he knowledge of long-term evolution of barrier - dune systems is a key issue to predict the morphological adaptation of those coastal systems to the predicted rise of the sea level. The geological inheritance plays a major role in the length, shape, type and behavior of most beach systems worldwide controlling the accommodation space of sediments and the nature of sediment availability. Along wave-dominated starved shelves the biogenic carbonates can be a relevant source of sediment for beaches. Here we present the long term evolution of a transgressive barrier-dune system with mixed bioclastic-siliciclastic sediments along a microtidal, wave-dominated starved shelf (San Giovanni beach, 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 long-term sediment budget between the beach compartments during the last 3 ka. The whole system accounts for ~2 Mm3 of sediments, about 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%). Sediments analysis allowed to classify carbonate grains into three classes, (i) relict (ii) stranded and (iii) Holocene grains, based on grain morphology, color and presence of diagenetic phases. Relict and stranded grains derive from the winnowing of older sediments transported during transgression and barrier rollover. The Holocene grains represent the present day production from coastal ecosystems. Sediments are transferred from the shoreface to the barrier-dune system with low average rates of sediment transport. The mass budget of carbonate grains among the beach components reveled that the IW is characterized by a mixture of stranded and Holocene grains whereas the dune were mainly composed by stranded grains. Those data highlight that the long term evolution of the barrier-dune system includes the following subsequent phases (i) migrations of the barrier through rollover with the rise of the sea level (ii) inland transport of sediment with development of the dune system (c) longshore sediment transport from the carbonate factories with formation of the IW. The proposed evolutionary model is discussed in relation to the adaptation of the barrier dune system to the future rise of the sea level.
Evolution of a transgressive barrier-dune system along a starved shelf: implication for beach adaptation to future sea level rise
MOLINAROLI, Emanuela;
2016-01-01
Abstract
he knowledge of long-term evolution of barrier - dune systems is a key issue to predict the morphological adaptation of those coastal systems to the predicted rise of the sea level. The geological inheritance plays a major role in the length, shape, type and behavior of most beach systems worldwide controlling the accommodation space of sediments and the nature of sediment availability. Along wave-dominated starved shelves the biogenic carbonates can be a relevant source of sediment for beaches. Here we present the long term evolution of a transgressive barrier-dune system with mixed bioclastic-siliciclastic sediments along a microtidal, wave-dominated starved shelf (San Giovanni beach, 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 long-term sediment budget between the beach compartments during the last 3 ka. The whole system accounts for ~2 Mm3 of sediments, about 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%). Sediments analysis allowed to classify carbonate grains into three classes, (i) relict (ii) stranded and (iii) Holocene grains, based on grain morphology, color and presence of diagenetic phases. Relict and stranded grains derive from the winnowing of older sediments transported during transgression and barrier rollover. The Holocene grains represent the present day production from coastal ecosystems. Sediments are transferred from the shoreface to the barrier-dune system with low average rates of sediment transport. The mass budget of carbonate grains among the beach components reveled that the IW is characterized by a mixture of stranded and Holocene grains whereas the dune were mainly composed by stranded grains. Those data highlight that the long term evolution of the barrier-dune system includes the following subsequent phases (i) migrations of the barrier through rollover with the rise of the sea level (ii) inland transport of sediment with development of the dune system (c) longshore sediment transport from the carbonate factories with formation of the IW. The proposed evolutionary model is discussed in relation to the adaptation of the barrier dune system to the future rise of the sea level.
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