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 beach–dune 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 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 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 sediment-supply 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.
Highlights
Carbonate beaches are widespread in tropical areas and locally, bioclastic carbonate sands may be the main component of sediments in beach–dune systems in the temperate zone (Short, 2000).The sediment budget of beach–dune systems is mainly influenced by the supply and delivery of sediments from the land and from the sea
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 few decades in coastal ecosystems following sea level rise
The digital terrain models (DTMs) of the marine area was derived from multibeam echosounder (MBES) data, acquired using a Reson SeaBat 7125 operating at a sonar frequency of 400 kHz, which yielded a DTM with a spatial resolution of 1 m and a vertical resolution of 0.01 m
Summary
Carbonate beaches are widespread in tropical areas and locally, bioclastic carbonate sands may be the main component of sediments in beach–dune systems in the temperate zone (Short, 2000).The sediment budget of beach–dune systems is mainly influenced by the supply and delivery of sediments from the land (fluvial, cliff erosion) and from the sea (nearshore). In areas where supply from the land is scarce or absent, marine sources of sediment became more important for the maintenance of the coastal system, and beaches become progressively more calcareous (Bird, 2008). This is the case with the coasts of southern and southwestern Australia, the largest temperate carbonate province in the world, where carbonates produced along the shelf and in seagrass meadows are transported landward, supplying extensive beach–dune systems (Short, 2000)
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