the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Ocean alkalinity enhancement in an open ocean ecosystem: Biogeochemical responses and carbon storage durability
Abstract. Ocean alkalinity enhancement (OAE) is considered for the long-term removal of gigatons of carbon dioxide (CO2) from the atmosphere to achieve our climate goals. Little is known, however, about the ecosystem-level changes in biogeochemical functioning that may result from the chemical sequestration of CO2 in seawater, and how stable the sequestration is. We studied these two aspects in natural plankton communities under carbonate-based, CO2-equilibrated OAE in the nutrient-poor North Atlantic. During a month-long mesocosm experiment, the majority of biogeochemical pools, including inorganic nutrients, particulate organic carbon and phosphorus as well as biogenic silica, remained unaltered across all OAE levels of up to a doubling of ambient alkalinity (+2400 µeq kg-1). Noticeable exceptions were a minor decrease in particulate organic nitrogen and an increase in the carbon to nitrogen ratio (C:N) of particulate organic matter in response to OAE. Thus, in our nitrogen limited system, nitrogen turnover processes appear more susceptible than those of other elements leading to decreased food quality and increased organic carbon storage. However, alkalinity and chemical CO2 sequestration were not stable at all levels of OAE. Two weeks after alkalinity addition, we measured a loss of added alkalinity and of the initially stored CO2 in the mesocosm where alkalinity was highest (+2400 µeq kg-1, Ωaragonite ~10). The loss rate accelerated over time. Additional tests showed that such secondary precipitation can be initiated by particles acting as precipitation nuclei and that this process can occur even at lower levels of OAE. In conclusion, on the one hand, our study under carbonate-based OAE where the carbon is already sequestered, the risk of major and sustained impacts on biogeochemical functioning may be low in the nutrient-poor ocean. On the other hand, the durability of carbon sequestration using OAE could be constrained by alkalinity loss in supersaturated waters with precipitation nuclei present. Our study provides evaluation of ecosystem impacts of an idealised OAE deployment for monitoring, reporting and verification (MRV) in an oligotrophic system. Whether biogeochemical functioning is resilient to more technically simple and economically more viable approaches that induce stronger water chemistry perturbations remains to be seen.
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RC1: 'Comment on egusphere-2024-417', Nicholas Ward, 08 Jun 2024
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General Comments:
Paul et al., describe a very interesting set of in situ OAE mesocosm experiments with varying levels of carbonate alkalinity added to large floating chambers. The study evaluates an impressive suite of bulk biogeochemical parameters allowing the authors to examine responses of multiple coupled carbon and nutrient cycle behaviors. The experiments were well designed to address the objective of the study – understanding whole system response to alkalinity addition.
The results show that fairly high alkalinity additions (up to doubling) have fairly minimal impacts on other biogeochemical cycles other than some unexpected bloom behaviors, which could just as likely be related to random mesocosm related issues. The main findings of note are that 10% of the alkalinity was lost to precipitation in the highest treatment and the carbon/nitrogen content of particles slightly shifted, though the mechanism for the latter is uncertain.
Overall this is a solid assessment of OAE impacts and a very useful body of work for this emerging field. I am supportive of this study’s publication and have mostly minor comments below.
Specific Comments:
Line 24: Consider making this statement quantitative, e.g., “X% of the added alkalinity was lost”
Abstract in general: The findings are described fairly generally and there’s little to any quantitative statements or comparisons. A lot of parameters were measured in this study, so I understand there is a lot to summarize. But after reading the abstract I only understand very high level observations. Adding quantitative elements to this summary would be useful, e.g. how much lower was PON in the high alkalinity treatment, how much alkalinity was lost (and perhaps what was the particle load that allowed nucleation to occur), etc.?
Line 51: Define NETs
Line 138: Can you give more details on the isotope tracer? E.g., what percentage 13C was the compound and what was the compound (e.g., CO2, HCO3, CO3)? Also instead of stating trace amounts, be specific about how much was added.
Line 140: Were the incubations at the end exposed to light or done in the dark?
Line 234: Can you explain this approach a bit more? I’m not following why you might have negative nutrient values, is this related to the data analysis approach or a negative value reported by an instrument (i.e., sample below the detection limit)?
Line 277: Is this sentence suggesting that the seawater used in the 2400 experiment may have had different particle loads than the other mesocosms? Or do you think 2400 just happened to be some important threshold in aragonite saturation in relation to the ambient particle load?
Line 314: Can you use some actual values in this section? For example, “POC:PON increased from X to Y across the OAE treatments.” Also, note in this section that this shift appears to be more related to a decrease in PON as opposed to increase in POC.
Line 335/Figure 7: For the biogeonic silica data it seems like data that’s below the instrument detection limit needs to be flagged in the figure if it’s being shown. A value below the detection limit means you can’t confidently say the value is a real number above zero.
Results in general: I don’t see any results related to the 13C tracer. Why was it mentioned in the methods if it’s not presented in the results?
Line 397: I understand this argument, but is it actually applicable to these results? The actual carbon content by mass didn’t seem to go up, there was just less nitrogen driving this shift. If molecular composition were analyzed perhaps the case could be made that alkalinity enhancement somehow resulted in the production/presence of more refractory carbon compounds, though I’m not sure what the mechanism of this would be.
Line 406: Can any statements be made about the physics of this study region? For example, is the flushing/residence time of Taliarte Harbour known, and would you expect that alkalinity additions would very quickly become diluted if performed here? I.e., would you ever expect to be able to achieve a +2400 alkalinity scenario that could yield similar precipitation in the environment?
Line 473-477: This is all very speculative and cherry picking observations from different mesocosms. “This suggests” seems like too definitive of a statement. It’s ok to speculate, but perhaps don’t tie this discussion point as much to your experimental findings.
Line 485: Perhaps clarify that the threshold isn’t just related to amount of OAE, the threshold seems to be related to both amount of OAE and in situ particle loads.
Citation: https://doi.org/10.5194/egusphere-2024-417-RC1
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