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| 19 Jun 2024
Review and syntheses: Ocean alkalinity enhancement and carbon dioxide removal through coastal enhanced silicate weathering with olivine
Abstract. Coastal enhanced silicate weathering (CESW) is increasingly receiving attention as a marine-based carbon dioxide removal (CDR) technology. The method aims to achieve ocean alkalinity enhancement (OAE) by introducing fast-weathering silicate minerals into coastal systems. The latter is envisioned to act as a large natural biogeochemical reactor, where ambient physical and biological processes can stimulate silicate dissolution, thus generating a concomitant alkalinity release and increasing the seawater’s capacity to sequester CO2. Olivine has been forwarded as the prime candidate mineral for CESW, but to the present, no results from larger-scale field studies in actual coastal systems are available, so all information is exclusively derived from idealized laboratory experiments. As a result, key uncertainties remain concerning the efficiency, CO2 sequestration potential, and impact of olivine-based CESW under relevant field conditions. In this review, we summarize recent research advancements to bridge the gap between existing laboratory results and the real-world environment in which CESW is intended to take place. To this end, we identify the key parameters that govern the dissolution kinetics of olivine in coastal sediments, and the associated CO2 sequestration potential, which enable us to identify a number of uncertainties that are outstanding with respect to the implementation and upscaling of olivine-based CESW, as well as the monitoring, reporting, and verification (MRV). From our analysis, we conclude that the current knowledge base is not sufficient to predict the outcome of in situ CESW applications. Particularly, the impact of pore water saturation on the olivine dissolution rate and the question of the additionality of alkalinity generation remain critical unknowns. To more confidently assess the potential and impact of olivine-based CESW, dedicated pilot studies under filed conditions are needed, which should be conducted at a sufficiently large spatial scale and monitored for a long enough time with sufficient temporal resolution. Additionally, our analysis indicates that the specific sediment type of the application site (e.g. cohesive versus permeable) will be a critical factor for olivine-based CESW applications, as it will significantly impact the dissolution rate by influencing the ambient pore water pH, saturation dynamics, and natural alkalinity generation. Therefore, future field studies should also target different coastal sediment types.
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RC1: 'Comment on egusphere-2024-1824', Anonymous Referee #1, 12 Jul 2024
The manuscript by Geerts et al. provides an overview of different types of coastal applications using enhanced silicate weathering as carbon dioxide removal method. Different coastal areas (ranging from bedload movement to permeable and cohesive sediments) are discussed with their respective advantages and disadvantages. The impacts of relevant geochemical parameters, such as pH and temperature, are modelled for the distinct scenarios and the uncertainties as well as knowledge gaps emphasized. While the manuscript provides a nice overview of the current state of knowledge, I’m missing an outlook for the most promising areas of application despite the current knowledge gaps. I would appreciate an effort to model the deposition of olivine in the different coastal environments taking also into account the accessibility of the deployment site and feasibility in the sense of political will and economics into account. This modelling effort could also take into account the different temperatures in the vicinity of the equator versus the lower solubility in comparison to northern and southern latitudes.
Next to this more elaborated outlook, I am missing some literature or would replace literature investigating enhanced rock weathering in the terrestrial environment with publications explicitly investigating the minerals of interest in the marine environment (see comments below). As this manuscript is a review, I would carefully check all references and verify their applicability, especially in the context of marine coastal OAE.
Nevertheless, I enjoyed reading the manuscript and recommend publication after revising the two major points mentioned above and minor comments below.
Minor comments:
Line 23: Typo field conditions
Line 45: With respect to the cited literature and to my best knowledge, the estimated lower potential is 0.5Gt CO2 yr-1 instead of 0.1Gt CO2 yr-1.
Line 48: I assume, Fig. 1a is not the correct reference here. These olivine beaches constitute more the exception on Earth’s surface than the norm as you also show in your map in Fig. 3. Either refer to Fig. 1b or put a picture of a mountain drained by rivers.
Line 50: What is meant by organic C sequestration? Blue carbon or enhanced primary production through fertilization? Please add a short explanation.
Line 53-56: The publications by Eisaman et al. (2023) and/or Rau et al. (2018) should also be cited here?
Line 72: The publication by te Pas et al. (2023) investigates ERW in the terrestrial environment and conducted experiments solely in the context of soil applications? I highly doubt that mineral dissolution rates are comparable in soils and seawater matrix and I definitely wouldn’t cite this publication here (and throughout the manuscript), in a study focusing explicitly on coastal marine applications. Also the other two cited papers don’t appear to be very well fitting for OAE. I would rather cite publications explicitly investigating different mineral in the context of OAE, such as Hartman et al. (2013), Bach et al. (2019), and Renforth and Henderson (2017).
