Methane: The Elephant in the Sea

March 18, 2024

Will depositing biomass in the anoxic Black Sea produce methane?


Methane molecules trap more heat than CO2 molecules (about 80x more, over the course of 20 years). In nature, methane is usually produced when organic matter decays in the absence of oxygen. This is why it is a topmost concern for us to make sure our CDR method does not turn the Black Sea into a methane source. The way we plan to do this is by minimizing the surface area of the deposited biomass, increasing the exposure to sulfate (preventing methanogenesis), locating storage sites at depths where produced methane will turn into clathrates (“methane ice”), and relying on the stagnancy of the water in the Black Sea (allowing thousands of years for any dissolved methane to come in contact with sulfate and oxidize). 

Let’s Dive In

Methane is a very potent greenhouse gas, commonly emitted from oil wells, livestock, landfills, and more. In recent years, methane has become a topmost concern for emissions reductions, as it has a dramatic short-term (20-year) heating effect, 80 times more potent than carbon dioxide. Methane is currently inaccurately measured worldwide, and is at the core of one of the most concerning climate change “points of no return”: the defrosting of the arctic permafrost.

Figure 1. Primary methane sources and sinks (Global Carbon Project, 2020). The red box on the top right shows that the total new methane persisting in the atmosphere every year is between 13 and 100 megatons (Mt=Tg), equivalent to 1 to 8 Gigatons of CO2

Since methane can be formed from the decomposition of biomass in the absence of oxygen, the team here at Rewind is often asked about this risk in our carbon dioxide removal method: Will storing terrestrial biomass in the deep anoxic Black Sea have the undesired byproduct of producing large amounts of methane, which may be re-emitted into the atmosphere? In this blog post, we’ll dive into the intricacies of methane, describing its fundamental characteristics, formation processes, and breakdown pathways. Most importantly, we’ll explain why we believe, based on the science available today, that storing biomass in the Black Sea will not lead to an increase in methane emissions into the atmosphere.

What is methane, and how is it formed?

Methane (CH₄), a simple hydrocarbon gas comprised of one carbon atom and four hydrogen atoms, plays a pivotal role in Earth's carbon cycle. Methane is the third-most abundant greenhouse gas in Earth’s atmosphere, after water vapor and carbon dioxide. Despite its atmospheric concentration being approximately 200 times lower than that of CO₂, methane emissions significantly impact climate change, as methane traps about 80 times more heat energy than CO₂ over the first 20 years in the atmosphere (IPCC 2007; Blasing 2013). 

In natural environments, methane is formed predominantly under two separate anaerobic conditions, where organic matter decomposes without access to oxygen. 

  • Biogenic Methane: Microorganisms, specifically methanogens, produce methane during the anaerobic digestion of organic material (a process called methanogenesis) in various environments such as wetlands, rice paddies, and aquatic sediments. Methane is produced only in the absence of oxygen (O2), as when microbes are starved of oxygen, they resort to other, less energetic forms of breakdown reactions (“food digestion”). Methanogenesis will happen only once other available molecules have also been depleted:  nitrate (NO3- ), manganese (Mn2+), iron (Fe2+), and sulfate (SO4−2). In seawater there’s actually quite a lot of sulfate.
  • Thermogenic Methane: this type of methane originates from the thermal decomposition of buried organic material under high pressure and temperature conditions, similar to the process of pyrolysis. Thermogenic methane differs from its biogenic counterpart by forming in sedimentary rocks over extended periods of time.
What happens to methane in nature? 

Nature has several ways of breaking down methane, also called methane sinks. Whether methane was formed biogenically or thermogenically , methane is consumed and broken down through chemical and microbial oxidation processes. Chemical oxidation occurs in both atmospheric and aquatic settings, while microbial oxidation only occurs in aquatic settings. Methanotrophic (methane eating) bacteria play a key role in converting methane to carbon dioxide and water. The oxidation processes can be aerobic, requiring oxygen, or anaerobic, with alternative molecules (electron acceptors, or oxidizing agents) such as nitrate or sulfate. 

Anaerobic methane oxidation in marine sediments is an important sink in the global methane budget, consuming between 20 Mt/yr (megatons per year) (Henrichs and Reeburgh, 1987) and 100 Mt/yr (Reeburgh et al., 1991) of methane, a significant amount when compared with  the yearly emissions of methane accumulating to 500-600 Mt/yr (IPCC, 2007). The following diagram explains the various pathways for methane breakdown (or consumption) in marine environments.

Figure 2. Methane consumption in the environment (source, page 36).
What happens to methane on the seafloor? 

To answer this question we must look at a few more parameters that will affect the fate of methane in the bottom sediment of the sea. These parameters include depth, pressure, temperature, and concentrations of oxygen, nitrate, sulfate, and methane. 

In shallow and anoxic sediments, methane can be biogenically produced from the decomposition of organic matter, and the discharged methane will be dissolved in the surrounding water (emphasis on dissolved and not yet forming bubbles). Only when the level of methane production increases enough for the methane to exceed the limit of its water solubility, methane gas bubbles can be formed. When these bubbles are formed at depths shallower than 100 meters, they may bubble up and reach the atmosphere. At water depths deeper than 100 meters, most methane bubbles tend to dissolve into the water before reaching the surface. Dissolved methane is then consumed by aerobic microbes, preventing it from ever reaching the atmosphere (Beaudoin et al., 2014). 

