long-term stability of buried woody biomass as a climate mitigation strategy. The close match between the multi-G kinetic model and real-world subfossil wood data validates that the model’s core assumption – a small labile fraction decays quickly, leaving a large refractory fraction that persists – holds true in practice. In other words, once the readily degradable components of wood are consumed (typically within the first months of burial), the remaining bulk of the carbon becomes extraordinarily stable. This outcome is consistent with previous research on wood in landfills: studies have concluded that wood disposed under anaerobic conditions effectively acts as a “long-term reservoir of carbon” with extremely slow decay. Ximenes et al. (2015), for instance, documented minimal decomposition of wood even after decades in well-managed landfills, confirming that most of the carbon is retained over time. Likewise, the observation of a >3,000-year-old buried log with only ~5% carbon loss underscores how durable buried biomass can be when oxygen and microbes are severely limited (Zheng et al. 2024). The reason is straightforward – lignin-rich wood in anoxic, water-saturated or clay-sealed environments does not readily support the microbial activity needed for decay. Our results reinforce that under such conditions, burial effectively “vaults” carbon out of the atmosphere for millennia.

For the release of carbon credits, we propose a monitoring period with a 1-year duration, with results that match evidence from subfossil wood and the multi-G kinetic model. The compiled literature and our analysis indicate that no more than 2.5% of carbon is lost in the first year of anoxic wood burial. If chemical data from storage batches align with literature and model estimates, then decay can be expected to match literature and model examples, which show 92% of wood carbon will stay sequestered on the timescale of thousands of years. The excellent agreement between the multi-G model and observed subfossil wood decay affirms storage of wood in anoxic sediment as a durable form of carbon removal.

Appendix C: 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 D 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 A3, Table A3). 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 A3: Summary of observation of furrowing and rippled scour depressions in literature

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