OS3.4 | Ocean alkalinity enhancement: a promising strategy for CO2 removal?
EDI
Ocean alkalinity enhancement: a promising strategy for CO2 removal?
Co-organized by BG4
Convener: Giulia Faucher | Co-conveners: Jens Hartmann, Phil Renforth, Miriam SeifertECSECS
Orals
| Fri, 28 Apr, 14:00–15:45 (CEST)
 
Room L3
Posters on site
| Attendance Fri, 28 Apr, 10:45–12:30 (CEST)
 
Hall X5
Posters virtual
| Attendance Fri, 28 Apr, 10:45–12:30 (CEST)
 
vHall CR/OS
Orals |
Fri, 14:00
Fri, 10:45
Fri, 10:45
Keeping global warming below 2°C will require drastic reductions in emissions together with large-scale removal of CO2 from the atmosphere that must be initiated within this decade to remove hundreds of gigatons of CO2 from the atmosphere over the coming decades. Ocean alkalinity enhancement (OAE) is a promising method to actively remove CO2 from the atmosphere whereby well-known chemical reactions, accelerate the ocean uptake of additional CO2 from the atmosphere, imitating geologic weathering processes that have sequestered trillions of tons of atmospheric CO2 in the ocean over millennia.
Compared to other carbon dioxide removal technologies, OAE has noticeable advantages since it is applicable to large regions of the coastal and open ocean and helps to mitigate ocean acidification. However, the effect of introducing gigatons of alkalinity, and potentially silicate, and dissolved metals on marine pelagic ecosystems remains unknown and the direct measurement of CO2 drawdown at scale from OAE is still unclear.
In this session, we welcome research ranging from field and laboratories experiments, theory, comparison with natural analogues and numerical modelling addressing the potential application of OAE and that could shed light on some still open questions: i) which are the ecological risks or co-benefit of OAE (ii) how can desired and undesired effects be identified, monitored, and mitigated; iii) under what conditions does OAE most efficiently sequester atmospheric CO2?

Orals: Fri, 28 Apr | Room L3

Chairpersons: Giulia Faucher, Miriam Seifert, Phil Renforth
14:00–14:05
14:05–14:15
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EGU23-12904
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OS3.4
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ECS
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Highlight
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On-site presentation
Francesco Pietro Campo, Stefano Caserini, and Mario Grosso

The global emission pathways which allow keeping the global temperature increase “well below 2°C” as set by the Paris Agreement require a rapid and drastic emission reduction reaching net zero emissions by 2050. Furthermore, hundreds to thousands of gigatonnes of CO2 need to be removed from the atmosphere cumulatively by 2100 to limit global warming to 1.5°C, which means that a portfolio of different Carbon Dioxide Removal (CDR) terrestrial and marine processes should be developed and upscaled.

Among CDR solutions, Ocean Alkalinity Enhancement (OAE) is the unique one which also counteracts the ongoing ocean acidification. OAE consists of spreading an alkaline material on the sea surface, enhancing the ocean alkalinity and consequently the sea uptake of atmospheric CO2, which is then stored in the form of dissolved bicarbonate ions (HCO3-).

To produce carbon emission-free Ca(OH)2 and consequently increase the overall process efficiency in CO2 removal, when slaked lime (Ca(OH)2) is used as alkaline material for OAE, the CO2 released from the limestone calcination should be captured and stored. Since the most developed storage alternatives are underground geological formations whose main limitations are the long time required for qualifying their suitability (on the order of years), the geographical uneven distribution and the uncertain injection rate, an alternative storage technology called Buffered Accelerated Weathering of Lime (BAWL) is under study.

BAWL stores CO2 from calcination in the form of HCO3- in seawater using a pipeline where CO2 dissolves in seawater and reacts with Ca(OH)2, forming dissolved Ca(HCO3)2 mimicking natural weathering but in an accelerated way. The use of Ca(OH)2 allows to discharge an ionic solution with the seawater pH, to avoid CO2 degassing and to store permanently CO2 in the form of bicarbonates. Since the raw materials are CO2, Ca(OH)2 and seawater, BAWL fits well with OAE.

To assess the overall process efficiency in CO2 removal, i.e., the effective CO2 removal net of life-cycle greenhouse gas (GHG) emissions, the Life Cycle Assessment (LCA) methodology was applied to a process whose system boundaries encompass limestone extraction, other raw materials supply, Ca(OH)2 discharge in the sea and CO2 storage through BAWL. In addition to climate change, 15 additional impact categories are considered for the environmental assessment according to the Environmental Footprint method, and the ecoinvent database was used for supporting the life-cycle inventory.

Electricity is considered the energy source for calcination, that requires 83% of the total energy demand, and its production from renewables results as the most impacting phase in most of the impact categories. Thus, the variation of climate change impact was analysed by varying the electricity emission factor. With renewables, the process efficiency is at least 85%, i.e. less than 15% of removed CO2 compensates the life-cycle GHG emissions removing 1.4 molCO2/molCa(OH)2.

Due to the lack of an impact category for assessing ecotoxicity on marine environments, further research is required to include in the LCA methodology the assessment of the benefits from the alkalinity enhancement and the contrast to ocean acidification, as well as the potential risks on the pelagic ecosystem.

How to cite: Campo, F. P., Caserini, S., and Grosso, M.: Assessment of the potential life-cycle environmental impacts of ocean alkalinity enhancement: from limestone extraction to slaked lime discharge in the sea, including carbon dioxide storage, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12904, https://doi.org/10.5194/egusphere-egu23-12904, 2023.

14:15–14:25
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EGU23-14128
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OS3.4
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ECS
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On-site presentation
Astrid Hylén, Matthias Kreuzburg, Saïd De Wolf, Laurine Burdorf, Géraldine Fiers, Cedric Goossens, Benjamin Van Heurck, Hannelore Theetaert, Silke Verbrugge, Veerle Cnudde, André Cattrijsse, and Filip Meysman

Enhanced silicate weathering (ESW) in coastal environments is a promising method for ocean alkalinity enhancement. The idea behind ESW is to generate alkalinity by application of silicate minerals in coastal areas, where waves, currents and bioturbation can speed up the weathering rate. Due to its potentially large CO2 sequestration capacity and relatively high technological readiness, allowing rapid upscaling, coastal ESW currently receives substantial interest from researchers and policymakers. However, the vast majority of studies on ESW have been conducted in idealised laboratory conditions, while research on the method in natural environments is lacking. As a result, the CO2 sequestration efficiency and environmental risks when applying ESW in the field remain largely unknown.

Here we present results from the first and longest-running mesocosm experiment investigating ESW and associated CO2 uptake in coastal marine sediments. Using tanks containing one square meter of natural seafloor each, we have studied biogeochemical cycling in sediment treated with the fast-weathering silicate mineral olivine. Lugworms (Arenicola marina) were added to some tanks to investigate the effect of bioturbation on the olivine dissolution rate, as well as the impact of olivine addition on biota. In the mesocosms, we quantified the sedimentary release of alkalinity and other weathering end-products (trace metals and dissolved silicate). Five years into the experiment, olivine dissolution is obvious from an elevated sedimentary alkalinity release and decreased average olivine grain size. The elevated alkalinity release has further led to higher CO2 sequestrations in tanks with olivine. Based on the results from this unique mesocosm setup, we will discuss the large-scale effect of ESW on biogeochemical cycling in coastal ecosystems.

How to cite: Hylén, A., Kreuzburg, M., De Wolf, S., Burdorf, L., Fiers, G., Goossens, C., Van Heurck, B., Theetaert, H., Verbrugge, S., Cnudde, V., Cattrijsse, A., and Meysman, F.: Ocean alkalinity enhancement through enhanced silicate weathering in coastal areas: a long-term mesocosm study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14128, https://doi.org/10.5194/egusphere-egu23-14128, 2023.