In deep-sea environments, under specific pressure and temperature conditions, usually in waters deeper than 500m, methane can take a solid form similar to ice, called methane hydrates, or methane clathrates. These are crystalline structures that trap methane within water molecules. The depth range in which conditions allow for methane hydrates to form is commonly called the “Methane Hydrate Stability Zone”, which refers to the range of pressure and temperature conditions where methane hydrates can maintain their solid, crystalline structure (Beaudoin et al., 2014). Methane hydrate is 164 times more condensed than methane gas, resulting in a higher storage capacity of methane in the form of hydrate compared to dissolved methane. Most commonly methane hydrates are formed in the existing pore structure of the sediment and are retained for geological time scales. Nevertheless, a pure methane hydrate is buoyant in seawater, so if hydrates disconnect from the sediment, e.g. in the case of an earthquake, they may float to the surface, potentially transporting methane to the atmosphere (Brewer et al., 2002).

 Figure 3: Methane hydrate retrieved during an expedition to the “hydrate ridge” off the coast of Oregon in the US (source

In even deeper marine environments, below the base of the Methane Hydrate Stability Zone, methane is present mostly as free gas. This free methane gas, if not trapped in “natural gas” reservoirs, could rise into the water column through geological formations, such as geological faults, fractures, and high permeability sediments. As the methane propagates through the geological layers, it can be incorporated into gas hydrates or can be released via cold seeps or mud volcanoes at the sediment’s surface.

Figure 4: Mud volcano discovered in the Barents Sea (source)
Is the Black Sea a methane source or sink? 

As the world's largest anoxic basin, the Black Sea, which spans 436,000 square kilometers, boasts unique methane-regulating mechanisms, allowing it to serve as a methane sink

  1. The formation of methane hydrates: The upper boundary of the Methane Hydrate Stability Zone (MHSZ) at the Black Sea starts at an average water depth of 675m (Merey, 2017), meaning that for the main part of the Black Sea, methane produced in the sediment will be transformed to methane hydrate and stay at the seafloor. 
  2. The anaerobic oxidation of methane: This mechanism converts methane to CO2 in the presence of sulfate, a common molecule in seawater which is replenished by the periodic inflow of sulfate-rich ocean water, uniquely flowing into the Black Sea through the Bosphorus Straits.  (AOM; Boetius et al., 2000).

Despite these important mitigation mechanisms, the Black Sea is also the largest surface-water reservoir of dissolved methane and contains 96 Mt of methane, a quantity equivalent to the global annual methane emission by wetlands or rice paddies (Cicerone and Oremland, 1988). Methane is emitted to the water column mainly from methane seeps, located on the Black Sea shelf, in a depth of a few hundred meters, above the Methane Hydrate Stability Zone. This means that the Black Sea may also be a source of methane emissions.

Figure 5. Distribution of gas seeps, mud volcanoes and gas hydrates in the Black Sea (OAIE et al., 2016)
Will the Storage of Biomass in the Black Sea increase methane emissions?

Now that we’ve established the methane dynamics, we can finally answer this question. Rewind's carbon sequestration solution, deploying biomass in the deep anoxic Black Sea, is designed in a way that will not increase the flux of methane into the atmosphere. Rewind deposits organic material in the deep Black Sea, under permanently anoxic conditions, while minimizing surface area, and leaving access to the abundant sulfate in seawater. Let’s break this sentence down to understand how methane re-emission can be mitigated.

  • Minimizing surface area: the deployed biomass is packed in organic burlap bags, preventing it from scattering on the seafloor, thus decreasing the surface area and contact with the sediment. The packed biomass pieces are shredded into medium-sized pieces, to further reduce its surface area with the environment. Decreasing the surface area will decrease the rate of any potential chemical reaction with the environment, including methanogenesis.
  • Maximizing sulfate exposure: The bags of organic material are placed on top of the sediment, maximizing exposure to seawater (and therefore sulfate), and minimizing coverage by the sediment. As we’ve established, in the presence of sulfate, methanogenesis will not take place since sulfate reduction is more energetically beneficial. Still, sulfate may be depleted, especially in the sediment around and below the biomass. Depletion of sulfate in the sediment will allow methanogenesis to commence, and this could lead to the presence of methane in the shallow layers of the sediment. This will be addressed in the following bullet point.
  • Promote formation of durable methane clathrates: Rewind’s storage sites will be located at depths between 1,000 to 2,000 meters in the Black Sea. At such depths produced methane is expected to transform to methane hydrate and stay in the sediment. As the deep zones of the Black Sea are known to be sinks for methane, rather than sources, we believe that this not only holds for the naturally deposited organic material but also for the material deposited by Rewind. 
  • Increase the likelihood of chemical breakdown of any dissolved methane: The last safety step is the absence of vertical currents in the deep Black Sea. Different from other seas and oceans on the globe, the Black Sea, due to its unique geomorphology, is not connected to the global deep-sea current system. This is one of the reasons for the Black Sea’s permanent anoxia. This also means, that even in the case methane is produced and dissolved in the deep water, it would propagate extremely slowly, and by diffusion, a very slow transport mechanism. Methane will therefore stay for a long duration in the water column, allowing enough time to come in contact with sulfate, and to chemically break down to CO2.

As we unravel the complex dynamics of methane in anoxic environments, our scientific scrutiny provides a reassuring perspective on the feasibility and safety of Rewind's carbon sequestration solution. Rewind’s biomass packaging and placement is designed to reduce overall bio-chemical interaction while leveraging natural sulfate to prevent biogenic methane formation, and leveraging proper depths to break down or turn into clathrates any produced methane. We are currently solidifying this approach with scientific experiments, and with a deep sea MRV that provides transparent data on the chemical and biological composition of the water. 

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