Abstract. Coastal ecosystems store carbonate particles, which play a significant role in the carbonate dynamics of coastal areas and may contribute to the sediment budget of adjacent beaches. In the nearshore seabed of temperate zones, marine biogenic carbonates are mainly produced inside seagrass meadows. This study quantifies the contribution of biogenic sediments, mainly produced in Posidonia oceanica seagrass meadows and secondarily in photophilic algal communities, to the sediment budget of a Mediterranean beach-dune system (San Giovanni beach, western Sardinia, western Mediterranean Sea). A set of geophysical, petrographic and sedimentological data were used to estimate the sediment volume and composition of the beach-dune system as a whole. The San Giovanni beach-dune system contains ~ 2106 m3 of sediments, about 83 % of which are located in the coastal wedge, 16 % in the dune fields and 1 % in the beachface. The sediments are composed of mixed modern bioclastic and relict biogenic and siliciclastic grains from various sources. The system receives a large input of modern bioclastic grains, mainly composed of Rhodophytes, Molluscs and Bryozoans, which derive from sediment production by present-day carbonate factories, particularly P. oceanica seagrass meadows. Radiocarbon dating of modern bioclastic grains indicated that they were produced during the last 4.37 ka. This value was used to estimate the long-term deposition rates of modern bioclastic sediments in the various beach compartments. The total deposition rate of modern bioclastic grains is 46 000 ± 5000 tons century−1, mainly deposited in the coastal wedge (85 %) and dunes (15 %). This deposition rate is equivalent to ~ 26 000 m3 century−1, and 26 000 m3 represents ~ 1.2 % of the total beach-dune sediment volume. Carbonate production from coastal ecosystems was estimated to be 132 000 ÷ 307 000 tons century−1, 28 % (15 % ÷ 34 %) of which is transported to the beach. The contribution to the beach sediment budget represents a further ecosystem service provided by P. oceanica, and our data can help quantify the value of this specific service in addition to the others provided by this seagrass. The dependence of the beach sediment budget on carbonate production associated with coastal ecosystems has several implications for the adaptation of mixed and carbonate beaches to the loss of seagrass meadows due to local impacts and the changes expected to occur over the next few decades in coastal ecosystems following sea level rise.
Biogenic sediments from coastal ecosystems to Beach-Dune Systems: implications for the adaptation of mixed and carbonate beaches to future sea level rise