Line 74-75: why did you choose to concentrate on CESW via olivine addition in your review and not on the other methods discussed before? Motivation is not clear.
Line 93: I wonder, if it is actually possible to compensate anthropogenic CO2 just by ‘sitting it through’, if emissions continue as business as usual. I would add a comment, that this scenario is only possible when emissions are cut drastically.
Line 148: Also Fuhr et al. (2024) investigated OAE with sediments in their experiments.
Line 189: Error in the anorthite formula and also here, I would add a comment that dissolution rates of these minerals were not investigated in seawater matrix and might be lower.
Line 238 ff: I doubt that this paragraph is very relevant, as the occurrence of olivine beaches is globally very low (as indicated in your Fig. 3). I don’t think, anyone is considering this option seriously for CDR, so I would delete this paragraph and also the two points on the map in Fig. 3.
Line 334: I would state more explicitly, that Rimstidt et al. defined a pH of 5.6 and higher as basic, as a pH below 7 is normally not described as basic.
Line 387 and 595: The experiments by Fuhr et al. (2023, 2024) were conducted at low T (~10°), alkaline pH, and in the 2024 publication also under anoxic conditions.
Line 479: Would be nice to include a comment about cable bacteria here: Fuhr et al. 2023 with reference to Meysman et al. (2019). In addition, the recent study by Li et al. (2024, https://doi.org/10.1016/j.scitotenv.2023.168571) investigates the impact on diatoms by olivine dissolution as well as the effect that diatoms have removing the passivating layer. This study should definitely be included here.
Line 580: Why not turn it around then? Saponite (former bowlingite).
Line 607 to 625 + 657: I don’t understand the separation of Section 3.3 from the previous sections, as it also deals with secondary precipitates. Also, the formation of secondary clays can play a major role during CESW, as described for example in Griffioen (2017) and Fuhr et al. (2023), so I wouldn’t exclude it here. The additionality problem described by Bach (2024) is indeed important and should be highlighted. As you do this in line 657ff, I would recommend to delete this paragraph here to avoid repetition.
Table 4: In this sequence, it is not clear why you use fayalite instead of forsterite here, as fayalite makes up ‘only’ 6-20mol%, as you mention in line 684? I would bring this argument (your lines 684 to 688) earlier or make a comment in the caption of Table 4.
Line 781ff: Personally, I’m not a big fan of referring to symbols instead of descriptions in flowing text. The repetitive look up for explanations disturbs the reading flow.
Figures:
Fig. 1: (a) As nice as this picture is, I don’t think it adds much value to the discussion and I would remove the discussion about olivine beaches completely from this manuscript. (b) what about alkalinity-consuming process, e.g. carbonate formation?
Fig. 2a: I would include phyllosilicates here as well.
Figure 3: Would be nice to include in this map, where currently dunite is mined and which of the deposits are realistic candidates for future exploration considering social, politic, and economic aspects.
Fig. 4a: is there a reason, why saline data points in the legend are much larger compared to the other? What are 75 Cl and 95Cl? Explanation missing in caption.
Citation: https://doi.org/10.5194/egusphere-2024-1824-RC1 - AC2: 'Reply on RC1', Luna J.J. Geerts, 27 Sep 2024
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RC2: 'Comment on egusphere-2024-1824', Anonymous Referee #2, 27 Aug 2024
The manuscript provides a review of coastal enhanced weathering with olivine minerals, and investigates the state of the art on many of the key factors which determine the rate of olivine dissolution, production of alkalinity, and conversion of alkalinity into actual CDR. I appreciate the effort but think the manuscript would be improved by a more complete overview of coastal enhanced weathering before focusing in on olivine. In addition, I think there are a number of important missing discussion points from the review. I also believe the manuscript would be improved with some restructuring. I elaborate on these points in detail below.
L8 - The authors use the term “coastal enhanced silicate weathering” (CESW) throughout the text. At this point, I think the field has essentially landed on “enhanced rock weathering” (ERW) as the name for this CDR strategy. If the authors wanted to distinguish coastal ERW specifically, I would therefore abbreviate it as CERW. I also would not reference “silicate” weathering specificalyl, given that CERW with non-silicate minerals is under consideration as well. Its unnecessarily limiting. And anyway “CESW with olivine” is redundant since olivine is a silicate. Altogether, my recommendation is “CERW with olivine” to be consistent with the field.
