The CARBONSINK survey made it possible to acquire 5 major results attested by publications in journals of the discipline:

1. Seismic interval velocity in the matte of Posidonia oceanica meadows: Towards a non-destructive approach for large-scale assessment of blue carbon stock.
This study describes the first methodology to estimate the seismic interval velocity in the matte. This approach performed on the eastern continental shelf of Corsica island (France, NW Mediterranean) is based on measurements of the vertical matte profile from high-resolution seismic reflection profiles (s TWTT) and from seafloor morpho-bathymetric DTM (multibeam echosounders - MBES and Light Detection and Ranging - LiDAR surveys) calibrated with ground-truthing data. A biogeosedimentological analysis of horizontal cores sampled in vertical matte escarpments has been undertaken to identify the potential relationship of sediment and environmental parameters with sound velocity. The cross-comparison and the data intercalibration show significant correlation of MBES (R² = 0.872) and LiDAR datasets (R² = 0.883) with direct underwater measurements. Seismic interval velocities (n = 367) have been found to range between 1 631.9 and 1 696.8 m s^-1 (95% confidence interval) and are estimated on average at 1 664.4 m s^-1, which is similar to the literature for unconsolidated marine sediments. The prediction map provided by the ordinary kriging method emphasized, however, a high variability of sound velocity within the study area. The results showed that changes in sound velocity in the matte are positively and strongly correlated with sand and gravel content and environmental factors such as distance to coastal river mouths and coastline. However, it was found that a negative relationship linked sound velocity with total and coarse organic content of matte deposits.

Figure 1: (A) Example of high-resolution seismic reflection profile (SBP-0087) recorded on the eastern coast of Corsica. (B) Section of the seismic profile displaying a continuous P. oceanica meadow (P.o.), the base of the matte (b.m.), a matte wall (m.w.) and a sand patch (s.p.). (C) Comparison with a seismo-acoustic profile (INM-006) where we can observe the rocky substrate (r.s.).

2. Sizing the carbon sink associated with Posidonia oceanica seagrass meadows using very high-resolution seismic reflection imaging.
In this study, high-resolution seismic reflection techniques were applied to obtain an accurate estimate of the potential size of the organic deposit underlying the meadows of the Mediterranean seagrass Posidonia oceanica (known as `matte'). Seismic profiles were collected over 1 380 km of the eastern continental shelf of Corsica (France, Mediterranean Sea) to perform a large-scale inventory of the carbon stock stored in sediments. The seismic data were ground-truthed by sampling sediment cores and using calibrated seismo-acoustic surveys. The data interpolation map highlighted a strong spatial heterogeneity of the matte thickness. The height of the matte at the site was estimated at 251.9 cm, being maximum in shallow waters (10 - 20 m depth), near river mouths and lagoon outlets, where the thickness reached up to 867 cm. Radiocarbon dates revealed the presence of seagrass meadows since the mid-Holocene (7000 - 9000 cal yr BP). Through the top meter of soil, the matte age was estimated at 1656 ± 528 cal yr BP. The accretion rate showed a high variability resulting from the interplay of multiple factors. Based on the surface area occupied by the meadows, the average matte thickness underneath them and the carbon content, the matte volume and total Corg stock were estimated at 403.5 ± 49.4 million m³ and 15.6 ± 2.2 million t Corg, respectively. These results confirm the need for the application of large-scale methods to estimate the size of the carbon sink associated with seagrass meadows worldwide.

Figure 2: (A) Prediction map of the Posidonia oceanica matte thickness in sector 2A, (B) sector 2B, (C) sector 2C, (D) sector 2D and (E), sector 2E.

3. Quantification of blue carbon stocks associated with Posidonia oceanica seagrass meadows in Corsica (NW Mediterranean)
In the Mediterranean, Posidonia oceanica (L.) Delile constitutes extensive meadows considered as long-term carbon sinks due to the development of an exceptional structure known as `matte', reaching several meters in height, which can be preserved over millennia. In order to specify the role of P. oceanica meadows in climate change mitigation, an estimate of carbon stocks has been conducted along the eastern coast of Corsica (NW Mediterranean). The approach is mainly based on the biogeochemical analysis of 39 sediment cores. Organic carbon (Corg; 327 ± 150 t ha^-1, mean ± SE) and inorganic carbon stocks (Cinorg; 245 ± 45 t ha^-1) show a high variability related to water depth, matrix (sandy vs rocky substrate) or the depositional environment (coastal vs estuary). The isotopic signature (?¹³C) revealed a substantial contribution of allochthonous inputs of organic matter (macroalgae and sestonic sources) mainly in estuarine environment and shallow areas. The carbon stocks in the first 250 cm of matte (average thickness) were estimated at 5.6 - 14.0 million t Corg (study site) and 14.6 - 36.9 million t Corg (Corsica), corresponding to 11.6 - 29.2 and 30.4 - 76.8 years of CO₂ emissions from the population of Corsica.

