the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jun 2024
Depositional controls and budget of organic carbon burial in fine-grained sediments of the North Sea – the Helgoland Mud Area as a test field
Abstract. The burial of organic matter (OM) within fine-grained continental shelf sediments represents one of the major long-term sinks of carbon. We investigated the key factors controlling organic carbon burial in sediments of the Helgoland Mud Area (HMA), which represents the most significant depocentre of fine-grained and organic-rich sediments in the German Bight (SE North Sea). The examined factors include sedimentation and accumulation rates, sediment mixing rates, grain size, total organic carbon (TOC) content and aerobic remineralisation rates. Highest sedimentation rates of up to ~4.5 mm yr-1 and average TOC contents of 2 wt% were found in the southern part of the HMA which is under the influence of the Elbe river outflow. The overall highest organic carbon burial efficiencies of >65 % were also determined in this area. Four times lower sedimentation rates and lowest TOC contents were found in the shallow, eastern part of the research area, with the lowest organic carbon burial efficiencies being 30 %. High sedimentation rates are known to limit oxygen exposure time and thereby enhance OM preservation. Our data support this finding and demonstrate that sedimentation rate is the key factor determining organic carbon burial efficiency and long-term sedimentary carbon storage. The area of the HMA is characterized by varying mixtures of OM from marine and terrestrial sources. In the southern part of the HMA, close to the outflow of the Elbe river, the OM being degraded is primarily of terrigenous origin, while in the central and northern part of the HMA a mixture of marine and terrigenous OM has been shown to be remineralised. At the sites dominated by the degradation of marine organic matter, as found in the western and northwestern HMA, the organic carbon burial efficiency is lower and fluctuates around 55 %. The burial efficiency of OM is highest in sedimentary habitats characterised by high sedimentation rates and OM of terrigenous sources. Our modelled sediment mixing rates were highest in the northwestern HMA, where also the highest bottom trawling activity is reported. The comparison of sites similar in depositional characteristics but different in bottom trawling intensity suggests that in the area of intense bottom trawling in the northwestern HMA the sequestration of OM is reduced by around 30 %. Furthermore, we have determined the annual burial flux of organic carbon in the HMA that amounted to an average of 22.5 g C m-2 yr-1. Considering the strong tidal currents in the shallow HMA, the burial flux is exceptionally high and even compares with those reported for the deeper Skagerrak and Norwegian Trough (~10 to 66 g C m-2 yr-1), which are the main depocentres for fine-grained and organic-rich sediments in the North Sea. For the entire HMA the determined burial flux results in a total annual organic carbon accumulation of 0.011 Tg C yr-1. These findings highlight the importance of depocentres for fine-grained sediments as important carbon sinks: while the area of the HMA represents only 0.09 % of the North Sea it stores 0.76 % of the total annual accumulated organic carbon in this shelf sea area.
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RC1: 'Comment on egusphere-2024-1632', Anonymous Referee #1, 20 Jul 2024
Muller et al., explores the carbon storage and accumulation in the Helgoland mud area and show high carbon accumulation especially in areas under the direct influence pf the Elbe. The rates of carbon accumulation in these muds is equivalent to those in the Skagerrak and Norwegian Trough two of the key depositional environment in the North Sea. This work highlights that the carbon storage and accumulation potentially of Helgoland mud area have been overlooked and shows the muddy depositional area need to be investigated more closely.
This manuscript will be of interest to a broad audience and after some minor revisions will be ready for publication.
Line 83 - As part of the BMBF-funded collaborative project APOC (Anthropogenic impacts on particulate organic carbon cycling in the North Sea), this is an acknowledgment and not required in the main text.
Line 86 -Should be in the methods. If you mention a detailed literature review, it is now expected that you provide search terms etc. in the supp mat. Since section 4.1. of the results is focused on this data compilation more details are required even with the data archived in Pangea.
Line 94 - (3) estimate the carbon budget of this significant depocentre in the German Bight of the North Sea. I agree that the first two objectives have been archived but the third on is a push, to make this stronger focus on objective 1 and 2.
Line 115 - Although the swept area ratio is low in the HMA the intensity outside is high. Though I suspect impossible to fully quantify how much sediment is resuspended and deposited in the HMA from these activities. Might be useful to mention this process at this point.
Line 135 - multiple corer (MUC) should this be multi-corer ?
Line 147 - Samples were taken at 1-cm-intervals in the top 10 cm and every second centimetre below. Can you clarify if this is the porewater sampling or the intervals the cores were sliced at. I assume it is the porewater if so add a sentence describing the sediment sampling.
Line 187 - Could more details be added to the grain size prep, sample mass, wet or dry, etc.
Line 189 - Sediment samples were freeze-dried and both, moisture and porosity, were calculated by the mass loss, assuming a sediment grain density (quartz) of 2.65 g cm-3 (e.g., Anderson and Schreiber, 1965). Was the grain size analysis preparation carried out on dry sample. If so move this be moved earlier in the paragraph.
Line 225 - Why was the CRS model chosen over the others, please provide clarification.
Results - The results section provide a good overview of the data and the figures are of high quality and communicate the work well.
