📄 earth_science

Integrating laboratory-based secondary precipitation thresholds with ocean biogeochemistry models to advance ocean alkalinity enhancement decision-making

This study combines controlled microcosm experiments across temperature and salinity gradients with regional biogeochemical modeling to establish new, higher precipitation thresholds for Ocean Alkalinity Enhancement, demonstrating that integrating these experimentally derived limits is essential for accurately predicting safe and efficient carbon dioxide removal outcomes.

Original authors: Allison Savoie, Kyle Hinson, Ashley Bird, Mallory Ringham, Emma Scott, Jessica Cross, Chinmayee Subban, Alexa Labossiere, Remi Pages, Jilian Xiong, Zhaoqing Yang, Nicholas Ward

Published 2026-08-26

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Allison Savoie, Kyle Hinson, Ashley Bird, Mallory Ringham, Emma Scott, Jessica Cross, Chinmayee Subban, Alexa Labossiere, Remi Pages, Jilian Xiong, Zhaoqing Yang, Nicholas Ward

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The ocean is a vast, natural sponge that has absorbed much of the heat and carbon dioxide humanity has released into the atmosphere. As the planet warms, scientists are looking for ways to help the ocean do even more to pull carbon out of the air. One promising idea is to make the seawater itself more alkaline, or less acidic. By adding a small amount of a basic substance to the water, the chemistry shifts, allowing the ocean to absorb more carbon dioxide from the atmosphere and store it safely as dissolved bicarbonate. This process, known as ocean alkalinity enhancement, could become a powerful tool for fighting climate change, but it carries a hidden risk. If the water becomes too alkaline too quickly, the minerals dissolved in the seawater can suddenly crash out of the solution and turn into solid rocks. This unwanted solid formation would waste the added alkalinity and potentially harm marine life, turning a solution into a problem.

To understand exactly when and where this dangerous solid formation might happen, a team of researchers at the Pacific Northwest National Laboratory and other institutions set up a series of controlled experiments. They wanted to find the precise tipping points where adding alkalinity to seawater stops being a safe way to store carbon and starts causing minerals to precipitate, or fall out of the water as solids. They worked with seawater from Sequim Bay in Washington state, placing it into tanks and carefully adjusting the temperature and saltiness to mimic different coastal environments around the world. In these tanks, they introduced an alkaline solution generated by an electrochemical process, similar to what might be used in a real-world project, and watched closely to see what happened over the next two days.

The researchers discovered that the conditions for this solid formation are far more specific and demanding than previously thought. They found that the water needs to reach a very high level of alkalinity before the minerals start to crash out, and that the temperature of the water plays a critical role. In warmer water, the minerals formed much faster and at lower levels of alkalinity than in cold water. In fact, the team observed a runaway effect where the formation of solids accelerated rapidly, but only when the water was at least sixteen degrees Celsius and the alkalinity was pushed very high. In colder water, the system remained stable even when the alkalinity was increased significantly. This suggests that in many cold coastal regions, the risk of this unwanted solid formation is much lower than in warm tropical or summer waters.

The type of solid that forms also depends heavily on how salty the water is. In very fresh, low-salinity water, the first solid to appear was a magnesium-based mineral that looked like a white powder. However, this mineral was unstable and eventually dissolved back into the water. As time passed, it was replaced by a calcium-based mineral that formed into distinct, needle-like crystals. In saltier water, the process skipped the initial magnesium stage and went straight to forming the calcium crystals. The researchers noted that the presence of tiny particles floating in the water, which act as seeds for crystal growth, did not significantly change the outcome in most cases, though it did make a difference in the coldest water tested. These findings provide a clear map of the boundaries within which ocean alkalinity enhancement can operate safely, showing that the risk of losing the added alkalinity to solid formation is highly dependent on local temperature and salinity.

To see how these laboratory findings would play out in the real world, the team plugged their new data into a sophisticated computer model of the Chesapeake Bay, a large estuary that experiences a wide range of temperatures and salt levels throughout the year. They simulated a large-scale release of alkalinity in two different locations: a low-salinity harbor in the north and a saltier southern bay. The model showed that the impact of solid formation would vary dramatically depending on the season and location. In the warm summer months, particularly in the southern part of the bay, the model predicted that a significant portion of the added alkalinity would be lost to solid formation, reducing the amount of carbon the water could absorb. In the cold winter, or in the fresher northern harbor, the loss was much smaller. The simulations indicated that in the worst-case summer scenario, the efficiency of the carbon removal could drop by up to 30% due to these solids, whereas in other conditions, the loss was minimal.

These results offer a crucial guide for anyone planning to deploy ocean alkalinity enhancement in the future. The study suggests that the technology is not a one-size-fits-all solution; instead, its success depends entirely on carefully matching the method to the local environment. Deployments in warm, salty waters will require much more careful engineering to avoid triggering the rapid formation of solids, perhaps by diluting the alkaline water more quickly or choosing different release times. Conversely, colder or fresher waters appear to offer a safer window for operation. The researchers emphasize that while their experiments used a specific type of alkaline solution, the principles they uncovered regarding temperature and salinity thresholds are likely to apply broadly. By integrating these real-world limits into planning and modeling, scientists and engineers can design projects that maximize carbon removal while minimizing the risk of unintended chemical reactions, ensuring that the ocean remains a reliable partner in the fight against climate change.

