the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Oct 2024
Cold-water coral mounds are effective carbon sinks in the western Mediterranean Sea
Abstract. Cold-water corals (CWC) build biogenic structures, so-called CWC mounds, that can store large amounts of carbon(ate). However, there is a lack of quantification studies on both recent as well as geological timescales, and knowledge is limited to the accumulation of carbonate (i.e., the inorganic carbon fraction), ignoring the organic carbon fraction. This hinders the calculation of total carbon accumulation rates and a wider understanding of the role CWC mounds play in the long-term carbon cycle. Here, we investigated two cores retrieved from CWC mounds in the Alborán Sea, Western Mediterranean Sea, comprising a ~400 kyr record of carbon accumulation. We calculated the accumulation of both inorganic and organic carbon within the CWC mounds. Further, we analysed the same parameters in two cores from the adjacent seafloor (~120 kyr record) to compare the mound records with the surrounding sedimentary deposits. Our results show that the studied CWC mounds accumulate up to 15 g C cm−2 kyr−1, of which 6–9 % is derived from the organic carbon fraction. Moreover, during enhanced mound formation phases, the mounds store up to 14–19 times more carbon than the adjacent seafloor deposits. We suggest that there is a selective enrichment of organic carbon on the mounds, with about an order of magnitude higher organic carbon accumulation rates than on the adjacent seafloor. Consequently, in phases of active mound formation, CWC mounds can be effective local sinks of both inorganic and organic carbon on geological timescales.
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RC1: 'Comment on egusphere-2024-2532', Anonymous Referee #1, 27 Oct 2024
Dear authors, dear editor,
In this paper, Greiffenhagen et al. calculated the amount of inorganic and organic carbon accumulation at two CWC mounds in the Mediterranean Sea using two sediment cores. They compared the results to two cores from the adjacent seafloor and showed that CWC mounds store much more carbon than the adjacent seafloor.
General comments
The manuscript is very well written and structured and I only have some minor suggestions for improvement (see below). Some more background information about CWCs and CWC reefs should be provided in the introduction, an explanation should be given why both off-mound sediment cores were collected in the same location and it should be clarified in the methods section which new information was added by this study and which data were taken from previous studies.
Specific comments
Abstract
The abstract provides a very nice summary of the study.
Introduction
The introduction provides background information on the carbon cycles and CWC mound formation. However, it lacks general information about CWCs (e.g. distribution, depth, biology etc.) and CWC reefs (e.g. reef structure, information about reef-forming species), which should also be included in the introduction in my opinion.
You state that previous studies have only focused on the accumulation of inorganic carbon on CWC mounds and state which information is still missing. However, can you also include some results of these previous studies to provide some more context?
Methods
Why were both off-mound cores collected so close to each other? Would it not be a better representation of the carbon accumulation rates in the region if they were chosen from different locations?
It is stated in the introduction that the new information provided in this study is the calculation of carbon accumulation on CWC mounds by taking into account both the inorganic and organic carbon content. However, it is stated in Table 1 that Wang et al. (2021) have already determined TIC and TOC values for two of the four cores. What is the new information provided by this study, except combining all information for all four cores in one manuscript and including some more data for some of the cores?
Please also provide some information here how the cores were prepared for CT scanning.
You cite a lot of previous studies that have been working on the same sediment cores. What exactly is the new information that is provided by this manuscript? This should be made more clear in the introduction and methods section.
Results
This section seems to be fine and the results are supported by nice figures.
Discussion
Line 519: However, the two off-mound cores were collected at the same location. Therefore, they do not provide a good range of “background” carbon accumulation rates. Therefore, how comparable are the coral-mound to off-mound carbon accumulation rates really?
Conclusion
This section provides a good summary of the main findings of this study.
Technical corrections
Consider including some of the more general information about the study region from section “study site” in the general introduction and not as a separate section and including the more specific information about the sampling locations in the methods.
Lines 73-75: Consider only citing some of these studies as examples.
Figure 1: Delete “Overview map”. A) is missing in figure legend (and describe location of red box in A, not in B). Can you change the orientation of the mound in C to be the same in both pictures (as it is the case in D)? That would make it easier for the reader to understand where on the mound the sediment core was taken. Consider using stars of different colours for sediment cores from coral mounds and off-mounds. A scale bar in B would help to better understand how far away both coral mounds are located.
What is the time period covered by core MD13-3457? This is only stated in Table 1 but not in the text.
As the methods section is rather long, consider moving some of it to the supplementary methods, e.g. methods that describe data collected by previous studies. This would also make it more clear which data were collected for this specific study and which data were used from previous studies.
Figure 3: Grey shaded area in the background that show the timepoint in the record for bars of carbon(ate) accumulation data is difficult to see, use darker colour or contours to make this more clear. The same is the case for light blue bares for glacial periods in top graph. Unclear which Y axis the bars and line in the top graph correspond to. I would suggest using two different colours for both to make the difference more clear.
Figure 5: Define acronym CWC in figure legend.
Isn’t section “5.4 Cold-water coral mounds as carbon sinks” the main finding of this paper and should therefore be stated at the beginning of the discussion?
Citation: https://doi.org/10.5194/egusphere-2024-2532-RC1 -
AC1: 'Reply on RC1', Luis Greiffenhagen, 31 Jan 2025
Dear Reviewer,
Thank you very much for providing your expertise and time to review our manuscript. Your comments and very helpful feedback most definitely improved this work.
From your general comments, we identify three main points raised. We will address these one by one, and then respond to all specific comments in detail (your comments in italics, new text in the manuscript in quotation marks).
1. More background information about CWCs and CWC reefs should be provided in the introduction
We appreciate your comment, and included a few more information on CWCs and CWC reefs in the first paragraph of the introduction (see specific comment response below.)
2. An explanation should be given why both off-mound sediment cores were collected in the same location
This study was conceived long after the research cruise campaign and we 100% agree that multiple off-mound core “background” locations would give a better representation. Unfortunately, there are no other cores from the East Melilla Coral Mound Province available, and the Southern Alborán Sea is rather scarce in sediment core data. However, upon your feedback and a similar question regarding this matter from Referee#2, we decided to include data from the only known publicly available drill core of the region, ODP 161-979A, into our discussion and interpretation. Our mean background carbon accumulation rates from the off-mound location match well with the average values from this (lower-resolution) core during the last > 100 kyr, suggesting that, apart from some variability, it is a representative record.
3. It should be clarified in the methods section which new information was added by this study and which data were taken from previous studies.
The agglomeration of data in this study is indeed very complex. Upon your feedback, we added a short clarification / narrative of the data’s origin to the beginning of the methods section, stating which part was fully measured for this study, and what has been available already. We also applied a few edits throughout the methods to make it all more clear. The main work of the study corresponds to the 70 m long Dragon Mound record of GeoB18116-2, where, except for the coral dating, everything was conducted for the purpose of this study. However, Dragon Mound lacks a record of Holocene mound formation, so to ensure a representative understanding of CWC mound formation across the entire region, we incorporate data from the Holocene formation phase on BRI (GeoB13729-1), which were luckily already available. Regarding the off-mound cores, corresponding Holocene off-mound core data (GeoB13731-1), are also available. In order to be able to compare the much older Dragon Mound record with off-mound data from the same time period, we also analysed the stratigraphically longer off-mound core MD13-3457, which is published for the first time in this study and is the second major work carried out exclusively for this study. The addition and processing of previously published data allows us to cover the full scope of CWC mound development in the East Melilla CWC Mound Province over the last 400,000 years. None of the data have been used specifically to calculate organic and inorganic carbon accumulation before, and all the methods presented have been carried out (at least once) solely for the purpose of this study. At the beginning of each methodological step, it is stated which core was previously analysed, and which one has been analysed for this study.
