An eighteenth century tunnel as possibile archivi for paleoclimate studies

The former Silva Lake (present “Pian del Lago”, Siena, Italy) developed during late Quaternary and formed as a poljie on the Triassiclimestones. The depression, nowadays completely drained, is N-S oriented, 4.5 km wide and 12 km long. The lake never exceeded 6m in depth, and it was mainly a grassy swamp during the dry season. The lake depression is filled with 20 to 30 m of a reddish siltyclayeysuccession. Starting from the Middle Age till late 18th century, the shallow waters of the lake and the humid area around actedas a swampy area infested by malaria.In 1766 a Sienese nobleman, Francesco Bindi Sergardi drained the lake excavating a drainage 2124m-long tunnel in Triassic limestonesto connect the Silva Lake with the closeby Rigo Creek. However, quite often the tunnel was filled with debris and the lake swampedup again. In 1780 Pietro Leopoldo Grand Duke of Tuscany definitively reclaimed the Silva Lake and completed the construction ofthe drainage tunnel by paving and extending it for an additional 197 m. Since then, the tunnel is called the ”Canale del Gran Duca”.The entrance altitude of the canal is at 252 m a.s.l., and the exit is at 247 m a.s.l. The altitude difference is therefore of 5 m, and thecanal floor has a slope of 0.2 %.The canal is for the most part paved but, in places, solid walls of Triassic limestone are still visible. Diffuse karst features are forminglocally. Stalactites have lengths varying from 5 to 10 cm, and flowstones occur along the tunnel walls. The presence of these speleothemshas allowed geochemical investigations to establish climatic variations of the last two centuries. The tunnel was probablycleaned and well maintained for sometime after its construction (1780), and it is likely that all the remaining speleothems have developedin the last two centuries with an estimated growth of a 0.5/6 mm per year. A petrographic investigation of a well laminated flowstonewith a parasitic stalagmite has been undertaken to determine the growth mechanisms. Oxygen and carbon isotope data (δ 18 O and δ 13 C values) were used as indirect proxies for palaeoenvironmental reconstructions. Preliminary, data show significant variationsalong the axis of the flowstone possibly related to environmental and climatic variations within and above the “canale”.