the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Dec 2025
Ideas and Perspectives: Max MACS – constraining the potential global scale of Marine Anoxic Carbon Storage for CO2 removal
Abstract. Marine Anoxic Carbon Storage (MACS) is a potential strategy for enhancing atmospheric CO2 removal (CDR) by sequestering organic carbon produced by terrestrial plants in stable, anoxic marine reservoirs. Initial results suggest that MACS could, in theory, operate at the gigatonne scale that would be required to impact global climate, with limited environmental risk and promising opportunities for co-benefits. However, several outstanding knowledge gaps make it challenging to quantify the actual potential global scale of MACS with confidence. To inform decisions about climate mitigation and trade-offs in the future, it is essential that we know how MACS implementation at scale would impact critical environmental and economic systems in the context of likely future scenarios.
Building on the results of a workshop in Bucharest, Romania in 2025, we discuss the potential impacts of MACS activities on the ecology, biogeochemistry, economy, and community around the Black Sea, seafloor brines, and other anoxic marine sites. Quantifiable limits to the potential maximum feasible scale of MACS for CDR are organized into five criteria: (1) Durable storage site capacity; (2) Biomass sources and logistics; (3) Greenhouse gas balance; (4) Oxygen and sulfide impacts at the redoxcline; and (5) Impacts on dissolved organic matter or nutrients in the oxic zone. For each criterion, we evaluate the factors that could limit scale, our current state of knowledge, and the priority knowledge gaps that, if addressed, would improve our ability to estimate the potential global scale of MACS for CDR. Research is needed to understand its potential impacts at scale, but MACS is nonetheless worthy of serious consideration as a potential pathway for climate mitigation in coming decades.
Competing interests: In addition to her primary role as UCSB faculty, MR serves as the Chief Science Officer for Carboniferous, a U.S.-based startup company exploring potential applications of MACS. Both the coordination of the Bucharest Workshop and the resulting analysis presented here were supported by philanthropic funds and/or grants to UCSB; an unconflicted PI is included on all CDR-related funds in the Raven group. Co-author TT is a member of the editorial team at Biogeosciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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- RC1: 'Comment on egusphere-2025-6086', Anonymous Referee #1, 31 Dec 2025 reply
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This paper addresses an important topic and, in part, builds on related work (e.g., Raven et al., 2024, 2025; Roberts et al., 2025) in the context of carbon dioxide removal (CDR). I agree that action is needed, and the paper is well written and very timely. While I have generally been cautious about relying on natural ecological systems to address anthropogenic problems (e.g., iron fertilization, species introductions), the hypersaline systems proposed here appear to be promising candidates with potentially minimal ecological disruption. However, before pursuing this approach, it would be useful to make a stronger case for why it may be preferable to geochemical CO₂ storage in deep geological reservoirs. While not perfect, geological storage leverages existing knowledge from fossil fuel exploration and avoids many ecological and logistical challenges associated with aquatic systems. I am not an expert, so I could be mistaken, but I believe this comparison should be emphasized before advocating the introduction of materials into aquatic environments. Overall, I recommend the paper for publication once this issue and some of the other more detailed points I have raised are addressed.
Lines 85-90; “Recent results suggest that most of this storage may be in the form of nonliving (soil) organic carbon; models suggest that global stocks may have increased by as much as 4.8 Gt CO2e/yr since 1993 (Bar-On et al., 2025).”
I think some clarification would be helpful here, as soil scientists have recognized this relationship for quite some time. Perhaps I am missing something, but as written it comes across as somewhat naïve. While I understand that these measurements were conducted at a global scale using satellite-based data, the underlying linkage aligns closely with what has long been established in the literature on long-term carbon storage in soils. The authors do discuss the role of organo-mineral protection elsewhere in the manuscript; it may be useful to explicitly note here that these findings reinforce and provide broader-scale support for concepts that have been well documented in the geochemical literature for decades, albeit from a more ecological perspective.
94-96: “For example, during events like Ocean Anoxic Event 2 (OAE-2; ~97 Ma), widespread anoxia supported the burial and preservation of largely marine OC, which contributed to global cooling (Jarvis et al., 2011).”
I really like the connection to OAE events, as it makes a strong case for the potential for global-scale MACS applications. While some OAE events involved hypersaline conditions, many did not, so it may be useful to clarify how hypersaline waters differ in terms of extreme ionic effects on terrestrially derived particulate material, and how this might influence deposition in bottom waters and sediments. Although reduced anoxic microbial decay under hypersaline conditions could favor greater OC storage, I am more concerned about the potential physical breakdown of POC and its dispersal, which could counteract enhanced preservation.
Line 101-103: “Under the right conditions, the preservation efficiency of terrestrial materials in coastal sediments can approach 100%, although it averages 20–44% today; Blair and Aller, 2012).” I would also cite the Galy et al. (2015) paper here (doi:10.1038/nature14400)
121-124: “Marine anoxic storage is only one of several proposed methods for sequestering terrestrial biomass; woody materials are also being stored in anoxic vaults on land (Zeng et al., 2024) and as slurries in deep wells (Snyder, 2022).”
