Construction waste for climate protection in the ocean?
New mesocosm experiment on Gran Canaria using ground concrete demolition waste
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Diver Isabell Hentschel is cleaning the outside of the mesocosms to prevent shadowing caused by biofouling.
Photo: Micha Sswat, GEOMAR
view moreCredit: Photo: Micha Sswat, GEOMAR
Over the next seven weeks, the harbour of the small community of Taliarte on the east coast of Gran Canaria will become an open-air laboratory. In twelve mesocosms – giant floating test tubes closed off from the surrounding seawater – an international research team led by the GEOMAR Helmholtz Centre for Ocean Research Kiel is testing for the first time whether concrete rubble is suitable for increasing the alkalinity of seawater. Ocean Alkalinity Enhancement (OAE) mimics the process of natural rock weathering and can increase the ocean’s ability to absorb CO2 from the atmosphere.
“This year’s experiment is about comparing a liquid source of alkalinity with ground concrete rubble and assess how well both substances are tolerated by the marine environment,” explains Emeritus Prof. Dr Ulf Riebesell, a marine biologist at GEOMAR and co-lead of the experiment. The experiment forms part of the international research project OceanAlkAlign, which aims to standardise measurement and assessment methods for OAE, thereby creating a robust basis for future decisions.
An open-air laboratory at PLOCAN
It is no coincidence that the experiment is taking place in Taliarte: the Canary Islands Marine Research Institute PLOCAN (Plataforma Oceánica de Canarias), a long-standing research partner of GEOMAR, is located right by the harbour there. It is from here that the mesocosms are deployed, filled and monitored throughout the entire duration of the experiment.
Why we need CO2 removal
The idea of adding extra alkalinity to the sea forms part of a wider context. Since the start of industrialisation, the CO2 content of the atmosphere has risen sharply; a significant proportion of the gas dissolves in the ocean, altering its chemistry. The result is progressive acidification, which can put particular pressure on organisms that form calcareous structures – such as mussels or corals. At the same time, the additional CO2 is driving global warming.
According to many current scenarios, emission reductions alone will not be sufficient to achieve the targets of the Paris Agreement. Consequently, methods for active CO2 removal (Carbon Dioxide Removal, CDR) are coming to the fore. OAE is regarded as an option with high potential, provided it can be implemented effectively and in an environmentally responsible manner.
Increasing the buffer capacity of seawater
Ocean alkalisation involves increasing the buffering capacity of seawater. Put simply, this reduces seawater pH which enables the ocean to absorb additional CO2 and store it in more stable dissolved forms. This is chemically straightforward; however, it remains unclear how marine ecosystems react to different sources of alkalinity, concentrations and forms of input, and where the tolerance limits lie. This is precisely where the mesocosm experiments come in: like giant test tubes, they replicate a section of the ecosystem, including planktonic food webs, microorganisms and biogeochemical processes, thereby enabling controlled comparisons.
Concrete rubble: waste with potential – and with questions
Concrete rubble is one of the largest waste streams worldwide: an estimated five billion tonnes are generated each year, and only a fraction of this has been reused to date. Because concrete contains cement, which has alkaline properties, finely ground material could, in principle, serve as a source of alkalinity. Model estimates are therefore exploring whether large quantities of CO2 could be sequestered in this way in the long term.
However, a material that appears unproblematic on land can have different effects in the sea: particles can increase turbidity or harm microorganisms, which in turn could have an impact on food webs. “A waste product does not automatically become a sustainable solution simply because it is available,” says Associate Professor Dr Kai Schulz of Southern Cross University (Australia), co-leader of the experiment. “We need data showing under what conditions OAE could be ecologically acceptable and where the limits lie.”
How the experiment works
Natural plankton communities are being observed over several weeks in the twelve mesocosms. The team is using a comparative approach: some of the systems are being fed ground concrete rubble in increasing quantities, whilst other mesocosms are treated with liquid sodium hydroxide (NaOH) as a reference for ‘pure’ alkalinity; there are also control systems with no additions.
