the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jun 2024
Dissolved Mn(III) is a key redox intermediate in sediments of a seasonally euxinic coastal basin
Abstract. Manganese (Mn) is an essential micronutrient and key redox intermediate in marine systems. The role of organically complexed dissolved Mn(III) (dMn(III)-L) as an electron acceptor and donor in marine environments is still incompletely understood. Here, we use geochemical depth profiles and a reactive transport model to reconstruct the seasonality in sedimentary dMn(III)-L dynamics and benthic Mn release in a eutrophic, seasonally euxinic coastal basin (Lake Grevelingen, the Netherlands). We find that dMn(III)-L is a major component of the dissolved Mn pool throughout the year. Our model indicates that, when O2 is present in the bottom water, there are three major sources of pore water dMn(III)-L, namely reduction of Mn oxides coupled to the oxidation of Fe(II), reduction of Mn oxides coupled to organic matter degradation and oxidation of Mn(II) with O2. Removal of pore water dMn(III)-L primarily takes place through reduction by dissolved Fe(II). When bottom waters are euxinic in summer, rates of sedimentary Mn cycling decrease strongly, because of a lower supply of Mn oxides. The dMn(III)-L transformations in summer mostly involve reactions with Fe(II) and organic matter. Benthic release of Mn mainly occurs as dMn(III)-L when bottom waters are oxic, as Mn(II) upon initial bottom water euxinia and as both Mn(II) and dMn(III)-L when the euxinia becomes persistent. Our findings highlight strong interactions between the sedimentary Fe and Mn cycles. Dissolved Mn(III)-L is a relatively stable and mobile Mn species, when compared to Mn(II), and is therefore more easily transported laterally throughout the coastal zone and possibly also to open marine waters.
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RC1: 'Comment on egusphere-2024-1706', Anonymous Referee #1, 02 Aug 2024
Review Klomp et al.
General comments
Klomp et al. investigate seasonal differences in the sedimentary Mn cycling in a eutrophic coastal basin in the Netherlands. Their study provides a comprehensive data set and combines geochemical water column, pore-water and solid-phase data with reactive transport modeling.
The authors emphasize that dissolved Mn(III)-L is an important component of the Mn cycle throughout the year. Depending on the seasonal oxygen concentration in the bottom water and the influx of Mn oxides, Mn is predominantly released to the overlying water as dissolved Mn(III)-L in winter (oxic bottom water and higher influx of Mn oxides), whereas in summer (euxinic bottom water and lower influx of Mn oxides), both dissolved Mn(II) and dMn(III)-L are released, but Mn(II) is the dominant species. In contrast to dissolved Mn(II), the relatively stable and mobile dMn(III)-L may be transported from coastal areas into the open ocean. In addition, the biogeochemical processes leading to the formation and removal of dMn(III)-L in the sediment are strongly linked to the Fe cycle. Therefore, this study is very helpful in improving our understanding of sedimentary Mn and Fe cycling, especially the coupling between the two cycles, in coastal areas.
Overall, the manuscript is well written, however some of the figures could be improved (see specific comments). I would recommend the publication of this manuscript after some minor revisions.
Specific comments
Abstract
Line 11: It is better to write “depth profiles of water, pore-water and solid-phase data” instead of just “depth profiles”. Otherwise, it is not clear which comprehensive data set has been the basis for this study.
Methods
Figure 1a: The map of Lake Grevelingen including the overview map of the Netherlands in the top right corner is too small. It is very difficult to identify the water depths, especially in the Scharendijke basin.
Results
Figure 3: In contrast to the water column profiles, the pore water profiles are relatively small. As some of the discussion relates specifically to the upper centimeters, it would be helpful if these were shown in more detail. In addition, the light grey dots and lines are hard to see.
Figure 3h-j: If I see it correctly, the sum of dMn(III)-L and Mn(II) does not correspond to the TD Mn concentrations. Is there any explanation for this?
Figure 4: Again, the light grey dots and lines are hard to see.
