Articles | Volume 23, issue 20
https://doi.org/10.5194/bg-23-7523-2026
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-23-7523-2026
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Research article
| 
26 Oct 2026
Research article |
| 26 Oct 2026
| 26 Oct 2026
Impacts of water advection and CO2 exchange on the carbon dioxide removal potential of ocean alkalinity enhancement
Charly Andre Moras
CORRESPONDING AUTHOR
Department of Earth System Sciences, University of Hamburg, Hamburg, Germany
Matias Saez Moreno
Department of Earth System Sciences, University of Hamburg, Hamburg, Germany
Peggy Bartsch
Department of Earth System Sciences, University of Hamburg, Hamburg, Germany
Jens Hartmann
Department of Earth System Sciences, University of Hamburg, Hamburg, Germany
Related authors
Abby Lunstrum, David Keller, Jens Hartmann, and Charly Andre Moras
EGUsphere, https://doi.org/10.70212/cdrxiv.2026517.v2, https://doi.org/10.70212/cdrxiv.2026517.v2, 2026
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Ocean alkalinity enhancement (OAE) is a proposed technology to remove CO2 from the atmosphere. The effectiveness of OAE is determined by several physical, biological, and geochemical processes, many of which are poorly constrained and not typically considered in recent assessments. To help align research and improve estimates of OAE efficiency, we provide a framework describing these processes, identify major knowledge gaps, and highlight research priorities.
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We tested three alkaline materials for removing carbon dioxide from seawater under different temperatures and salinities. Sodium bicarbonate worked reliably in all cases. Calcium hydroxide performed best in cold water but lost efficiency in warmer, saltier conditions because of carbonate formation. Magnesium hydroxide dissolved more slowly and was strongly hindered by high salinity. These results show that local ocean conditions strongly affect the performance of ocean alkalinity enhancement.
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Alkalinity leakage limits the efficiency of ocean alkalinity enhancement. Drivers of this process remain unquantified, restricting accurate assessments. The induced runaway process can be modeled using surface area and aragonite oversaturation as key factors. This study proposes a framework for improving predictability of alkalinity loss due to runaway precipitation, emphasizing the need for field experiments to validate theoretical models concerning dilution and particle sinking processes.
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Ocean Alkalinity Enhancement (OAE) is an approach to sequester atmospheric CO2 in the ocean and may alleviate ocean acidification. A large-scale mesocosm experiment in Norway tested calcium- and silicate-based OAE, increasing total alkalinity (TA) by 0–600 µmol kg-1 and measuring CO2 gas exchange. While TA remained stable, we found mineral-type and/or pCO2/pH effects on coccolithophorid calcification, net community production and zooplankton respiration, providing insights for future OAE trials.
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Recent studies described the precipitation of carbonates as a result of alkalinity enhancement in seawater, which could adversely affect the carbon sequestration potential of ocean alkalinity enhancement (OAE) approaches. By conducting experiments in natural seawater, this study observed uniform patterns during the triggered runaway carbonate precipitation, which allow the prediction of safe and efficient local application levels of OAE scenarios.
Charly A. Moras, Tyler Cyronak, Lennart T. Bach, Renaud Joannes-Boyau, and Kai G. Schulz
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We investigate the effects of mineral grain size and seawater salinity on magnesium hydroxide dissolution and calcium carbonate precipitation kinetics for ocean alkalinity enhancement. Salinity did not affect the dissolution, but calcium carbonate formed earlier at lower salinities due to the lower magnesium and dissolved organic carbon concentrations. Smaller grain sizes dissolved faster but calcium carbonate precipitated earlier, suggesting that medium grain sizes are optimal for kinetics.
Charly A. Moras, Lennart T. Bach, Tyler Cyronak, Renaud Joannes-Boyau, and Kai G. Schulz
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This research presents the first laboratory results of quick and hydrated lime dissolution in natural seawater. These two minerals are of great interest for ocean alkalinity enhancement, a strategy aiming to decrease atmospheric CO2 concentrations. Following the dissolution of these minerals, we identified several hurdles and presented ways to avoid them or completely negate them. Finally, we proceeded to various simulations in today’s oceans to implement the strategy at its highest potential.
Abby Lunstrum, David Keller, Jens Hartmann, and Charly Andre Moras
EGUsphere, https://doi.org/10.70212/cdrxiv.2026517.v2, https://doi.org/10.70212/cdrxiv.2026517.v2, 2026
Short summary
Short summary
Ocean alkalinity enhancement (OAE) is a proposed technology to remove CO2 from the atmosphere. The effectiveness of OAE is determined by several physical, biological, and geochemical processes, many of which are poorly constrained and not typically considered in recent assessments. To help align research and improve estimates of OAE efficiency, we provide a framework describing these processes, identify major knowledge gaps, and highlight research priorities.
