the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Oct 2026
Alkalinity generation and trace metal dynamics during olivine-based marine enhanced rock weathering in bioturbated sediments
Abstract. Ocean alkalinity enhancement (OAE) via marine enhanced rock weathering (mERW) of olivine is a proposed strategy for atmospheric carbon dioxide removal (CDR). However, the potential co-release of trace metals to marine ecosystems, most notably nickel (Ni) and chromium (Cr), raises environmental concerns.
Here, we investigated total alkalinity (AT) release, trace metal dynamics and metal bioaccumulation during mERW using two complementary approaches. First, a 28-day microcosm experiment showed that bioturbation by the lugworm Arenicola marina in pure olivine sand enhanced AT release nearly fivefold and more than doubled the release of dissolved Ni (DNi). Second, in a long-term (> 1 year) mesocosm experiment, we added fine-grained olivine at sediment loading levels of 6% and 10%, and coarse-grained olivine at 10%, to natural intertidal sediments inhabited by A. marina. We quantified sediment–water fluxes and porewater concentrations of AT, DNi, and DCr, and assessed metal accumulation in lugworm tissues. Olivine amendment increased sediment–water AT fluxes approximately two- to threefold, without significant effects of grain size or loading level. Fine-grained olivine treatments resulted in the highest DNi fluxes and the greatest Ni bioaccumulation in lugworms. Lugworm Ni concentrations increased up to tenfold relative to controls. In contrast, olivine amendment did not increase DCr fluxes and Cr bioaccumulation was minor compared to Ni.
These results show that bioturbation can enhance olivine weathering and that olivine amendment can increase AT release from natural coastal sediments. However, finer grain size disproportionately increased Ni release and bioaccumulation without a detectable additional AT benefit. This decoupling suggests that AT generated by olivine weathering may have been partly offset by suppression of natural sedimentary AT generation. Under the conditions tested here, coarse olivine therefore provided a more favourable balance between net AT release and ecotoxicological risks from Ni exposure during mERW.
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Status: open (until 19 Nov 2026)
- RC1: 'Comment on egusphere-2026-5968', Anonymous Referee #1, 29 Oct 2026 reply
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- 1
General comments
Van Heurck et al. combine a 28-day microcosm experiment in pure olivine sand with a 22-month outdoor mesocosm experiment in natural intertidal sediment inhabited by the lugworm Arenicola marina. They report sediment–water fluxes of total alkalinity (AT), dissolved Ni and Cr, endpoint porewater profiles and tissue metal concentrations in lugworms. The authors conclude that fine olivine increases Ni release and Ni bioaccumulation without an additional AT benefit and that coarse olivine therefore offers a more favorable balance between CO2 removal and ecological risk.
The study addresses a clear gap in the current literature, as long-term experiments that combine natural sediment, resident infauna and olivine are rare, and the tissue data for a deposit-feeding species are a valuable addition to the ecotoxicological evidence base for marine enhanced rock weathering.
The manuscript is well written, the statistical models are documented in detail and several limitations to the interpretation of the results are already acknowledged by the authors.
However, I have some comments that focus on the ecotoxicological interpretation and the experimental design. My main concerns are the following:
1. Individual worms are treated as independent replicates
1.1 L. 248: It is unclear to me if the number of 108 lugworms refer to the total number summed up from all mesocosms or if that refers to the exact number of sampled from each mesocosm individually. Please report the exact number of lugworms collected per mesocosm and per treatment.
1.2 L. 439–452: Olivine treatments were applied to mesocosms (n = 3–4 per treatment), but the Kruskal–Wallis tests appear to use individual lugworms as replicates (N = 108, χ²(3) = 79.37). With 14 mesocosm values, the test statistic could not exceed approximately 12, so a value of 79.37 is only possible when individual worms are treated as replicates. Worms from the same mesocosm share their exposure and are not independent, so the reported p values are likely too small. I recommend a mixed model with mesocosm as a random effect. In addition, unlike the flux analyses (Tables C1–C8), the tissue statistics are not reported in a table. Please provide such a table with n per treatment and mesocosm, medians and the post-hoc contrasts to make this analysis traceable.
1.3 Fig. 6a, b and Fig. A1a, b: Please report the number of worms per mesocosm and treatment in the figure captions or integrate it into the figures.