Specific Comment Responses:
The introduction provides background information on the carbon cycles and CWC mound formation. However, it lacks general information about CWCs (e.g. distribution, depth, biology etc.) and CWC reefs (e.g. reef structure, information about reef-forming species), which should also be included in the introduction in my opinion.
Response: As outlined above we followed your suggestion and add the following text (L.43): “CWCs occur circumglobally, with only a few scleractinian CWCs, i.e. carbonate framework-forming corals such as Lophelia pertusa (syn. Desmophyllum pertusum) and Madrepora oculata being capable of building entire CWC reefs (Roberts et al., 2009; Wienberg and Titschack, 2017). With their three-dimensional framework, these CWCs provide habitat and the foundation for some of the most biodiverse ecosystems of the deep sea (Henry and Roberts, 2007; 2017). The occurrence and proliferation of CWC reefs depends on a certain range of environmental conditions such as temperature, pH, and oxygenation (e. g., Davies and Guinotte, 2011), combined with sufficient nutrient supply through bottom currents and primary production (e. g., Portilho-Ramos et al., 2022; Maier et al., 2023).”
You state that previous studies have only focused on the accumulation of inorganic carbon on CWC mounds and state which information is still missing. However, can you also include some results of these previous studies to provide some more context?
Response: Upon your suggestion, we decided to include a range (0.3 – 2114 g CaCO3 cm-2 kyr-1) of all published CWC mound carbonate accumulation values into the introduction (L.66), while we go into detail regarding previous studies in the discussion.
Why were both off-mound cores collected so close to each other? Would it not be a better representation of the carbon accumulation rates in the region if they were chosen from different locations? // Line 519: However, the two off-mound cores were collected at the same location. Therefore, they do not provide a good range of “background” carbon accumulation rates. Therefore, how comparable are the coral-mound to off-mound carbon accumulation rates really?
Response: Agreed and changes applied, see point 2) in the main comment section.
Please also provide some information here how the cores were prepared for CT scanning.
Response: Done, information added in 3.1 (L.162), stating: “The coral-bearing core sections of GeoB18116-2 were frozen at -20°C and split length-wise with a stone saw, into a “work” half for sediment sampling, and an “archive” half (undisturbed) for CT scanning.”
It is stated in the introduction that the new information provided in this study is the calculation of carbon accumulation on CWC mounds by taking into account both the inorganic and organic carbon content. However, it is stated in Table 1 that Wang et al. (2021) have already determined TIC and TOC values for two of the four cores. What is the new information provided by this study, except combining all information for all four cores in one manuscript and including some more data for some of the cores? // You cite a lot of previous studies that have been working on the same sediment cores. What exactly is the new information that is provided by this manuscript? This should be made more clear in the introduction and methods section.
Response: Agreed that this may be confusing, see also point 3) in the main comment section. Text added to L.153: “While the raw data measurements in this study were performed on the Dragon Mound core (GeoB18116-2) and the long off-mound core (MD13-3457), (raw) data from BRI (GeoB13729-1) and the shorter off-mound core (GeoB13731-1) have been previously published. The latter data have been obtained using the same protocols, and were eventually processed for the specific purpose of this study. The detailed origin of all raw data is outlined for each methodological step, and in Table 1.” Table 1 gives a good insight into the origin of all data, and we added several more clarifying edits to the methods section about where the data are from throughout.
Consider including some of the more general information about the study region from section “study site” in the general introduction and not as a separate section and including the more specific information about the sampling locations in the methods.
Response: Thanks for your suggestion. We agree and moved the more specific information to the Methods section (L.111), which makes the structure clearer indeed. The more general information remains as a small chapter now called “Regional Setting” following the standard manuscript structure of similar publications in this specific field (e. g., Wienberg et al., 2022; Corbera et al., 2021; Hebbeln et al., 2019; Fink et al., 2013).
Lines 73-75: Consider only citing some of these studies as examples.
Response: Agreed, we removed some of the references.
Figure 1: Delete “Overview map”. A) is missing in figure legend (and describe location of red box in A, not in B). Can you change the orientation of the mound in C to be the same in both pictures (as it is the case in D)? That would make it easier for the reader to understand where on the mound the sediment core was taken. Consider using stars of different colours for sediment cores from coral mounds and off-mounds. A scale bar in B would help to better understand how far away both coral mounds are located.
Response: All suggestions were addressed in the new version of the manuscript.
What is the time period covered by core MD13-3457? This is only stated in Table 1 but not in the text.
Response: We added the age range of our off-mound record to the results section of the new version, and its entire record is now also presented in the discussion and Fig. 6.
As the methods section is rather long, consider moving some of it to the supplementary methods, e.g. methods that describe data collected by previous studies. This would also make it more clear which data were collected for this specific study and which data were used from previous studies.
Response: Thank you for your thoughtful comment, we fully agree that the methods section is rather long. However, we have streamlined the text in earlier versions of the manuscript, and believe that including the remaining details in the main methods section is important for ensuring clarity and context for the reader. In our opinion, keeping them in the main text will help readers follow the logical flow of the study without needing to refer to the supplementary materials. We did, however, remove already published information under 3.4. and added some clarifications on the data origin, please refer here to comment above on data origin. This should also clarify that, whilst some raw data have been produced by previous studies, the compilation and all the methods presented have been carried out (at least once) solely for the purpose of this study.
Figure 3: Grey shaded area in the background that show the timepoint in the record for bars of carbon(ate) accumulation data is difficult to see, use darker colour or contours to make this more clear. The same is the case for light blue bares for glacial periods in top graph. Unclear which Y axis the bars and line in the top graph correspond to. I would suggest using two different colours for both to make the difference more clear.
Response: Done.
Figure 5: Define acronym CWC in figure legend.
Response: Done.
Isn’t section “5.4 Cold-water coral mounds as carbon sinks” the main finding of this paper and should therefore be stated at the beginning of the discussion?
Response: We appreciate the comment, and are aware of this consideration. We have heavily discussed this multiple times amongst the co-authors and the initial draft’s discussion indeed had the structure as suggested here by Referee #1. However, after careful considerations, all co-authors agreed to show the main findings at the end. From our perspective, leading towards the main finding at the end of the discussion feels more natural in this specific case, whereas its importance would get lost if it was already presented at the beginning of the discussion.
The Response is also supplied as PDF format (text identical, attached).
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AC1: 'Reply on RC1', Luis Greiffenhagen, 31 Jan 2025
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RC2: 'Comment on egusphere-2024-2532', Evan Edinger, 18 Dec 2024
Evaluation.
This MS synthesizes existing research and presents summary figures (with details in supplementary material) on rates of sediment accumulation, carbonate accumulation, and organic matter accumulation in cold-water coral mounds of the western Mediterranean sea, and compares these with adjacent and broadly contemporaneous off-mound sediments.
The basic premise of the paper is that greatly enhanced mound accretion rates during mound-building phases of cold-water coral mounds, sequester vast amounts of inorganic carbon and organic carbon. Inorganic carbon accumulates in the form of either coral skeletal calcium carbonate or other sedimentary inorganic carbon. Furthermore, sedimentary organic carbon is trapped within the mound sediments when mounds are growing. The organic carbon content of mound sediments is about 25% higher than that of off-mound sediments, but because mound accretion rates are roughly an order of magnitude higher than rates in contemporary and adjacent off-mound sediments, the mounds function as much more effective carbon sinks than background sedimentation.
This paper is an important demonstration of the concept at a local scale, using two different records from cold-water coral mounds from the Younger Dryas back to the mid-Pleistocene (MIS stage 11, roughly 400 ka). The paper should most definitely be published, following some minor revisions.
General comments.
The paper most definitely merits publication, following a few minor corrections. Overall, the paper is scientifically sound and demonstrates both the mound accretion rates and the carbonate and organic carbon accumulation rates in great detail.
