GNGTS 2017 - 36° Convegno Nazionale

658 GNGTS 2017 S essione 3.2 GPR devices, georeferencing each ping or trace with a metric precision. Standard processing techniques were applied on the raw datasets. On the Sirio Lake, all the raw sub-bottom profiles showed one or more multiples of the rock basement, acting as an acoustic mirror (Fig. 1a). Only after further processing, involving time and spatial filtering and autocorrelation, a quite uniform thin carpet of lake sediments could be observed along some profiles (Fig. 1b). These sediments show an average thickness of about 2.5 m, which increases to approximately 4 m in a few short profile segments. Similar results are observed on the raw profiles of the Candia Lake, where multiples are even more visible because of the shallower water depth (Fig. 2a). Also in this case, after processing, a thin (1÷1.5 m) layer of sediments just below the water-sediment interface which acts as an acoustic mirror is highlighted (Fig. 2b). This layer is also visible somewhere within the GPR profiles (Fig. 3) where water depths are lower than about 3 m. Discussions. Considering the similar sedimentation rates estimated on core samples of both lakes for the last 2 kyr BP, and the sediment thickness reconstructed from the presented geophysical surveys a puzzling situation appears. In particular, the reconstructed lacustrine sediment thicknesses (2.5-3 m for Sirio Lake and 1.5-2 m for Candia Lake, on average) appear to be non-coherent with the measured sedimentation rates. For Sirio Lake, it is not clear which kind of climatic evolution could have generated 1.7 m of sediments in the last 2 kyr, compared to only 1-1.3 m in the preceding 13 kyr. Considering the total lacustrine thickness as the result of 15 kyr of sedimentation, a very lowmean sedimentation rate [approximately 0.2 mm/yr, less than half the rate estimated by Facchinelli et al. (2005)] would be obtained. Such a low value could be only partially explained with the absence of an inflow and the scarce sediment supply from the lake surroundings, which are totally composed of granulitic rock with very thin and loose terrigenous covers. Fig. 2 - Sirio subbottom profile: a) raw data and b) processed.