Figure 8 - The IDW should be introduced in the statistical section of the methods.
Line 527 - Section heading ?
Figure 12 - were the burial efficiencies calculate in the same manner as this study are they comparable ?
Section 5.4 - A new paper by Diesing et al, 2024 will be useful in further highlighting the importance of the HMA in comparison to the wider north west European shelf. https://www.nature.com/articles/s43247-024-01502-8
Line 644 - total organic carbon (TOC), why is this being defined in the conclusion, has it not be used before ?
Citation: https://doi.org/10.5194/egusphere-2024-1632-RC1 - AC1: 'Reply on RC1', Daniel Müller, 24 Aug 2024
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RC2: 'Comment on egusphere-2024-1632', Anonymous Referee #2, 23 Jul 2024
Review of the manuscript egusphere-2024-1632
Muller et al. aim to estimate the capacity of the Helgoland Mud Area (HMA) to store and recycle organic carbon and the factors controlling the efficiency of this preservation. The study site is particularly interesting to achieve this objective because the different zones of the area are subject to different forcing (e.g., trawling, river inputs, origin of organic matter). The authors finally showed the importance of such systems for organic carbon sequestration by estimating the relative contribution of the HMA to organic carbon burial at the scale of the North Sea.
The manuscript presents an interesting data set to improve the understanding and the budget of organic carbon burial on continental shelves. Overall, it is well written but sometimes confusing. It lacks of organization and some figures are not relevant. Although the data set is interesting, the work has too many deficiencies and inaccuracies to be suitable for publication in Biogeosciences. Moreover, it lacks of comparisons with similar environments to be of an international interest.
General comments
The paper is mainly of a regional interest and I think that an entire section of the discussion would have be devoted to a comparison with other environments. In the Figure 12, the burial efficiencies calculated in the HMA are compared with those measured in other marine environments. This comparison is interesting but too general. Indeed, the burial of organic carbon on continental shelves has been studied for some decades and several studies have quantified it in environments that are morphologically similar to the HMA. A comparison with such systems would have been relevant to complement the global comparison presented in the Figure 12.
Another major concern is related to the estimates of organic carbon accumulation rates and burial efficiencies. Firstly, TOC accumulation rates are estimated from a mean TOC value for each site. However, such estimates are usually made using either surface organic carbon contents to calculate a part of organic carbon inputs to the sediments or organic carbon contents measured at the core bottom to estimate organic carbon burial rates.
Secondly, the organic carbon inputs, used to estimate organic carbon burial efficiencies, are calculated as the sum of aerobic respiration and organic carbon burial rates. For this calculation, it is not clearly indicated which values of TOC are used (i.e., the bottom value, an average of values below the oxygen penetration depth, an average of the last points of the profiles). In addition, such calculations are typically made by summing organic carbon burial rates, calculated from bottom TOC contents, and total organic carbon remineralization rates. However, aerobic respiration rates do not represent the total benthic remineralization. The data show that aerobic respiration is limited to the first 1-2cm of sediment. However, the degradation of organic matter continues below this surface layer as indicated by the profiles of DIC shown in the Figure 3. In addition, since the relative importance of the different pathways of organic matter remineralization is not presented, there are no data to justify that aerobic respiration is the main pathway for organic matter remineralization in these sediments. Accordingly, the values of organic carbon burial efficiencies and the subsequent interpretations are not reliable.
Finally, the efficiency of TOC preservation is partly discussed by comparing the mean TOC contents of different stations. As it is indicated that the area is fueled with particles of different origins, this approach is not reliable. For example, the influence of bottom trawling on organic carbon preservation is estimated by comparing TOC contents at three stations (W, Cdeep, NW). However, the three sites have different grain-size and the site with the lowest TOC contents is the one where the sediments are coarser (D50~58µm versus 26-30µm for the others). Therefore, it is likely that the difference in TOC contents is due to the difference in grain size rather than to an influence of bottom trawling on organic matter remineralization.
Specific comments
Title : I suggest to replace “test field” by “case study” or to modified the title such as: “Depositional controls and budget of organic carbon burial in fine grained sediments of the the Helgoland Mud Area (North Sea)”
l. 20 – « lowest TOC contents ». Precise the range of values between brackets.
l. 55 – 57 – The authors only mentioned the free-energy yield of aerobic respiration to justify that it is a key process for organic carbon preservation. However, some labile compounds are as efficiently degraded in oxic as in anoxic conditions. The sentence should be modified to precise that the effect of oxygen exposure time is mainly relevant for refractory organic compounds.
l. 58: The bottom trawling should be mentioned as a process that could enhance the exposure time of organic matter to dissolved oxygen.
l. 62 - 65: The sentence from “Natural sediment” to “prolonged aerobic respiration” develops the same idea that the sentence at the beginning of the paragraph. The paragraph should be revised to avoid repetitions.
l. 75 – 77 : The sentence “In these cohesive sediments diffusion is the dominant transport process and oxygen only penetrates a few millimetres into the sediments, leading to the establishment of anoxic conditions at shallow sediment depth that enhance the build-up of OM” develops an idea with no links with the previous and following sentences and so confuses the paragraph. I recommend to suppress this sentence to clarify the text.
l.79: “these seafloor habitats characterized by high organic carbon burial efficiencies”. Precise the typical range of burial efficiencies observed in these environments to justify this affirmation.
l.100: Precise the tidal range and the mean significant wave height to justify that the area is subject to a high hydrodynamics.
