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 -
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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