Calculation of Net Carbon Removed
At its simplest level, the net CO₂ removed from this multi-pathway system can be calculated as:
Where:
= CO₂ Removal by Sinking of Terrestrial Biomass
= CO₂ Removal by Alkaline Mineral Dissolution
= CO₂ Removal by Macroalgae
= End-to-End System Emissions
Initial deployments will focus on the sinking of terrestrial woody biomass. This pathway is fully operationalized and accounted for today via:
Direct measurement of the carbon content of representative terrestrial biomass samples contained in the carbon buoys, validated by laboratory testing of biomass characteristics.
The adoption of existing ocean models for predictive modeling of expected carbon buoy transport behavior in the open ocean, also based on assessments of the physical behavior of carbon buoys in lab-replicated ocean environments and empirical data from GPS sensors released alongside deployment.
Direct observation of biomass sinking from a subset of carbon buoys via in-situ image analysis from observational camera systems released alongside carbon buoy deployment.
Quantification of emissions associated with biomass material inputs, production, transport, and deployment.
Initial deployments will also utilize a mixture of calcium carbonate and lime kiln dust for dissolution in the surface ocean. This pathway is operationalized for initial deployments and will be used to buffer any acidity generation that occurs during terrestrial biomass floatation, but will not be used to generate credits through the ocean alkalinity enhancement pathway until the quantification and monitoring methodology is refined in the .
The primary objective in quantifying alkalinity enhancement is to determine what quantity of alkalinizing material has been successfully dissolved into the mixed layer in a given region of the ocean. The amount of CO₂ sequestered by the alkalinity addition will be discounted by the amount of CO₂ released by any reprecipitation of carbonate minerals that occurs in the surface waters, which will be estimated from laboratory experiments and measurements of key parameters in the field (salinity, temperature, pH). To-date, testing of ocean alkalinity enhancement has primarily been conducted in laboratory settings to characterize how quickly alkaline materials dissolve in water. In the laboratory setting, dissolution reactors continuously log temperature, pH, and salinity of water and pCO₂ of air at the surface, alkalinity is measured with an auto titrator, and water samples are sent to independent labs for elemental analysis. Experiments have also been conducted to characterize the effect of organic acid leaching from wood and the mitigation of this by alkaline mineral dissolution, namely lime kiln dust. Supplementing research by external collaborators, additional experiments are planned to further quantify rates of acid leaching from terrestrial biomass and neutralization of this acidity by dissolution of alkaline minerals, and to elucidate the degradation of dissolved organic carbon released by the macroalgae and terrestrial biomass.
Weather permitting, surface water conditions will be monitored and sequentially sampled during deployments to help establish a solid empirical baseline for key parameters related to CO₂ removal, including pH, salinity, total alkalinity, and other chemical constituents as needed. Both the interactions observed from laboratory experiments and surface water sampling during deployments will be incorporated into predictive modeling of the perturbation against baseline seawater carbonate chemistry to evaluate net CO₂ sequestration.
Over time and with scale, direct observation of the carbon chemistry perturbation and how it evolves in time may be possible, and in-situ imaging of alkaline mineral dissolution via instruments such as those shown above may be used to validate results in the open ocean.
Geochemical dynamics embedded within global circulation models will be used to quantify the amount of additional CO₂ transferred from the fast to the slow carbon cycle. Although these models are adapted and developed on an ad hoc basis today, best practices for modeling should be developed at an industry level.
Ongoing research related to OAE quantification includes studying the impact of surface-ocean carbon uptake or alkalinity addition on the air-sea gas exchange of CO₂. While this air-sea flux is an important and well studied area of carbon cycle research, it is primarily a fast cycle transfer of atmospheric CO₂ into surface waters as the dissolved CO₂ that was reallocated to the bicarbonate reservoir is replaced by novel CO₂ exchange across the air-sea boundary. The slow cycle transfer that is the primary activity of carbon removal occurs via the reallocation of fast cycle carbon to the larger and more stable bicarbonate reservoir. While the time horizon to complete the exchange of CO₂ between the atmosphere and ocean may vary, it will eventually equilibrate, completing the fast cycle transfer. The air-sea exchange time horizon for CO₂ relative to the time it takes for surface waters to be subducted to the deep ocean [Sabine et al., 2004] may be relevant to establishing total carbon removed via natural ocean mixing – i.e. if full equilibration has not yet occurred at the time of mixing to the deep ocean, there may be an impact on the fast-to-slow transfer of carbon from that natural pathway associated with project activities. Determining and refining how this is considered in OAE quantification remains an active area of discussion amongst practitioners and the research community as the OAE field matures.