On the Figure 1a, the blue point representing the dumping site is not well visible because the bathymetry is also represented in blue.
In the introduction, it is indicated that sedimentation rates have already been estimated in the studied area. The range of mean sedimentation rates previously estimated has to be presented in the section “Study area”.
l. 113: Precise the discharge of the Elbe and Weser rivers to give an idea of their potential importance on sediment inputs.
l. 123-124: The reason of the deposition of fine sediments in the study area is discussed with no clear conclusions. These sentences should be revised based on recent literature. For example, Walsh and Nittrouer (2009) have detailed several types of fine-grained depositional environments influenced by rivers and presented systems with the same morphology of the HMA.
l.134-135: Precise what are the different depositional environments studied, at least in brackets.
It is not necessary to precise the place where the analyses were performed. This makes the text heavy. The description of the methods is enough.
There is no indication that oxygen concentrations were monitored during the measurements of oxygen profiles. Was the bottom water saturation at 100%? If not, the oxygen saturation in the overlying water of the core has to be controlled/monitored to ensure that in situ
conditions are reproduced.l. 184: Please add a symbol on the Figure 1a to highlight the stations selected for grain size analyses. However, it is quite difficult to visualize the transects.
It is indicated that the samples for particulate analyses were freeze-dried (l. 189, l. 200) but they were stored at 4°C and not frozen (l. 152-153).
Precise the reproducibility of radionuclides measurements between the gamma and the alpha detector.
l. 230-234: The reproducibility of TOC measurements has to be precise.
l. 250-253: This has already been indicated in the introduction. As the sentences do not bring additional information, they can be suppressed.
I recommend placing the Figure 2 in supplementary material. Moreover, a table synthetizing the parameters measured at each station would be useful because all the parameters were not measured at all sites.
The Results section is confusing partly because of the order in which the data are described. I recommend starting with a description of the physical properties of the sediments, then the age models, then the TOC contents and finally the origin of organic matter and aerobic remineralization.
Rather than presenting oxygen profiles in supplementary materials, it would be more relevant to make a synthetic figure with the profiles and to include it in the results section.
Table 2: Explicit the unit of porosity because porosity usually has no unity.
Figure 5: Explain more clearly the reasons why there are subsurface Pbxs maximums on several cores.
The profiles of 210Pbxs are sometimes noisy or present several maximums (e.g., N, W, Cdeep, SC, SE). Some anomalies may be related to grain size changes. Accordingly, the profiles of grain-size should be presented next to the 210Pbxs profiles or, if they are not available, the profiles of porosity. Due to these anomalies that are not explained in the manuscript, it would be better to estimate a sedimentation rate from a linear regression of the 210Pbxs profiles and to adjust the model thanks to Cs data rather than estimating sedimentation rates according to ages as it is done in the Figure 6. Indeed, these sedimentation rates likely integrate the noise of the profiles and do not represent realistic variations of the sedimentation. If the sedimentation is fairly constant, the linear regression give robust sedimentation rates that could be extrapolated on the entire core.
l. 382-384: Describe the typical shape of the profiles affected by mixing.
l. 394-395 : It is indicated that no prediction on changes on sedimentation rates can be made with the method used. However, it is what it is represented in the Figure 6. According to the discussion, I recommend to remove the Figure 6. The values of interest are already presented in the Table 3.
l. 365-403: These paragraphs would be move in the material and methods or results sections. Indeed, while they could clarify the section 4.5 they confuse the discussion.
The interpolation of the Figure 8b is too extensive toward the north and the south where there are no data point.
l. 478: It is mentioned the potential occurrence of an advective transport at some stations. However, it is inconsistent with the use of a diffusive model to estimated fluxes of dissolved oxygen at the sediment-water interface from oxygen profiles. In such a case, permeability measurements have to be presented to justify the use of this method to estimate oxygen fluxes
On the Figure 9, the significant correlation between MAR and TOC seems mainly related to one extreme point (I supposed the station SC). Is the correlation still significant without this extreme point? Moreover, the linear relationship between the parameters was tested using a Pearson correlation without indicating if the data follow a normal distribution. This has to be indicated prior performing a parametric analysis. Otherwise, the correlation has to be tested with a non-parametric analysis like a Spearman correlation.
l. 529 : The title of the section has no number.
l. 550: The section is numbered 5.2.1 but there is no section 5.2.2.
l. 521-564: Variations in TOC contents may also be related to grain size changes. It would be relevant to discuss this point in this paragraph, which is rather speculative.
Technical corrections
l. 147: Replace “second centimeter” by “two centimeters”’
l. 189: Use “water content”’ rather than “moisture”
Citation: https://doi.org/10.5194/egusphere-2024-1632-RC2 - AC2: 'Reply on RC2', Daniel Müller, 24 Aug 2024
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