Storing Carbon the Natural Way

Clarifying the known risks, their magnitude, and what we are doing to monitor and detect even the slightest effect (positive or negative). This blog post comes to complement Puro.earth’s MACS (Marine Anoxic Carbon Storage) Methodology, and to prevent misconceptions about storing biomass in anoxic basins, such as the Black Sea.

1. Is storing biomass on the bottom of the Black Sea considered “dumping”?

No. The term “dumping”, as defined by all international conventions involving environmental protection of the seas and oceans, applies to “deliberate disposal of waste at sea”, most commonly dredged sediments from ports and waterways. Placing material in the sea for any purpose which is not “mere disposal”, is excluded from the definition of placement. Furthermore, Annex 4 and 5 of the London Protocol specifically define marine CDR as “placement activities”. See more details on our Marine CDR Policy blog post. 

It’s also important to note that rivers have always carried organic material like wood and terrestrial plant material into the ocean — it’s a natural part of the Earth’s carbon cycle. Terrestrial biomass is carbon-rich, nitrogen-poor organic matter, not waste, and its use here follows nature’s own long-standing model for carbon storage.

Of course, if storing biomass in the Black Sea would harm marine life, that would not be allowed. This question is addressed below.

2. Can any type of biomass be used for MACS?

No. The MACS protocol specifies woody biomass only, with stringent criteria such as high carbon-to-nitrogen (C:N) ratios, high lignin and cellulose content, and absence of contaminants. This means that:

1. The biomass does not introduce harmful chemicals to the ocean (i.e., it is not treated), 
2. The biomass contains a high amount of carbon, which was captured from the atmosphere and will be sequestered for thousands of years. 
3. The biomass has low nitrogen content, meaning less nutrients and less valuable for composting and returning to the field/forest.
4. Lignin is highly recalcitrant against degradation due to its high molecular weight, relative insolubility, and complex aromatic structure.

3. Is there scientific support for MACS?

Yes. The MACS methodology is backed by years of scientific knowledge regarding marine biology, geochemistry, and oceanography. Dozens of researchers support the concept and believe it to be a viable solution for removing atmospheric CO₂. You can read our scientific literature review, our reports, and the enthusiasm expressed in the international MACS Workshop, hosted by GeoEcoMar in Bucharest, and supported by the Grantham Foundation and the DOORS Black Sea Project.

4. Is there evidence that biomass preserves for over 1,000 years?

Yes. There are well-documented examples of ancient wood—from shipwrecks and natural sources—preserved for thousands of years in anoxic water basins like the Black Sea as well as within marine sediments. We have a research project that  focuses on retrieving ancient samples of wood, identifying the species living on and in them, locating fresh samples of the same type, and comparing the biochemical changes that took place over thousands of years. The results which will be published in Q3 of 2025 demonstrate remarkable preservation.

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5. Are anoxic zones truly “devoid of life”?

No. Life is amazing and can be found in many remarkable places. In aquatic environments without oxygen, there are microorganisms of various kinds that live off (i.e. get their energy from) other molecules such as sulfate (SO₄), which is abundant in seawater. 

6. Will MACS harm life in the Black Sea?

No. We have not found any scientific research demonstrating that the placement of uncontaminated, nutrient poor, terrestrial organic matter harms microbial life. The reason for concern comes from “what we don’t know that we don’t know”, and from the precautionary principle that assumes that most impacts of humans on the environment are negative. To address what we don’t know, we start with a “first principles” risk analysis:

• In nature, biomass is carried out to the sea by rain and rivers all the time. The Danube river carries a million tons of organic carbon to the Black Sea every year
• Organic matter is a source of energy for microorganisms
• Microorganisms can’t get crushed by biomass
• Terrestrial microorganisms in the deposited biomass that naturally grow in oxygenated environments (where the biomass comes from) will not survive in the oxygen-free, highly sulfidic environment on the Black Seafloor and will not adapt or evolve fast enough to survive in the harsh conditions in the deep Black Sea.

And to complement the risk analysis, we design experiments, operational procedures, and monitoring systems to measure the impact on biodiversity.