Matias Saez Moreno, Jens Hartmann, Janine Börker, Peggy Bartsch, and Charly Andre Moras
EGUsphere, https://doi.org/10.5194/egusphere-2026-3277, https://doi.org/10.5194/egusphere-2026-3277, 2026
Short summary
Short summary
We tested three alkaline materials for removing carbon dioxide from seawater under different temperatures and salinities. Sodium bicarbonate worked reliably in all cases. Calcium hydroxide performed best in cold water but lost efficiency in warmer, saltier conditions because of carbonate formation. Magnesium hydroxide dissolved more slowly and was strongly hindered by high salinity. These results show that local ocean conditions strongly affect the performance of ocean alkalinity enhancement.
Niels Suitner, Jens Hartmann, Selene Varliero, Giulia Faucher, Philipp Suessle, and Charly A. Moras
Biogeosciences, 23, 3965–3980, https://doi.org/10.5194/bg-23-3965-2026, https://doi.org/10.5194/bg-23-3965-2026, 2026
Short summary
Short summary
Alkalinity leakage limits the efficiency of ocean alkalinity enhancement. Drivers of this process remain unquantified, restricting accurate assessments. The induced runaway process can be modeled using surface area and aragonite oversaturation as key factors. This study proposes a framework for improving predictability of alkalinity loss due to runaway precipitation, emphasizing the need for field experiments to validate theoretical models concerning dilution and particle sinking processes.
Arthur Vienne, Patrick Frings, Jet Rijnders, Lucilla Boito, Jens Hartmann, Harun Niron, Reinaldy Poetra, Miguel Portillo Estrada, Tom Reershemius, Laura Steinwidder, Tim Jesper Suhrhoff, and Sara Vicca
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Our study explores Enhanced Weathering (EW) using basalt rock dust to combat climate change. We treated maize-planted mesocosms with varying basalt amounts and monitored them for 101 d. Surprisingly, we found no significant realized inorganic CO2 removal. However, rock weathering was evident through increased exchangeable bases. While inorganic CO2 removal was not realized within this experiment, basalt amendment may enhance soil health and potentially long-term carbon sequestration.
Julieta Schneider, Ulf Riebesell, Charly André Moras, Laura Marín-Samper, Leila Richards Kittu, Joaquín Ortíz-Cortes, and Kai Georg Schulz
Biogeosciences, 23, 137–153, https://doi.org/10.5194/bg-23-137-2026, https://doi.org/10.5194/bg-23-137-2026, 2026
Short summary
Short summary
Ocean Alkalinity Enhancement (OAE) is an approach to sequester atmospheric CO2 in the ocean and may alleviate ocean acidification. A large-scale mesocosm experiment in Norway tested calcium- and silicate-based OAE, increasing total alkalinity (TA) by 0–600 µmol kg-1 and measuring CO2 gas exchange. While TA remained stable, we found mineral-type and/or pCO2/pH effects on coccolithophorid calcification, net community production and zooplankton respiration, providing insights for future OAE trials.
Jens S. Hammes, Jens Hartmann, Johannes A. C. Barth, Tobias Linke, Ingrid Smet, Mathilde Hagens, Philip A. E. Pogge von Strandmann, Tom Reershemius, Bruno Casimiro, Arthur Vienne, Anna A. Stoeckel, Ralf Steffens, and Dirk Paessler
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To test enhanced weathering's efficacy, we ran a two-year greenhouse study under warm, wet conditions, comparing several rock additives across farm soils. We tracked alkalinity and cation soil pools. Soil type was decisive: acidic, low-buffer soils exported more additional alkalinity, while alkaline or pH neutral soils retained it in cation pools. The results point to where enhanced weathering can deliver durable carbon removal and underscore the need for long, well-controlled trials.
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Ocean alkalinity enhancement (OAE) is being assessed for its potential to absorb atmospheric CO2 and store it for a long time. OAE still needs comprehensive assessment of its safety and effectiveness. We studied an idealised OAE application in a natural low-nutrient ecosystem over 1 month. Our results showed that biogeochemical functioning remained mostly stable but that the long-term capability for storing carbon may be limited at high alkalinity concentration.
Niels Suitner, Giulia Faucher, Carl Lim, Julieta Schneider, Charly A. Moras, Ulf Riebesell, and Jens Hartmann
Biogeosciences, 21, 4587–4604, https://doi.org/10.5194/bg-21-4587-2024, https://doi.org/10.5194/bg-21-4587-2024, 2024
Short summary
Short summary
Recent studies described the precipitation of carbonates as a result of alkalinity enhancement in seawater, which could adversely affect the carbon sequestration potential of ocean alkalinity enhancement (OAE) approaches. By conducting experiments in natural seawater, this study observed uniform patterns during the triggered runaway carbonate precipitation, which allow the prediction of safe and efficient local application levels of OAE scenarios.