L46 - Rather than citing papers that state storage timescales by referencing yet other papers, I recommend citing the actual papers that establish such estimates of timescale, e.g. David Archers work, Jack Middelburgs work, etc.
Figure 1 - I would cut images a) and b) from this figure. With regards to a), the image of the beach in Hawai’i and natural olivine beaches aren’t discussed in the text and so no context is given. Plus you cant see any olivine in this picture anyway, so what is it adding? With regards to b) the natural weathering process has been covered at length elsewhere. For a review like this, I would simply assert that CERW is based on the natural process, provide references and move on. Its not a meaningful contribution to this article.
L88-89 - “The chemical dissolution of silicate minerals in terrestrial and coastal environments releases alkalinity into the surface ocean, where it drives the uptake of CO2 from the atmosphere” This is an incorrect summary of how ERW works on land. On land, the alkaline minerals react with, and sequester atmospheric CO2 directly and the ocean is simply a holding tank for the resulting alkalinity. This is different from ERW in the ocean, where the alkaline minerals react with, and sequester seawater CO2, which ultimately then drives CO2 from the atmosphere through air-sea gas exchange.
L100 - Remove Schuiling and Krijgsman, 2006 reference. This paper did not suggest the application of silicates to coastal and shelf environments, only land.
L100 - 119 - I take issue with the framing the authors present here that there are three distinct CERW scenarios. It does not appear to be based on an understanding of sediment transport and coastal geomorphology. To begin, for the “bedload scenario” there is essentially no real-world scenario where a coastal environment is made of only gravel. Coastlines exist with a gradient of particle sizes. There will always be some fine grained material, and any olivine sand that was added to a rocky coastline with almost immediately sort, based on the brazil nut effect, downwards to those finer native particles. Regarding the permeable and cohesive sediment scenarios, I would argue that the permeability (and porosity) of a sediment are simply factors which should be discussed as controls on advection, bioirrigation, as well as diffusion (the latter which was excluded altogether from this manuscript). Coastal sediment will have some component of all three of these processes occurring. Permeability and porosity influence how the relative importance, and magnitude, of each of these three processes differ across coastlines. And of course, other factors will influence advection, bioirrigation, as well as diffusion as well, such as local oceanographic conditions and organic matter content. Taken together, I think the presented framing is overly simplistic and somewhat inaccurate (e.g. diffusion), and the authors would be better served by instead including a discussion of the impact of permeability further on in the paper when they discuss other controls on dissolution rates.
Additionally, this framing leaves out entirely the role for water column dissolution in CERW. Any CERW that uses sufficiently small particle sizes will have some fraction of the olivine dissolution occur in the water column through particle resuspension (i.e. suspended sediment). Furthermore, some scientists are working on understanding CERW project sites to intentionally increase the amount of water column dissolution so as to avoid the messy complications of sediment dynamics. To date, much of the CERW modeling has actually assumed water column dissolution for small particle sizes (e.g. Feng et al. 2017). Water column dissolution - and what, for example, water column pH means for dissolution - should be added to this review.
Finally, this discussion also leaves out that many coastal environments under consideration for CERW are not beaches or similar sandy coastlines, but rather wetlands - marshes, mangroves, etc. There is a funded field trial of this in the US right now. This should be included in the discussion.
https://oceanacidification.noaa.gov/funded-projects/tidal-wetlands-as-a-low-ph-environment-for-accelerated-and-scalable-olivine-dissolution/
Figure 2. - a) should include a depiction of clays as secondary minerals as well, beyond carbonates and metal oxides.
L135 - 140 - I would reference here the extensive amount of work on ecology and ecotox that has been done, but simply state that it is outside the scope of the review.
L142 - You reference here “Vesta, 2023” which is a monitoring report, but in the next sentence you say there are no reports on the outcomes of field trials. It seems like there is a report on outcomes, since you reference it. Additionally, Vesta (and their collaborators) have generated a lot of conference abstracts, which I recommend you reference as well. The USGS also did a field trial of olivine in a coastal environment (see link below), the PI on this field trial is Kevin Kroeger. His team has also published a lot of abstracts on their findings at AGU and OSM. Both this field trial and key abstracts should be mentioned. I would also reference the field trial conducted by Planetary Technologies. They added the mineral brucite to the seafloor in Halifax Harbor, Canada. Its not olivine, but it is nonetheless a CERW field trial that warrants mention given how few are being conducted. Theyve published abstracts as well with Dalhousie University. In general, given that coastal enhanced weathering field trials are ongoing, I am surprised how little attention they receive in this manuscript, particularly given that “field trials are needed” is a primary conclusion.
https://www.usgs.gov/media/images/usgs-and-partners-collect-a-soil-core-massachusetts
L145 - You said these laboratory experiments are done under “idealized conditions”. I would argue that these are definitely not “ideal” given that they are closed systems and subject to batch effects, as discussed later in the manuscript.
