the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Ideas and perspectives: The benthic iron flux from sandy advective bioturbated sediments
- School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, 11794, NY, USA
- School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, 11794, NY, USA
Abstract. Multiple investigators have suggested that the benthic flux of dissolved iron (Fed) from continental shelf sediments represents an important source of this micronutrient to ocean waters. The magnitude, biogeochemical controls, and seasonal dynamics of Fed fluxes to date, however, have mostly been studied for muddy cohesive sediments dominated by molecular diffusion. Data from these studies have been included in global biogeochemical models to determine the contribution of this Fe source to the ocean. Fed fluxes from sandy advective sediments have received little consideration, although these sediments cover 50–60 % of the continental shelves. Sandy permeable deposits function as dynamic catalytic filters characterized by the rapid exchange of solutes and infiltration of particles —including labile Corg and reactive metal oxides— and high biogeochemical reaction rates. In this article, we discuss how the fundamentally different modes of solute and particle transport in sands affect the sedimentary Fe cycle and Fed flux. We present a case study in which we simulate bioirrigation in sands in summer and winter. In our experiments, Fed fluxes from non-irrigated sediments under diffusive conditions did not exceed 6 and 13 μmol Fe m-2 d-1 in winter and summer, respectively. Fluxes from irrigated cores reached values of 150 μmol Fe m-2 d-1 (winter) and 115 μmol Fe m-2 d-1 (summer). The results indicate that the pumping activity of the benthic macrofauna plays a key role in controlling the extent of the benthic Fed flux from permeable sediments, and that both biogenic and physical advection enhance fluxes. We argue that bioturbated sandy advective sediments constitute an important benthic Fe source to coastal waters and advocate for a more differentiated treatment of sediment type (muddy diffusive vs. sandy advective) and macrofaunal activity -reflecting different functional groups of the macrobenthos- in global biogeochemical Fe models. A better understanding of the benthic Fe cycle in sandy advective sediments is particularly important to help predict how anthropogenic effects such as changes in the deposition patterns of Corg and metals, the expansion of oxygen minimum zones, and changes in benthic biodiversity will affect the tightly coupled benthic-pelagic ecosystem along continental shelves.
Laura M. Wehrmann et al.
Status: open (until 23 Feb 2023)
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RC1: 'Comment on bg-2022-247', Sebastiaan van de Velde, 14 Jan 2023
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Wehrmann et al. argue in this MS that dissolved iron (Fed) fluxes from sandy sediments – which have received little attention – are an important source of iron for the ocean. They present a case study to illustrate that Fed fluxes from sandy sediments with simulated bioirrigation are several times higher than in the absence of irrigation, and that these fluxes are in the higher range of fluxes measured from muddy deposits.
While I agree with the overall message of the perspective paper, and do not question the quality of the presented results, I have one main comment on the experimental design and conclusions drawn from the results. Because the way the experiment is designed (and the authors acknowledge this during the discussion, e.g. L243), you end up with a sandy deposit that is diffusion dominated. In reality however, as discussed in the introduction, sandy deposits are advection dominated – irregardless of the presence or absence of bio-irrigators. As a results, when you compare your irrigated results with your non-irrigated results, you can say something about the the importance of irrigation relative to diffusion – but you cannot make an accurate assessment of how important irrigation would be under natural and advective conditions (which is what you do on L281 and L363), since you have no control that includes advection without irrigation. I would ask the authors to consider this in their discussion – and either provide a quantitative argument as to why irrigation is important in sandy deposits relative to advection, or rephrase the discussion so it focuses on advective sandy sediments in general, and does not go into too much detail with respect to the different types of irrigators or other benthic fauna. In essence, this would link better with the introduction, where you do explain how sandy deposits w/o bioturbators are distinctly different from muddy deposits.
A second concern is the comparison of Fed fluxes from this study with the literature (L280-283). Since you use a novel method, the discrepancy is likely a consequence of your new method to measure Fed fluxes. Dale et al. base themselves on the available data, which is almost exclusively collected by the traditional flux incubation methods. They then calibrate their model on the available fluxes, which are lower than what you would have measured with your extractor-method.
So you could also make the argument that both muddy as sandy deposits are roughly comparable as Fed sources, but our older methods underestimate the actual flux. Please also consider this difference in methods in the discussion.I am however confident that the authors can address these concerns and the smaller remarks listed below. The message of the manuscript is an important and valid one, and worthy to be published. I will happily read a revised version.
Kind regards
Sebastiaan van de Velde
Minor comments:L32: or through acidic dissolution of FeS in sediments colonized by cable bacteria (Rao et al., 2016; Seitaj et al., 2015; Sulu-Gambari et al., 2016; van de Velde et al., 2016)
L40: could use a reference
L59: I would perhaps also say that this is due to practical limitations regarding the in-situ sampling and processing of sandy sediments.
L60: and the fact that most of these sands are dominated by aerobic respiration and denitrification, thus little dissolved Fe(2+) is expected to accumulate and be released from these sediments.
L63: clarify if this is µmol cm-3 sediments or solid phase
L59-70: This argument is based on the extracteable Fe contents – but you would also need to take into account the oxygenation state of sandy versus muddy deposits (how deep is the sediment oxygenated?).
L101: or anthropogenic activities (trawling, dredging) (van de Velde et al., 2018)
L197: does this compare to the natural populations? (I now see you say this at L296, I would move this upward)
L301ff: Or if Corg gets too high, most iron will be rapdily precipitated to FeS(2) and not recycled to the overlying water (van de Velde et al., 2020a, 2020b)
Fig. 1: there is no label on the x-axis
Fig. 2: and what is happening in the rest of the sediment? I also assume this conceptuel model is based on actual measurements of the O2 distribution, which you might either refer to or show alongside the image.
