Overview
Throughout Earth history, one of the principal mechanisms by which atmospheric CO2 concentrations have declined from hothouse conditions is the burial of organic carbon in anoxic marine basins. Carboniferous’s Anoxic Biological Carbon Sequestration (ABCS) process is designed to safely accelerate the natural process of organic carbon burial by collecting and processing agricultural byproducts and storing them in deep, hypersaline, anoxic basins, our first being below the oxygenated ocean in the Gulf of Mexico.,
Our CDR approach is designed to maximize the efficiency of biomass preservation through both our choice of biomass sources and our targeted storage location. Most terrestrial biomass and crop residues are composed of lignin, cellulose, and other structural polymers that are relatively resistant to chemical and microbial breakdown. Some of these materials, like lignin, are particularly difficult to degrade in the absence of O2. Anoxia also slows the breakdown of terrestrial materials by restricting the activity of animals like worms that physically degrade biomass, and it reduces the energy available to microbes. Deep hypersaline anoxic basins take this effect even further, generally slowing metabolic rates and limiting life forms to extremophilic bacteria and archaea. Because of this terrestrial biomass storage in anoxic environments should represent an optimal scenario for efficient biomass preservation.
The sources of carbon for the ABCS process are crop byproducts, including sugarcane bagasse (the pulp remaining after sugarcane processing) and corn stover (stalks, leaves, etc.). These materials fix large amounts of carbon every year, with one tonne of dry terrestrial biomass embodying approximately 1.6t of CO2e. Currently, most non-grain biomass is tilled back into the soil to decompose, or burned, releasing the fixed carbon back into the atmosphere. Our approach mitigates the return of part of this biomass to the atmosphere.
Sequestration Process
Biomass is gathered and transported to a local facility where it is processed to enable more efficient transportation, loaded onto a barge, transported down the Mississippi, and placed into a deep, hypersaline anoxic basin located 2,400 m below the surface of the ocean and 130 miles south of Louisiana. This is considered a biomass carbon removal and storage (BiCRS) pathway and is favorable because it leverages existing resources and is grounded in well-established fundamental science, giving it a high probability of achieving gigatons of sequestration. The approach is scalable, durable, safe, affordable, and equitable.
Scalability
ABCS can be deployed at scale through existing crop wastes infrastructure. Of any CDR technique being actively considered, terrestrial biomass sequestration in anoxic basins has the highest potential for both short-term implementation and medium-term scaling to the gigaton scale. Additionally, sufficient anoxic basin volume exists (>500,000 km3) to sequester climatically-relevant quantities of carbon. This is an advantage relative to terrestrial storage applications that are often limited by available volume (each gigaton of biomass has a volume of approximately 1 cubic kilometer).
Durability
Durable (>1000-year) storage of carbon is is fundamentally controlled by the hypersaline waters in the basins inability to mix into overlying water on short timescales; brines in the basin today have been trapped there for at least 7,900 years because the excess salt creates a very strong density gradient at 2250 m depth. Due to the long residence time of brine in the basin, the water is fully anoxic, which also reduces rates of biomass breakdown. Importantly, even the fraction of the biomass carbon that is slowly respired within the brine is trapped by the lack of significant mixing between the brine and the overlying seawater.
Effectively 100% of the biomass carbon that reaches the deep basin is trapped for at least 1,000 years.
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Minimization of Ecological Risk
ABCS minimizes risks to benthic and deep-ocean animal communities by avoiding the environments in which they can live. Anoxic basins are physically restricted, which means that any environmental impacts are localized and unable to spread over a large area. This dramatically reduces project risks relative to other pathways that operate in the open, interconnected, more physically dynamic ocean. Additionally, because microbes in this environment primarily respire sulfate, the environment will not experience major changes in oxygenation or pH, which are critical concerns in oxic systems. Finally, in contrast with CDR approaches that use marine biomass, ABCS does not significantly perturb the marine nutrient cycle. On land, approaches are well established to manage the loss of the small quantities of nutrients trapped in the biomass materials.
Affordability
To reach gigaton sequestration levels the cost per tonne of CO2 removed must be as low as possible. Carboniferous’s process has distinct price advantages over other BiCRS and CDR pathways in carbon utilization, transportation efficiency and minimal processing. High carbon efficiency is enabled by not burning the biomass. For each tonne of biomass, processes like biochar and bio oil sequestration burn off ~50% of the carbon contained in the biomass, while Carboniferous uses the biomass as a sequestration medium without “wasting” half of the carbon, nearly doubling our impact. Secondly, by using predominantly maritime transportation, the cost per tonne for transportation is avery low even over long distances. And finally, the lack of extensive processing increases affordability. Where most other BiCRS pathways need dry material, the ABCS process can utilize either wet or dry biomass which increases the types and conditions of biomass available for CDR.
Equitability
The ABCS process uses crop biomass from near the Mississippi and maritime transportation along the Mississippi river, creating jobs predominantly in rural communities. Sourcing crop wastes for sequestration generates financial opportunities for farmers and other industries in the agricultural supply chain. And, by purchasing annual crop biomass, food prices are decreased because the increased revenue to farmers acts as a food subsidy. Finally, our selection of a storage site within the U.S. EEZ but far from the coasts or any human populations avoids concentrating risks or other negative impacts in local communities.
Verifiability
Deep hypersaline anoxic basins are effectively isolated from the open, well-oxygenated ocean due to very slow mixing rates. This means that our sequestration site has boundaries to contain any breakdown products and limit the spatial area relevant for monitoring.
Conclusion
There direct benefits of carbon sequestration for climate mitigation coupled with a scalable, low cost, low environmental risk, and highly equitable process make ABCS one of the best options for carbon sequestration.
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