L178 - Also cobalt.
L190 - 195 - Since you have expanded the scope beyond just silicates here (e.g. carbonates) I would also mention brucite (particularly since Planetary already did a field trial with this mineral, as mentioned above), and anthropogenic minerals too. I think that would produce a better synthesis of the CERW field generally.
Figure 3 - On this figure you have a large olivine deposit labeled in Colorado. Is this supposed to be the Twin Sisters deposit? If so, the Twin Sisters deposit is in Washington State, not Colorado.
L251 - 253 - Transport (shipping) of the olivine to the project site is also an important step between production of olivine (mining and grinding) and spreading the olivine. Please include.
L257 - The equation here presents Rdiss as if there is a single dissolution rate attached to the feedstock applied to the seafloor, but further down (L268) the text acknowledges that any feedstock has accessory minerals that dont contribute alkalinity, though the text should include the possibility that the accessory minerals simply contribute alkalinity with a ratio g/g ratio and a different rate. In other words, the feedstock (not a singular mineral) applied to the seafloor will be a mixture of different minerals, with different CDR efficiencies and different dissolution rates but this is not made explicit.
L274 - Should also include a function that accounts for the potential of alkalinity loss in the water column to biotic carbonate formation, for example. This framework also leaves out the potential for incomplete air-sea gas exchange and a key factor that can reduce FCO2.
L284 - In general, this section and the overall manuscript should include a discussion of porewater CO2 as a limiting factor for olivine dissolution. If porewater exchange is low, or respiration is low (low organic matter), etc CO2 consumption may be the rate limiting factor for olivine dissolution.
L298 - Section 3.1.1 - Generally speaking, I think this section would be improved by a discussion of the reasons why there is so much spread in the dissolution rate data (and therefore, how we can improve on it). For example, mineral impurities in the tested materials or the method of rate quantification (e.g. the use of Si concentrations which gives net dissolution rate) https://www.sciencedirect.com/science/article/abs/pii/S0016703719307811)
L506 - In general, its not clear to me why carbonate precipitation isnt included in this section on secondary mineral precipitation. Why distinguish between clays + metal oxides and carbonates?
L513 - “Clay formation has been proposed as a third potential mechanism…” but sepiolite is a clay. Youve just mentioned clay formation in the prior sentences.
L579 - This section is titled “formation of other clay minerals” but then talks about metal oxides. Though metal oxides are also discussed in section 3.3.2. Combine.
L607 - This “secondary reactions” section (3.3) is a combination of mineral formation (though excluding clays, which is section 3.2) plus a discussion of carbonate dissolution and additionality. My recommendation is to combine section 3.2 and the mineral formation part of section 3.3 into one section on secondary mineral formation, and then make section 3.3 strictly about mineral dissolution and additionality. I think this would make for a better logic flow.
L658 - I think this discussion of carbonate dissolution would benefit from a brief review of where carbonate dissolution and the additionality question are likely to be significant (or not), as discussed in Bach, 2024. I think in the present manuscript, important context is missing that this additionality consideration is not likely to be important everywhere.
L672 - 682 - This paragraph, and discussion of water column processes, comes a bit out of nowhere. I would recommend that consideration of the water column should represent a unique step that must be assessed for CERW efficiency. This paragraph ends with the assertion that “The slow alkalinity release from the seabed during CESW implies a substantial dilution in the overlying water, thus avoiding local alkalinity excursions, and hence the chances of immediate CaCO3 precipitation in the overlying water are minimal.” but this assertion isnt based on anything quantative. This would be substantially improved with even a back-of-the-envelope calculation. I’ll also add that referencing biotic calcification (which is not said specifically here) would benefit the manuscript.
L704 - This section should include a discussion of how water mass sinking and incomplete air-sea gas exchange also play a major role in determining CO2 drawdown - references such as He and Tyka, 2022 or Ho et al., 2023.
Citation: https://doi.org/10.5194/egusphere-2024-1824-RC2 - AC1: 'Reply on RC2', Luna J.J. Geerts, 27 Sep 2024
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