By Kirby Simon • Dr. Dariia Atamanchuk • Dr. Will Burt

Relying purely on emissions reductions is no longer sufficient to limit global warming to 1.5 or 2° C. To meet the annual global carbon dioxide (CO₂) emissions targets set by the Intergovernmental Panel on Climate Change, CO₂ removal strategies must be explored in parallel to reduce atmospheric concentrations of greenhouse gases. The ocean is a key player in many of these strategies as it naturally absorbs CO₂ and sequesters stable carbon in the deep ocean for a long time. Marine carbon dioxide removal (mCDR) has therefore emerged as a critical tool in the fight against climate change.

Mineral-based ocean alkalinity enhancement (OAE) is a promising mCDR technique that enhances natural oceanic processes where alkaline materials dissolve over time and react with CO₂ and water (H₂O) to form stable bicarbonate. The addition of alkaline materials, such as olivine or magnesium hydroxide, to seawater (“dosing”) promotes this natural reaction, and as more dissolved CO₂ is neutralized into stable carbon, more CO₂ from the air can dissolve into the ocean. This acceleration of a natural ocean carbon cycle has a co-benefit of mitigating ocean acidification associated with CO₂ uptake, with the added alkaline material increasing the buffer capacity of the seawater. These aspects of OAE, along with the favorable economic assessment, make it one of the more promising mCDR techniques, as evidenced by the rapid advancement in research and prevalence of field trials in recent years.

Through increased research efforts in the laboratory, in mesocosms, and in the field, significant progress has been made to demonstrate safety and viability for OAE. As research continues to transition toward more field trials and small-scale pilot projects, remaining knowledge gaps related to the safety and efficacy of these techniques become increasingly important to address.

Why Monitor Particles?

Key ongoing OAE research involves the fate of alkaline material added to the water, as well as potential environmental impacts associated with dosing. Particles play a critical role here; whether it is particles added directly to the water or interactions between the alkaline material and particles already in the water, particle properties govern many interactions that impact the efficacy and safety of OAE.

In the case of mineral-based OAE, the size, shape, and concentration of the minerals are important parameters that govern material dissolution rates and transport in situ. Interactions between particles, as well as processes at particle surfaces, can lead to particle aggregation and seabed deposition or secondary mineral precipitation, either of which can reduce the potential CO₂ uptake of an OAE intervention. Additionally, these potential secondary effects can result in the generation of new particles and aggregates that should be monitored for environmental impacts.

Complex ocean modeling coupled with laboratory and mesocosm experiments have been used to study these scenarios to predict and assess alkaline material fate. As these research efforts grow and OAE deployments move out of the lab to the field, it is increasingly important to validate the results of these studies with in-situ measurements and sampling in real-world conditions.

Collecting these real-world measurements is not trivial, however. On top of the costs and complications of typical field work, OAE research has added complexities related to environmental permitting, public perception, and other considerations that make it difficult for any one entity to take on. This limits the number of opportunities to test OAE in the field and the scope of such research endeavors. Open science questions, such as those related to in-situ particle dynamics, are therefore difficult to answer without broader cross-sector collaboration and novel mechanisms to support research investigations.

Planetary Technologies of Dartmouth, Canada, a company at the forefront of OAE research and deployment, and the ocean researchers at Dalhousie University in Halifax, Canada, have partnered to conduct several OAE field trials in Halifax Harbor, Nova Scotia. Since 2023, alkaline materials (fine-grained magnesium hydroxide and magnesium oxide) have been added to an existing permitted powerplant outflow by Planetary, with the intervention heavily monitored by sensors and through discrete sampling at the dosing site and throughout the harbor.

As research progressed and alkalinity dosing ramped up, the desire to expand this collaboration grew. In the summer of 2024, the Carbon to Sea Initiative and the Centre for Ocean Ventures & Entrepreneurship, or COVE, in Dartmouth, Canada, announced a Joint Learning Opportunity (JLO) that invited new teams to participate in the field trial. The goal of the JLO was to provide access to the field trial and resources for deploying new technologies, engaging with local communities, and exploring complementary research to maximize the trial outcomes. Given both its importance and complexity, in-situ particle dynamics was listed as a JLO strategic research priority.

Sequoia Scientific Inc. of Bellevue, Washington, a leading manufacturer of submersible optical sensors for in-situ particle size and optical property analysis, saw the JLO as a perfect opportunity to use its sensors to contribute to fundamental particle research in OAE. Sequoia was selected as one of the four JLO project leads, and from the fall of 2024 through the spring of 2025 Sequoia worked closely with Planetary and Dalhousie-based researchers from the Ocean Alk-Align project (https://alkalign.ocean.dal.ca) to use the company’s sensors for laboratory experiments and in-situ monitoring to study alkalinity feedstock dissolution, transport, and accumulation.