The presence of a long core (drilled, with the MeBo) through 6 phases of (interglacial) mound growth back to MIS 11, is highly valuable. It would be even more valuable if the complete record or carbon(ate) accumulation rates from the longer off-mound cores were presented, rather than just the comparisons with the two mound-forming intervals (YD, and MIS 5). The raw data and the age models for the off-mound cores are already in the supplementary material. It is important to know how glacial-interglacial cycles affected non-coral carbonate accumulation, and sedimentary organic matter accumulation in off-mound sediments. This should be an easy addition. Similarly, they could refer to published off-mound cores from elsewhere in the region that might extend back to MIS 11, should they exist (they probably do).
The textbook interpretation is that cold-water corals, and mounds, are relatively unimportant in global carbonate and carbon budgets because the mounds are small and not widespread, and they occur sporadically in the rock record. The title of this paper appears to challenge that textbook view. The next logical step that should be taken, but probably not in this manuscript, would be to integrate the inorganic and organic carbon accumulation rates through space and time. Specifically, it would be valuable to calculate the area of the similar depth sediments, and the length of time of mound accumulation vs. the total time of background sedimentation (with varying rates between glacial and interglacial stages). This integration through time and space would actually indicate the total contribution of cold-water coral mounds in this region to carbonate and organic carbon sequestration. While this analysis is beyond the scope of the current paper, the authors should acknowledge the importance of extrapolating their results beyond the current analysis.
Technical Corrections
Marginal comments are included throughout the marked-up version of the MS.
Methods, Data, analyses, and interpretations.
The methods are appropriate; the data and analyses are well-presented and the interpretations are consistent with the data.
Writing.
The manuscript is well-written and well-referenced.
The authors should remove the phrase “so-called” from the manuscript (it is used, inappropriately, in the first line of abstract and in at least 2 other parts of the MS. This phrase has a perjorative connotation that implies that the term (in this case, CWC mounds) is in some way incorrect.
Similarly, the term “time-averaging” should not placed in quotes. Time-averaging is a well-established concept in paleontology and sedimentary geology.
At a few points in the marked-up MS, writing corrections are identified that should be addressed. For example, lines 526-258 are not a complete sentence. Furthermore, the statements in those lines require greater clarification scientifically).
Figures.
In general, the figures are of high quality, and most do not require revisions.
The inset maps in Fig 1 C and D should have latitude, longitude, and scale added.
Figure 5 is particularly important for summary ideas, but would really benefit from individual marker labels. Because there is such high variation in aggradation rates and carbonate accumulation rates among the different mound growth phases in the DM core, labelling the individual growth phases DM1-DM6 would help the reader. Similarly, labelling the other data points, so that we know whether the variation is spatial or temporal (or both) would be very helpful.
Additional marginal comments throughout the MS should be considered by the authors.
My identity may be revealed to the authors, if journal policies so permit.
Evan Edinger
Geography / Biology / Earth Sciences
Memorial University of Newfoundland, Canada
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AC2: 'Reply on RC2', Luis Greiffenhagen, 31 Jan 2025
Dear Professor Evan Edinger,
Thank you for dedicating your time and expertise to reviewing our manuscript. Your comments and constructive feedback have been very helpful and we believe that addressing your suggestions has significantly improved the quality of this work.
From your general comments, we identify three main points. We will address these first in a summary, and then reply to your specific comments in detail (your comments in italics, new text in the manuscript in quotation marks).
- It would be even more valuable if the complete record or carbon(ate) accumulation rates from the longer off-mound cores were presented […]
- Similarly, they could refer to published off-mound cores from elsewhere in the region that might extend back to MIS 11, should they exist (they probably do) / we need a longer record from the off-mound site.
- what about the integral of the total accumulation through time?
We are grateful for this suggestion which has led to a few additions to the manuscript (see also new text provided in the detailed comment section), and extended the interpretation of our study. First, we now show the full range of our off-mound carbon accumulation, and present mean and min-max ranges. Most importantly, we have identified and integrated the only known published sediment core from the Southern Alborán Sea (ODP161-979A) that extends back to MIS11, to allow for a full comparison between the history of Dragon Mound and the background sedimentation, i.e., the common seafloor. Due to IODP standard procedures, organic and inorganic carbon content, as well as sediment density were measured and are available online, along with a biostratigraphic age-depth model. The organic and inorganic carbon accumulation rates from our off-mound core record match well with the background rates from the ODP core, which suggests that the off-mound cores are representative for the area and that it is reasonable to use the ODP core as off-mound reference site for older coral mound formation phases. Compiling the ODP data underlines that also during earlier, enhanced formation stages, the CWC mound accumulates more carbon than the background environment. By adding the background carbon accumulation rates from ODP161-979A, we are now able to make a fully integrated comparison of total carbon accumulation comparison over the entire time span of 400 kyr of mound formation. We first outline how our off-mound cores can only document part of the mound formation history, and then, through ODP161-979A, provide evidence that, even across multiple glacial cycles, despite > 200 kyrs of missing on-mound sedimentation, the total carbon accumulation is higher on the CWC mound than in the background record. All these outlined additions are also illustrated in Fig. 6 of the new manuscript.
We also appreciate your perspective on further spatial upscaling beyond the scope of this paper; indeed, this aspect is already planned to be addressed as a deliverable of our next research project and a further motivation.
Specific Comment Responses:
All minor, non-content related suggestions, e.g., general phrasing, abbreviations, references, were accepted and integrated into the new manuscript. These and all in-line comments that refer to a concern that has already been addressed before, are not listed individually in this response in order to make the response letter more manageable.
remove “so-called” from text, as well as quotation marks for common terms
Response: We fully agree and have applied the suggested changes throughout.
L53: [CWC mound carbonate accumulation is in the same range as tropical coral reefs etc.] This is arguable - I think a caveat is appropriate - they can be in the same range, that is overlap the range, but in general, we are looking at g/ m2/ y, not kg/ m2/y, like G, as used for warm-water coral reef carbonate budgets.
Response: We added “can be”, instead of “is / are”.
L142: keV instead of kV?
Response: In fact, x-ray source generator units are given in kV.
L160: This sounds analogous to the core "decompression" based on kinematics, like what IFREMER has been doing. Similarly, should publish whatever kinematic or other data are used to calculate revised depths.
Response: For clarification, we applied some minor changes to the section (see L.184f). Here we need to deal with post-drilling core expansion, and apply a “linear squeeze”, i. e. core compression scaling, where each core data point from the CT scan is recalculated by linear interpolation to a new depth through the MeBo-specific standard length of 235 cm (100% recovery) per barrel. It is necessary here to assume that the expansion was proportional to the total length of the core barrel. As suggested, we publish the depth model along with our data points in PANGAEA.
L247: why not overlaps? Two records are better, no ?!
Response: Yes, a full analysis of both cores (TIC/TOC and DBD) would generally be better. Yet, there was certainly a budget limitation to double-measuring the same record twice. Based on the matching coordinates of the two cores, we define the two cores to be in the same location. Secondly, as the ages / stratigraphies do overlap, we can be confident to create one continuous “off-mound” record, with the longer core MD13-3457 from 12 kyrs BP onwards. However, also upon your helpful suggestions later in the review, we decided to compare our off-mound results to the only other known core from the region (ODP161-979A) and find that our mean off-mound rates on carbon accumulation lie in the same range, suggesting our record is representative for the purpose of this study (see above).
L266: Remind us what is the origin of the 12.01/100.09 value at the end, since it has not units. Is this based on the atomic weight of carbon, vs CaCO3?
Response: Yes indeed, this is the atomic weight ratio of carbon vs. carbonate (in molar mass g mol-1, unit added to text in new version). Since we quantify the coral (i.e., carbonate) volume and mass in the analysis / equation first, it is necessary to convert to carbon at the end. Slight correction added.