Measurements include changes in the water’s carbon dioxide system (including pH and alkalinity), CO2 uptake, and biological parameters: the composition and productivity of phytoplankton, zooplankton responses, microbial processes, and indications of shifts in the food web. This enables both the effectiveness and any potential side effects along the food chain to be assessed.
Objective: to define a ‘safe operating space’
The results from Taliarte are intended to help determine threshold values: which dosages alter the water chemistry in the desired way – and at what point do ecological effects become apparent? What differences are evident between solid particles and dissolved alkalinity? And how can findings from laboratory, mesocosm and field studies be combined in such a way that they serve as a sound scientific basis for decision-making? “Understanding before scaling up – that is the crux of the matter,” emphasises Schulz. “If OAE is ever to be discussed on a larger scale, it must be based solely on transparent data regarding benefits and risks.”
About: KOSMOS Mesocosms
Since 2006, GEOMAR has been using its self-developed “Kiel Off-Shore Mesocosms for Future Ocean Simulations” (KOSMOS) to investigate questions of ocean change under realistic conditions. In 23 experiments to date, the focus has included ocean acidification, warming, nutrient dynamics and potential countermeasures such as artificial upwelling or various OAE approaches.
Physics based AI unlocks first global predictions of carbon cycling in ocean sediments
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Underwater view beneath the ocean waves.
view moreCredit: Photo by Fernando Jorge
Researchers at The University of Manchester have developed a new physics‑based artificial intelligence approach that, for the first time, enables accurate global‑scale predictions of how dissolved organic carbon moves between seawater and marine sediments, a crucial but previously unquantifiable component of the planet’s carbon cycle. The work, led by Dr Peyman Babakhani from the Department of Civil Engineering and Management and carried out in collaboration with Dr Majid Sedighi, reveals how relatively simple AI algorithms can successfully emulate complex mechanistic environmental models that are normally too computationally demanding to run on a planetary scale.
Solving mechanistic models of natural environments is notoriously time‑consuming and often unstable under diverse real‑world conditions. To overcome this, the team trained AI “emulators” to reproduce the behaviour of an existing mechanistic model that describes carbon cycling in ocean sediments. Once trained, these emulators could then be applied globally to predict dissolved organic carbon behaviour at a resolution and scale that were not feasible using the original numerical model alone.
The study reveals that 11% of the particulate organic carbon arriving at the seafloor is returned to seawater as dissolved organic carbon, while 24% is sorbed onto minerals. Strikingly, about half of all solid‑phase organic carbon in the upper metre of marine sediments appears to originate from dissolved carbon that has been sorbed onto minerals. These findings provide the first global quantification of dissolved organic carbon cycling within sediments and highlight its significance within Earth’s long‑term carbon budget.
In developing the modelling framework, the researchers compared deep learning architectures, random forest models and simpler feedforward artificial neural networks. Unexpectedly, the simplest algorithms produced the most accurate predictions. The team confirmed these results by validating emulator outputs against low‑resolution global maps, where the mechanistic model remained numerically solvable, as well as against algebraic solutions for variables with known analytic expressions. They also found that increasing the complexity of the neural network structures consistently reduced prediction accuracy, offering rare empirical support for the Principle of Parsimony, also known as Occam’s Razor, within AI model development.
These insights have important implications for climate science. Quantifying carbon budgets across the sediment–water interface is essential for understanding global climate dynamics but has historically been hindered by computational limitations. By providing a fast, scalable and accurate way to represent sediment carbon processes, the new AI‑based framework can be integrated into global circulation models and used to explore potential ocean‑based climate change mitigation strategies. The research opens new avenues for simulating and testing how marine carbon reservoirs may respond to environmental change in the coming decades.
Dr Peyman Babakhani, Lecturer in Geoenvironmental Engineering said "The modelling framework developed in this study can play a substantial role in testing potential ocean‑based climate change mitigation scenarios in silico. With this approach, we can finally explore global‑scale carbon cycling processes that were previously impossible to quantify."
Journal
The Innovation
Article Title
Global cycling of dissolved organic carbon between seawater and sediments quantified using physics-based artificial intelligence
Article Publication Date
8-Sep-2026