Figure 3: Contribution of the top 100 cm (A) and 250 cm (B) of P. oceanica matte as CO₂ sink (in green) or CO₂ source (in red) based on the balance between C_org stock and C_inorg stock. Calculated by multiplying the Cinorg stock value in each DTMraster cell (Appendix S6) by 0.6 to meet the assumption that 0.6 mol of CO₂ is released permole of CaCO₃ precipitated, then deducting this adjusted C_inorg value from the C_org stock (Appendix S6) to get the net CO₂ stock.

4. Long-term dynamics of a Cladocora caespitosa bank as recorded by a Posidonia oceanica millenary archive
Along most Mediterranean coasts, the endemic seagrass species Posidonia oceanica builds extensive meadows and complex peat-like bioconstruction known as `mattes'. These belowground deposits are recognized as a valuable long-term archive allowing the reconstruction and the study of palaeo-climatic and palaeo-ecological changes in the coastal environment over the Holocene period. One of the P. oceanica matte cores sampled during a coring survey along the eastern continental shelf of Corsica Island (France, NW Mediterranean) revealed the unprecedented finding of a dead bank of the scleractinian coral Cladocora caespitosa embedded in the matte. Measurement of the morphological and biometrical features of corallite fragments coupled to biogeosedimentological analysis and radiocarbon dating contributed to provide a basis for the reconstruction of the stratigraphic sequence since the mid-Holocene (last 4750 years). The study of the sediment core enabled identification of three major phases: (i) the settlement of the C. caespitosa colonies (~4750-3930 cal yr BP), (ii) the coexistence of the C. caespitosa bank and the P. oceanica meadow (~3930-1410 cal yr BP), followed by (iii) the death of the coral bank and the development of only the P. oceanica meadow (~1410 cal yr BP-present). The sclerochronological analysis completed on the well-preserved corallite fragments revealed that the mean annual growth rate of the coral ranged between 1.9 and 3.1 mm yr-1 with a mean value estimated at 2.3 ± 0.8 mm yr-1. Trend analysis showed semi-millennial to millennial oscillations in annual growth rates which are probably related to environmental climatic changes since the Cold Phase of the Subatlantic period (2925-2200 cal yr BP). During the Roman Warm Period (2200-1500 cal yr BP), the decline and the death of the bank (~1410 cal yr BP) was probably due to the combined effect of a prolonged increase in summer temperatures and an increase in the competition with the P. oceanica meadow.

Figure 4: Lithology, calibrated radiocarbon dates, content in organic matter, mineral and calcium carbonate in the total fraction, content in Cladocora caespitosa in the coarse calcium carbonate fraction and biogeosedimentological units identified in the core.

5. Major regression of Posidonia oceanica meadows in relation with recreational boat anchoring: A case study from Sant'Amanza bay

The anchoring of large recreational craft constitutes one of the main threats in shallow marine habitats. In theMediterranean, this activity has seen constant development during the last decades, causing major physical disturbances in Posidonia oceanica meadows and associated ecosystem services. In this context, the main aims of the present study are to estimate the impact of this anchoring on P. oceanica meadows surface areas and carbon fixation and sequestration capacities, in a particularly highly-frequented area (Sant'Amanza bay, Corsica Island). Accurate benthic marine habitats mapping was performed yearly (2019¿2021) using a drone coupled with ground-truthing data and side scan sonar images from a previous oceanographic survey (CARBONSINK, 2018). The evaluation of carbon fixation and sequestration by the plant was measured at 12 stations within the bay (-5 m to -30 m). The maps of the marine habitats reveal a great number of scars linked to anchoring (73 ha) and in a particularly frequented area, an extensive regression of P. oceanica meadows (7.5 ha) between 2019 and 2020. This destruction represents a 9% decline in the total carbon fixation and sequestration performed each year by P. oceanica meadows within the bay. The related ecosystem services loss is estimated at 4.72 million € yr-1. Although an overall decline of boat anchoring in the seagrass meadow has been observed (e.g. recent enforcement of anchoring regulations), other solutions should be experimented to manage this major carbon sink.

Figure 5: Distribution of main biocenoses and associations (a) and location of the main anchoring tracks (b) in Sant'Amanza bay.