14:25–14:35
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EGU23-7254
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OS3.4
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On-site presentation
Isabel Mendes, Julia Lübbers, Alexandra Cravo, Joachim Schönfeld, Cátia Correia, Patricia Grasse, A. Rita Carrasco, and Ana Gomes

Reducing atmospheric carbon dioxide (CO2) concentrations to combat global warming is one of the greatest challenges of humanity.

Marine alkalinity enhancement is a promising carbon dioxide removal measure with high potential to increase oceanic carbon uptake and storage. The natural processes of weathering on land sustains the alkalinity of the ocean and thereby removes CO2 from the atmosphere on geological time scales. The weathering can be enhanced by deploying fine-grained alkaline minerals to coastal areas, to directly supply more alkalinity to near-coastal waters. Nevertheless, the rate of CO2 consumption depends on the minerals used, grains size, temperature, pH, and salinity. During mineral dissolution, nutrients and trace elements are also released, which may affect marine biota. In order to evaluate the CO2 sequestration potential, ensuing biogeochemical and ecological impacts of alkalinity enhancement in intertidal environments, a novel in-situ experiment was installed in the Ria Formosa Coastal Lagoon, southern Portugal.

The Ria Formosa is a highly dynamic lagoon system, with daily renewal of water and nutrients through multiple tidal inlets. A succession of salt marshes with varying zonation and faunal communities fringes the lagoon. The experiment was installed in an undisturbed zone colonized with Spartina maritima, in September 2022. The experimental set-up includes three replicate treatments with coarse olivine, fine olivine, coarse basalt, fine basalt, and an untreated control. Lagoonal, supernatant, and porewater waters are sampled from each treatment every month and analysed for temperature, salinity, oxygen concentration, pH, total alkalinity, nutrients, and trace metals. Preliminary data show an increase in total alkalinity in the supernatant and porewaters shortly after minerals deployment, by 0.36 and 2.05 mmol kg-1 on average, relative to the control. Lower values of total alkalinity were recorded in December 2022, followed by markedly lower salinities after heavy rainfall in the study area. The experiment will run over two years and monthly sampled for water properties. For monitoring potential biodiversity changes, sediment samples are analysed for faunal and floral composition. Results of this novel field experiment will provide strategic knowledge on the benefits and risks of alkalinity enhancement in intertidal environments.

Acknowledgement. Research supported by the Portuguese Science Foundation, with the projects PTDC/CTA-CLI/1065/2021, UID/00350/2020CIMA and contracts DL57/2016/CP1361/CT0009, DL57/2016/CP1361/CT0002 and CEECINST/00146/2018/CP1493/CT0002.

How to cite: Mendes, I., Lübbers, J., Cravo, A., Schönfeld, J., Correia, C., Grasse, P., Carrasco, A. R., and Gomes, A.: Alkalinity enhancement in intertidal environments: preliminary results of a field experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7254, https://doi.org/10.5194/egusphere-egu23-7254, 2023.

14:35–14:45
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EGU23-330
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OS3.4
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ECS
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On-site presentation
Charly Moras, Lennart Bach, Tyler Cyronak, Renaud Joannes-Boyau, and Kai Schulz

Ocean Alkalinity Enhancement (OAE) is a carbon dioxide (CO2) removal technology with one of the largest potentials, which simultaneously decreases the pressure of ocean acidification. Most of the current understanding of OAE stems from numerical models. However, two recent studies have shown that secondary calcium carbonate (CaCO3) precipitation can occur at unexpectedly low saturation state, when using particle based OAE feedstocks. This is undesirable as it reduces OAE efficiency and can lead to “runaway CaCO3 precipitation”.

Both mineral dissolution kinetics and secondary CaCO3 precipitation are influenced by the physical and environmental properties of mineral feedstock and seawater. For example, the surface area of particles in suspension is an important factor for dissolution kinetics of minerals, as well as secondary CaCO3 precipitation kinetics. Furthermore, CaCO3 precipitation depends directly on the concentrations of calcium (Ca2+) and carbonate ions (CO32-) in seawater. The higher their concentrations, the more likely CaCO3 will precipitate. Since Ca2+ concentration have a positive correlation with salinity in the open ocean, variations in seawater salinity could be an important modifier.

Here, we present experimental data on the effects of grain size and salinity on the kinetics of brucite (magnesium hydroxide) dissolution and secondary CaCO3 precipitation. Preliminary results on the effect of grain size suggest that CaCO3 precipitation for medium sized particles (63-180 µm) is slower. At larger grain size, the slower dissolution rate, as of the smaller surface area, leads to more quickly measurable CaCO3 precipitation. At smaller grain size, it is the greater surface area that seems to increase the CaCO3 precipitation rate.

For salinity, first results suggest that dissolution rates increase towards lower salinities, while CaCO3 precipitates quicker. The former finding is most likely related to higher brucite dissolution at lower ambient magnesium concentrations, due to lower salinity. The quicker CaCO3 precipitation is also likely due to the lower magnesium concentration in lower salinity seawater. Magnesium ions are known to inhibit CaCO3 precipitation, hence CaCO3 precipitation is less likely inhibited at lower rather than higher salinities. Therefore, both feedstock grain size and seawater salinity are two key parameters for real-world OAE applications.

How to cite: Moras, C., Bach, L., Cyronak, T., Joannes-Boyau, R., and Schulz, K.: Effects of grain size and seawater salinity on brucite dissolution and secondary calcium carbonate precipitation kinetics: implications for Ocean Alkalinity Enhancement, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-330, https://doi.org/10.5194/egusphere-egu23-330, 2023.

14:45–14:55
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EGU23-13889
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OS3.4
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ECS
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On-site presentation
Niels Suitner, Giulia Faucher, Carl Lim, Ulf Riebesell, and Jens Hartmann

To ensure a safe and efficient application of Ocean Alkalinity Enhancement (OAE) it is crucial to investigate its impact on biogeochemical systems. While various theoretical studies have shown promising results, there has been a lack of practical research to test the applicability of this technology in natural environments. Recent studies by Moras et al. (2022) and Hartmann et al. (2022) described the effect of runaway precipitation in the context of OAE. During this process Ca-carbonate formation is triggered, leading to a loss of the initially added alkalinity and counteracting the whole idea of OAE.

At a field campaign at the Espeland Marine Biological Station (Bergen, Norway) we examined the characteristics of runaway precipitation by using local natural seawater and storing the reactor bottles in a flow-through incubation chamber, mimicking the real-time temperature and light conditions of the Raunefjord. Conducted lab experiments lasted between 20-25 days, and tested CO2-equilibrated and non-CO2-equilibrated addition of alkalinity. The temporal development of the carbonate chemistry parameters was monitored after alkalinity addition and the triggered Ca-carbonate precipitation process was described in detail. We found that above upper critical limits of alkalinity addition in natural seawater, immediate precipitation prohibited an enhancement to higher alkalinity levels.  Our results could be helpful to guide the definition of upper limits of alkalinity for the safe and efficient application of OAE in an open sea scenario. In addition, the precipitates were analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy, to characterize the formed particles and follow their growth patterns.

