FORAMINIFERAL RECORD AND HIGH RESOLUTION SEISMIC STRATIGRAPHY OF THE LATE HOLOCENE DELTAIC SUCCESSION OF THE OMBRONE RIVER (NORTHERN TYRRHENIAN SEA, ITALY).

The marine sector between Piombino-Elba Island to the north and Argentario-Giglio Island to the south is dominated by the Ombrone River delta. Such river has a considerable load discharge in comparison to the limited drainage basin area and is responsible for the natural processes of transport and coastal evolution of the entire basin. Its fine sediments are dispersed along a wide area between the Elba Island and the Argentario Promontory. During the last low-stand, the continental shelf was largely exposed and the Ombrone River flowed into the Tyrrhenian Sea several kilometres to the west of the present mouth (Carboni et al., 2005). The Holocene evolution of the Ombrone delta was characterised by flooding of the coastal plain during the rapid sea-level rise, thus giving rise to brackish marshes or lagoons. Sea-level rise was punctuated by minor still stands, during which the river was able to build fluviodeltaic bodies into the paleovalley (Bellotti et al., 2004). At the end of postglacial sea-level rise (about 6000 yr BP), the Ombrone River flowed in a large lagoon, partly closed by prograding barrier-beaches (Bellotti et al., 2004). Sediment deposits referable to Etruscanearly Middle Ages (2800 to 700 yr BP), are considerable, as fluvial supplies were very high during cool-humid periods. At 700 yr BP, sedimentation values were low because of anthropic impact on this area (Bellotti et al., 2004). From 500 to 200 years BP, a new phase of high sedimentation rate started, related to a cool-humid climatic oscillation kwon as the Little Ice Age (LIA) coinciding with the maximum progradation of the delta. Successively an erosive phase affected the delta area caused by a decrease in solid supply, related to human actions (dams, wetland reclamations) or to a rainfall decrease. The submarine portion of the Ombrone River delta can be subdivided into delta front and prodelta slope. The prodelta slope develops between 20 m and about 90-100 mwd. Single-channel, very high resolution seismic profiles show three distinct seismic units (A, B and C), overlying the LGM unconformity, and formed during the last sea-level rise and highstand. The most recent unit (unit A) has a distinct seismic facies characterised by highamplitude and high-continuity reflectors generally affected by soft-sediment deformation (creep). This unit formed during the highstand phase and covers almost all the shelf area, exceeding 46 m in thickness off the Ombrone River mouth. It can be subdivided into several sub-units (at least six) possibly related to distinct phases of delta construction. Unit B lies above unit C and is characterised by a transparent seismic facies. It is distributed over the shelf with thickness of about 10-14 m and possibly formed during the late transgression-early highstand, before the construction of the wave-dominated delta. In this study we discuss temporal and spatial distribution patterns of the benthic foraminifera collected in cores comprising sediments of Holocene age. The specific aim of this paper is to document the paleoenvironmental changes in the Ombrone River delta area, by comparison with appropriate modern analogues. In this area, recent benthic foraminiferal assemblages have been extensively studied and a bathymetrical zonation parallel to the coast was defined (Frezza and Carboni, 2009). The quantitative and qualitative changes in foraminiferal assemblages reflect the impact of factors as organic matter and oxygen content, sediment type, and presence of seagrass. Seven cores collected in 1996 on the delta front (11-25.1 mwd) and on the outer continental shelf (76.4-105 mwd), were considered in this study. The cores mainly sampled sediment of unit A. The upper part of unit B was found only in one case. Two radiocarbon dates from cor