L420: The timing is pretty interesting - MIS 5c, MIS7b - so not at the peaks of either integlacial, and during the cooling limbs of MIS 9 and MIS 11. Presumably the timing is discussed somewhere - if in Wienberg et al. 2022, then that should be referenced, and a short summary repeated.
Response: Indeed, the timing is heavily discussed in Wienberg et al. 2022. Following your comment, we add a very brief summary of the interpretation of CWC mound formation timing in the Alborán Sea in the “Regional Setting” introduction section in L. 93ff. of the new manuscript: “[…] Mound formation phases in the EMCP correspond mainly to interglacial periods, but also to one glacial period (MIS 10). While there is no clear link between phases of active CWC mound formation and ice age-paced climate oscillations, there seems to be a strong coupling with changes in the African hydroclimate (see Wienberg et al., 2022 for a detailed description)..” We also reference Wienberg et al. (2022) in the Age Model and Results section (L.190ff and L.354ff, respectively) when describing the results for Dragon Mound and list the marine isotope stages that correspond to the mound formation phases presented.
Jointly listed:
L461, 501, 602 / Fig. 4: Here, it is REALLY important to present the full history of C burial in the off-mound core.
L490.2: But, what about the integral of the total accumulation through time? […] Which one sequesters more C over time?
Response: This is a very valuable point raised. As outlined above, we aim to address all these concerns through adding ODP core 161-797A to our interpretation and integrating total carbon accumulation over the full ~400 kyr of record. We also include the full range of our off-mound accumulation into our interpretation now (GeoB13731-1 and MD13-3457) and all TIC/TOC data of the entire record will be published in PANGAEA. While a detailed analysis and the palaeoceanographic interpretation of the background carbon accumulation dynamics in the Southern Alborán Sea goes beyond the scope this paper, we provide a first compilation, which may also be helpful for further studies on regional marine carbon cycles. We compare, as suggested, the total integrated carbon accumulation over ~400 kyr in Dragon Mound vs. background seafloor. We applied changes as outlined above, which involved a restructuring of the discussion section “5.4 Cold-water coral mounds as carbon sinks.”
The new addition (L. 691ff) to the text is provided here: “In addition to comparing carbon accumulation rates during the mound formation phases, assessing the total long-term net carbon burial throughout a mound’s entire existence (i.e., Dragon Mound, ~400 kyr) in both on- and off-mound settings may provide further insight into the potential role of CWC mounds in the marine carbon cycle. Notably, there were extensive periods without any mound formation or carbon accumulation (see hiatuses, Fig. 3), whereas off-mound accumulation is expected to be continuous. However, our off-mound record is comparably short (~126 kyr) and only overlaps with one mound formation phase on Dragon Mound. A stratigraphically longer off-mound record, preferably spanning the full record of Dragon Mound from initiation until present, would allow a much more meaningful comparison. Sediment core records documenting ~400 kyr of carbon accumulation are scarce in the Southern Alborán Sea – The only available off-mound sediment core record that provides the necessary data is ODP Leg 161 Hole 979A (Comas et al., 1996; ODP Shipboard Scientific Party, 1996), at 1062 mbsl (35°43.427'N, 3°12.353'W; data sources: https://web.iodp.tamu.edu/OVERVIEW/?&exp=161&site=979, biostratigraphic age model from De Kaenel et al., 1999). To assess the applicability of this ODP core as a valuable off-mound counterpart to our on-mound record from Dragon Mound, we first compare it to the off-mound record analysed in this study. Over the last >100 kyr, the herein presented off-mound record (originating from cores GeoB13731-1 and MD13-3457) with a mean organic carbon accumulation rate of 0.2 g Corg cm-2 kyr-1 (range: 0.03 – 0.7 g Corg cm-2 kyr-1) is comparable to the 0.3 g Corg cm-2 kyr-1 (range: 0.1 – 0.5 g Corg cm-2 kyr-1) in the ODP core. Similarly, mean inorganic carbon accumulation of 0.9 g Cinorg cm-2 kyr-1 (range: 0.3 – 3.6 g Cinorg cm-2 kyr-1) at our off-mound site is very close to 1.0 g Cinorg cm-2 kyr-1 (range: 0.7 – 1.3 g Cinorg cm-2 kyr-1) in the ODP core. Consequently, also the mean total carbon accumulation rates in both cores are quite similar with 1.1 g C cm-2 kyr-1 (range: 0.3 – 4.3 g C cm-2 kyr-1) and 1.3 g C cm-2 kyr-1 (range: 0.9 – 1.7 g C cm-2 kyr-1), respectively, highlighting the regional consistency of off-mound net carbon burial through time. Correspondingly, our off-mound core record represents not only local, but also regional off-mound accumulation dynamics well suited to the purpose of this study.
Finally, the integration of net carbon burial for on- and off-mound (GeoB18116-2 vs. ODP161-979A, respectively) over the complete evolution of Dragon Mound since mound initiation, i.e., the last ~400 kyr, covering several glacial-interglacial cycles, reveals that total carbon accumulation at Dragon Mound (649 g C cm-2) is 20% higher than at the off-mound site ODP161-979A (544 g C cm-2; Fig. 6 in the new manuscript). Importantly, this is observed despite >200 kyr of non-sedimentation on the mound. In this direct comparison the extensive BRIfinal mound formation phase, widely recorded in the EMCP (see Wienberg et al., 2022), cannot be considered as it is not documented at Dragon Mound. Thus, for other CWC mounds in the EMCP, the surplus on-mound carbon accumulation might be even higher.”
L490: What about the differences in space? How much of the Alboran sea had mounds in it during the Holocene or in MIS 5?
Assessing the volumes of the mounds (or even their footprints only) would require adding another somewhat complex methodology here – morphometrics. This would significantly increase the length of this manuscript and presents a study in itself. However, the very different mound formation histories for the individual mounds, see Wienberg et al. (2022), would add another level of complexity to do such a spatial analysis. To our feeling, this would overload the paper. Generally, the relative fraction of the seafloor covered by CWC mounds is likely to be minor. On the other hand, large stretches of the continental slopes’ seafloor off NW Africa that have been mapped, do have CWC mounds present (for example, > 1500 km² off Atlantic Morocco, Hebbeln et al., 2019), and many parts of the South Alborán seafloor are unexplored. We further mention the limitations of spatial upscaling in our conclusion (L.753).
L490 / Table 4: Since comparing the organic carbon concentrations becomes important later on in the discussion, it would be important to report the number of measurements included, and to present an average, standard error, and maybe even a statistical comparison of mound vs off-mound.
Response: We added the number of measurements and the standard deviation (SD) to Table 4. Beyond that, a statistical comparison would indeed be a great idea, which would however need more measurements and go beyond the scope of this study.
L526: This is not a complete sentence. Some were locally absent, and some were locally displaced. Those two are quite different.
Response: Thank you for spotting this! We adapted the sentence (L. 541f) and added references. “For instance, present-day shallow-water reefs are carbonate factories that swing between states of active formation and extinction, being locally absent or displaced due to periodic sea-level fluctuations and associated environmental change (e. g., Schlager, 1981; Milliman, 1993; Camoin and Webster, 2015; Wood et al., 2023).”
L561: This graph could be improved in two ways. First, all of the points in this study should have labels (for the different DM phases, and for the 1 phase from BRI. Similarly, the 6 points from previous studies should also have location labels added to them. Finally, adding the background values from the off-mound core here would be valuable, and would show that the low accumulation rate portions are still somewhat elevated above "background" (off-mound) carbon accumulation rates.