Hartmann, J., Suitner, N., Lim, C., Schneider, J., Marín-Samper, L., Arístegui, J., Renforth, P., Taucher, J., and Riebesell, U. (2022). Stability of alkalinity in Ocean Alkalinity Enhancement (OAE) approaches – consequences for durability of CO2 storage, Biogeosciences Discuss. [preprint], https://doi.org/10.5194/bg-2022-126

Moras, C. A., Bach, L. T., Cyronak, T., Joannes-Boyau, R., & Schulz, K. G. (2022). Ocean alkalinity enhancement–avoiding runaway CaCO 3 precipitation during quick and hydrated lime dissolution. Biogeosciences, 19(15), 3537-3557, https://doi.org/10.5194/bg-19-3537-2022

How to cite: Suitner, N., Faucher, G., Lim, C., Riebesell, U., and Hartmann, J.: The nature of runaway precipitation and consequences for the safe applicability of OAE, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13889, https://doi.org/10.5194/egusphere-egu23-13889, 2023.

14:55–15:05
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EGU23-10342
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OS3.4
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Highlight
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On-site presentation
Daniela Basso, Arianna Azzellino, Piero Macchi, Chiara Santinelli, Emilio Fernández, Pablo Serret, Giancarlo Bachi, Giovanni Checcucci, Alexandra Diaz, Eva Teira, Guido Raos, Silvia Valsecchi, Selene Varliero, Pietro Bazzicalupo, Karen Gariboldi, and Jose Gonzalez

The project OLCAPP was aimed at exploring the response of a natural, eutrophic system to ocean liming by slaked lime (calcium hydroxide) dispersal in the wake of ships. The main objectives were:

- to monitor and model the slaked lime dissolution kinetics and the carbonate equilibrium (alkalinity, pH spikes/alteration, Dissolved Inorganic Carbon);

- to assess Dissolved Organic Matter changes in quantity and composition;

- to assess possible changes in primary production, photosynthetic efficiency and phytoplankton abundance and associations;

- to assess the short and medium-term response of planktonic and benthic calcareous primary producers (calcareous red algae = maerl) to alkalinization and potential precipitation of carbonate crystals induced by the treatments.

In the framework of the Transnational Access provided by AQUACOSM-plus, at the ECIMAT-UVIGO facility (Vigo, Spain) we had the opportunity to test ocean liming in nine mesocosm tanks. A sediment trap and a basket of calcareous algae (maerl) were positioned at the bottom of each tank. Mearl was previously collected in the Ria de Vigo at 7m depth and prepared for the experiment. Successively, each tank was filled with natural coastal seawater (~1m3).

Out of the nine mesocosms, three tanks were treated with calcium hydroxide 0.02g/L (High) and three tanks with 0.006g/L (Low) per treatment, repeated on days 1, 3 and 5 (multiple exposure). The remaining three tanks were kept as control. A record of pH, O2, salinity, temperature and PAR was performed in the mesocosms during the experiment with a ten-minutes frequency.

Nutrient concentration is monitored on a long-term basis in the Ria de Vigo, and was also tested on days 1, 3 and 5 for all treatments and controls. Seawater samples were collected from the mesocosms before (pre-treatment), and after 1h, 4h, and 24h from each treatment, with a 5L Niskin bottle. Dissolved Organic Matter as Dissolved Organic Carbon (DOC) and Chromophoric Dissolved Organic Matter (CDOM) were analysed, along with the Dissolved Inorganic Carbon (DIC). Moreover, samples collected after 1h from treatments were used for assessing also the bacterial association, the size-fractionated Chl-a concentration and the plankton primary production and photosynthetic efficiency. Gross primary production, community respiration and net community production were measured by changes in oxygen concentrations after 24 h light-dark bottle incubations. Dissolved oxygen was measured by Winkler titration. A total of 165 samples were obtained from filtering 2L of mesocosm water from pre-treatments, 4h and 24h samples, for the collection of the phytoplankton community, to be analysed under optical and scanning electron microscope.

Preliminary observations during the experiment and the first data on the plankton community suggest that the High treatment leads to important flocculation and sedimentation affecting both the transparency of the water and the bottom environment, with significant and stable pH increase and decrease in phytoplankton production and efficiency. The mineralogical nature of the flocculation, the response of benthic calcareous algae and phytoplankton community, in term of composition and abundance of the major components, is here discussed.

How to cite: Basso, D., Azzellino, A., Macchi, P., Santinelli, C., Fernández, E., Serret, P., Bachi, G., Checcucci, G., Diaz, A., Teira, E., Raos, G., Valsecchi, S., Varliero, S., Bazzicalupo, P., Gariboldi, K., and Gonzalez, J.: Ocean liming in eutrophic water: a mesocosm scale approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10342, https://doi.org/10.5194/egusphere-egu23-10342, 2023.

15:05–15:15
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EGU23-9528
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OS3.4
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Highlight
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On-site presentation
Allanah Paul, Mathias Haunost, Silvan Goldenberg, Nicolas Sanchez Smith, and Ulf Riebesell

Ocean alkalinity enhancement (OAE) is one approach under investigation to remove carbon dioxide (CO2) from the atmosphere and durably sequester it in the ocean. Currently, little is known about possible biogeochemical or ecological changes that may result from this increase in seawater alkalinity from experimental data. To address this gap, we carried out a in situ mesocosm experiment to investigate how a plankton community in the subtropical eastern North Atlantic Ocean responds to enhancement of ocean alkalinity over a 33 day study period. A gradient of nine alkalinity treatments ranging from ambient (~2400 µmol kg-1) to 2x ambient (~4800 µmol kg-1) was implemented where the seawater CO2 was already equilibrated with atmospheric CO2. Here, we focus on changes in the biogeochemical element pools to determine if organic matter partitioning and nutrient cycling may be sensitive changes in seawater chemistry induced by deployment of OAE in an oligotrophic plankton community. Overall, only 3 out of 15 sampled biogeochemical pools displayed measurable changes. Nitrogen turnover processes in the surface ocean appear to be more susceptible than other elements to OAE as two of the impacts were on nitrogen-related pools and a significant phytoplankton bloom (3.5 – 5 µg L-1) occurred in selected mesocosms where alkalinity was enhanced despite nitrate-limited growth in primary producers. However, overall this study suggests that as long as no additional nutrients are added (silicate, nitrogen, trace metals) in the process of enhancing seawater alkalinity, and the carbon is already sequestered (equilibrated with atmospheric pCO2), the risk of negative impacts on primary producer biomass and biogeochemical functioning, appears to be low on time scales of up to 30 days, even with a doubling of seawater alkalinity. 

How to cite: Paul, A., Haunost, M., Goldenberg, S., Sanchez Smith, N., and Riebesell, U.: Testing the response of natural plankton community to ocean alkalinity enhancement in the subtropical North Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9528, https://doi.org/10.5194/egusphere-egu23-9528, 2023.

15:15–15:25
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EGU23-15436
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OS3.4
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ECS
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Highlight
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On-site presentation
Nicolás Sánchez, Silvan Urs Goldenberg, Daniel Brüggemann, Merlin Weichler, Scott Dorssers, and Ulf Riebesell

In light of the climate crisis and the necessity to meet the Paris Agreement goal of staying well below the 2°C temperature increase whilst respecting other UN sustainable development goals, an array of technologies to absorb and store atmospheric , named Carbon Dioxide Removal (CDR) technologies, are being proposed, developed and researched. One such technologies is Ocean Alkalinization, or Ocean Alkalinity Enhancement (OAE), which stems from the natural process of rock weathering. Several alkalinity sources and deployment options have been proposed, each associated with differing biological drivers. Among these, a carbonate-based, dissolved, equilibrated addition stands as the most optimistic deployment scenario, both from a carbon sequestration verification and an ecosystem impact stand point.

