the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Mar 2026
Millennial-Scale Carbon Accumulation in Mediterranean Rhodolith Deposits
Abstract. Rhodolith and maërl beds are globally relevant biogenic habitats whose long-term carbon storage capacity remains poorly quantified, particularly in the Mediterranean. To fill this gap, we investigated the formation, structure, and carbon content of a sediment deposit underlying a rhodolith bed in the Menorca Channel (Western Mediterranean). High-resolution seismo-acoustic profiling revealed a highly heterogeneous biogenic sedimentary deposit at ~60 m depth, with thickness ranging from a few centimeters to 3.7 m (mean = 0.95 m). Seven vibrocores extracted from the thickest sediment deposits were analyzed for grain size, carbonate content, bioclast composition, organic carbon, and radiocarbon age. Radiocarbon dating indicates that sediment accumulation began during the early Holocene (11,700–9,000 yr BP), when post-glacial sea-level rise transitioned the area from subaerial exposure to shallow-marine conditions. Early deposits were dominated by bivalves and dispersed coralline fragments. The establishment of modern sea level around 7,000–6,500 yr BP led to the development of dense rhodolith–maërl facies that persist today. Sediment accretion rates are low (median = 8.54 cm kyr⁻¹), reflecting very low external sediment supply, and slow growth of coralline algae. Organic carbon content in the upper 50 cm, representing the most dynamic and recently deposited carbon pool, averaged 0.57 % (± 0.22), with an estimated organic carbon stock of 32.04 (± 4.18) Mg C ha⁻¹. These results show that Mediterranean rhodolith beds act as long-term organic carbon stores, forming spatially complex Holocene deposits whose contribution to carbon storage has been largely overlooked.
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Status: open (until 06 May 2026)
- RC1: 'Comment on egusphere-2026-1394', Anonymous Referee #1, 07 Apr 2026 reply
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- 1
This manuscript addresses an important topic and presents a potentially valuable dataset combining seismo-acoustic mapping, vibrocores, radiocarbon dating, sediment characterization, and organic carbon measurements from a Mediterranean rhodolith deposit in the Menorca Channel. The study has clear potential, particularly because millennial-scale sedimentary development of rhodolith systems remains poorly constrained, and because carbon storage in such calcifying habitats is still under-documented. However, in its present form, the manuscript overstates several conclusions relative to the data presented. The central problem is a mismatch between the scale of the claims and the scale of the measurements. The paper repeatedly frames the system as a millennial-scale organic carbon reservoir, yet organic carbon was only measured in the upper 50 cm of sediment, while chronology is available for only four of seven cores and includes reworked material in at least some basal horizons. As a result, several conclusions about long-term preservation, sequestration, and carbon-reservoir function are currently more interpretive than demonstrative. A second major issue is conceptual framing. The manuscript uses blue-carbon language for a calcifying system, but only later acknowledges the unresolved balance between sedimentary organic carbon storage and calcification-related CO₂ release. That caveat is fundamental, not secondary, and needs to be built into the framing from the introduction onward. At present, the manuscript reads as more certain about climate-mitigation significance than the evidence supports. In addition, the manuscript would benefit from more rigorous treatment of representativeness, uncertainty, and inference. The geophysical mapping demonstrates strong spatial heterogeneity in deposit thickness, yet the stock estimates are based on a small number of targeted cores collected from thicker deposits. That does not invalidate the study, but it does mean that statements about the broader Menorca Channel rhodolith bed need to be more tightly bounded.
For these reasons, I think the manuscript has merit, but it requires substantial revision before it is ready for publication.
Main comments
Claims exceed direct evidence
The manuscript’s strongest direct results are: highly heterogeneous Holocene biogenic deposits, plausible onset of persistent dense rhodolith–maërl facies after sea-level stabilization, and measured organic carbon stocks in the upper 50 cm. Those are worthwhile findings. But several broader claims—especially around millennial-scale carbon preservation and long-term reservoir function—go beyond what was directly measured. The authors need to distinguish much more carefully between what is measured, what is inferred, and what remains hypothetical.
Blue-carbon framing is conceptually incomplete
The introduction foregrounds organic carbon burial in coastal ecosystems, then extends that framing to rhodolith beds, but the manuscript does not adequately distinguish carbon storage from net carbon sequestration in a calcifying carbonate-producing habitat. This issue is acknowledged only near the end of the discussion. That is too late. The manuscript should explicitly state early on that the study quantifies sedimentary organic carbon stocks and depositional history, but does not resolve net ecosystem carbon balance.
