GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 659 Fig. 3 - GPR profile on Candia Lake. For Candia Lake, the sediment thickness observed by the past CVES surveys (10-15 m) could be motivated by the sedimentation rate observed by Lami et al. (2000). However, considering the acoustic results, it is not clear the nature of sediments having a resistivity of about 30 Ωm and acting as an acoustic mirror. At present, the most probable interpretation is the presence of a thin but widespread shallow layer of high-porosity lacustrine sediments hosting gases released from the decomposition of organic matter. This latter was found to be particularly abundant (12-37% of sample dry weight) in the topmost sediments sampled by Lami et al. (2000). These quantities, together with the anoxic conditions of the lake bottom and the absence of seasonal water mixing or inflow circulation, could probably support the hypothesis of gas formation in the shallower lacustrine sediments, approximately at 1.5-2 m from the lake bottom, inhibiting the acoustic penetration. Conversely, another explanation to electrical and acoustic results would be to consider these acoustic impenetrable sediments not as lacustrine deposits but as sub-glacial silts, settled before the glacier retreat and consolidated by the ice weight. However, in this case, analogously to Sirio Lake, a marked change in the sedimentation rate would be implied. In particular, lacustrine sediments would be represented by the thin layer of sediments observed at the top of the lake bottom. This sediment pack (1.5-2 m) would have deposited in the last 15 kyr, with the shallowest meter deposited only in the last 2 kyr [considering the sedimentation rate estimated by Lami et al. (2000)], and thus with deeper 0.5-1 m representing the previous 13 kyr of sedimentation. Motivations for this variation do not appear to be explained by literature evidences on particular climatic transition at that time. Literature data on sedimentation velocity in lakes without inflow (Tab. 1) do not justify either such low sedimentation rates in the period from 2 kyr to 15 kyr or such an eventual dramatic change in sedimentation rate. In addition, the observations made on these two lakes are not common to all the lakes within the IMA. Sub- bottom profiles acquired on a third IMA lake (Viverone Lake, located between Candia and Sirio lakes, at a distance of approximately 20 km from both) gave a clear imaging of acoustically transparent lacustrine sediments, up to 30-m tick. Unfortunately, no literature information on drill cores and sedimentation rates on the Viverone Lake was found for a critical comparison with the two investigated lakes. Further studies are therefore needed to understand if the sediment nature and thickness observed by the presented surveys confute the sedimentation rate estimated on the Sirio Lake and highlight the widespread presence of water and gas saturated sediments on the bottom of the Candia Lake or are indeed potential indicators of a particular climatic transition.