Response: Following your suggestion, we revised this graph and added the labels to the graph and it is now much more informative. We also tried to add the two off-mound records, however, while we agree that it would be good to see them there too, they are not part of the regression curve, and also would overload the lower part of the graph. As this section only focusses on the mounds themselves, we think it is appropriate to only present the mound records here. Yet, through changes outlined above, our new graph including the much longer published off-mound core shows how low mound accumulation rates and off-mound rates relate. Notably, upon reviewing the graph, we also decided to remove the data point from Urania Bank, since it comes from a “CWC talus deposit” and not a mound sensu stricto. However, this increased the R² to 0.97.
L595: yes, unless submarine canyons are a major source of sediment, as in some regions of the NE Atlantic.
Response: Great point, we replaced “naturally” by “generally”.
The Author Response is also provided in PDF format (identical text, attached).
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AC2: 'Reply on RC2', Luis Greiffenhagen, 31 Jan 2025
Status: closed
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RC1: 'Comment on egusphere-2024-2532', Anonymous Referee #1, 27 Oct 2024
Dear authors, dear editor,
In this paper, Greiffenhagen et al. calculated the amount of inorganic and organic carbon accumulation at two CWC mounds in the Mediterranean Sea using two sediment cores. They compared the results to two cores from the adjacent seafloor and showed that CWC mounds store much more carbon than the adjacent seafloor.
General comments
The manuscript is very well written and structured and I only have some minor suggestions for improvement (see below). Some more background information about CWCs and CWC reefs should be provided in the introduction, an explanation should be given why both off-mound sediment cores were collected in the same location and it should be clarified in the methods section which new information was added by this study and which data were taken from previous studies.
Specific comments
Abstract
The abstract provides a very nice summary of the study.
Introduction
The introduction provides background information on the carbon cycles and CWC mound formation. However, it lacks general information about CWCs (e.g. distribution, depth, biology etc.) and CWC reefs (e.g. reef structure, information about reef-forming species), which should also be included in the introduction in my opinion.
You state that previous studies have only focused on the accumulation of inorganic carbon on CWC mounds and state which information is still missing. However, can you also include some results of these previous studies to provide some more context?
Methods
Why were both off-mound cores collected so close to each other? Would it not be a better representation of the carbon accumulation rates in the region if they were chosen from different locations?
It is stated in the introduction that the new information provided in this study is the calculation of carbon accumulation on CWC mounds by taking into account both the inorganic and organic carbon content. However, it is stated in Table 1 that Wang et al. (2021) have already determined TIC and TOC values for two of the four cores. What is the new information provided by this study, except combining all information for all four cores in one manuscript and including some more data for some of the cores?
Please also provide some information here how the cores were prepared for CT scanning.
You cite a lot of previous studies that have been working on the same sediment cores. What exactly is the new information that is provided by this manuscript? This should be made more clear in the introduction and methods section.
Results
This section seems to be fine and the results are supported by nice figures.
Discussion
Line 519: However, the two off-mound cores were collected at the same location. Therefore, they do not provide a good range of “background” carbon accumulation rates. Therefore, how comparable are the coral-mound to off-mound carbon accumulation rates really?
Conclusion
This section provides a good summary of the main findings of this study.
Technical corrections
Consider including some of the more general information about the study region from section “study site” in the general introduction and not as a separate section and including the more specific information about the sampling locations in the methods.
Lines 73-75: Consider only citing some of these studies as examples.
Figure 1: Delete “Overview map”. A) is missing in figure legend (and describe location of red box in A, not in B). Can you change the orientation of the mound in C to be the same in both pictures (as it is the case in D)? That would make it easier for the reader to understand where on the mound the sediment core was taken. Consider using stars of different colours for sediment cores from coral mounds and off-mounds. A scale bar in B would help to better understand how far away both coral mounds are located.
What is the time period covered by core MD13-3457? This is only stated in Table 1 but not in the text.
As the methods section is rather long, consider moving some of it to the supplementary methods, e.g. methods that describe data collected by previous studies. This would also make it more clear which data were collected for this specific study and which data were used from previous studies.
Figure 3: Grey shaded area in the background that show the timepoint in the record for bars of carbon(ate) accumulation data is difficult to see, use darker colour or contours to make this more clear. The same is the case for light blue bares for glacial periods in top graph. Unclear which Y axis the bars and line in the top graph correspond to. I would suggest using two different colours for both to make the difference more clear.
Figure 5: Define acronym CWC in figure legend.
Isn’t section “5.4 Cold-water coral mounds as carbon sinks” the main finding of this paper and should therefore be stated at the beginning of the discussion?
Citation: https://doi.org/10.5194/egusphere-2024-2532-RC1 -
AC1: 'Reply on RC1', Luis Greiffenhagen, 31 Jan 2025
Dear Reviewer,
Thank you very much for providing your expertise and time to review our manuscript. Your comments and very helpful feedback most definitely improved this work.
From your general comments, we identify three main points raised. We will address these one by one, and then respond to all specific comments in detail (your comments in italics, new text in the manuscript in quotation marks).
1. More background information about CWCs and CWC reefs should be provided in the introduction
We appreciate your comment, and included a few more information on CWCs and CWC reefs in the first paragraph of the introduction (see specific comment response below.)
2. An explanation should be given why both off-mound sediment cores were collected in the same location
This study was conceived long after the research cruise campaign and we 100% agree that multiple off-mound core “background” locations would give a better representation. Unfortunately, there are no other cores from the East Melilla Coral Mound Province available, and the Southern Alborán Sea is rather scarce in sediment core data. However, upon your feedback and a similar question regarding this matter from Referee#2, we decided to include data from the only known publicly available drill core of the region, ODP 161-979A, into our discussion and interpretation. Our mean background carbon accumulation rates from the off-mound location match well with the average values from this (lower-resolution) core during the last > 100 kyr, suggesting that, apart from some variability, it is a representative record.
3. It should be clarified in the methods section which new information was added by this study and which data were taken from previous studies.
The agglomeration of data in this study is indeed very complex. Upon your feedback, we added a short clarification / narrative of the data’s origin to the beginning of the methods section, stating which part was fully measured for this study, and what has been available already. We also applied a few edits throughout the methods to make it all more clear. The main work of the study corresponds to the 70 m long Dragon Mound record of GeoB18116-2, where, except for the coral dating, everything was conducted for the purpose of this study. However, Dragon Mound lacks a record of Holocene mound formation, so to ensure a representative understanding of CWC mound formation across the entire region, we incorporate data from the Holocene formation phase on BRI (GeoB13729-1), which were luckily already available. Regarding the off-mound cores, corresponding Holocene off-mound core data (GeoB13731-1), are also available. In order to be able to compare the much older Dragon Mound record with off-mound data from the same time period, we also analysed the stratigraphically longer off-mound core MD13-3457, which is published for the first time in this study and is the second major work carried out exclusively for this study. The addition and processing of previously published data allows us to cover the full scope of CWC mound development in the East Melilla CWC Mound Province over the last 400,000 years. None of the data have been used specifically to calculate organic and inorganic carbon accumulation before, and all the methods presented have been carried out (at least once) solely for the purpose of this study. At the beginning of each methodological step, it is stated which core was previously analysed, and which one has been analysed for this study.
Specific Comment Responses:
The introduction provides background information on the carbon cycles and CWC mound formation. However, it lacks general information about CWCs (e.g. distribution, depth, biology etc.) and CWC reefs (e.g. reef structure, information about reef-forming species), which should also be included in the introduction in my opinion.
Response: As outlined above we followed your suggestion and add the following text (L.43): “CWCs occur circumglobally, with only a few scleractinian CWCs, i.e. carbonate framework-forming corals such as Lophelia pertusa (syn. Desmophyllum pertusum) and Madrepora oculata being capable of building entire CWC reefs (Roberts et al., 2009; Wienberg and Titschack, 2017). With their three-dimensional framework, these CWCs provide habitat and the foundation for some of the most biodiverse ecosystems of the deep sea (Henry and Roberts, 2007; 2017). The occurrence and proliferation of CWC reefs depends on a certain range of environmental conditions such as temperature, pH, and oxygenation (e. g., Davies and Guinotte, 2011), combined with sufficient nutrient supply through bottom currents and primary production (e. g., Portilho-Ramos et al., 2022; Maier et al., 2023).”