We decided to test this implementation in the first community-level mesocosm experiment to be done in the field. Pelagic mesocosms were deployed in Taliarte (Gran Canaria), enclosing 8 m³ of oligotrophic coastal waters with their associated natural plankton community. Nine OAE addition scenarios were simulated, increasing alkalinity (TA) in steps of 300 µeq/L from ambient levels up to its doubling, using a mixture of sodium carbonate and bicarbonate. Particular focus was placed on the impacts of enhanced TA on the ecosystem service (ES) of food production. This was addressed via zooplankton properties pertaining to ecosystem stability, food quantity and nutritional quality. Zooplankton diversity, functional composition, biomass, CN stoichiometry, population size structure, secondary production, trophic length, and a number of fatty acid nutritional indexes and trophic markers were monitored throughout the 33-day experiment.

Here, only 4 out of the over 30 different stability, food quantity and quality proxies were significantly affected by enhanced TA. Out of these, two were interpreted as negative impacts: a shorter-term halving in small copepod production, coinciding with a halving in copepod nauplii biomass, with a doubling in TA. These responses could be partly explained by the halving in large microplankton, an assumed preferred food source for copepods, detected right after treatment. However, none of these were sustained until the end of the experiment, thus suggesting no longer-term consequences. All in all, this study provides evidence for a low impact risk of enhanced TA on zooplankton, and ultimately the ES of food production. These findings set a promising stage towards the safe implementation of CO2-equilibrated OAE in oligotrophic coastal waters.

How to cite: Sánchez, N., Goldenberg, S. U., Brüggemann, D., Weichler, M., Dorssers, S., and Riebesell, U.: Ecosystem impacts of Ocean Alkalization in an oligotrophic marine plankton community: A mesocosm study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15436, https://doi.org/10.5194/egusphere-egu23-15436, 2023.

15:25–15:35
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EGU23-11876
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OS3.4
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On-site presentation
Tommi Bergman, Timothée Bourgeois, Jörg Schwinger, Spyros Foteinis, Phil Renforth, and Antti-Ilari Partanen

Negative emission technologies (NETs) are an integral part of most climate change mitigation scenarios limiting global warming to 1.5 °C above preindustrial levels. Several different NETs have been proposed, including ocean alkalinization that has been considered as one method with high carbon removal potential. To date, most studies on NETs with Earth System Models have been based on idealized scenarios where atmospheric carbon is either simply removed by prescribed amount or some NET is deployed at magnitudes that would be extremely challenging to reach if any economic, technical, or political constraints were considered. 

Here, we present a more realistic global deployment scenario for ocean alkalinization with Ca(OH)2 dispersed at ocean surface in the exclusive economic zones of US, EU, and China, based on their respective production capacities. The dispersion scenario is based on current excess capacities in the lime and cement industries, and high-end projections on how they could evolve until 2100. We use the high-overshoot SSP5-3.4-OS as the socioeconomic background scenario. We simulate the deployment scenario with two different Earth System Models: EC-Earth and NorESM2-LM. In addition to this sophisticated scenario, we carry out an idealized scenario with a uniform addition of 0.5 Gt Ca(OH)2 per year in the same coastal areas. 

The preliminary results show that the idealized 0.5 Gt Ca(OH)2 flux decreased the atmospheric CO2 concentration by 7 ppm in the first 15 years. The effects on ocean carbon uptake and surface ocean pH were strongly localized near the dispersion regions. The early version of the dispersion zone also included the Baltic Sea and the Mediterranean Sea, which led to significant increase in the alkalinity in these sea regions as the water exchange with the wider oceans are limited there. 

By providing a more realistic scenario for ocean alkalinization, we can give also more realistic assessment of climate effects and explore new research questions such as detectability of local changes in pH or carbon fluxes with slowly increasing deployment rates.  

How to cite: Bergman, T., Bourgeois, T., Schwinger, J., Foteinis, S., Renforth, P., and Partanen, A.-I.: Earth system impacts of a realistic ocean alkalinization deployment scenario, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11876, https://doi.org/10.5194/egusphere-egu23-11876, 2023.

15:35–15:45
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EGU23-9305
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OS3.4
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ECS
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Highlight
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On-site presentation
Feifei Liu, Ute Daewel, Jan Kossack, and Corinna Schrum

Increased ocean alkalinity reduces the activity of CO2 in seawater and prompts an enhanced flux of CO2 from the atmosphere into the ocean, thereby provides a promising means to reduce the atmospheric CO2 concentrations. However, due to the high complexity of physical and biological processes in coastal waters, the possible effects of coastal alkalinity enhancement (AE) are unclear yet. We thereby aim to set up a model framework to simulate the carbon cycles in the North Sea, which further allows scenario studies to disentangle the efficiency of various forms of coastal AE measures as well as their side effects and ecosystem impacts on the Northwest European Shelf (NWES) system. In two scenarios, the same quota of alkalinity is added into two designated areas, the European coast and the middle North Sea along with the ship tracks, respectively. The alkalinity is distributed continuously and evenly into the two areas. Our results indicate that the North Sea is quickly adjusted to both AE deployments, given that the AE efficiency shows no significant trend since the second year of these deployments. Seasonally, AE is more efficient in wintertime. The efficiency reaches the lowest level in spring, implying that the ocean uptake of the atmospheric CO2 is dominated by biological processes during this season. This modelling assessment will serve as a guide for coastal management and policy making that allows reconciling the application of Carbon Dioxide Removal (CDR) techniques with the maintenance of a good environmental status. It thereby offers an important yet unprecedented case study for a regional to local CDR deployment in the proximal coastal ocean of a temperate shelf sea.

How to cite: Liu, F., Daewel, U., Kossack, J., and Schrum, C.: Atmospheric CO2 removal by alkalinity enhancement in the North Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9305, https://doi.org/10.5194/egusphere-egu23-9305, 2023.

Posters on site: Fri, 28 Apr, 10:45–12:30 | Hall X5

Chairpersons: Miriam Seifert, Phil Renforth, Giulia Faucher
X5.344
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EGU23-16677
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OS3.4
Shannon Sterling, Edmund Halfyard, and Kristin Hart

Effective carbon dioxide removal (CDR) strategies are urgently needed to reduce risks of climate change. Here we propose a new strategy for Ocean Alkalinity Enhancement that targets the land-to-ocean component of the inorganic carbon cycle: river-based alkalinity and weathering enhancement (RAWE). RAWE adapts freshwater acidification mitigation technology to capture CO2 through mineral weathering and by increasing rivers’ capacity to retain and transport bicarbonate to long-term storage in the ocean. Field experiments in Nova Scotia rivers demonstrate the proof of concept, and global-scale modelling of RAWE indicates a potential millions of tonnes of CDR per year. Results suggest that RAWE meets CDR criteria, such as scalability, permanence, safety, and ability to simply quantify the CO2 removed, whilst delivering ecological co-benefits. 

How to cite: Sterling, S., Halfyard, E., and Hart, K.: Addition of Alkalinity to Rivers: a novel strategy for Ocean Alkalinity Enhancement, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16677, https://doi.org/10.5194/egusphere-egu23-16677, 2023.

X5.345
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EGU23-1259
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OS3.4
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ECS
Felipe S. Freitas, Sebastiaan van de Velde, Katharine R. Hendry, Filip Meysman, and Sandra Arndt

On geological timescales, continental silicate weathering plays a crucial role regulating Earth’s climate. Accelerating this slow thermostat might be the key to help mitigate present-day global warming and ocean acidification through increased alkalinity generation produced by enhanced marine silicate weathering. Laboratory studies show that benthic dissolution of olivine minerals can stimulate oceanic CO2 uptake by increasing seafloor alkalinity release. Although enhanced benthic silicate weathering is an attractive solution to both CO2 problems, until now its efficiency remains unclear. This is because the intrinsic dissolution rate of silicates in the seafloor remains poorly constrained, while also the impact of secondary reactions such as carbonate precipitation and reverse silicate weathering (authigenic clay formation) remains poorly quantified. Thus, we first need to develop a detailed understanding of natural benthic silicate dissolution and the feedbacks on carbon and silicon cycles.

