Sediment supply and accommodation space dictate the adaptation of microtidal barrier-lagoon systems to sea level rise.

Study The response of coastal barrier - lagoon systems to late Holocene high frequency sea level oscillations can be useful to infer the answer of such systems to the sea level rise expected in the next century. We analysed the late Holocene evolution of two coastal systems located in a low lying microtidal alluvial plane (Gulf of Oristano, western Sardinia, Mediterranean Sea): the Mistras barrier-lagoon and the Tirso river coastal plane. A set of geophysical data (high resolution seismic data, multibeam data) coupled to stratigraphic and radiocarbon data derived from 27 sediment cores were analyzed to create a paleo-environmental reconstruction of the two coastal systems during the late Holocene. The two systems differs for sediment supply and accommodation space. The Mistras barrier-lagoon system is characterised by a low rate of sediment input, mainly of marine biogenic origin, and evolved over a Miocene to Pleistocene substrate located 3-5 m below the present sea level. The system developed as transgressive during the final stages of the Holocene sea level rise (6300–6000 cal y BP), and become prograding from about 2500 cal y BP, when sea level reached the present elevation. The regression of the coast was, however, discontinuous characterized by distinct Transgressive-Regressive phases (T-R), associated to precise climatic fluctuations, tied with global eustatic and climatic phases. The Tirso river coastal system is the result of the infilling of an incised valley with formation of coastal barrier and backbarrier lagoon progressively filled by riverine sediments. Several order of barrier formation where related to changes in sea level rise rates over the last 6 ka. Both area will be drowned following the expected sea level rise at 2100. In view of their evolutionary history we can evaluate the way of adaptation of such systems to the predicted sea level rise.