You state that previous studies have only focused on the accumulation of inorganic carbon on CWC mounds and state which information is still missing. However, can you also include some results of these previous studies to provide some more context?
Response: Upon your suggestion, we decided to include a range (0.3 – 2114 g CaCO3 cm-2 kyr-1) of all published CWC mound carbonate accumulation values into the introduction (L.66), while we go into detail regarding previous studies in the discussion.
Why were both off-mound cores collected so close to each other? Would it not be a better representation of the carbon accumulation rates in the region if they were chosen from different locations? // Line 519: However, the two off-mound cores were collected at the same location. Therefore, they do not provide a good range of “background” carbon accumulation rates. Therefore, how comparable are the coral-mound to off-mound carbon accumulation rates really?
Response: Agreed and changes applied, see point 2) in the main comment section.
Please also provide some information here how the cores were prepared for CT scanning.
Response: Done, information added in 3.1 (L.162), stating: “The coral-bearing core sections of GeoB18116-2 were frozen at -20°C and split length-wise with a stone saw, into a “work” half for sediment sampling, and an “archive” half (undisturbed) for CT scanning.”
It is stated in the introduction that the new information provided in this study is the calculation of carbon accumulation on CWC mounds by taking into account both the inorganic and organic carbon content. However, it is stated in Table 1 that Wang et al. (2021) have already determined TIC and TOC values for two of the four cores. What is the new information provided by this study, except combining all information for all four cores in one manuscript and including some more data for some of the cores? // You cite a lot of previous studies that have been working on the same sediment cores. What exactly is the new information that is provided by this manuscript? This should be made more clear in the introduction and methods section.
Response: Agreed that this may be confusing, see also point 3) in the main comment section. Text added to L.153: “While the raw data measurements in this study were performed on the Dragon Mound core (GeoB18116-2) and the long off-mound core (MD13-3457), (raw) data from BRI (GeoB13729-1) and the shorter off-mound core (GeoB13731-1) have been previously published. The latter data have been obtained using the same protocols, and were eventually processed for the specific purpose of this study. The detailed origin of all raw data is outlined for each methodological step, and in Table 1.” Table 1 gives a good insight into the origin of all data, and we added several more clarifying edits to the methods section about where the data are from throughout.
Consider including some of the more general information about the study region from section “study site” in the general introduction and not as a separate section and including the more specific information about the sampling locations in the methods.
Response: Thanks for your suggestion. We agree and moved the more specific information to the Methods section (L.111), which makes the structure clearer indeed. The more general information remains as a small chapter now called “Regional Setting” following the standard manuscript structure of similar publications in this specific field (e. g., Wienberg et al., 2022; Corbera et al., 2021; Hebbeln et al., 2019; Fink et al., 2013).
Lines 73-75: Consider only citing some of these studies as examples.
Response: Agreed, we removed some of the references.
Figure 1: Delete “Overview map”. A) is missing in figure legend (and describe location of red box in A, not in B). Can you change the orientation of the mound in C to be the same in both pictures (as it is the case in D)? That would make it easier for the reader to understand where on the mound the sediment core was taken. Consider using stars of different colours for sediment cores from coral mounds and off-mounds. A scale bar in B would help to better understand how far away both coral mounds are located.
Response: All suggestions were addressed in the new version of the manuscript.
What is the time period covered by core MD13-3457? This is only stated in Table 1 but not in the text.
Response: We added the age range of our off-mound record to the results section of the new version, and its entire record is now also presented in the discussion and Fig. 6.
As the methods section is rather long, consider moving some of it to the supplementary methods, e.g. methods that describe data collected by previous studies. This would also make it more clear which data were collected for this specific study and which data were used from previous studies.
Response: Thank you for your thoughtful comment, we fully agree that the methods section is rather long. However, we have streamlined the text in earlier versions of the manuscript, and believe that including the remaining details in the main methods section is important for ensuring clarity and context for the reader. In our opinion, keeping them in the main text will help readers follow the logical flow of the study without needing to refer to the supplementary materials. We did, however, remove already published information under 3.4. and added some clarifications on the data origin, please refer here to comment above on data origin. This should also clarify that, whilst some raw data have been produced by previous studies, the compilation and all the methods presented have been carried out (at least once) solely for the purpose of this study.
Figure 3: Grey shaded area in the background that show the timepoint in the record for bars of carbon(ate) accumulation data is difficult to see, use darker colour or contours to make this more clear. The same is the case for light blue bares for glacial periods in top graph. Unclear which Y axis the bars and line in the top graph correspond to. I would suggest using two different colours for both to make the difference more clear.
Response: Done.
Figure 5: Define acronym CWC in figure legend.
Response: Done.
Isn’t section “5.4 Cold-water coral mounds as carbon sinks” the main finding of this paper and should therefore be stated at the beginning of the discussion?
Response: We appreciate the comment, and are aware of this consideration. We have heavily discussed this multiple times amongst the co-authors and the initial draft’s discussion indeed had the structure as suggested here by Referee #1. However, after careful considerations, all co-authors agreed to show the main findings at the end. From our perspective, leading towards the main finding at the end of the discussion feels more natural in this specific case, whereas its importance would get lost if it was already presented at the beginning of the discussion.
The Response is also supplied as PDF format (text identical, attached).
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AC1: 'Reply on RC1', Luis Greiffenhagen, 31 Jan 2025
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RC2: 'Comment on egusphere-2024-2532', Evan Edinger, 18 Dec 2024
Evaluation.
This MS synthesizes existing research and presents summary figures (with details in supplementary material) on rates of sediment accumulation, carbonate accumulation, and organic matter accumulation in cold-water coral mounds of the western Mediterranean sea, and compares these with adjacent and broadly contemporaneous off-mound sediments.
The basic premise of the paper is that greatly enhanced mound accretion rates during mound-building phases of cold-water coral mounds, sequester vast amounts of inorganic carbon and organic carbon. Inorganic carbon accumulates in the form of either coral skeletal calcium carbonate or other sedimentary inorganic carbon. Furthermore, sedimentary organic carbon is trapped within the mound sediments when mounds are growing. The organic carbon content of mound sediments is about 25% higher than that of off-mound sediments, but because mound accretion rates are roughly an order of magnitude higher than rates in contemporary and adjacent off-mound sediments, the mounds function as much more effective carbon sinks than background sedimentation.
This paper is an important demonstration of the concept at a local scale, using two different records from cold-water coral mounds from the Younger Dryas back to the mid-Pleistocene (MIS stage 11, roughly 400 ka). The paper should most definitely be published, following some minor revisions.
General comments.
The paper most definitely merits publication, following a few minor corrections. Overall, the paper is scientifically sound and demonstrates both the mound accretion rates and the carbonate and organic carbon accumulation rates in great detail.
The presence of a long core (drilled, with the MeBo) through 6 phases of (interglacial) mound growth back to MIS 11, is highly valuable. It would be even more valuable if the complete record or carbon(ate) accumulation rates from the longer off-mound cores were presented, rather than just the comparisons with the two mound-forming intervals (YD, and MIS 5). The raw data and the age models for the off-mound cores are already in the supplementary material. It is important to know how glacial-interglacial cycles affected non-coral carbonate accumulation, and sedimentary organic matter accumulation in off-mound sediments. This should be an easy addition. Similarly, they could refer to published off-mound cores from elsewhere in the region that might extend back to MIS 11, should they exist (they probably do).
