Carbon accumulation in Mediterranean rhodolith beds during the Holocene

Rhodolith and maërl beds are globally relevant biogenic ecosystems 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 cores 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 (11 700–9000 yr BP), when post-glacial sea-level rise transitioned the area from subaerial exposure to shallow-marine conditions. Despite the spatial limitation of collected data, several conclusions could be drawn. Early deposits produced during the last sea-level rise were dominated by bivalves and dispersed coralline fragments. The following establishment of modern sea level around 7000–6500 yr BP marks a change to the development of more stable 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 rhodolith beds can act as long-term organic carbon stores, forming spatially complex Holocene deposits that have been largely overlooked.