Here, we couple two well-tested diagenetic model set-ups that resolve benthic carbon, redox and pH dynamics (organic matter degradation, re-oxidation of reduced species, equilibria reactions, carbonate dissolution and precipitation) and benthic silicon dynamics (biogenic silica dissolution, and authigenic silica precipitation) in the uppermost sediments. We use this new framework to resolve natural basalt and/or olivine weathering by explicitly accounting for the dissolution of key basalt constituents (basaltic glass, plagioclase, pyroxene, and olivine). We also account for reverse weathering through illite authigenic formation. The newly coupled model captures the observed shifts in porewater pH and carbonate system, and the dynamics of benthic alkalinity production and consumption associated with marine silicate weathering. We assess the impact of benthic redox state and basalt compositions on benthic alkalinity fluxes by performing an extensive sensitive study over the entire plausible parameter and bottom water forcing space. We find complex links between rates of benthic silicate weathering and net alkalinity production. Ultimately, we will apply the new model framework to explore the geological context of Iceland. As such, we will fully constrain natural rates of benthic silicate weathering associated to the substantial inputs of natural basalts to coastal and shelf sediments.

How to cite: Freitas, F. S., van de Velde, S., Hendry, K. R., Meysman, F., and Arndt, S.: Controls on benthic alkalinity fluxes from natural and enhanced silicate weathering in coastal and shelf sediments: new diagenetic model insights, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1259, https://doi.org/10.5194/egusphere-egu23-1259, 2023.

X5.346
|
EGU23-1494
|
OS3.4
Murugan Ramasamy and Nils Moosdorf

One strategy for lowering atmospheric CO2 levels is enhanced weathering, which involves dispersing rock powder to accelerate natural weathering. One obvious application area would be seawater. Because weathering is a chemical reaction, it is influenced by environmental properties like temperature and physical properties like the reaction area.

This study looks into the variability in sequestration rates from spreading of olivine at 13 distinct regional coasts around the world, including those with warm and temperate climates. Furthermore, sensitivity analysis was performed with various combinations of influencing parameters (grain size and seawater temperature) to determine the effects of individual parameter combinations. A 100-year simulation was conducted using geochemical thermodynamic equilibrium modeling (PHREEQC).

According to the simulations, over a 100-year period, CO2 uptake from atmosphere varies significantly between the seas, ranging from 0.13 (Black Sea) to 0.94 (Banda Sea) tonne (t) CO2 per t of olivine at a grain size of 100 μm. The difference between warm and temperate region’s atmospheric CO2 uptake is 0.4 t CO2 per tonne of olivine dissolve in seawater. A subsequent sensitivity study of parameter combinations reveals that the Black Sea can reach 0.8 t CO2 consumption rates per tonne of olivine if the material was ground to a grain size of 19 μm while the Banda Sea can reach the same amount of consumption rates at a grain size of 150 μm. The study results suggest that there are large differences of enhanced weathering speeds between different regions.

How to cite: Ramasamy, M. and Moosdorf, N.: Regional variations in the potential for CO2 removal through enhanced rock weathering in aquatic environments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1494, https://doi.org/10.5194/egusphere-egu23-1494, 2023.

X5.347
|
EGU23-2245
|
OS3.4
Claudia Hinrichs, Judith Hauck, Christoph Völker, and Peter Köhler

We evaluated the large-scale alkalinity distribution in 14 CMIP6 models against the observational data set GLODAPv2 and found that most models as well as the multi-model-mean underestimate alkalinity at the surface and in the upper ocean, while overestimating alkalinity in the deeper ocean. The dissection of the global mean alkalinity biases into their contribution from physical processes (preformed alkalinity), remineralization, and carbonate formation and dissolution showed that the bias stemming from the physical redistribution of alkalinity is dominant. However, below the upper few hundred meters the bias from carbonate dissolution can become similarly important as physical biases, while the contribution from remineralization processes is negligible. In light of ongoing and planned projects involving ocean alkalinity enhancements experiments using ESMs, a back-of-the-envelope calculation was conducted with each model’s global mean surface alkalinity and dissolved inorganic carbon (DIC) as input parameters. It was shown that the degree of compensation of DIC and alkalinity biases at the surface matters more than the alkalinity biases themselves for additional CO2 uptake capacity. The global mean surface alkalinity bias relative to GLODAP ranges from -3.6% to +2.1% with a mean of -1.1%, while for DIC the relative bias ranges from -2.6% to +2.5% with a mean value of -0.6%. Because of this partial compensation, all but two of the here evaluated CMIP6 models overestimate the Revelle factor at the surface and thus overestimate the CO2-draw-down after an alkalinity addition of 100 µmol kg-1  by up to 13% and the pH increase by up to 7.2%. This probable overestimate has to be taken into account when reporting on efficiencies of ocean alkalinity enhancement experiments using CMIP6 ESMs.

How to cite: Hinrichs, C., Hauck, J., Völker, C., and Köhler, P.: Alkalinity and Sensitivity to Alkalinity Enhancement in CMIP6 Earth System Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2245, https://doi.org/10.5194/egusphere-egu23-2245, 2023.

X5.348
|
EGU23-7154
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OS3.4
|
ECS
Tanvi Nagwekar, Cara Nissen, and Judith Hauck

Subduction regions play an important role in transferring carbon from the surface to the deep ocean and sequestering it on a multi-decadal to centennial timescale. Hence, we test the hypothesis that a Carbon Dioxide Removal (CDR) method, namely Ocean Alkalinity Enhancement (OAE) based on olivine addition, is more efficient in deep and bottom water formation region in terms of enhancing the ocean CO2 uptake.

Using an ocean-only setup of the physical-biogeochemical model FESOM2.1-REcoM3, we quantify the responses to the spatially uniform and continuous addition of olivine (alkalinity, silicic acid and iron) over the period of 2030-2100 under the SSP1-2.6 and SSP3-7.0 emission scenarios in a global (3 Pg olivine/yr) and a regional application (0.22 Pg olivine/yr). For the regional case, we deposit olivine in the major deep and bottom water formation areas of the Southern Ocean, in the Labrador Sea and the Norwegian Sea.

Under the SSP1-2.6 (SSP3-7.0) scenarios, CO2 uptake increases by 1.2 (1.3) Pg C/yr by the end of the 21st century in the global case, whereas it increases by 0.2 (0.2) Pg C/yr in the regional case. The area of uniform olivine deposition is significantly smaller in the regional case compared to the global case, yet the regional OAE has a 2.3-fold higher CDR potential compared to the global OAE in both emission scenarios. The high CDR potential in the regional case is largely (80%) attributed to enhanced biological activity resulting from nutrient fertilization in the Southern Ocean, while only 20% is due to enhanced alkalinity. However, the nutrient effect decreases over time. Furthermore, nutrient addition promotes small-phytoplankton calcification in global and regional OAE cases, leading to lower surface alkalinity by the end of the century. Interestingly, CDR potential of adding alkalinity only (without nutrients) is also 4% and 6.6% higher in the subduction regions than in global OAE under the SSP1-2.6 and SSP3-7.0 scenarios. Overall, the subduction regions, hence show higher CDR efficiency in both cases (alkalinity+nutrients, only alkalinity addition). This effect is two orders of magnitude larger when nutrients are included as this essentially includes Southern Ocean iron fertilization.

How to cite: Nagwekar, T., Nissen, C., and Hauck, J.: Effects of Ocean Alkalinity Enhancement in deep and bottom water formation regions on the 21st century CO2 uptake under low and high emission pathways., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7154, https://doi.org/10.5194/egusphere-egu23-7154, 2023.