• We have an ongoing experiment in collaboration with GeoEcoMar, with real seawater and sediment from the Black Sea (meaning, it has real microorganisms from the Black Sea), measuring the impact on microbial life due to the placement of wood. The results will be published when the experiment is concluded (by end of the year), but so far the measurements support our assumptions that the life is unharmed by the biomass
• Scaling MACS will be in a stepwise fashion, with enough time in between steps to allow for careful evaluation of the environmental impacts.
• We are designing our monitoring system, in accordance with the MACS methodology, to monitor for changes in microbial life where biomass is placed and stored.

7. Is there absolutely no degradation of biomass in anoxic water?

No. Since there is microbial life in anoxic marine environments, there is some decomposition of organic matter. This is how microbes get their “food”. In addition, some organic compounds are soluble in water, releasing organic carbon without microbial interventions. The important questions are how much degradation is there, at what rate, and how does it differ with different types of biomass. This is the reason why we decided to focus on woody biomass, which has a high percentage of lignin. Lignin is a complex molecule and the best available science supports the assumption that microbes cannot break lignin down in the absence of oxygen. That is why there is living proof of thousand years old wood (see above) in the Black Sea, buried in sediments, and in other anoxic environments. But even wood breaks down to a small extent (as you can see in our report on our Black Sea experiment), and this breakdown results in degradation products such as hydrogen sulfide (H₂S), CO₂, and methane (CH₄). Our experience in several experiments shows that about 3% of the wood breaks down in anoxic environments during the first three-months, followed by no decomposition in the following year.

8. Can degradation products cross the chemocline and re-enter the atmosphere?

Yes, but the rate, extent, and timescale of that transport are key to determining the overall effect. The Black Sea is an almost fully enclosed body of water, with strong stratification and very limited circulation or vertical mixing. Because of that, it's extremely unlikely that degradation products would reach the surface and re-enter the atmosphere within 1,000 years. 

We mentioned that only a small fraction of the biomass would decompose, and to answer the question of “where will the decomposition products end up?”, we need to examine several factors. The first and most influential is the physical dynamics of the Black Sea, or, in simple words, how the water moves. There are 3 ways to get an indication of how the water moves, and the MACS research community is doing all 3:

1. Use a computer model to calculate and predict the movement of the seawater. This work is led by Prof. Ilicak and Prof. Tutak from the Istanbul Technical University. Additional work is carried out at the Middle East Technical University (METU) and at the University of Liege.
2. Get samples of seawater from the depths and date the carbon dissolved in them. The carbon dating gives an indication of when the water came in contact with the atmosphere and absorbed carbon from it. This has been done by several researchers in the past, and shown to range between 500 years to 2,000 years.
3. Measure the movement of the water, and this is, of course, part of Rewind’s monitoring approach.

Just by forecasting the physical movement of the water, it is possible to deduce that the small fraction of decomposed biomass will take over a thousand years to propagate to the surface of the Black Sea. But if that is not enough, then the second layer of analysis looks at biogeochemical processes that will impact the dissolved molecules moving in the water. The most concerning would be methane (CH₄), which over such a long time frame will break down to CO₂ and water (via anaerobic oxidation of methane, AOM).

9. Will biomass sinking cause turbulent mixing or heat that collapses stratification?

No. Hypothetically, as biomass sinks in the water, the water around it will move. And hypothetically, as the small fraction of the biomass breaks down, it will create heat (just like compost creates heat). But the devil is in the details. Biomass sinking freely in the water, which is how Rewind’s patented sinking method works, will sink very slowly due to its density, somewhere around walking speed (7 km/h = 4.4 mph). In addition, the volume of water in the Black Sea is 540,000 km³ (1 km³ of water weighs 1 gigaton). So assuming a worse case scenario with 0.5 gigatons of biomass per year, sinking at walking speed will not generate significant turbulence. As for the question of heat, using similar assumptions of scale and minimal breakdown, the heat generated will still be about 10x smaller than the natural geothermal heat released from the bottom of the Black Sea, and when compared to the huge volume of water, it will not have a measurable impact on the temperature of the water. By now you are probably already guessing, yes, temperature of the water is of course part of the storage site monitoring system. 

10. Will sinking biomass disturb the seabed upon impact and release toxic dissolved substances like  H₂S and CH₄?

Yes, but at very small amounts that will quickly be resuspended. We already mentioned that the biomass will be sinking at walking speed, and here we should add that the bottom of the Black Sea has been described as a soft, fluffy mud, in which concentrations of H₂S and CH₄ are already present and significant (due to the natural sinking of organic matter and its anaerobic breakdown). To ensure this floatation and resuspension of seabottom sediment is not affecting the concentration of H₂S and CH₄ in the water column, Rewind’s monitoring system includes visual documentation of the sinking process as well as ongoing monitoring of the chemical composition in the water column and sediment. 