Technical Summary: Integrating Laboratory-Based Secondary Precipitation Thresholds with Ocean Biogeochemistry Models

Problem Statement
Ocean Alkalinity Enhancement (OAE) is a marine carbon dioxide removal (mCDR) strategy that increases seawater alkalinity to enhance atmospheric CO₂ uptake. However, its efficacy is compromised by secondary abiotic precipitation, where added alkalinity reacts to form solid minerals (primarily calcium carbonate, CaCO₃, and magnesium hydroxide, Mg(OH)₂). This precipitation removes alkalinity from the solution, reducing the net carbon uptake and potentially causing environmental issues such as increased turbidity. Current regional ocean biogeochemistry models used to assess OAE often lack explicit parameterizations for these secondary precipitation dynamics, potentially leading to overestimations of carbon sequestration. There is a critical need to define operational thresholds for precipitation under varying coastal conditions (temperature and salinity) and to integrate these thresholds into predictive modeling frameworks to ensure accurate carbon accounting.

Methodology
The study employed a two-pronged approach combining controlled laboratory microcosm experiments with regional ocean modeling:

Laboratory Experiments:
- Setup: 48-hour microcosm experiments were conducted using natural seawater from Sequim Bay, WA. The study utilized an electrochemically generated NaOH solution (0.5 mol L⁻¹) as the alkalinity source, simulating electrochemical OAE pathways.
- Variables: Two experimental series were conducted to isolate environmental factors:
  - Temperature Variation: Salinity was held constant (~32.5) while temperature was stepped from 7.5°C to 29.5°C.
  - Salinity Variation: Temperature was held constant (~9.9°C) while salinity was adjusted from 6.5 to 32.5.
- Conditions: Each condition included "filtered" (0.45 µm) and "unfiltered" seawater to assess the impact of suspended particulates and biological activity. Alkalinity was added to raise Total Alkalinity (TA) by +1500, +2500, and +3500 µmol kg⁻¹.
- Measurements: Continuous monitoring of temperature, salinity, turbidity, and pH was performed. Discrete samples for TA and Dissolved Inorganic Carbon (DIC) were collected at specific intervals (t0, t1, t3, t24, t48). Solid precipitates were collected for X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Spectroscopy (EDS) to identify mineralogy and morphology.
Model Integration:
- Platform: A regional biogeochemical model of the Chesapeake Bay (ROMS-ECB) was used, chosen for its wide range of temperature and salinity variability compared to the experimental site.
- Parameterization: A logistic regression model was developed using the experimental data (and literature data from Ringham et al.) to define a temperature- and salinity-dependent aragonite saturation state threshold ( $\Omega_{thresh}$ ) for precipitation.
- Simulation: The model simulated a hypothetical, large-scale OAE deployment (surface flux of 4.05 mmol m⁻² s⁻¹) in January and June 2024. Two scenarios were compared: one with alkalinity addition only (OAE) and one including the secondary precipitation module (OAE + 2° Precipitation). The module triggered precipitation when local $\Omega_{ar}$ exceeded the derived threshold, removing carbonate from the dissolved pool and simulating particle sinking.

Key Results

Precipitation Thresholds: The experiments revealed that precipitation thresholds were substantially higher than previously reported in some literature. Significant precipitation generally required $\Omega_{ar}$ $Ω_{a r}$ values > 30–36 for high alkalinity additions (+3500 µmol kg⁻¹).
- Temperature Effect: Higher temperatures accelerated precipitation rates and lowered the $\Omega_{ar}$ threshold required to trigger it. "Runaway" precipitation (where TA and DIC losses exceeded initial additions) occurred only at temperatures ≥ 16°C under high alkalinity conditions.
- Salinity Effect: Lower salinities generally increased precipitation rates but shifted mineralogy. At very low salinities (S = 6.5 and 12.5), precipitation was initially dominated by brucite (Mg(OH)₂) rather than CaCO₃, and the total amount of precipitation was limited by the availability of Ca²⁺ ions. At S = 18.5 and above, aragonite was the dominant precipitate.
Mineralogy: Aragonite was the primary mineral formed across most temperature and moderate-to-high salinity conditions. Brucite formed initially at low salinities but was often replaced by aragonite over time (48 hours), with brucite crystals acting as nucleation sites for aragonite growth.
Model Outcomes: Integrating the precipitation module into the Chesapeake Bay model resulted in spatially and temporally variable reductions in added alkalinity and atmospheric CO₂ uptake.
- In January (cold, low salinity), TA losses ranged from ~18% (Baltimore Harbor) to ~116% (Southern Bay).
- In June (warm, low salinity), losses increased significantly, with TA decreasing by ~31% in Baltimore Harbor and ~438% in Southern Bay.
- Overall, secondary precipitation reduced net air-sea CO₂ uptake by approximately 3.5% to 30% over the one-month simulation period.

Significance and Claims
The paper claims to provide a framework for three specific advancements in OAE science:

Operational Bounds: It establishes conservative, experimentally derived secondary precipitation thresholds for NaOH-based OAE across coastal temperature and salinity gradients, offering guidance for engineering controls to limit precipitation.
Model Integration: It demonstrates the feasibility and necessity of integrating explicit precipitation parameterizations into regional biogeochemistry models. The study highlights that without these dynamics, models may significantly overestimate carbon removal efficiency, particularly in warm, low-salinity environments.
Future Guidance: The work underscores the need for further refinement of laboratory and field observations, specifically regarding induction times, the role of suspended nuclei, and the scaling of precipitation dynamics from point-source releases to regional models.

The authors emphasize that these thresholds represent operational limits for optimizing OAE delivery and minimizing alkalinity loss, rather than biological safety limits. They conclude that while the specific high-flux scenario used in the model was hypothetical and likely infeasible for current regulatory standards, the methodology provides a critical tool for assessing alkalinity loss as OAE projects scale up and move toward realistic deployment scenarios.

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