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 core NK2 and one from NK3 were performed at the CEDAD (University of Salento, Italy). The analyses were carried out on benthic foraminifera at the bottom (sample NB1) and at -32.5 cm (sample NB16) of the core NK2, and at -136.5 cm (sample NC14) of the core NK3. All the results showed Late Holocene ages: NB1: 2815±100 cal yr BP; NB16: 302±45 yr BP; NC14: 656±93 cal yr BP. The micropaleontological analysis was carried out on a total of 96 samples. The Q-mode Hierarchical Cluster Analysis (HCA) was applied to the results of quantitative analysis. The samples of the 4 cores from the delta front were considered into a single statistical analysis, whereas the 3 cores from the prodelta slope-continental shelf were analysed separately. On the whole, seven clusters were recognised by the HCA. Each cluster is characterised by rather homogeneous foraminiferal content, which may be considered as corresponding to a distinct environment, ranging from infralittoral to lower circalittoral zones. Ammonia parkinsoniana assemblage (20.3-55.4%) was found in 6 samples from the NK7 core and indicates an infralittoral environment characterised by fresh-water input. Ammonia tepida, Elphidium decipiens and Elphidium granosum assemblage was found in 23 samples from the cores NK7, NK8, BOK5, and BOK10. This is a typical infralittoral assemblage characterised by taxa normally found in shallow-marine deposits near fluvial deltas, and it is comparable to the modern Ammonia spp. and Elphidium spp. assemblage (Frezza and Carboni, 2009). Bolivina catanensis assemblage (9.3-16.7%) was found in six samples from core NK8 and it is characterised by a dominance of Bolivina spp. (17.3-29.7%), that could be representative of relatively prolonged periods of dysoxia at the sea bottom. In the recent sediments of this area Bolivina spp. assemblages are not present. However, core NK8 was collected very close to the Ombrone mouth, whereas no recent samples studied by Frezza and Carboni (2009) were located in the same area. Valvulineria bradyana "high dominance" assemblage, comprising 10 samples from core NK4, is dominated by V. bradyana with very high percentages (56.2-72.4%), denoting stressed environmental conditions, related to very elevated input of organic matter by the Ombrone River. This foraminifer shows an opportunistic behaviour in organic matter enriched sediments, and it dominates assemblages corresponding to a transition between infralittoral and upper circalittoral environment, in the Mediterranean areas characterised by the presence of river mouths. Valvulineria bradyana "low dominance" assemblage consists of 10 samples from core NK2, 9 from core NK4, and 4 from core BOK10. Valvulineria bradyana is the dominant species, but with percentages lower than 50% (11-49%). The structure of this assemblage (as regards the percentages of dominance) allows comparison with the modern association found in the recent sediments (Frezza et al., 2005; Frezza and Carboni, 2009). Bulimina marginata and Melonis spp. assemblage includes 7 samples from core NK2 and 9 from NK3. These species are characteristic of circalittoral muds with high organic matter, but Melonis spp. does not seem to tolerate strong ecological stress. This assemblage is comparable to B. marginata assemblage present in recent sediments of the lower circalittoral zone (Frezza and Carboni, 2009). The circalittoral assemblage Uvigerina spp. and Bigenerina nodosaria is characteristic of 12 samples from the core NK3. Uvigerina spp. comprise two species (U. peregrina and U. mediterranea) recorded from the circalittoral zone and very abundant in bathyal muds, as well as B. nodosaria. On the whole, this association shows a high similarity with the circalittoral U. mediterranea assemblage, today present in front of the Ombrone River delta, at water depths exceeding 100 m (Frezza and Carboni, 2009).