2. Exposure duration and body size differ between and within the experiments. Lugworms were added at several time points and juveniles recruited during the experiment, so individual exposure durations are unknown. This is correctly stated, but it has several consequences that require further elaboration:
2.1 L. 177–178: “The lugworm population gradually declined during the experiment, and so additional lugworms were collected and added to the mesocosms in June and October 2022.” This description is rather vague and requires further information. How many lugworms were added at these time points and was the number equal for each mesocosm? Were all added lugworms adults within the same size class?
2.2 L. 114 versus L. 436: The microcosm worms were adults (mean dry weight 276 mg), whereas the sampled mesocosm worms were juveniles (median dry weight 19.8 mg). Metal concentrations in invertebrates often depend on body size and life stage. This should be taken into account when tissue data of both experiments are discussed together (L. 642–643) and with other species (L. 650–669).
2.3 L. 655: The statement that the mesocosm worms were exposed for much longer than 35 days might not be supported for recently recruited juveniles.
3. The conclusion of lower Ni risk of coarse olivine is based on two early sessions (3 and 6), in which the fine treatments released roughly ten times more Ni than the coarse treatment, whereas the endpoint data indicate a smaller difference between grain sizes.
3.1 L. 179–182: I could not find any explanation for why sessions 3 and 6 were selected for trace metal analysis. Please explain why you chose these sessions and not later ones. If there are no data for other sessions (as Fig. 1e suggests), the resulting limitations for your interpretation should be discussed.
3.2 L. 419–420, Fig. 5b: At the end of the experiment, overlying-water DNi was ~0.5 µmol L⁻¹ in the fine and ~0.25 µmol L⁻¹ in the coarse treatment, a difference of about a factor of two rather than ten, although these values stem from only one or two mesocosms per treatment. Similarly, in L. 439–444, median tissue Ni differed by less than a factor of two between fine and coarse olivine (1.45 versus 0.84 µmol g⁻¹). This may indicate that coarse olivine dissolves more slowly but over a longer period, so that its relative contribution to Ni exposure increases with time. I recommend limiting the conclusion in L. 742–743 that coarse olivine was associated with substantially lower ecological risk to the period covered by the trace metal sessions.
4. The experimental design does not resolve population-level effects
4.1 L. 453–459: Adult pseudofeces counts declined to near zero in all treatments, including the control, despite restocking. Do you have any evidence or data on the fate of the adult lugworms? Did they die within the sediment and decompose without any possibility to trace them after the experiment was terminated? Based on other literature for this species, was the mortality expected due to the organism’s life span or was it likely caused by the conditions of the enclosure? If this was unexpected, possible causes (for example temperature extremes in a shallow outdoor system or food supply) should be discussed, together with potential implications for the semi-natural character of the mesocosms.
4.2 L. 460–463, Fig. A4: The juvenile pseudofeces show that recruitment occurred in all treatments, which is a relevant observation. However, because counts were capped at 100, the data cannot reveal whether juvenile abundance differed between treatments, and the body size of the sampled worms cannot be interpreted as growth without known age. I therefore suggest rephrasing the conclusion in L. 702–704, 726–728 that there was no evidence of an olivine-induced effect on the lugworm population. The data support that recruitment was not prevented, but they cannot exclude differences in abundance or condition. If the number of worms recovered per sampled sediment area is available, a rough density estimate per treatment would strengthen this point.
5. The emergence and bleeding of lugworms after fine olivine addition is a clear adverse biological effect in the study and deserves more attention
5.1 L. 455–457, Fig. A3: If possible, please quantify the observation, for example the number of worms and mesocosms affected and the duration of the response. If quantifiable, you could consider integrating this into the abstract.
6. The microcosm experiment is informative, but its design limits the inferences on enhanced olivine dissolution and on Ni uptake.
6.1 L. 104–106: There is no bioturbated treatment without olivine (for example lugworms in natural or quartz sand). The worm’s own contribution to AT and solute export therefore cannot be separated from enhanced dissolution.
6.2 L. 237–238: The tissue controls were collected in the field and not held under the same laboratory conditions. With n = 3 exposed worms, the ~30-fold difference (L. 348–350) should be interpreted with caution.
6.3 L. 482, 721: I suggest describing the microcosm results as consistent with, rather than as proof of concept for, the benthic weathering engine hypothesis.
I recommend major revision. The requested changes concern a reanalysis of the tissue data, a clearer description of the exposure conditions and a more cautious wording of the ecological conclusions. None of them requires new experiments, and I consider the manuscript a valuable contribution once these points are addressed.