X5.349
|
EGU23-8111
|
OS3.4
Limits and CO2 equilibration of near-coast alkalinity enhancement
(withdrawn)
Michael Tyka and Jing He
X5.350
|
EGU23-13428
|
OS3.4
Jörg Schwinger and Timothée Bourgeois

Ocean Alkalinization deliberately modifies the chemistry of the surface ocean to enhance the uptake of atmospheric CO2. Here, we quantify, using idealized and scenario Earth system model (ESM) simulations, changes in carbon cycle feedbacks and in the seasonal cycle of the surface ocean carbonate system due to ocean alkalinization. We find that both, the sensitivity to changes in atmospheric CO2 concentration (carbon-concentration feedback) as well as the sensitivity to temperature changes (carbon-climate feedback) are enhanced. While the temperature effect, which decreases ocean carbon uptake, remains small in our model, the carbon-concentration feedback enhances the uptake of carbon due to alkalinization by more than 20% compared to the carbon sequestration that alkalinity addition would facilitate at constant CO2 levels. This effect depends on the trajectory of atmospheric CO2 concentration, and leads to an increased loss of carbon from the ocean if net emissions become negative. The seasonal cycle of air-sea CO2 fluxes is enhanced due to an increased buffer capacity in an alkalinized ocean. The seasonal cycle of H+-ion concentration is also enhanced, although it remains smaller than under preindustrial conditions. This, together with an increased seasonal cycle of the aragonite saturation state in some regions, has the potential to adversely affect ecosystem health.

How to cite: Schwinger, J. and Bourgeois, T.: Ocean carbon cycle feedbacks and the seasonal cycle of the carbonate system under ocean alkalinization, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13428, https://doi.org/10.5194/egusphere-egu23-13428, 2023.

X5.351
|
EGU23-14592
|
OS3.4
Reiner Steinfeldt and Monika Rhein

The efficiency of open ocean alkalinization is estimated for four regions in the Atlantic: the subpolar North Atlantic, the northern subtropics, the equatorial region and the Southern Ocean. Therefore, a simple one-dimensional model is applied. First, observational based surface values of temperature, salinity, alkalinity, mixed layer depth (MLD), air-sea gas exchange velocity and pCO2 for each region are derived from the ARMOR3D data set (Guinehut et al., 2012) and the surface pCO2 data product by Landschützer et al. (2020). The model is run for 26 years, using the data from 1994 to 2019. Alkalinity is added to the mixed layer, which leads to enhanced oceanic carbon uptake, depending on the change in pCO2 and the gas transfer velocity. When the mixed layer shallows, parts of the added substances remain in the deeper layer of the model (below the mixed layer). They can either be exported, or can be entrained into the mixed layer again when it deepens. In this way, the efficiency of alkalinization (mole of absorbed CO2 per mole of added alkalinity) for the four regions and varying model parameters (alkalinity, pCO2, gas transfer velocity, mixed layer depth, fraction of exported carbon/alkalinity) is computed. In addition, the cases with permanent and monthly alkalinity supply (repeated every year) are investigated.

Guinehut, S., A.-L. Dhomps, G.Larnicol and P.-Y. Le Traon (2012). High resolution 3-d temperature and salinity fields derived from in-situ and satellite observations, Ocean Sci., 8, 845–857, https://doi.org/10.5194/os-8-845-2012.

Landschützer, P., N. Gruber and D.C.E. Bakker (2020). An observation-based global monthly gridded sea surface pCO2 and air-sea CO2 flux product from 1982 onward and its monthly climatology (NCEI Accession 0160558). Version 6.6. NOAA National Centers for Environmental Information. Dataset. [2021-27-09]

How to cite: Steinfeldt, R. and Rhein, M.: Estimating the efficiency of open ocean alkalinization, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14592, https://doi.org/10.5194/egusphere-egu23-14592, 2023.

X5.352
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EGU23-15984
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OS3.4
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ECS
|
Highlight
Neha Mehendale, Hao-wei Wey, Tronje Kemena, David Keller, and Andreas Oschlies

To assess the potential of coastal ocean alkalinity enhancement as a CO2 removal option for climate change mitigation, the Flexible Ocean and Climate Infrastructure (FOCI) earth system model was used to simulate alkalinity addition at the European coast open to the North Atlantic. FOCI has a global ocean resolution of 0.5° which can be regionally refined to 0.1° with a two-way nesting approach. The model was run in emission driven mode, starting with a linear ramp up from 2025 to 2035, after which alkalinity equivalent of 1Gt Ca(OH)2 per year was added in both resolution configurations from year 2035 to 2100 along the European coastline in a high and low emission scenario. To assess regional efficacies, the coast was divided into subsections. We illustrate the importance of adequate model resolution for simulating coastal alkalinity deployment, and show how each coastal region has a different CO2 uptake efficiency that is caused by differences in the regional environmental and hydrodynamic conditions.

How to cite: Mehendale, N., Wey, H., Kemena, T., Keller, D., and Oschlies, A.: Simulating Ocean Alkalinity Enhancement along the European Coast in an Earth System Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15984, https://doi.org/10.5194/egusphere-egu23-15984, 2023.

X5.353
|
EGU23-17368
|
OS3.4
Laura Bastianini, Kristina Peterson, and Phil Renforth

Removing large volumes of CO2 from the atmosphere, as well as rapid and deep emission reductions, may be required to meet the goals of the Paris Agreement. This has catalyzed recent attention on carbon dioxide removal (CDR) approaches that can remove more CO2 from the atmosphere than they emit. The oceans absorb approximately 25% of the CO2 that is emitted to the atmosphere, which causes acidification and adds to the stress experienced by some shell-forming organisms. A relatively inexpensive process for creating a hydrated calcium carbonate, ikaite, could be used to mimic the effect of natural carbonate weathering. This process uses high pressure CO2 (~15 bar) in an aqueous reactor to dissolve crushed limestone within minutes. The calcium rich water is passed to a low-pressure reactor (~0.1 bar) that evolves and recycles gaseous CO2 and forces the precipitation of ikaite over 30 – 80 minutes at temperatures <15°C. Experimental results suggest complete dissolution of ikaite can increase seawater alkalinity and thus potentially ameliorate the effects of ocean acidification. The focus of this study is on material characterisation and geochemical kinetics. In particular, we are using Raman spectroscopy coupled with X-Ray Diffraction for identification of the materials synthetically formed and we are exploring experimentally the precipitation and dissolution kinetics of ikaite and other hydrated carbonate minerals such as amorphous calcium carbonate (ACC). Preliminary results show that ikaite might be a precursor of ACC and synthetic can be stable for days at low temperature (sufficient time to be added to the ocean) and that it dissolves in seawater and stoichiometrically increases alkalinity.

            This technology could be scaled up to have a meaningful impact on climate change, and the costs could be comparable to other CO2 removal approaches. That is possible within the next 20 to 30 years, particularly as the raw materials are abundant.

How to cite: Bastianini, L., Peterson, K., and Renforth, P.: Novel method of ocean alkalinity enhancement using ikaite and other hydratedcarbonate minerals, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17368, https://doi.org/10.5194/egusphere-egu23-17368, 2023.

X5.354
|
EGU23-6292
|
OS3.4
|
ECS
|
Michael Fuhr, Andy W. Dale, Klaus Wallmann, Isabel Diercks, Mark Schmidt, Habeeb Thanveer Kalapurakkal, and Sonja Geilert

Abstract

 

The natural dissolution of mafic silicate rocks (e.g. dunite) and carbonate minerals in the marine environment increases alkalinity and draws down CO2. Consequently, large-scale manual dispersal of such minerals has been proposed as a potential measure to alleviate rising atmospheric CO2 levels through ocean alkalinity enhancement (OAE). This study investigates the effects of biogeochemical processes on alkaline mineral dissolution in surface sediments in a controlled experimental environment. Dunite and calcite were added to the surface of organic rich sediments from the Baltic Sea in order to simulate mineral dissolution and OAE under oxic conditions. Eight sediment cores were incubated with ~20 cm of overlying Baltic Sea bottom water over a period of 4 months; three replicates were treated with calcite, three with dunite, and two served as unamended controls.

