Efforts to boost the ocean’s carbon-storing capacity by making seawater more alkaline could wreak havoc on the marine food web, researchers have warned.
Ocean alkalinity enhancement is a proposed method of removing excess carbon dioxide from the atmosphere. It involves adding alkaline substances, such as basalt or calcium oxide, to ocean water to increase its pH. This helps to convert dissolved CO2 in seawater into carbonates and bicarbonates, unlocking capacity for the oceans to draw down further CO2 from the atmosphere.
The oceans already hold around 38,000 billion tonnes of carbon. In theory, ocean alkalinisation could remove many billions of tonnes more CO2 from the atmosphere, helping to limit the impacts of climate change.
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But releasing vast quantities of alkaline substances into the ocean could have catastrophic consequences for the marine food web.
at the Georgia Institute of Technology in Atlanta and his colleagues investigated how the release of alkaline minerals would affect the cycling of carbon in the ocean. Using computer models, they assessed the impact of alkaline mineral release on “marine snow”, the particles of organic matter that drift down from upper layers to the deep ocean.
“This marine snow is really central to the ecological and biological function of the ocean,” says Reinhard. “If we take minerals and we dump them into the ocean, they’re going to intersect with this background particle field in ways that may or may not be predictable, and may or may not be beneficial. No one has really explored that in any real detail.”
The study also modelled how effective alkaline minerals would be at boosting carbon absorption in the oceans.
The research yielded some worrying findings. Naturally occurring minerals such as basalt and olivine were found to be ineffective at boosting ocean carbon absorption, because they don’t dissolve well in ocean water. They may also seriously disrupt the composition of marine snow, reducing the availability of organic particles many marine species rely on for sustenance.
“If you have to search 30 times as hard to find the same amount of organic matter to fuel your metabolism, you are changing the energetics of the entire food web,” says Reinhard. “In some cases, you’re really wreaking havoc on the background particle field.”
This could have serious ramifications across the marine food web, even affecting the number of fish in the ocean, warns Reinhard. “Some of those impacts might be pretty profound,” he says.
Manufactured forms of alkaline minerals such as calcium oxide and magnesium oxide dissolve much more quickly in ocean waters than natural minerals like basalt and olivine and so are more effective for carbon removal and have less impact on ocean ecology, the study found. Synthetic minerals may therefore be a better option.
Companies around the world are kicking off trials of various forms of ocean-based carbon removal, including ocean alkalinity enhancement, as investors flock to the fast-developing market for carbon drawdown.
Canadian firm Planetary Technologies is preparing to release 200 to 300 tonnes of magnesium hydroxide in its mineral form, brucite, .
Such field trials should be monitoring not just their effectiveness at removing CO2, but also their impact on marine snow, says Reinhard. “If you’re putting a solid mineral phase into the ocean, you need to be monitoring how the particle engine is responding to this,” he says.
“It is unlikely that our addition of magnesium hydroxide in Cornwall will impact marine snow formation and additional accumulation on the sea floor, especially given the nature of our study – which is so small,” says William Burt at Planetary Technologies. “Regardless, scientific integrity, transparency and accountability are at the heart of what we do. That’s why we have an extensive monitoring programme for the seafloor and sedimentation, and if we identify any unusual precipitation, for any reason including marine snow, we will pause the trial and examine the situation before proceeding.”
at the National Oceanography Centre in Southampton, UK, says the study raises some interesting questions, but more research is needed to see if the findings apply in real-world tests. Shallow coastal waters may respond differently to alkalinisation compared with the open-ocean settings the study modelled, he says.
Environmental Research Letters