In the Snyder (2022) paper, the authors describe a biomass slurry fracture injection approach, in which biogenic wastes are injected into fractures created within permeable saline formations, rather than into water-filled wells, unless I am misunderstanding the phrasing here.
189-193: “Carbon storage in a third type of anoxic environment, rapidly-accumulating sediment at (e.g.) major river mouths, is less well understood but has the advantages of engaging multiple sites across the global North and South and having vast potential storage capacity.”
I think that anoxic events near major river mouths are much more dynamic (e.g., presence of mobile muds, and coastal currents) than many other deep basin systems (e.g., hypersaline and fjords) mentioned in the paper and would NOT be good sites for MACS. While I realize that this was not the focus in this paper, I think there has been much published on these environments to make the case against it (e.g., Aller et al., 2008, doi:10.1029/2006JF000689; Bianchi et al., 2011; doi.org/10.1016/j.scitotenv.2009.11.047). Major river fans are a different case, as the authors discuss later in the paper.
Line 218-220 “Sediments from Orca Basin exhibit exceptional organic matter preservation
efficiencies, with reports of unprecedented levels of seaweed preservation for thousands of years (Kennett and Penrose, 1978).”
This is an interesting point regarding the preservation of brown macroalgae and could support a strong case for using such sites to dispose of nuisance Sargassum blooms. It would also be valuable to mention the role of fucoidan in brown macroalgal preservation, as highlighted in recent studies. There has been substantial work on this topic in recent years, and it remains an active area of research in the context of carbon sequestration (e.g., Sichert et al., 2020, doi.org/10.1038/s41564-020-0720-2; Li et al., 2024, doi.org/10.1016/j.ijbiomac.2024.137944).
249-251: Additional sites may be valuable scientific resources and analogs for the development of MACS datasets and MRV 250 systems. Fjords with anoxic bottom water, for example in Norway, British Columbia, and Alaska, provide insights into anoxic organic carbon burial over centuries to millennia.
Once again, while I understand the motivation for mentioning these estuarine systems, I would argue against considering them as potential MACS sites due to their dynamic nature, albeit less so than many estuaries, and their status as some of the world’s most iconic and valued coastal regions.
292-295:The seawater or brine volumes of the reservoirs discussed here (Black Sea, Orca Basin) are well known. The abyssal Black Sea as a whole is not physically storage-limited at the Gt scale (Murray et al., 1991; Raven et al., 2024), but sub-regions of the Black Sea with particularly favorable conditions for storage remain to be quantified.
This is an excellent point, as the depth of the pycnocline can vary over time. A related issue, which I mentioned earlier, is the potential for physical disruption of POC at the density interface. It is unclear how much terrestrially derived organic carbon would be able to penetrate this high-density layer, or how it would need to be “packaged” to do so. For example, sinking marine particles and materials transported from the Mississippi River are retained at this interface, where they undergo aging and substantial degradation (Diercks et al., 2019). As a result of this prolonged accumulation of organic matter breakdown products, the brines of Orca Basin are enriched in DOM (Diercks et al., 2019). Another issue is that lignocellulosic biomass is generally less soluble in seawater than biomass from marine primary producers, which could disperse at the pycnocline.
375-376: Additionally, both the Black Sea and Orca Basin are naturally methane-rich environments (Wiesenburg et al., 1985; Reeburgh et al., 1991), and changes to natural rates of methane release would contribute to the net greenhouse gas impacts of a hypothetical MACS project.
I agree that this point needs to be carefully evaluated, as most methane is retained below the pycnocline due to limited vertical mixing; however, any methane that does enter the overlying waters is likely to be rapidly oxidized by microbial communities in the oxic waters of the Gulf of Mexico (Kessler et al., 2011).
454-456. “Over decadal timescales, increases in dissolved inorganic carbon are observed in the oxic layers (Voynova et al., in prep).” Perhaps citing Dorofeev and Sukhikh (2024) here would be useful: ISSN 2413-5577, Ecological Safety of Coastal and Shelf Zones of Sea, 2024, No. 3, pp. 36–48.
Line523-525: example, both iron oxide and iron sulfide minerals can contribute to organic carbon preservation in sediments because organic molecules within and on the surfaces of these minerals may be protected from microbial breakdown (Lalonde et al., 2012; Nabeh et al., 2022).
Perhaps it would be helpful to add a few more lines and references on this topic here, as there is a substantial body of literature addressing it.
Line 566-569: “…From the perspective of marine organisms, the vast pool of molecules categorized as DOM includes energy sources (food), building blocks for biosynthesis (e.g., amino acids), and key reactants in cellular processes (e.g., B vitamins, signalling molecules). Additionally, some DOM molecules are acidic and can cause pH change (e.g., carboxylic acids), while others can complex and stabilize trace metals…” Need to add some references here.
584-587: In contrast, the overlying water column is generally oligotrophic, although surface nutrients and primary productivity vary in response to seasonal cycles and mesoscale eddies (Damien et al., 2021). What about long lived dissolved species that escape laterally at the pycnocline to other regions???