11. Could seabed disturbance reduce oxygen-rich habitat zones?

No. Putting all the pieces together: a very small fraction of biomass will decompose, a negligible amount of sulfide and methane will be released to the water upon impact with the sediment, and with the modeled and measured movement of the water, these chemical changes will not propagate to the surface layer and impact its oxygen content. The real risks to the oxygenated top layer of the Black Sea is eutrophication from fertilizer runoff via the Danube and Dnieper, and the warming of the top layer due to climate change (which is what we’re trying to mitigate).

12. Could re-emission of GHGs from MACS cause a catastrophe?

No. As explained above, even in worst-case scenarios, some CO₂ which was captured by the biomass is re-emitted and returned to the atmosphere, reducing the carbon removal efficiency of the method. As a matter of fact, some hypothetical re-emission is already baked into the carbon accounting of MACS, resulting in a carbon credit which “insures” for the worst case re-emission assumption. 

Real catastrophic outcomes will occur, however, if we continue to not take action in the face of a warming planet.

13. Could a CO₂ bubble pop out of the Black Sea like in the Lake Nyos incident?

No. The Lake Nyos incident involved a pressurized CO₂ bubble from volcanic sources, which bubbled up to the surface because lake Nyos is relatively shallow (208m max depth). The source of CO₂ in Lake Nyos was not decomposing biomass, and the depth and currents of the lake are not comparable geochemically, oceanographically, or geographically.

14. Could microbial adaptation increase degradation rates over time?

Unlikely. Theoretically, evolution is magical, but it hasn’t happened in the 10,000 years that the Black Sea exists or the billions of years that swamps and wetlands exist (which are natural organic carbon sinks). Storing terrestrial organic matter in oxygen-free water is a common process that occurs naturally all around the planet. Though MACS intends to accelerate it, it is unlikely to accelerate evolution that hasn’t happened until now. Small as this risk may be, it is addressed through multi-year in situ incubations by Rewind, GeoEcoMar, and Prof. Raven, and will be monitored closely as part of Rewind’s monitoring system.

15. Is MACS energy-intensive and inefficient?

No. Far from it. Peer-reviewed life-cycle analysis, performed independently to the MACS research community, shows that biomass sinking is the most energy-efficient CDR method among currently proposed approaches. See the comparison in the chart below:

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16. Will lab testing of many biomass batches incur unreasonable costs?

No. Testing adds some cost, but it’s manageable based on Rewind’s operational experience. Technological improvements will further reduce these costs over time, and are part of Rewind’s technology roadmap.

17. Is large-scale deployment of MACS possible?

Yes. The Black Sea region alone produces ~300 Mt/year of agricultural biomass that can be used for mCDR, with additional feedstocks from forestry and agriculture from much of Europe accessible via the Danube-Bosphorus transport corridor.

18. Shouldn’t there be small-scale, multiyear trials first?

Absolutely — and that’s exactly the plan. Rewind intends to start with a reversible pilot of 100 tons, monitor it carefully for a year, and after peer-reviewed analysis of the collected data and publication in open access journals (so everyone can read about it), gradually scaling up 10x year on year, while following a similar process of meticulous monitoring, peer-reviewed analysis and publications. One more thing we’d like to emphasize is that this is an extremely cautious approach, and what may be perceived to us as a large amount (e.g. 100 tons, or 300 m³), can in fact be a microscopic amount when compared to the size of the Black Sea (300 m³ biomass << 540,000,000,000,000 m³ water) and will most likely not cause any significant change (which we intend to monitor closely). Balancing speed and safety is very important as the seas and oceans, Black Sea included, are already facing a huge risk due to rising water temperatures.

Last words from Rewind

Rewind was founded in 2022, following 3 years of research, analyzing various methods to remove CO₂, with the goal of finding a method that would be permanent, scalable, affordable, and environmentally safe. We decided to focus on the Black Sea because it met all of our requirements, and we remain committed to creating a better future for ourselves and our children, backed by science and strengthened by honesty, transparency, and courage. Please reach out to us if you have any further questions, suggestions, or corrections.