Carbon removal by macroalgae will not be operational for initial deployments and will be introduced in subsequent deployments based on the successful recruitment and growth of macroalgae in the open ocean and continued advancements in modeling algal growth versus observed growth.
Most macroalgae species are coastal organisms adapted to thrive in environments with high nutrient concentrations and low wave energy relative to the open ocean. Research on open ocean cultivation of macroalgae has focused on species that will sink intact through the water column, particularly Ulva lactuca and Saccharina latissima. Foundational work has been performed on macroalgal genomics and life cycle, including extraction of high molecular weight DNA from Saccharina latissima and various Ulva species. Isolation, cultivation, and banking of various developmental stages of these species have been performed, and this basic research, conducted in collaboration with Los Alamos National Laboratory, should lead to the first published long-read genomes for Ulva lactuca and Saccharina latissima. Surveys of natural variation at initial deployment locations in West Iceland have also been performed to collect genotype and phenotype data for a variety of macroalgae species in that region.
Alongside this foundational research, a series of coastal macroalgae growth experiments have been performed to establish technical capacity for macroalgae growth and image-based phenotyping alongside environmental time series data collection at the experimental growth site. Initial experiments included measurements of temperature, pH, salinity, light intensity at depth, photosynthetic active radiation (PAR) at depth, and chlorophyll-a content of the ambient water. Later experiments were expanded to include fluorescent dissolved organic matter (fDOM), ammonium, dissolved oxygen, nitrate, turbidity, and subsurface ocean currents. Ongoing coastal experiments afford the opportunity to explore methods of seeding carbon buoys with macroalgae, and observe and measure macroalgae growth rates in an ocean setting. The technology developed in this setting will be applied to the open ocean to support the quantification of macroalgae growth offshore.
Open ocean tests of integrated prototypes have also been conducted, placing macroalgae seed on instrumented buoys in the open ocean to observe open ocean macroalgae seed recruitment and test verification hardware. Critically, the recruitment and growth of macroalgae was successfully imaged in the open ocean.
The results of controlled laboratory experiments are being used to develop biological growth models for macroalgae carbon accumulation — based on inputs related to genetics, macroalgae development stage, and environmental parameters — to predict growth (and thus carbon sequestered) along an ocean deployment trajectory. Laboratory experiments replicate open ocean light, temperature, and nutrient conditions to enable robust models and predictions. Models developed are in part based on existing macroalgae growth models [Broch and Slagstad, 2012]; however, these pre-established models are not specific to macroalgae grown in the open ocean, and as such require significant modification, and must be built and refined against open ocean conditions via quantification outputs from both controlled and test deployments.
Models are trained with imaging data, where predictive quantification of biomass growth is compared with observed growth in the open ocean validated by in-situ imaging, and discrepancies are used to refine model outputs. Machine vision enables the generation of texturized, three-dimensional models from a set of two-dimensional buoy and macroalgae images, providing additional data on macroalgae biomass accumulation and carbon content.
Following initial deployments, it is expected that the ratio of macroalgae to woody biomass will be gradually increased, with the long-term (decadal) goal of achieving a 5:1 ratio of macroalgae mass to carbon buoy mass on a carbon basis. Yields of 50:1 have been observed in fixed research locations with low wave energy, high-nutrient conditions.
With that context, CO₂eTerrestrial, CO₂eOAE, CO₂eMacroalgae, and CO₂eEmissions can be broken down into their component parts.
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