The textbook interpretation is that cold-water corals, and mounds, are relatively unimportant in global carbonate and carbon budgets because the mounds are small and not widespread, and they occur sporadically in the rock record. The title of this paper appears to challenge that textbook view. The next logical step that should be taken, but probably not in this manuscript, would be to integrate the inorganic and organic carbon accumulation rates through space and time. Specifically, it would be valuable to calculate the area of the similar depth sediments, and the length of time of mound accumulation vs. the total time of background sedimentation (with varying rates between glacial and interglacial stages). This integration through time and space would actually indicate the total contribution of cold-water coral mounds in this region to carbonate and organic carbon sequestration. While this analysis is beyond the scope of the current paper, the authors should acknowledge the importance of extrapolating their results beyond the current analysis.
Technical Corrections
Marginal comments are included throughout the marked-up version of the MS.
Methods, Data, analyses, and interpretations.
The methods are appropriate; the data and analyses are well-presented and the interpretations are consistent with the data.
Writing.
The manuscript is well-written and well-referenced.
The authors should remove the phrase “so-called” from the manuscript (it is used, inappropriately, in the first line of abstract and in at least 2 other parts of the MS. This phrase has a perjorative connotation that implies that the term (in this case, CWC mounds) is in some way incorrect.
Similarly, the term “time-averaging” should not placed in quotes. Time-averaging is a well-established concept in paleontology and sedimentary geology.
At a few points in the marked-up MS, writing corrections are identified that should be addressed. For example, lines 526-258 are not a complete sentence. Furthermore, the statements in those lines require greater clarification scientifically).
Figures.
In general, the figures are of high quality, and most do not require revisions.
The inset maps in Fig 1 C and D should have latitude, longitude, and scale added.
Figure 5 is particularly important for summary ideas, but would really benefit from individual marker labels. Because there is such high variation in aggradation rates and carbonate accumulation rates among the different mound growth phases in the DM core, labelling the individual growth phases DM1-DM6 would help the reader. Similarly, labelling the other data points, so that we know whether the variation is spatial or temporal (or both) would be very helpful.
Additional marginal comments throughout the MS should be considered by the authors.
My identity may be revealed to the authors, if journal policies so permit.
Evan Edinger
Geography / Biology / Earth Sciences
Memorial University of Newfoundland, Canada
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AC2: 'Reply on RC2', Luis Greiffenhagen, 31 Jan 2025
Dear Professor Evan Edinger,
Thank you for dedicating your time and expertise to reviewing our manuscript. Your comments and constructive feedback have been very helpful and we believe that addressing your suggestions has significantly improved the quality of this work.
From your general comments, we identify three main points. We will address these first in a summary, and then reply to your specific comments in detail (your comments in italics, new text in the manuscript in quotation marks).
- It would be even more valuable if the complete record or carbon(ate) accumulation rates from the longer off-mound cores were presented […]
- Similarly, they could refer to published off-mound cores from elsewhere in the region that might extend back to MIS 11, should they exist (they probably do) / we need a longer record from the off-mound site.
- what about the integral of the total accumulation through time?
We are grateful for this suggestion which has led to a few additions to the manuscript (see also new text provided in the detailed comment section), and extended the interpretation of our study. First, we now show the full range of our off-mound carbon accumulation, and present mean and min-max ranges. Most importantly, we have identified and integrated the only known published sediment core from the Southern Alborán Sea (ODP161-979A) that extends back to MIS11, to allow for a full comparison between the history of Dragon Mound and the background sedimentation, i.e., the common seafloor. Due to IODP standard procedures, organic and inorganic carbon content, as well as sediment density were measured and are available online, along with a biostratigraphic age-depth model. The organic and inorganic carbon accumulation rates from our off-mound core record match well with the background rates from the ODP core, which suggests that the off-mound cores are representative for the area and that it is reasonable to use the ODP core as off-mound reference site for older coral mound formation phases. Compiling the ODP data underlines that also during earlier, enhanced formation stages, the CWC mound accumulates more carbon than the background environment. By adding the background carbon accumulation rates from ODP161-979A, we are now able to make a fully integrated comparison of total carbon accumulation comparison over the entire time span of 400 kyr of mound formation. We first outline how our off-mound cores can only document part of the mound formation history, and then, through ODP161-979A, provide evidence that, even across multiple glacial cycles, despite > 200 kyrs of missing on-mound sedimentation, the total carbon accumulation is higher on the CWC mound than in the background record. All these outlined additions are also illustrated in Fig. 6 of the new manuscript.
We also appreciate your perspective on further spatial upscaling beyond the scope of this paper; indeed, this aspect is already planned to be addressed as a deliverable of our next research project and a further motivation.
Specific Comment Responses:
All minor, non-content related suggestions, e.g., general phrasing, abbreviations, references, were accepted and integrated into the new manuscript. These and all in-line comments that refer to a concern that has already been addressed before, are not listed individually in this response in order to make the response letter more manageable.
remove “so-called” from text, as well as quotation marks for common terms
Response: We fully agree and have applied the suggested changes throughout.
L53: [CWC mound carbonate accumulation is in the same range as tropical coral reefs etc.] This is arguable - I think a caveat is appropriate - they can be in the same range, that is overlap the range, but in general, we are looking at g/ m2/ y, not kg/ m2/y, like G, as used for warm-water coral reef carbonate budgets.
Response: We added “can be”, instead of “is / are”.
L142: keV instead of kV?
Response: In fact, x-ray source generator units are given in kV.
L160: This sounds analogous to the core "decompression" based on kinematics, like what IFREMER has been doing. Similarly, should publish whatever kinematic or other data are used to calculate revised depths.
Response: For clarification, we applied some minor changes to the section (see L.184f). Here we need to deal with post-drilling core expansion, and apply a “linear squeeze”, i. e. core compression scaling, where each core data point from the CT scan is recalculated by linear interpolation to a new depth through the MeBo-specific standard length of 235 cm (100% recovery) per barrel. It is necessary here to assume that the expansion was proportional to the total length of the core barrel. As suggested, we publish the depth model along with our data points in PANGAEA.
L247: why not overlaps? Two records are better, no ?!
Response: Yes, a full analysis of both cores (TIC/TOC and DBD) would generally be better. Yet, there was certainly a budget limitation to double-measuring the same record twice. Based on the matching coordinates of the two cores, we define the two cores to be in the same location. Secondly, as the ages / stratigraphies do overlap, we can be confident to create one continuous “off-mound” record, with the longer core MD13-3457 from 12 kyrs BP onwards. However, also upon your helpful suggestions later in the review, we decided to compare our off-mound results to the only other known core from the region (ODP161-979A) and find that our mean off-mound rates on carbon accumulation lie in the same range, suggesting our record is representative for the purpose of this study (see above).
L266: Remind us what is the origin of the 12.01/100.09 value at the end, since it has not units. Is this based on the atomic weight of carbon, vs CaCO3?
Response: Yes indeed, this is the atomic weight ratio of carbon vs. carbonate (in molar mass g mol-1, unit added to text in new version). Since we quantify the coral (i.e., carbonate) volume and mass in the analysis / equation first, it is necessary to convert to carbon at the end. Slight correction added.
L420: The timing is pretty interesting - MIS 5c, MIS7b - so not at the peaks of either integlacial, and during the cooling limbs of MIS 9 and MIS 11. Presumably the timing is discussed somewhere - if in Wienberg et al. 2022, then that should be referenced, and a short summary repeated.
Response: Indeed, the timing is heavily discussed in Wienberg et al. 2022. Following your comment, we add a very brief summary of the interpretation of CWC mound formation timing in the Alborán Sea in the “Regional Setting” introduction section in L. 93ff. of the new manuscript: “[…] Mound formation phases in the EMCP correspond mainly to interglacial periods, but also to one glacial period (MIS 10). While there is no clear link between phases of active CWC mound formation and ice age-paced climate oscillations, there seems to be a strong coupling with changes in the African hydroclimate (see Wienberg et al., 2022 for a detailed description)..” We also reference Wienberg et al. (2022) in the Age Model and Results section (L.190ff and L.354ff, respectively) when describing the results for Dragon Mound and list the marine isotope stages that correspond to the mound formation phases presented.