 

 

Optical Sensors for Monitoring

Sequoia’s LISST (laser in-situ scattering and transmissometery) instruments were uniquely suited to support particle investigations in the OAE field trial. The LISST-Portable|XR, LISST-200X, and LISST-RTSSV (real-time size and settling velocity) were used in the laboratory to characterize Planetary’s alkaline material feedstock. These laboratory measurements were critical to interpreting field measurements with the contributed sensors: for example, measuring the particle size distribution (PSD) of the alkaline feedstock provided a particle “fingerprint” to look for in the in-situ data to distinguish between signatures of the alkaline material and other particles (e.g., sediment, plankton) or bubbles in the water that similarly scattered light. Experiments using a recirculating sample chamber could also be used to validate modeled alkalinity dissolution rates or study secondary precipitation mechanisms by measuring changes in the PSD (shape and magnitude) over time as particles recirculate and dissolve.

For in-situ measurements, Sequoia worked with Planetary to mount its LISST-Tau and LISST-OST (optical sediment trap) to a mooring at the dosing site in the mixing zone for near-continuous measurements of optical transmission in the water. These sensors were deployed from September 2024 to January 2025, performing measurements every 5 min., after which they were exchanged for a LISST-200X particle size analyzer through the end of the JLO in March. The LISST-200X measured mean particle size, PSD and concentration every 15 min. The high-frequency measurements from these sensors supported investigations into how quickly the particle environment changed when dosing was active versus paused, as well as how the alkalinity diluted and dissolved after addition to the turbulent water.

Additionally, a LISST-200X was deployed by Dalhousie researchers while performing approximately biweekly boat surveys for environmental monitoring. They hand-profiled the sensor at discrete locations in and around the mixing zone, as well as at several fixed sites throughout Halifax Harbor, to get depth-correlated particle measurements. These deployments aimed to study and constrain the spatiotemporal dynamics of alkalinity fate and transport.

Data Insights

Analysis of the data sets is ongoing; however, preliminary interpretations have provided meaningful insights into the in-situ particle environment. For example, when dosing was active, the optical transmission measured by the LISST-Tau and LISST-OST was lower in magnitude with a higher variance than times when dosing was inactive. When alkalinity dosing was paused, the measured transmission generally increased back to the approximate pre-dosing value quickly (on the order of tens of minutes). The trend implies a similarly short residence time of the alkalinity in the mixing zone, although this time is influenced by alkalinity dissolution, transport mechanisms (e.g., sinking or currents), and other environmental variables, making it difficult to quantify without additional sensor measurements and physical sampling to deconvolve these effects.

 

 

Measurements from the mooring-mounted LISST-200X indicated a similar trend with dosing state. Median PSDs binned over a >40-hr. period of active dosing showed an elevated concentration of particles in the alkalinity “fingerprint” region compared to measurements from a similar period when dosing was paused. Importantly, these measurements demonstrated that the dosed alkalinity could be detected in situ in realistic dosing scenarios, which is a critical step toward directly monitoring alkalinity and observing its fate (e.g., dissolution, sinking, aggregation) in the near-field through optical measurements.

In contrast, there did not appear to be clear signatures of the alkaline material in the PSDs measured while profiling across Halifax Harbor further away from the dosing site. This may suggest rapid dissolution of the alkaline feedstock; more analysis is needed (e.g., correlation with other sensor data and physical samples) to support this theory. Across time, space, and depth, natural variability of the in-situ measured PSDs made it difficult to discern and assign any signatures to the alkaline material independent of whether dosing was active or paused. This result emphasized the importance of establishing an environmental baseline for each location such that natural versus alkalinity-induced perturbations can be identified in the measurements.

Outcomes and Roadmap for Research

The use of Sequoia’s optical sensors in the Halifax Harbor field trial demonstrates a critical step toward closing knowledge gaps surrounding particles in OAE research. The preliminary results highlight the importance of feedstock characterization for interpreting in-situ measurements as it relates to both detecting (e.g., identifying the alkalinity “fingerprint” in the PSD) and monitoring (e.g., measuring spatiotemporal changes in the PSD magnitude and shape) alkalinity in the environment. The particle measurements provide evidence that alkalinity can be detected in situ in real-world environments and dosing conditions, which is a critical first step to monitoring and parameterizing the dissolution and accumulation of the feedstock in the field to validate laboratory experiments and models.

With the collected data sets now published to the open-access Ocean Carbon and Acidification Data System (OCADS) hosted by NOAA, additional analysis and interpretation is possible by the broader scientific community. This data transparency is critical to maximizing the impact that these observations have on answering open questions in OAE research. As interest in OAE continues to accelerate and more work moves from the lab to the field, it is paramount to continue addressing knowledge gaps through collaborative, transparent, rigorous, and technologically advanced research investigations.

 

 

Acknowledgments

The authors thank the Carbon to Sea Initiative and COVE for providing funding for the collaborative deployment of Sequoia’s sensors under the JLO. Additionally, the authors thank the teams at Dalhousie University, particularly CERC.OCEAN laboratory at the Department of Oceanography, and Planetary Technologies for supporting sensor integration, deployment, and maintenance throughout the field trial.

References

For a full list of references, contact Kirby Simon at: kirby.simon@sequoiasci.com.

 

Kirby Simon is the science and technology lead at Sequoia Scientific Inc.

 

 

 

Dr. Dariia Atamanchuk is an oceanographer and ocean technology researcher at Dalhousie University, Canada.

 

 

 

Dr. Will Burt is the vice president of science and product at Planetary Tech.