Experiments Test the Side Effects of an Ocean Antacid

All the forests in the world pale in comparison to the ocean when it comes to absorbing carbon dioxide from the atmosphere. The ocean is the world’s largest natural carbon sink, absorbing about a quarter of all CO2 emissions every year (and most of the excess heat trapped in Earth’s atmosphere from greenhouse gases).

But there are two problems.

For one, there are natural limits to how much carbon the ocean can absorb and how quickly. Second, as CO2 dissolves in the ocean, it reacts with seawater. Those reactions release hydrogen ions, which make the water more acidic. Since the Industrial Revolution, the surface ocean has become 30% more acidic on average, to the point that it’s making life hard for organisms that build chalk-like shells and skeletons — including corals, coccolithophores, and young oysters.

Adding minerals that alter the chemistry of seawater, in a process called ocean alkalinity enhancement (OAE), is a proposed solution to address both of these challenges. As a “marine carbon dioxide removal” strategy, OAE would amplify natural processes in the ocean to increase its ability to absorb CO2. An added benefit would be countering acidification.

Again, there is a caveat. As a relatively new idea that hasn’t been tested widely at scale, there are still significant questions about how these chemical changes would impact marine life. That includes the plant-like phytoplankton that form the base of the marine food web and drive many of the ocean's most important biogeochemical processes.

Steve Archer, a senior research scientist at Bigelow Laboratory for Ocean Sciences, is working with a team of researchers from around the world to help fill those knowledge gaps. Over the last five years, they’ve run several large-scale experiments using an isolated system called a mesocosm that replicates natural conditions without influencing the ocean directly. These experiments are helping scientists understand the potential biological impacts and nuances of OAE, including its effect on plankton.

“There are already companies proposing to do this, but they’re going into it without sufficient information on what’s going to happen,” Archer said. “There are some important questions that remain unknown — that we’re trying to answer — to ensure we have informed, science-based decision-making when it comes to OAE and marine carbon dioxide removal.”

MIMICKING MOTHER NATURE

OAE works on a rather basic principle. The goal is to accelerate natural rock weathering by adding crushed minerals to seawater. Archer describes it as “an antacid for the ocean.”

“It’s just mimicking a natural process, which is how the pH of the ocean has been maintained over geologic times,” Archer explained. “What’s happening now is that atmospheric CO2 emissions are outpacing weathering, but if you throw in a bunch of crushed rock, for example, it’ll restore the alkalinity. It’s quite a tractable problem in theory.”

For example, in one of the group’s experiments in Kiel, Germany, they tested the differences — in terms of how fast the minerals dissolve and what impact they have — of quicklime and brucite. Both products are ingredients for cement and widely produced by existing industries.

One of the challenges, though, is the amount of minerals that experts expect it will take to make an appreciable, lasting impact (according to Archer, “it’s a heck of a lot of rock.”).

For ocean scientists, the bigger concern is the lack of knowledge of how a chemical change could influence biology as there have only been a handful of small, ocean-based tests on OAE so far. That’s why the mesocosm experiments Archer has been involved in with colleagues in Europe are so valuable.

CONTAINING THE OCEAN

Archer has been working with colleagues at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany, Universidad de Las Palmas de Gran Canaria, and University of Malaga in Spain. Since 2021, they’ve run five experiments: two in Gran Canaria, one in Bergen in Norway, and two in Germany (in Kiel, on the Baltic Sea, and in Heligoland, on the North Sea).

Each experiment has tested the addition of different quantities of different minerals, sometimes in different forms, to understand how OAE could be done. Though the sites are relatively close (compared to the ocean as a whole), they also each reflect a different marine system, ranging from the open subtropics to the cold waters of the North Atlantic to the semi-enclosed seas of northern Europe that are heavily diluted by inflowing rivers. For the work in Kiel in 2024, they were also able to run several smaller, seasonal experiments. Together, the data will provide insight on whether OAE may be more effective in certain parts of the ocean or at certain times of year.

The experiments represent the first major attempt to evaluate the impacts of OAE on plankton communities specifically under natural conditions.

The team has looked at everything from how changing alkalinity alters plankton physiology to whether changes in the phytoplankton community cascade to higher levels of the food web. Archer has also examined the mechanics of photosynthesis. He has a tool for measuring fluorescence that allows him to quantify how fast and effectively plankton capture photons of sunlight under different conditions and how that relates to the amount of CO2 they absorb through photosynthesis.

"We’re testing how effective some of these minerals might be and, more importantly, what impact and knock-on effects the change has on the ecosystem,” Archer said. “A big piece of the work is also about tracing the actual drawdown of CO2 to see if the excess CO2 that is absorbed as you increase alkalinity actually gets sequestered long term.”

To answer these questions, the team is relying on mesocosms, a fancy way of describing what is essentially a large bag (a potentially very large bag; the ones they’ve used for the Bergen experiment were over 60 feet long).

Each mesocosm isolates over 8,000 liters of water, including all the little organisms living there. The bag is placed in the ocean and left there for upwards of a month. None of the water — and more importantly, the added minerals — can get out, but it ensures the conditions inside match those of the surrounding water.

A RESILIENT OCEAN

“I’ve been consistently impressed by how robust these phytoplankton are.” That’s Archer’s main takeaway from preliminary results across the experiments so far.

As expected, the experiments suggest plankton respond differently based on the kinds and amount of minerals added. In some cases, the experiments show that the plankton community is negatively impacted but only at the highest levels of alkalinity, which Archer explains would be difficult to replicate in the real world.

The team has published two papers so far based on data from the Gran Canaria experiments. The first showed no concerning effect from the experiment on primary production or the makeup of the plankton community (in fact, the authors observed some evidence of a positive response since there’s more carbon available for organisms to feed on). Likewise, the second paper showed that OAE didn’t cause any noticeable stress at the cellular level or decline in fitness on the plankton they studied, at least over the experiments’ relatively short timescales.

“Not surprisingly, as you change pH, there’s initially a quick negative response, but it’s not as big as you might expect,” Archer said. “Like we often see, there’s a threshold. You get to the point, an alkalinity level, where the effect becomes deleterious, but, up to that point, organisms seem to adjust.”

Of course, as with any study, there are caveats. There’s also just a lot more data to analyze from the other experiments. And the team is actually returning this fall to Gran Canaria for yet another round of studies in which Archer hopes to see if there’s any differences in how the plankton respond when they’re differentiated by size. Each of these experiments, Archer says, answer some questions — and usually opens up a whole suite of new ones.

“As a scientist, knowing that you’re providing useful information for some big decisions that are going to have to be made, is interesting and exciting,” he said. “My take is that OAE might be something that can help our planet, but only if we do it at the right scale in the right place in the right way.”

Photo Captions

Photo 1: Kai Schulz of Southern Cross University adds rock flour to a mesocosm (Credit: Michael Sswat).

Photo 2: Senior Research Scientist Steve Archer in Gran Canaria in 2021 (Credit: Michael Sswat).

Photo 3: Mesocosm are set up at the pier in 2024 (Credit: Sarah Kaehlert).

Photo 4: Mesocosms are shown during an experimental setup in Gran Canaria in 2021 (Credit: Michael Sswat).

Photo 5: Leila Kittu of GEOMAR samples a mesocosm (Credit: Michael Sswat).