Within the EC funded PALICLAS Project, aimed at the reconstruction of the palaeoenvironmental evolution of italian crater lakes and adjoining Adriatic sea areas, we analysed the geochemistry of PAL94-8 and PAL94-66 cores coming from the western margin and the deepest part of the Meso-Adriatic Depression (MAD) respectively. The area is located south-east of the shelf-edge break, in a position corresponding to the late Glacial Po delta front and the coring positions were chosen in order to provide sedimentary sequences with expanded scales of accumulation for time intervals spanning the last 20ka. Sixty-one samples from PAL 94-8 core and eighty samples from PAL 94-66 core were collected every 10 cm, and with higher resolution (1-2 cm) where magnetic measurements suggested the occurrence of tephra layers. The analyses were carried out by XRD and XRF on samples washed with deionized water to remove sea-water salts.
The main mineral phases in the sediments are: quartz, calcite, dolomite, feldspars, micas, clay minerals (illite, smectite, chlorite, serpentine) and amphibole; the comparison of XRD patterns suggests regular downward increase of dolomite coupled to a regular decrease of calcite and quartz; in core PAL94-8 these last phases display a moderate increase in the lower part (below 360 cm). The bulk chemistry of the two cores is rather uniform and consistent with middle Adriatic sediments, and variations depend on the changes in the silicate/carbonate ratios and on the silicates supply; in the depth profiles some spikes of elements typical of alkaline volcanic rocks (K, Rb, Sr, Zr, Nb, Y, Ba, La) mark the occurrence of tephra layers. Major elements depth profiles are similar in both cores, with general increase of "silicate" elements (Si, Al, Fe, Na and K) towards the lower portions. In the depth curves of K2O (fig. 1a-b) three broad zones (bottom-360 cm, 360-240 cm, 240-0 cm) can be clearly indentified in the PAL94-8 core and three tephra layers (64, 221, 358 cm) stand out in the PAL94-66 core. Among trace elements, some follow "silicate" pattern (Rb, Ba, and Zn) and some others (Cr, Ni, Co and Sr) display higher values in the upper portion of both cores. In the depth curves (fig. 1c-d) Ni content (60 ppm) starts to increase gradually at about 240 cm depth with stabilizations (150 ppm) from 150 cm upward. The variations in chemistry (lower K2O, Rb, higher Cr, Ni, Co, and Cr/V, Ni/Mg upward from 240 cm depth) fit with the biostratigraphic records of the Full Glacial-Late Glacial-Holocene transitions marked by the Adriatic sea transgression, and testify the increase, since Late Glacial, of more mafic/ultramafic sediment supply in the MAD area from sources located in the Dalmatian region. The peculiarity of Sr depth profiles can be explained in terms of early diagenetic mobilization, even if provenance induced variations (Sr and Sr/CaO patterns, Fig. 1e-f, are coherent with mafic elements) cannot be excluded.
On the basis of the bulk sediment composition the evolution of sedimentation in the MAD can be evaluated. Several features characterize the Full Glacial-Late Glacial-Holocene transition, and Cr, Ni, Co and Sr contents, and Cr/V, Ni/Mg and Sr/Ca ratios appear as promising geochemical indexes of the palaeoenviromental changes in the Adriatic sea during the late Quaternary.