First results indicate that the addition of the two materials directly increased benthic fluxes of alkalinity (from 1.3 to 2.5 µmol/cm2/d) and other respective weathering products such as calcium and silicate compared to the control experiments. These enhanced fluxes vanished into the strong natural benthic background after ~4 weeks. The main driver for enhanced and natural weathering is undersaturation with respect to the dissolving minerals which appears to be governed by microbial activity.

As the experiment progressed, porewater pH profiles in sediment cores where the sulfur oxidizing bacteria Beggiatoa spp. were visible shifted towards profiles that were more characteristic of sediments displaying cable bacteria activity. Very low pH values (~5.6) produced by presumably cable bacteria at ~1-3 cm depth in the sediment led to strong calcium carbonate dissolution. Additionally, their metabolism provides alkalinity to the bottom water by the formation of water directly from oxygen and protons, hence without addition of corresponding cations. This microbial activity produced high pH values in the upper millimeters of the sediments (~8.5) leading to Ωcalcite values >15 that might promote CaCO3 precipitation. Enhanced dunite weathering is indicated by slightly enhanced sedimentary Si fluxes, although this proved difficult to discern from the natural background flux arising from biogenic opal dissolution.

The overall natural complexity of the sediment chemistry combined with the alteration of the sediments during the incubation complicate a clear disentangling of natural and enhanced mineral weathering. Further investigation of these sedimentary systems along with field experiments will be necessary to provide estimates on the feasibility of benthic weathering as a realistic OAE and climate change mitigation measure.

How to cite: Fuhr, M., Dale, A. W., Wallmann, K., Diercks, I., Schmidt, M., Kalapurakkal, H. T., and Geilert, S.: Benthic dunite and calcite weathering as a method for ocean alkalinity enhancement, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6292, https://doi.org/10.5194/egusphere-egu23-6292, 2023.

X5.355
|
EGU23-7285
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OS3.4
Jørgen Bendtsen, Niels Daugbjerg, Kristina Vallentin Larsen, Rasmus Dyrberg Dahms, Katherine Richardson, and Minik Rosing

Glacial rock flour (GRF) is a felsic, silicate sediment that originates below the Greenland Ice Sheet, where the ice abrades basement rocks to a very fine powder. This is then transported by meltwater rivers or subglacial discharge into fjords and coastal waters. Thus, GRF is a naturally occurring component of the oceans around Greenland. The grain size of GRF typically ranges <1 - 100 µm with a median of 2-5 µm. The material behaves colloidally in water and distributions in fjords and coastal waters show that it has a residence time in the surface layer of up to several weeks. Glacial rock flour deposits are voluminous and common along the coast of Greenland and therefore it has the potential to be applied in geoengineering efforts on a global scale. The potential alkalinization from conservative cation release is estimated to be ~5,000 moles of alkalinity produced per ton of dissolved GRF. Additionally, GRF contains silica and phosphate that may contribute with macronutrients for phytoplankton growth together with various trace metals, e.g., iron and manganese. Hence, adding GRF to ocean surface waters has the potential to influence phytoplankton growth and, at the same time, increase alkalinity. However, the physical and chemical cycling of GRF in the water column, its implications for ecosystem services, and the chemical impact on the carbonate system are not well understood.

 

The first results from incubation experiments with GRF in the field and from controlled laboratory experiments are presented here. Incubation experiments of GRF added to seawater collected in the Canary Current system showed a significant increase in photosynthetic activity during short term (~1 week) incubations. The positive influence from GRF on phytoplankton biomass and photosynthetic activity is also found in incubation experiments with a monoculture of a green planktonic alga and shows that trace metals mobilized within a few weeks have a significant positive effect on phytoplankton growth. Laboratory experiments of the settling rate of GRF show that the residence time is relatively long but also that flocculation of GRF particles, caused by salinity increases, may be an important process to consider in future field studies. Our results show that GRF has significant potential for increasing alkalinity, and that trace metals are mobilized from GRF in seawater, which can stimulate photosynthesis. We propose that GRF has the potential to impact ecosystem structure and increase biological productivity when applied to the ocean.

How to cite: Bendtsen, J., Daugbjerg, N., Vallentin Larsen, K., Dyrberg Dahms, R., Richardson, K., and Rosing, M.: Impact from glacial rock flour on phytoplankton growth and the carbonate system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7285, https://doi.org/10.5194/egusphere-egu23-7285, 2023.

X5.356
|
EGU23-10281
|
OS3.4
|
ECS
Rasesh Pokharel, Guangnan Wu, Helen E. King, Peter Kraal, Gert-Jan Reichart, and Jasper Griffioen

Every year about 10-14 million m3 of sediments are dredged in the Port of Rotterdam (PoR) as part of harbor maintenance.  Approximately 10% of these sediments are stored in a confined disposal facility (CDF) Slufter due to high levels of toxic metals (cadmium, zinc, mercury, lead, etc.) and persistent organic pollutants, whereas over 90% of the sediments that are not contaminated are currently being relocated to the sea. Whilst the majority of these sediments have the potential to be used in nature-based environmental management projects, there are concerns that the oxidation of these sediments will release greenhouse gases and contaminants to the environment.

The idea of spreading ground olivine in terrestrial and coastal environments to capture CO2 is becoming increasingly popular due to the urgency to combat climate change. This technique (termed artificially enhanced olivine weathering, EOW) capitalizes on the natural process of olivine weathering that encourages gaseous CO2 to transform into dissolved bicarbonate ions (HCO3-).  In addition, the dissolution of olivine increases soil water pH and allows precipitation of secondary minerals (e.g. Fe oxyhydroxides) that can immobilize toxic metals through adsorption and co-precipitation mechanisms. As a result, EOW could be a promising geo-engineering solution for sediment management at PoR and reduce the negative environmental impacts associated with dredging. However, the specific controls on the drawdown of CO2 and toxic metal dynamics via silicate weathering are not well constrained.

Through laboratory experiments and field trials, we aim to investigate whether the addition of various commercial olivine-rich mineral mixtures (Greensand, Sibelco sand, etc.) can transform the dredged material from the PoR into a sustainable resource. Several bulk sediment and intact core samples, representing the majority of sediment supplied to the Slufter, were collected from the fluvial stretch of the PoR area. Laboratory batch experiments using artificial seawater were conducted for 90 days (at 1 bar and 12oC) with (1) only fine-grained (10 – 30 μm) Greensand containing ~62 weight-% forsteritic olivine, 2) only fluvial harbor sediment, and 3) mixtures of Greensand and fluvial harbor sediment. Our results show that olivine dissolution caused significant increases in alkalinity, dissolved inorganic carbon (DIC), and seawater-pH. Nickel concentrations in the aqueous phase remained below the environmental standards in most of the experiments and only slightly exceeded the standard value in experiments with the highest solid/liquid ratio. Furthermore, the mobilization of toxic metals like Zn and Mn from the harbor sediment to the solution was limited in the olivine-sediment mixed experiments, most likely due to adsorption with olivine or with precipitated byproducts of olivine dissolution. Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM/EDS) analysis of the reacted olivine samples shows the presence of Ca carbonates precipitation but no clear evidence of Mg carbonates or secondary Mg silicate phases (in contrast to results from thermodynamic calculations using PHREEQC). ­Overall, these preliminary laboratory findings indicate that EOW applications in PoR are likely to be viable from an environmental geochemical point-of-view, but further testing in long-term experiments and field trials planned in the project will provide a more accurate assessment.

