Evolution of a transgressive barrier-dune system along a starved shelf: implication for beach sediment budget related to current trends in seawater acidification

Along wave-dominated starved shelves the biogenic carbonates can be a relevant source of sediment for beaches. The budget of carbonate systems depends on the biogenic carbonate production associated to ecosystems which act as carbonate factories. In this respect, current trends in seawater acidification represent a threat to these processes and can potentially alter the sediment budget of carbonate systems. Here we present the evolution of a transgressive barrier-dune system along a microtidal, wave-dominated starved shelf (Western Sardinia, Mediterranean Sea). The beach is located in a wide embayment with rocky outcrops which control the accommodation space for the dunes, the beachface (BF) and the infralittoral wedge (IW). A set of geophysical and sedimentological data, coupled with archaeological evidences were used to evaluate the sediment budget during the last 3 ka. The whole system accounts for about 2 Mm3 of sediments, 83% located in the IW, 16% in the dune and 1% in the BF. Dune and IW sediments are mainly bioclastic medium-fine sands (CaCO3 76±13% and 68±12% respectively) whereas BF sediments are mixed coarse sands (CaCO3 44±18%). The carbonate grains were classified into three classes, (i) relict (ii) stranded and (iii) Holocene grains, based on grain morphology, color and presence of diagenetic phases. The IW is characterized by a mixture of stranded and Holocene grains whereas the dunes were mainly composed by stranded grains. The evolutionary model of the barrier-dune system includes (i) the migrations of the barrier with the rise of the sea level (ii) the inland transport of sediment with development of the dunes (c) the longshore sediment transport from the carbonate factories with formation of the IW. This evolutionary model is discussed in relation to the adaptation of the barrier system to the future rise of the sea level and to the reduction of carbonate sediment production following acidification