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High-resolution seismic profiles acquired on November 2009 in the Lago Fagnano (Tierra del Fuego) have shown the presence of shallow gas layers in the south-easternmost sector of this basin. Data have been acquired in the frame of an Italian-Argentinean scientific project funded by the Italian Foreign Ministry. The survey consisted of single-channel seismic reflection profiles acquired using a Boomer source and a single-channel streamer (10-hydrophones selectable array). These data complement and complete the bathymetric and seismic surveys carried out previously in the Lago Fagnano (Lodolo et al., 2007; Waldmann et al., 2010). Moreover, during this Campaign, some gravity piston cores have been collected to analyze the stratigraphy of the most recent (Middle to Late Holocene) sedimentary cover in the eastern sector of the basin. The high-resolution seismic investigation has revealed an extensive area marked by poor seismic penetration that is caused by the presence of shallow gas (Fig. 1). The gas-related features observed on the seismic profiles include typical acoustic turbidity with a strong phase reversal reflector on top that creates multiple reflections (Best et al, 2004) . The gassy sediments exhibit high attenuation (blanking) that hide geological sub-surface structures. The lake-floor morphology does not reveal any evidence of clear gas escape from the floor. The top of the acoustically turbid layer is located between 0-1 and 7-10 m below the lake-floor surface. It generally forms a sharp boundary, often marked by a varying offset probably due to different levels of gas penetration which could be related to the lithology (poorly consolidated muddy layers) of the overlying sediments. Shallow gas horizons are all located in the south-eastern sector of Lago Fagnano where water depths vary from 20 to 50 m and where a presence of a ground moraine in the vicinity of the lake shore is reported (Coronato et al., 2009). This geographical distribution may be in some ways conditioned by shallow structural lineaments associated to the left-lateral transform system which separates the continental South American plate from the Scotia plate (Lodolo et al., 2003; Menichetti et al., 2008). Lago Fagnano itself occupies a segment of the transform system, and is considered an example of pull-apart basin developed in a series of graben-shaped, asymmetrical tectonic sinks disposed in an en-echelon arrangement along the transform boundary (Lodolo et al., 2002; Lodolo et al., 2003). The tectonic structure of the Lago Fagnano formed presumably during the Paleogene and was subsequently modified by glacial erosion, especially during Late Quaternary (Menichetti et al., 2007). Seismic characteristics of the profiles where shallow gas layers have been individuated seem to suggest a low concentration of gas, most likely less than 1%. To confirm the actual presence of gas, some gravity coring have been performed in correspondence of the high-resolution seismic profiles in both places where the blanking effect was most relevant, and in areas where shallow gas was not evident. These cores were then analyzed in laboratory in order to quantify and define the type of gas. Gas was not sampled from the cores, mostly because it was almost completely released in the lake water during the core recovery. It can be thought of as the reduction of hydrostatic pressure from 40 m depth at which the samples have been retrieved to atmospheric pressure, has favoured the immediate volatilization of gases in the water column. Further laboratory analyses will be carried on the recovered sediments to eventually detect the presence of heavier gases. As a preliminary interpretation, we may assume that the main origin of the gas could to some extent be linked to the presence of a shallow, thin peat-rich layer of Middle-Late Holocene age. To date, this is the first evidence of shallow gas layers in Tierra del Fuego lakes.

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