Charly A. Moras, Tyler Cyronak, Lennart T. Bach, Renaud Joannes-Boyau, and Kai G. Schulz
Biogeosciences, 21, 3463–3475, https://doi.org/10.5194/bg-21-3463-2024, https://doi.org/10.5194/bg-21-3463-2024, 2024
Short summary
Short summary
We investigate the effects of mineral grain size and seawater salinity on magnesium hydroxide dissolution and calcium carbonate precipitation kinetics for ocean alkalinity enhancement. Salinity did not affect the dissolution, but calcium carbonate formed earlier at lower salinities due to the lower magnesium and dissolved organic carbon concentrations. Smaller grain sizes dissolved faster but calcium carbonate precipitated earlier, suggesting that medium grain sizes are optimal for kinetics.
Annika Nolte, Ezra Haaf, Benedikt Heudorfer, Steffen Bender, and Jens Hartmann
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Nele Lehmann, Hugues Lantuit, Michael Ernst Böttcher, Jens Hartmann, Antje Eulenburg, and Helmuth Thomas
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Riverine alkalinity in the silicate-dominated headwater catchment at subarctic Iskorasfjellet, northern Norway, was almost entirely derived from weathering of minor carbonate occurrences in the riparian zone. The uphill catchment appeared limited by insufficient contact time of weathering agents and weatherable material. Further, alkalinity increased with decreasing permafrost extent. Thus, with climate change, alkalinity generation is expected to increase in this permafrost-degrading landscape.
Mingyang Tian, Jens Hartmann, Gibran Romero-Mujalli, Thorben Amann, Lishan Ran, and Ji-Hyung Park
Biogeosciences Discuss., https://doi.org/10.5194/bg-2023-131, https://doi.org/10.5194/bg-2023-131, 2023
Manuscript not accepted for further review
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Effective water quality management in the Elbe River from 1984 to 2018 significantly reduced CO2 emissions, particularly after Germany's reunification. Key factors in the reduction include organic carbon removal and nutrient management, with nitrogen control being more critical than phosphorus for the restoration of ecosystem capacity. Unpredictable influxes of organic carbon and the relocation of emissions from wastewater treatment can cause uncertainties for CO2 removals.
Matteo Willeit, Tatiana Ilyina, Bo Liu, Christoph Heinze, Mahé Perrette, Malte Heinemann, Daniela Dalmonech, Victor Brovkin, Guy Munhoven, Janine Börker, Jens Hartmann, Gibran Romero-Mujalli, and Andrey Ganopolski
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In this paper we present the carbon cycle component of the newly developed fast Earth system model CLIMBER-X. The model can be run with interactive atmospheric CO2 to investigate the feedbacks between climate and the carbon cycle on temporal scales ranging from decades to > 100 000 years. CLIMBER-X is expected to be a useful tool for studying past climate–carbon cycle changes and for the investigation of the long-term future evolution of the Earth system.
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CO2 can be stored in the ocean via increasing alkalinity of ocean water. Alkalinity can be created via dissolution of alkaline materials, like limestone or soda. Presented research studies boundaries for increasing alkalinity in seawater. The best way to increase alkalinity was found using an equilibrated solution, for example as produced from reactors. Adding particles for dissolution into seawater on the other hand produces the risk of losing alkalinity and degassing of CO2 to the atmosphere.
Shuang Gao, Jörg Schwinger, Jerry Tjiputra, Ingo Bethke, Jens Hartmann, Emilio Mayorga, and Christoph Heinze
Biogeosciences, 20, 93–119, https://doi.org/10.5194/bg-20-93-2023, https://doi.org/10.5194/bg-20-93-2023, 2023
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We assess the impact of riverine nutrients and carbon (C) on projected marine primary production (PP) and C uptake using a fully coupled Earth system model. Riverine inputs alleviate nutrient limitation and thus lessen the projected PP decline by up to 0.7 Pg C yr−1 globally. The effect of increased riverine C may be larger than the effect of nutrient inputs in the future on the projected ocean C uptake, while in the historical period increased nutrient inputs are considered the largest driver.
Charly A. Moras, Lennart T. Bach, Tyler Cyronak, Renaud Joannes-Boyau, and Kai G. Schulz
Biogeosciences, 19, 3537–3557, https://doi.org/10.5194/bg-19-3537-2022, https://doi.org/10.5194/bg-19-3537-2022, 2022
Short summary
Short summary
This research presents the first laboratory results of quick and hydrated lime dissolution in natural seawater. These two minerals are of great interest for ocean alkalinity enhancement, a strategy aiming to decrease atmospheric CO2 concentrations. Following the dissolution of these minerals, we identified several hurdles and presented ways to avoid them or completely negate them. Finally, we proceeded to various simulations in today’s oceans to implement the strategy at its highest potential.
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Short summary
Ocean alkalinity enhancement (OAE) is a method of choice to capture carbon dioxide. This research investigates the role of advection and CO2 invasion on OAE efficiency for a period of 6 months. Overall, alkalinity remained mostly stable due to carbon dioxide invasion and the derived gas-transfer velocities corresponded to open ocean wind speeds of ~3 m s-1. It highlights that hydrodynamics strongly influences the carbon capture efficiency of ocean alkalinity enhancement.
Ocean alkalinity enhancement (OAE) is a method of choice to capture carbon dioxide. This...
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