How to cite: Pokharel, R., Wu, G., King, H. E., Kraal, P., Reichart, G.-J., and Griffioen, J.: Two Birds With One Stone: Artificially Enhanced Olivine Weathering for Sediment Management and CO2 Sequestration in the Port of Rotterdam, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10281, https://doi.org/10.5194/egusphere-egu23-10281, 2023.

X5.357
|
EGU23-11269
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OS3.4
|
ECS
|
Highlight
Jakob Rønning, James S. Campbell, Phil Renforth, and Carolin Löscher

Climate change continues to have escalating effects worldwide. Multiple solutions are needed, one of which is carbon dioxide removal (CDR), where CO2 is removed directly from the atmosphere.

One approach to CDR is ocean alkalinity enhancement (OAE), whereby finely ground alkaline minerals are added to the ocean, increasing pH and total alkalinity and enhancing the ocean’s ability to draw down CO2 from the air. This effect also helps counter ocean acidification, a phenomenon problematic to marine biodiversity and biogeochemistry.

Here a similar process is investigated but using rivers or fjords settings instead of the coasts. Rivers are proposed to be used as conveyors of finely crushed olivine (10-30 μm) mixed in river water. The goal is for the river to have higher alkalinity and pH before entering the chosen ocean region.
In this work, a closed mesoscale laboratory flume is used to study the feasibility of treating three different conditions, river, fjord, or saltwater, with finely crushed olivine, for alksalisation and CO2 absorption.

During the experiments, we examined weathering of olivine (details on physical and chemical composition and mineralogy needed) in flowing freshwater, brackish water, and saltwater with a flow rate of 1.25 -1.4 m3/s and the solids-to-liquid ratio of 0,00015 kg. Preliminary results indicate that freshwater is an optimal candidate as a conveyor. Furthermore, using rivers as one of the long-term solutions as an output of alkaline and pH-rich water to targeted regions is suitable.

How to cite: Rønning, J., Campbell, J. S., Renforth, P., and Löscher, C.: Enhanced Weathering of Olivine in Rivers for Carbon Dioxide Removal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11269, https://doi.org/10.5194/egusphere-egu23-11269, 2023.

X5.358
|
EGU23-12209
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OS3.4
|
ECS
|
Anna Groen, Leila Kittu, Joaquin Ortiz-Cortes, Kai Schulz, and Ulf Riebesell

Ocean Alkalinity Enhancement (OAE) is one of the most promising ocean-based negative emissions technologies (NETs) currently discussed. Dissolution of alkaline minerals such as olivine or quicklime in the surface seawater elevates total alkalinity (TA), thereby increasing the oceanic CO2 uptake capacity. Depending on the mineral used, increased TA may have different consequences for marine pelagic ecosystems and water column biogeochemistry that are still unknown, creating a need for empirical data. Our study reports on the particulate matter stoichiometry of a pelagic ecosystem in response to a gradient of elevated alkalinity levels and under different alkaline mineral applications. Ten offshore mesocosms were deployed in the mesotrophic waters of the Raunefjord off Bergen, Norway, from May – July 2022. Using NaOH, a delta alkalinity gradient was created (∆TA = 0, 150, 300, 450, and 600 µmol·L-1), resulting in two sets of five alkalinity levels each. To simulate the application of olivine (Mg2SiO4) vs. calcium-based minerals for OAE, corresponding amounts of MgCl2 and CaCl2 were each added to the respective treatments. Each mesocosm of the silicate-based OAE treatments additionally received 70 µmol·L-1 Si(OH)4, simulating the concomitant release of silicate under silicate-based mineral dissolution. We found significant differences in the production of biogenic silica between the two mineral simulations, indicating beneficial conditions for diatoms when silicate-based minerals are dissolved. However, the hypothesis of calcium-based mineral dissolution being favorable for calcifying organisms was not supported in our study. Neither the concentrations of particulate inorganic carbon (PIC) nor its ratio to particulate organic carbon (POC) was significantly different between TA treatments and mineral type. Additionally, increased TA had a negative effect on particulate organic nitrogen (PON) and phosphorus (POP) concentrations resulting in increased POC:PON and POC:POP ratios with higher alkalinity in both mineral simulations, yet more evident in the silicate-based treatments. Hence, the interaction of OAE and mineral effect on the particulate matter stoichiometry is possibly not induced by a single factor, yet by a variety of drivers, e.g. phytoplankton species specific physiology or food web interactions, such as grazing pressure. These results provide useful insights for better assessing ecological risks and co-benefits of OAE, like possible CO2 limitation of primary producers or ecosystem restructuring, thus will help to inform the practical implementation of large-scale OAE applications for carbon dioxide removal.

How to cite: Groen, A., Kittu, L., Ortiz-Cortes, J., Schulz, K., and Riebesell, U.: Assessing the response of particulate matter stoichiometry to ocean alkalinity enhancement, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12209, https://doi.org/10.5194/egusphere-egu23-12209, 2023.

Posters virtual: Fri, 28 Apr, 10:45–12:30 | vHall CR/OS

Chairpersons: Jens Hartmann, Phil Renforth
vCO.16
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EGU23-5878
|
OS3.4
|
ECS
Jennifer Weeks, Samar Khatiwala, Liam Bullock, and Aidong Yang

Global mitigation commitments which aim to limit global warming to less than 2ºC require dramatic and rapid reductions in atmospheric carbon dioxide (CO2) over the coming century. Carbon Dioxide Removal (CDR) technologies, whereby CO2 is actively taken out of the atmosphere and “durably” stored terrestrially, geologically or in the ocean could be employed to help reduce or counter-balance CO2 emissions to meet national net zero and net negative mitigation targets. Weathering processes would naturally draw atmospheric CO2 down towards pre-industrial levels over hundreds of thousands of years. One such CDR approach involves accelerating the uptake of CO2 through “enhanced weathering” (EW). CDR through EW of silicate minerals such as olivine or carbonate minerals can be achieved, for example, by spreading pulverized rocks on soils or employing mine tailings in specialized reactors to increase the weathering rate and hence carbon sequestration on decadal timescales. Here, we explore the efficiency of using mining waste to achieve CDR through EW. We exploit the results of a recent study by Bullock et al. (Science of the Total Environment, 2022) which produced one of the first comprehensive assessments of the global and country level suitability of mine tailings, accounting for reaction kinetics, and their potential for CO2 drawdown. While the overall CDR from EW of mine tailings is relatively modest, such an approach may still help individual countries meet their net zero goals and it is useful to investigate the broader implications of the deployment of this approach. EW leads to the production of alkalinity and bicarbonate ions (through CDR). We use Bullock et al.’s estimates of the annual generation of these quantities over the coming century to force an ocean biogeochemical model to investigate the impact of the release of these constituents into the ocean on atmospheric CO2 and ocean chemistry under various emission scenarios. In our simulations, alkalinity and dissolved inorganic carbon (DIC) are injected into the Exclusive Economic Zones of each suitable country or region as identified in Bullock et. al (2022). We examine in particular the competing effects of alkalinity (which increases CO2 solubility) and outgassing of CO2 (both due to the injection of DIC and reduction of atmospheric CO2) on CDR and find that the latter can substantially reduce the efficiency of carbon dioxide removal (by as much as 25% in the lowest emission scenario).

How to cite: Weeks, J., Khatiwala, S., Bullock, L., and Yang, A.: Efficiency of carbon dioxide removal by ocean alkalinity enhancement via enhanced weathering of mine tailings, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5878, https://doi.org/10.5194/egusphere-egu23-5878, 2023.