Line 237 + Figure 4a: As the Corg content usually refers to the total sediment weight, it is better to speak of Corg contents rather than Corg concentrations.
Line 245-311: The description of the model results already contains many possible interpretations or approaches for discussion. It would be good if these were directly linked with the discussion part.
Figure 5: At first glance, the figure is a little confusing due to many numbers/rates. One suggestion here would be to split the figure in two: (a) situation in March, (b) situation in September. In this way, the size of the arrows could be adjusted to the respective rates to better highlight the differences in the rates between March and September. In addition, the oxygen conditions in the bottom water could then be integrated for both situations.
Discussion
Line 334: Again, it would be very good if the top 0 to 10 cm were shown in more detail to see the strong counter gradients.
Line 394-395: At this point it should be described in more detail to what extent the transport of trace metals is decoupled from that of Mn when it is mainly present as dMn(III)-L. Were trace metals measured in the water column samples that could confirm the mentioned hypothesis?
Technical corrections
Line 16: Since manganese (Mn) is introduced in line 11, oxygen should be introduced in the same way: Our model indicates that, when oxygen (O2) is present in the bottom water, there are three major sources of pore water dMn(III)-L (…).
Line 43: Again, please also introduce oxygen for consistency.
Line 307-308: Here the authors are certainly referring to Figure 8, as Figure 9 does not exist.
Line 370: The sentence structure is a bit awkward. It would be better: When bottom water O2 re-establish in October, the influx of Mn and Fe oxides, the rates of sedimentary Mn cycling and the benthic flux of Mn all increase.
Line 388: Instead of saying “when it meets O2”, it is more appropriate to write “when it is exposed to O2”.
Citation: https://doi.org/10.5194/egusphere-2024-1706-RC1 -
RC2: 'Comment on egusphere-2024-1706', Aubin Thibault de chanvalon, 30 Aug 2024
General comment
The manuscript written by Klomp et al. describes the manganese cycle in a coastal sediment with extremely high accumulation rates (> 10 cm yr-1) and overlaid by a seasonally anoxic saline water. The manuscript recycles part of the data published by Żygadłowska et al. (2023) with the addition of new data on manganese speciation (called Mn(II) and Mn(III)=L). The main originality of the manuscript is the use of dissolved manganese speciation measurements into a reactive transport model. Most of the kinetic parameters concerning Mn(III)=L are deduced from the model fit, which allows the author to discuss the reactivity of Mn(III)=L and its importance in manganese efflux from the sediment. Beside the clear importance of this topic, the real novelty of this approach for Mn speciation and the good quality of the dataset, it seems that in the discussion, the authors feel too confident about the model and the analytical results and avoid discussing the underlying hypothesis and limitations. Ultimately, it leads to a general overinterpretation of the data with many direct affirmative sentences not supported by detailed argumentation. Model results are taken as true while they rely on many cases of hypotheses hidden by the complexity of the model, preventing the reader to appreciate the model's limitations and thus its scope.
In particular, a) the model fit to Mn(III)=L is not properly discussed while it fits mainly to one unique Mn(III)=L measurement (March, 0-1 cm depth) and fails to fit the deeper part of the Mn(III)=L profile; b) the analytical demonstration of the true occurrence of Mn(III)=L is not detailed while caution have been published on this method since the Madison et al. (2011) paper (Kim et al., 2022) which requires a particular effort of clarity; c) it seems that most of the model output are not produced by the modelled chemical reaction but mainly result from the model input i. e. by the strong seasonality of the manganese oxides deposition rate chosen by the authors but not discussed; d) Important parts of the sedimentary Mn cycle are not discussed, neither mentioned, in particular the interaction with the nitrogen cycle and the role of adsorbed Mn2+. These reservations are detailed below.
