Double tombolo formation by regressive barrier widening and landside submergence: The case of Orbetello, Italy
Résumé
The double tombolo of Orbetello, in Italy, has formed during the Holocene around an ancient central tombolo. Earlier models consider that its sand barriers formed as sand spits that stretched from the mainland to a coastal island before enlarging seawards. This evolution, however, remains speculative. In order to test these models, we conducted the first study of a double tombolo that combines coring of its sand barriers and comprehensive imaging of its internal structure using sub-bottom acoustic surveys offshore and in the back-barrier. Sediment ages were constrained by 14 C, luminescence, and U/Th dating. Acoustic images below the lagoon show that the barriers are in fact broad regressive strandplains that initiated on the flanks of the preexisting central isthmus when sea level was -7 ± 1 m lower than today. The strandplains then rose upwards and outwards, tracking sea level rise over the past 7 kyr. The oldest and lowest parts of the strandplains were flooded into the shallow intervening lagoon. The central isthmus is composed of regressive sand barriers accreted around a MIS 5.5 core during subsequent stages MIS 5.3 and MIS 5.1. The emplacement of the isthmus interrupted longshore drift between the mainland and the coastal island, converting the flanks of the initial tombolo into terminal sinks in which sand accretion accelerated, spurring early and rapid regression during the Holocene. A review of the environmental parameters conducive to double tombolo formation suggests that double tombolos may represent a frequent, albeit short-lived stage during the enlargement of single tombolos.
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