Representativeness and extrapolation need stronger caution
Seven cores were retained from twelve collected, and core sites were selected on the basis of maximum sediment thickness. This is a sensible strategy for reconstructing deposit history, but it also biases the dataset toward thicker accumulations. That sampling design needs to be discussed explicitly when presenting carbon stock estimates and any implication for the wider bed.
Chronostratigraphic interpretation is plausible but needs more restraint
The manuscript uses reworked basal dates to interpret pre-Holocene material and then infers onset and consolidation of rhodolith-rich facies during the early to mid-Holocene. This is reasonable, but the text sometimes presents the sequence rather too smoothly, given the limited number of dated horizons and evidence of reworking. The depositional history should be framed as a best-supported reconstruction rather than as a tightly resolved chronology.
The manuscript needs major language revision
There are several grammatical, syntactic, and stylistic problems that they materially affect readability and precision. Several sentences are difficult to understand or ungrammatical, and some interpretive passages become less convincing because of this. The paper would benefit from careful editing by a fluent English speaker before resubmission.
Specific comments
Abstract / lines 24–27
The abstract currently presents the results as showing that Mediterranean rhodolith beds “act as long-term organic carbon stores.” This should be toned down. The study demonstrates organic carbon stocks in the upper 50 cm within a Holocene depositional framework; it does not directly measure long-term preservation throughout the full deposit.
Introduction / line 52
There are so many papers that give definitions of rhodoliths, so no need to use this in-review citation (Cabrito et al under review) from one of the co-authors. Check articles by Basso, Bosence, etc.
Introduction / lines 60–67
This paragraph needs a clearer conceptual distinction between organic carbon storage in sediment and net climate mitigation in calcifying systems. Without that distinction, the framing is incomplete. For organic carbon, it is also important to refer to floridean starch commonly produced by many coralline algae, which might play an important role in the blue carbon realm (see Pyko et al 2025 in Aquatic Conservation: Marine and Freshwater Ecosystems, doi:10.1002/aqc.70045)
Introduction / lines 93–98
The sentence ending “reconstruct the depositional history through in relation with the last glacio-eustatic oscillation” is ungrammatical and should be rewritten. More importantly, this section should state explicitly that the OC measurements are limited to the upper 50 cm.
Methods / lines 136–139
The manuscript should report uncertainty or resolution limits for the thickness estimates derived from the acoustic interpretation. This is especially important because the manuscript emphasizes fine-scale patchiness.
Methods / lines 141–146
Why were 5 of the 12 collected cores excluded? This should be stated clearly. At present, the omission is insufficiently explained.
Methods / lines 165–169
Please clarify whether age reversals or other stratigraphic inconsistencies were evaluated formally, particularly given the presence of reworked older material.
Methods / lines 178–196
The OC methods need more detail. For highly carbonate-rich sediments, the acidification protocol, mass correction, and QA/QC steps matter. The present description is too brief for full reproducibility.
Results / lines 222–230
The mapping result is important, but the manuscript should explicitly connect this heterogeneity to the limits of stock extrapolation. The current text highlights heterogeneity descriptively but does not fully integrate its implications.
Results / lines 253–254
The figure is extremely difficult to read.
Results / lines 259–277
The age interpretation would be clearer if the authors separated reworked/pre-Holocene ages from depositional ages more explicitly in both the text and Table 2.
Results / lines 288–293
The derivation of annual carbon accumulation from the 50 cm stock and median accretion rate needs to be shown more transparently. At present, this estimate appears precise, but the underlying assumptions are not fully explained.
Results / lines 306–315
The mixed-effects model is potentially useful, but the interpretation is stronger than warranted by the limited sample size. With seven cores and upper 50 cm subsamples only, the modelling results should be presented more cautiously.
Discussion / lines 337–338
“when sea-level lowered down its rise almost reaching its actual level” should be rewritten entirely.
Discussion / lines 362–369
The interpretation of no depth trend as evidence for mixing, burial, and textural control is plausible, but still speculative. The authors should label it as such.
Discussion / lines 377–384
The comparison to Scottish rhodolith beds is potentially useful, but it needs more caveats. Differences in depth interval, age structure, hydrodynamics, sediment composition, and analytical design make direct comparison difficult.
Discussion / lines 398–401
This is the conceptual caveat the manuscript most needs, yet it appears only near the end. It should be moved earlier into the framing and discussion.
Conclusions / lines 403–413
The conclusions are currently overstated. “Actively storing carbon over millennia” and broader references to global stability are too expansive relative to the evidence presented here. The authors should conclude more narrowly: this deposit preserves measurable organic carbon in the upper sediment and sits within a dated Holocene depositional framework consistent with long-term accumulation.