Main reservations
- a) The model fit to Mn(III)=L and Mn(II) seems insufficient to deduce fluxes, production and constant rates with a high level of confidence. First, it seems very dangerous to base most of the paper interpretation only on one unique Mn(III)=L (March, 0-1 cm depth) measurement since contamination or analytical errors are always possible. Even is the data is validated, the sampling uncertainty on one point should obviously produce important uncertainties in the model results. For example, the authors suppose that the observed maximum is at 0.5 cm depth, while it could also occur at 0.2 cm, given the centimeters resolution of the sampling. Does such difference significantly change the model results ? Many additional sampling uncertainties described in the literature should prevent the author to stand most of their results on only one measurement (spatial heterogeneity from macro organism, erosion during sampling with a gravimetric core, loss of any fluffy layer on the top of the sediment, …). Second, the fit favors the Mn(III) maximum in the 0-1 cm depth layer, at the cost of a bad fit at depth. Could it be possible to ignore the high value at the top to favor a good fit at depth ? What would be the model result in this case ? Why do you not select these results?
- b) The identification of Mn(III)=L needs to be strengthened since skeptical points of view have been published about this method (Kim et al., 2022). The credibility of the competitive ligand exchange kinetic methods requires more information about the deconvoluting of the kinetic signal and the precise conditions of the essay. In particular, the kinetic of manganese complexation is very sensitive to the chlorite content during the measurement, as detailed in (Thibault de Chanvalon and Luther, 2019). The oxygen concentration during the essay is also critical and should be discussed (Kim et al., 2022). I recommend publishing as supplementary material some examples of the time series of Mn=porphyrin formation rate including the March 0-1 cm depth sample, together with detailed essay conditions (salinity, oxygen), the strategy developed to overcome the method known limitations and the profile of apparent rate constants obtained for Mn(II) and Mn(III)=L.
- c) Most of the model output is not produced by the modeled chemical reaction, but by the model input: it seems that the Mn-ox concentration in the settling particles varies from 9.6 µmol/g in winter to 0.4 µmol/g during euxinic condition. Such important forcing needs to be discussed in detail, along with the most important geochemical reaction constraining the system. In particular, 1 - Zygadlowska et al. 2023 measured suspended material concentration and demonstrated that the bottom Mn concentration in particles does not change so much between seasons; 2 - if important Mn oxide consumption in euxinic water is credible (e.g. Thibault De Chanvalon et al., 2023) why should it be the same for Mn carbonate ? I was expecting an increase of Mn-carbonate in this case as it occurs in the euxinic sediment and because primary production favors carbonate precipitation; 3 – there is no direct proof of H2S in September; 4 – why assuming that anoxic water is necessary euxinic while transitory period with dominance of dissolved manganese has been observed over months in similar environment (Shaw et al., 1994) ? 5 – the discussion should clearly underline that most of the seasonality is driven by settling particle composition, while it is currently suggested by the topic discussed in section 4.2 that the “sediment becomes depleted in Mn oxides” because of sediment efflux and OM oxidation. 6 - Nice oscillations for Mn-carbonate and Corg content in the sediment are reported and present phase opposition (maximum on one fit with the minimum of the other); is there any possibility to explain them because of geochemical reaction rather than because of input seasonality ? Something in the model switches the oscillation from phase opposition to in-phase oscillation below 65 cm depth, what it is ? 7 - The observed loss of 4 umol/g of Mn oxides between March and September in the top 5 cm sediment would require a sediment efflux of approximately 4 umol/g x 2 600 g/dm3 x (1-0.90) x 0.5 dm / 6 months = 87 umol/dm2/month = 290 umol/m2/d which approximately fits with the observed gradient and the calculated flux of 210 umol/m2/d in March. So, on one hand, the rapid calculation suggests biogeochemical processes strong enough to produce the observed MnO2 depletion between March and September. And on the other hand, the elaborated model requires very strong forcing in the Mn content input to fit the data. Is the rapid calculation I propose wrong ? why ? 8 - Why does the published model decrease the benthic efflux as soon as the water column becomes anoxic? I expected the opposite, the absence of oxygen should favor Mn efflux (since there is no more MnO2 precipitating in the oxygenated layer) until reactive MnO2 is consumed (which should take about 6 months). Why is it not modelled ? This counterintuitive result should be underlined and discussed. I also suggest comparing the sedimentary Mn efflux taking into account Mn(III)=L (figure 7) with those calculated without Mn(III)=L as probably done in Żygadłowska et al., 2023.
