Sizing the carbon sink associated with Posidonia oceanica seagrass meadows using very high-resolution seismic reflection imaging

Among blue carbon ecosystems, seagrass meadows have been highlighted for their contribution to the ocean carbon cycle and climate change mitigation derived from their capacity to store large amounts of carbon over long periods of time in their sediments. Most of the available estimates of carbon stocks beneath seagrass meadows are based on the analysis of short sediment cores in very limited numbers. 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 1380 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.This work would not have been possible without the participation of the oceanographic research vessel L'Europe (IFREMER) and its crew (GENAVIR), provided by the Flotte Océanographique Française for the CoralCorse, PosidCorse and Carbonsink surveys. This research was financially supported by the Office Français de la Biodiversité (AAMP/15/065 and UCPP 2510-AFB/2018/274), the Collectivité de Corse (PADDUC-CHANGE program; 17-DESR-SR-87), the Office de l’Environnement de la Corse (UCPP 2019-156) and the Direction Régionale de l’Environnement, de l’Aménagement et du Logement de Corse (2015073–0001). This research was part of the Interreg Italy-France Marittimo 2014–2020 cooperation program - GIREPAM project (E76J16001050007). The authors would like to express their gratitude to PhD researcher C. Luzzu (Biosurvey Company), and researchers from the University of Palermo for their contributions to the seismic acquisition during Sismat survey as well as the University Grant program of the Information Handling Services company (IHS Inc. www.ihs.com) by providing the access to the Kingdom PAKaged Suite + software.Peer reviewe