Coastal and estuarine carbon removal technique may backfire when pushed too far

Scientists investigating a proposed way to remove carbon dioxide from the atmosphere using seawater have found that adding too much alkalinity to neutralize acids can trigger chemical reactions that undermine the process.

The study, published in Frontiers in Marine Science, examined a form of marine carbon dioxide removal known as ocean alkalinity enhancement. The approach aims to increase the ocean's capacity to absorb carbon dioxide by adding alkaline substances that shift seawater chemistry and encourage more carbon dioxide to move from the atmosphere into the ocean.

Using dissolved calcium carbonate (the main mineral found in limestone and seashells), the researchers found that there are clear limits to how much alkalinity can be added before the chemistry becomes unstable. At high doses, calcium carbonate rapidly forms solid mineral particles, effectively undoing some of the intended carbon storage benefits. The findings help define practical boundaries for the technique and highlight the importance of tailoring it to local conditions.

Balancing ocean chemistry

The ocean already absorbs around a quarter of human-generated carbon dioxide emissions, making it one of Earth's most important natural carbon sinks. Ocean alkalinity enhancement seeks to increase this capacity by raising the alkalinity of seawater, allowing it to take up additional carbon dioxide from the atmosphere.

In the new study, Amanda Beatriz Melendez-Perez, of Georgia Southern University, and colleagues dissolved calcium carbonate in seawater that had been enriched with carbon dioxide. The resulting water contained higher levels of alkalinity, which in principle should help lock away more carbon in dissolved forms, such as bicarbonate ions.

However, there is a catch. If conditions become too favorable for minerals to form, dissolved calcium carbonate can begin to crystallize and precipitate back out of the water. This removes some of the added alkalinity before it has had time to draw down atmospheric carbon dioxide and, in extreme cases, could even release carbon dioxide back into the atmosphere.

Previous work has identified unwanted carbonate precipitation as one of the key challenges facing ocean alkalinity enhancement technologies.

Clear thresholds emerge

To investigate where these limits lie, the researchers tested three different alkalinity additions under a range of temperatures and carbon dioxide conditions. They also explored how mixing treated seawater with natural river water affected stability.

The lowest dose remained stable for more than a month, while the highest dose consistently caused calcium carbonate precipitation within a day. An intermediate level showed mixed behavior, with the timing of precipitation strongly influenced by temperature and the precise chemical conditions of the water.

The results revealed a threshold effect: Below certain levels, the enhanced seawater remained chemically stable, but above them the risk of precipitation increased sharply. Although the highest additions led to rapid mineral formation, the researchers did not observe a worst-case "runaway" scenario in which more alkalinity was lost than had originally been added.

The team also found that mixing treated seawater with natural estuarine water improved stability. Mixtures containing at least 60% estuarine water were less prone to precipitation, suggesting that local salinity and water chemistry could have a major influence on how and where the technique might be used.

Guidance for future projects

The findings do not rule out calcium carbonate-based ocean alkalinity enhancement, but they suggest that more is not necessarily better. Instead, there appears to be a chemical "sweet spot" in which additional alkalinity can be introduced without triggering counterproductive reactions.

The researchers also showed that a parameter known as the aragonite saturation state (a measure of how close seawater is to forming calcium carbonate minerals) can help predict when precipitation is likely to occur. However, they found that changes in calcium concentration also must be taken into account when calcium carbonate itself is used as the source of alkalinity.

As interest grows in technologies designed to remove carbon dioxide from the atmosphere, understanding their limitations is becoming increasingly important.

Marine-based carbon removal approaches have attracted growing attention because the ocean has naturally helped regulate Earth's climate over millions of years, storing carbon through a combination of physical, chemical and biological processes. These techniques aim to enhance those natural processes, but their effectiveness depends on maintaining the right chemical balance.

Rather than applying a one-size-fits-all approach, future projects may need to account for factors such as temperature, salinity and local water chemistry to maximize carbon storage while avoiding unintended consequences.

Further, the researchers caution that the chemical thresholds identified in the study should not be taken as indicators of ecological safety. In natural environments, factors including seasonal changes, river-borne sediments and biological activity can influence how seawater responds to added alkalinity.

Microbial communities and plankton also may affect whether calcium carbonate forms, meaning the impacts on local ecosystems will need to be carefully considered. Additional testing in real-world settings is essential to determine how the approach performs in the more complex and variable conditions found in the ocean.

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