O’Estrucan Ports, Where Are Thou ? Multiproxy sedimentological investigation of the Orbetello Lagoon

Technical improvements at the end of the Bronze Age led to the rise of a 1rst generation of major sea powers around the Mediterranean Sea, such as Etruria in modern Italy. The Etruscan coast was the fringed by a series of large lagoons. Only one survives today: the lagoon of Orbetello. The lagoon is preserved by two subparallel sand spits that connect former Argentario island to mainland Italy, as situation that today protects it from rapid infilling. A third sand spit, in the middle of the lagoon, hosts the Etruscan city of Orbetello. Today, three canals connect the lagoon to the sea. A massive phase of eutrophication driven by the ingress of fertilizers has plagued the lagoon in late 20th Century. Eutrophication at times has spurred fish and bird kills, and the release of mercury in the water column. Major contingency plans have been implemented to fight off eutrophication, with various success. 

              However little is known of the lagoon management and the evolution of Orbetello before the 17th century CE. Nonetheless, the wealth of the city and the health of its lagoon have been tightly related during the past three millennia. To track this coevolution, a large team of researcher has been assembled to conduct an analysis of the lagoon sediments using XRF scanning of cored sediments, SMIR, Rock Eval, hyperspectral imaging of chromatic pigments, analysis of mercury and phosphorus content, ostracods and pollen assemblages, to document the links between sediment facies, eutrophication and salinity crises, as a result of successive phases of rise and demise of lagoon management over the past three millennia. Here, we focus on the sub-bottom imaging conducted in the very shallow (< 1.5m) waters of this extensive (30 km2) lagoon. The Exail Echoes 10 000 sub-bottom profiler reveals individual layers that can be traced across the lagoon, allowing stratigraphic correlations between cores, and highlighting the environmental significance of the sedimentary facies. Acoustic imaging using a 3.5 kHz Chirp systems from Exail (Haliotis R/V) was conducted offshore to document the architecture of the sand spits protecting the lagoon. The architecture of the deposits, 14C, OSL, and U-Th dating reveal that the lagoon results from the drowning of strandplains that started forming on both side of the older, central sand spit, at the end of the postglacial transgression. Drowning accompanied the final rise in sea level over the past 6.5 ka, forming two lagoons on both sides of the central spit. These initial lagoons eventually coalesced after drowning the central sand spit. Continuation of the lagoon level rise since Antiquity led to the flooding of Bronze Age, Etruscan and Roman settlements. Sub-bottom imaging in the lagoon reveals buried structures possibly used for navigation and salinity control. Sedimentation is marked by an alternation of black, shelly organic silty clays and decimeter-thick layers of broken shells. Radiocarbon dating indicates that the cores capture up to five millennia of sedimentation, with a sharp decrease in sedimentation rates four millennia ago.