Jointly listed:
L461, 501, 602 / Fig. 4: Here, it is REALLY important to present the full history of C burial in the off-mound core.
L490.2: But, what about the integral of the total accumulation through time? […] Which one sequesters more C over time?
Response: This is a very valuable point raised. As outlined above, we aim to address all these concerns through adding ODP core 161-797A to our interpretation and integrating total carbon accumulation over the full ~400 kyr of record. We also include the full range of our off-mound accumulation into our interpretation now (GeoB13731-1 and MD13-3457) and all TIC/TOC data of the entire record will be published in PANGAEA. While a detailed analysis and the palaeoceanographic interpretation of the background carbon accumulation dynamics in the Southern Alborán Sea goes beyond the scope this paper, we provide a first compilation, which may also be helpful for further studies on regional marine carbon cycles. We compare, as suggested, the total integrated carbon accumulation over ~400 kyr in Dragon Mound vs. background seafloor. We applied changes as outlined above, which involved a restructuring of the discussion section “5.4 Cold-water coral mounds as carbon sinks.”
The new addition (L. 691ff) to the text is provided here: “In addition to comparing carbon accumulation rates during the mound formation phases, assessing the total long-term net carbon burial throughout a mound’s entire existence (i.e., Dragon Mound, ~400 kyr) in both on- and off-mound settings may provide further insight into the potential role of CWC mounds in the marine carbon cycle. Notably, there were extensive periods without any mound formation or carbon accumulation (see hiatuses, Fig. 3), whereas off-mound accumulation is expected to be continuous. However, our off-mound record is comparably short (~126 kyr) and only overlaps with one mound formation phase on Dragon Mound. A stratigraphically longer off-mound record, preferably spanning the full record of Dragon Mound from initiation until present, would allow a much more meaningful comparison. Sediment core records documenting ~400 kyr of carbon accumulation are scarce in the Southern Alborán Sea – The only available off-mound sediment core record that provides the necessary data is ODP Leg 161 Hole 979A (Comas et al., 1996; ODP Shipboard Scientific Party, 1996), at 1062 mbsl (35°43.427'N, 3°12.353'W; data sources: https://web.iodp.tamu.edu/OVERVIEW/?&exp=161&site=979, biostratigraphic age model from De Kaenel et al., 1999). To assess the applicability of this ODP core as a valuable off-mound counterpart to our on-mound record from Dragon Mound, we first compare it to the off-mound record analysed in this study. Over the last >100 kyr, the herein presented off-mound record (originating from cores GeoB13731-1 and MD13-3457) with a mean organic carbon accumulation rate of 0.2 g Corg cm-2 kyr-1 (range: 0.03 – 0.7 g Corg cm-2 kyr-1) is comparable to the 0.3 g Corg cm-2 kyr-1 (range: 0.1 – 0.5 g Corg cm-2 kyr-1) in the ODP core. Similarly, mean inorganic carbon accumulation of 0.9 g Cinorg cm-2 kyr-1 (range: 0.3 – 3.6 g Cinorg cm-2 kyr-1) at our off-mound site is very close to 1.0 g Cinorg cm-2 kyr-1 (range: 0.7 – 1.3 g Cinorg cm-2 kyr-1) in the ODP core. Consequently, also the mean total carbon accumulation rates in both cores are quite similar with 1.1 g C cm-2 kyr-1 (range: 0.3 – 4.3 g C cm-2 kyr-1) and 1.3 g C cm-2 kyr-1 (range: 0.9 – 1.7 g C cm-2 kyr-1), respectively, highlighting the regional consistency of off-mound net carbon burial through time. Correspondingly, our off-mound core record represents not only local, but also regional off-mound accumulation dynamics well suited to the purpose of this study.
Finally, the integration of net carbon burial for on- and off-mound (GeoB18116-2 vs. ODP161-979A, respectively) over the complete evolution of Dragon Mound since mound initiation, i.e., the last ~400 kyr, covering several glacial-interglacial cycles, reveals that total carbon accumulation at Dragon Mound (649 g C cm-2) is 20% higher than at the off-mound site ODP161-979A (544 g C cm-2; Fig. 6 in the new manuscript). Importantly, this is observed despite >200 kyr of non-sedimentation on the mound. In this direct comparison the extensive BRIfinal mound formation phase, widely recorded in the EMCP (see Wienberg et al., 2022), cannot be considered as it is not documented at Dragon Mound. Thus, for other CWC mounds in the EMCP, the surplus on-mound carbon accumulation might be even higher.”
L490: What about the differences in space? How much of the Alboran sea had mounds in it during the Holocene or in MIS 5?
Assessing the volumes of the mounds (or even their footprints only) would require adding another somewhat complex methodology here – morphometrics. This would significantly increase the length of this manuscript and presents a study in itself. However, the very different mound formation histories for the individual mounds, see Wienberg et al. (2022), would add another level of complexity to do such a spatial analysis. To our feeling, this would overload the paper. Generally, the relative fraction of the seafloor covered by CWC mounds is likely to be minor. On the other hand, large stretches of the continental slopes’ seafloor off NW Africa that have been mapped, do have CWC mounds present (for example, > 1500 km² off Atlantic Morocco, Hebbeln et al., 2019), and many parts of the South Alborán seafloor are unexplored. We further mention the limitations of spatial upscaling in our conclusion (L.753).
L490 / Table 4: Since comparing the organic carbon concentrations becomes important later on in the discussion, it would be important to report the number of measurements included, and to present an average, standard error, and maybe even a statistical comparison of mound vs off-mound.
Response: We added the number of measurements and the standard deviation (SD) to Table 4. Beyond that, a statistical comparison would indeed be a great idea, which would however need more measurements and go beyond the scope of this study.
L526: This is not a complete sentence. Some were locally absent, and some were locally displaced. Those two are quite different.
Response: Thank you for spotting this! We adapted the sentence (L. 541f) and added references. “For instance, present-day shallow-water reefs are carbonate factories that swing between states of active formation and extinction, being locally absent or displaced due to periodic sea-level fluctuations and associated environmental change (e. g., Schlager, 1981; Milliman, 1993; Camoin and Webster, 2015; Wood et al., 2023).”
L561: This graph could be improved in two ways. First, all of the points in this study should have labels (for the different DM phases, and for the 1 phase from BRI. Similarly, the 6 points from previous studies should also have location labels added to them. Finally, adding the background values from the off-mound core here would be valuable, and would show that the low accumulation rate portions are still somewhat elevated above "background" (off-mound) carbon accumulation rates.
Response: Following your suggestion, we revised this graph and added the labels to the graph and it is now much more informative. We also tried to add the two off-mound records, however, while we agree that it would be good to see them there too, they are not part of the regression curve, and also would overload the lower part of the graph. As this section only focusses on the mounds themselves, we think it is appropriate to only present the mound records here. Yet, through changes outlined above, our new graph including the much longer published off-mound core shows how low mound accumulation rates and off-mound rates relate. Notably, upon reviewing the graph, we also decided to remove the data point from Urania Bank, since it comes from a “CWC talus deposit” and not a mound sensu stricto. However, this increased the R² to 0.97.
L595: yes, unless submarine canyons are a major source of sediment, as in some regions of the NE Atlantic.
Response: Great point, we replaced “naturally” by “generally”.
The Author Response is also provided in PDF format (identical text, attached).
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AC2: 'Reply on RC2', Luis Greiffenhagen, 31 Jan 2025
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