- d) Some known reactions important in the sedimentary Mn cycle are not modeled or discussed. The revised manuscript should explain and justify why it seems negligible in your site. For example, Mn(III)=L oxidation by nitrite studies (Luther et al., 1997; Luther et al., 2021; Karolewski et al., 2020); debates on Mn-annamox (Hulth et al., 1999; Thamdrup and Dalsgaard, 2000); or the role of adsorbed Mn2+ (Richard et al., 2013; van der Zee et al., 2001; Canfield et al., 1993).
Bibliography
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Hulth, S., Aller, R. C., and Gilbert, F.: Coupled anoxic nitrification/manganese reduction in marine sediments, Geochim. Cosmochim. Acta, 63, 49–66, https://doi.org/10.1016/S0016-7037(98)00285-3, 1999.
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Kim, B., Lingappa, U. F., Magyar, J., Monteverde, D., Valentine, J. S., Cho, J., and Fischer, W.: Challenges of Measuring Soluble Mn(III) Species in Natural Samples, Molecules, 27, 1661, https://doi.org/10.3390/molecules27051661, 2022.
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Luther Iii, G. W., Karolewski, J. S., Sutherland, K. M., Hansel, C. M., and Wankel, S. D.: The Abiotic Nitrite Oxidation by Ligand-Bound Manganese (III): The Chemical Mechanism, Aquat. Geochem., 27, 207–220, https://doi.org/10.1007/s10498-021-09396-0, 2021.
Richard, D., Sundby, B., and Mucci, A.: Kinetics of manganese adsorption, desorption, and oxidation in coastal marine sediments, Limnol. Oceanogr., 58, 987–996, https://doi.org/10.4319/lo.2013.58.3.0987, 2013.
Shaw, T. J., Sholkovitz, E. R., and Klinkhammer, G.: Redox dynamics in the Chesapeake Bay: The effect on sediment/water uranium exchange, Geochim. Cosmochim. Acta, 58, 2985–2995, https://doi.org/10.1016/0016-7037(94)90173-2, 1994.
Thamdrup, B. and Dalsgaard, T.: The fate of ammonium in anoxic manganese oxide-rich marine sediment, Geochim. Cosmochim. Acta, 64, 4157–4164, https://doi.org/10.1016/S0016-7037(00)00496-8, 2000.
Thibault de Chanvalon, A. and Luther, G. W.: Mn speciation at nanomolar concentrations with a porphyrin competitive ligand and UV–vis measurements, Talanta, 200, 15–21, https://doi.org/10.1016/j.talanta.2019.02.069, 2019.
Thibault De Chanvalon, A., Luther, G. W., Estes, E. R., Necker, J., Tebo, B. M., Su, J., and Cai, W.-J.: Influence of manganese cycling on alkalinity in the redox stratified water column of Chesapeake Bay, Biogeosciences, 20, 3053–3071, https://doi.org/10.5194/bg-20-3053-2023, 2023.
van der Zee, C., van Raaphorst, W., and Epping, E.: Absorbed Mn2+ and Mn redox cycling in Iberian continental margin sediments (northeast Atlantic Ocean), J. Mar. Res., 59, 133–166, https://doi.org/10.1357/002224001321237407, 2001.
Żygadłowska, O. M., Venetz, J., Klomp, R., Lenstra, W. K., Van Helmond, N. A. G. M., Röckmann, T., Wallenius, A. J., Martins, P. D., Veraart, A. J., Jetten, M. S. M., and Slomp, C. P.: Pathways of methane removal in the sediment and water column of a seasonally anoxic eutrophic marine basin, Front. Mar. Sci., 10, 1085728, https://doi.org/10.3389/fmars.2023.1085728, 2023.
Citation: https://doi.org/10.5194/egusphere-2024-1706-RC2
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