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.