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References:
Rao, A. M. F., Malkin, S. Y., Hidalgo-Martinez, S. and Meysman, F. J. R.: The impact of electrogenic sulfide oxidation on elemental cycling and solute fluxes in coastal sediment, Geochim. Cosmochim. Acta, 172, 265–286, doi:10.1016/j.gca.2015.09.014, 2016.
Seitaj, D., Schauer, R., Sulu-gambari, F., Hidalgo-martinez, S., Malkin, S. Y., Burdorf, L. D. W., Slomp, C. P. and Meysman, F. J. R.: Cable bacteria generate a firewall against euxinia in seasonally hypoxic basins, Proc. Natl. Acad. Sci., 112(43), 13278–13283, doi:10.1073/pnas.1510152112, 2015.
Sulu-Gambari, F., Seitaj, D., Behrends, T., Banerjee, D., Meysman, F. J. R. and Slomp, C. P.: Impact of cable bacteria on sedimentary iron and manganese dynamics in a seasonally-hypoxic marine basin, Geochim. Cosmochim. Acta, 192(2016), 49–69, doi:10.1016/j.gca.2016.07.028, 2016.
van de Velde, S. J., Lesven, L., Burdorf, L. D. W., Hidalgo-Martinez, S., Geelhoed, J. S., Van Rijswijk, P., Gao, Y. and Meysman, F. J. R.: The impact of electrogenic sulfur oxidation on the biogeochemistry of coastal sediments: a field study, Geochim. Cosmochim. Acta, 194, 211–232, doi:10.1016/j.gca.2016.08.038, 2016.
van de Velde, S. J., Van Lancker, V., Hidalgo-Martinez, S., Berelson, W. M. and Meysman, F. J. R.: Anthropogenic disturbance keeps the coastal seafloor biogeochemistry in a transient state, Sci. Rep., 8(1), 1–10, doi:10.1038/s41598-018-23925-y, 2018.
van de Velde, S. J., Reinhard, C. T., Ridgwell, A. and Meysman, F. J. R.: Bistability in the redox chemistry of sediments and oceans, Proc. Natl. Acad. Sci., 117(52), 33043–33050, doi:10.1073/pnas.2008235117, 2020a.
van de Velde, S. J., Hidalgo-Martinez, S., Callebaut, I., Antler, G., James, R., Leermakers, M. and Meysman, F.: Burrowing fauna mediate alternative stable states in the redox cycling of salt marsh sediments, Geochim. Cosmochim. Acta, 276, 31–49, doi:10.1016/j.gca.2020.02.021, 2020b.
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RC2: 'Comment on bg-2022-247', Anonymous Referee #2, 24 Jan 2023
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The manuscript by Wehrmann et al. focuses on the problem of benthic iron fluxes on continental margins. The scientific background to this problem is well described in the introductory chapter. The text then describes experiments where measurements of dissolved iron fluxes at the water-sediment interface were performed with permeable sands. In one case, pore water does not flow through the sediment, in the other case, a pumping system forces water flow and simulates the bio-irrigation activity of a common annelid in West Atlantic margin sediments. The experiment was conducted under both winter and summer temperature conditions. The idea of experimentally examining iron flux from a bioturbated sediment is interesting, however, I do not understand why the manuscript is submitted to Biogeosciences under "ideas and perspectives", since the manuscript describes and discusses the results of an experiment.
The experiments here lasting 7 and 12 days show that benthic fluxes of dissolved iron are higher for permeable bio-irrigated sediments than for non-irrigated sediments. Oxygen levels remained high in the water column during the experiments. At the end of the experiments, the dissolved ammonium inventory in the pore water was measured. This was 4 to 9 times higher in the non-irrigated sediments. In view of the samples that were collected and the discussion, it is surprising not to see some additional results that would have strongly supported the conclusions: only iron fluxes were measured, only oxygen was monitored in the water column, and only ammonium concentrations were measured in the pore water. Thus many questions remained unanswered.
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Since the samples were available, why were dissolved nitrogen compounds not monitored in the water column? Why were the fluxes of other biogenic compounds not measured? What was the inventory of dissolved iron in the pore water? What is the proportion of iron that crossed the water-sediment interface compared to the initial inventory of pore water iron? The experiments last 7 and 12 days, what justifies this duration?
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The dimensions of the benthic chambers indicate that the volume of pore water was 1100 ml. The water injected daily into the sediment to simulate bioirrigation was 281 ml, suggesting a turnover time of about 4 days. Thus I wondered if the experiment simply flushed the pore water initially rich in products of benthic biogeochemical processes. The experiment seems to have mimicked an environment where one would start from an initial situation without bioturbation followed by a situation with bioturbating organisms. Figure 1 shows the evolution of iron fluxes as a function of time. It shows a decrease with time: this suggests that pore waters are progressively replaced by seawater. The duration of the experiment did not allow to reach a stationary state. The measured fluxes thus seem to be essentially dictated by the initial state where dissolved iron in the pore water was abundant before irrigation. If the experiment seeks to simulate continental shelf sands colonized by benthic organisms, I believe that water should first be circulated for a sufficient amount of time before benthic fluxes are considered. One suggestion is to allow time for three times the volume of pore water to be renewed before making flux measurements, to avoid simply measuring fluxes from flushing the water prior to bioirrigation. In other words, I would tend to think that the results of the experiment should have been considered from the moment it was stopped here, i.e. after a dozen days. Thus, the extrapolation of the data produced here for the entire continental shelf presented in Figure 3 seems to me insufficiently justified. I therefore suggest that the authors better justify the durations of the experiments and better describe the volumes of water involved and the initial state. I would be happy to review a corrected version of the manuscript in which my remarks have been considered.
Laura M. Wehrmann et al.
Laura M. Wehrmann et al.
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