Biogenic sediments from coastal ecosystems to beach–dune systems: implications for the adaptation of mixed and carbonate beaches to future sea level rise

Abstract

Abstract. Coastal ecosystems produce and 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 (e.g. Mediterranean Sea and South Australia), 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 beachdune system (San Giovanni beach, western Sardinia, western Mediterranean Sea). A set of geophysical, petrographic and sedimentological data was used to estimate the sediment volume and composition of the beach-dune system as a whole. The San Giovanni beach-dune system contains 3 797 000 ± 404 000 t of sediment, 83 % (3 137 000 ± 404 000 t) of which is located in the coastal wedge, 16 % (619 000 ± 88 000 t) in the dune fields and 1 % (41 000 ± 15 000 t) in the subaerial beach. The sediments are composed of mixed modern bioclastic and relict bioclastic and non-bioclastic 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 of 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 kyr. This value was used to estimate the longterm deposition rates of modern bioclastic sediments in the various beach compartments. The total deposition rate of modern bioclastic grains is 46 000±5000 t century −1 , mainly deposited in the coastal wedge (39 000 ± 4 000 t century −1 ) and dunes (7000 ± 1000 t century −1 ), and 46 000 t represents ∼ 1.2 % of the total beach-dune sediment mass. Carbonate production from coastal ecosystems was estimated to be 132 000/307 000 t century −1 , 28 % (15 % / 34 %) of which is transported to the beach-dune system, thus significantly contributing to the beach sediment budget.The contribution to the beach sediment budget represents a further ecosystem service, which our data can help quantify, provided by P. oceanica. The value of this sedimentsupply service is in addition to the other important ecological services provided by seagrass meadows. 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.

Data set of submerged sand deposits organised in an interoperable spatial data infrastructure (Western Sardinia, Mediterranean Sea)

“…The mixed fine and medium sands derive from the mixing and winnowing of siliciclastic riverine sediments (De Falco et al, 2015) with bioclastic sediments derived from marine benthic ecosystems (Posidonia oceanica and coralligenous assemblages) (De Falco et al, 2017). The fine and medium sand deposits are associated with submerged depositional ter-races characterised by a distinct clinoform pattern and were considered as transgressive deposits related to the last sea level rise (Carboni et al, 1989;De Falco et al, 2015).…”

“…The compatibility of SSDs with beach sediment of the adjacent coastal area is limited to a few sectors due to the great heterogeneity of beach sediment grain size and composition (De Falco et al, 2017. In particular, beaches located along the coast, ranging from the northern sector of the Gulf of Oristano to Cape Mannu, generally show a mixed composition with variable percentages of biogenic carbonate sediments.…”

“…The coastal sector located between Cape Mannu and Cape San Marco is characterised by wide mixed bioclastic and siliciclastic beaches (in the proximity of Capo Mannu), coarse siliciclastic beaches (Sinis Peninsula) and, in the proximity of San Marco, embayed and semi-embayed mixed bioclastic terrigenous beaches. Grain sizes vary from medium-tocoarse sand in the proximity of Cape Mannu, from coarse sand to very fine gravel along Sinis Peninsula and from fine sand to very coarse sand in the proximity of Cape San Marco (Simeone et al, 2018;De Falco et al, 2017.…”

“…In all samples, grainsize distribution was measured using dry sieving for the gravel/sand fraction between 4000 and 90 µm at half-phi intervals. Multivariate statistical techniques were used to classify sediment samples into sedimentary facies (De Falco et al, 2015;Brambilla et al, 2016;De Muro et al, 2017). The statistical analysis of sediments data was performed by the software GRADISTAT (Blott and Pye, 2011).…”

Dataset of Submerged Sand Deposits organised in an interoperable Spatial Data Infrastructure (Western Sardinia, Mediterranean Sea)

“…The continental shelf is sediment-starved, with limited land to sea sedimentary run-off. The transgressive deposits associated with the last sea level rise are characterized by siliciclastic sands and calcareous bioclastic sands (Carboni et al, 1989;De Falco et al, 2008;De Falco et al, 2010, De Falco et al, 2015, De Falco et al, 2017 along the inner shelf as well as clays and calcareous mud along the outer shelf (Carboni et al, 1989).…”