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Geomorphic Records along the General Carrera (Chile)–Buenos Aires (Argentina) Glacial Lake (46°–48°S), Climate Inferences, and Glacial Rebound for the Past 7–9 ka : A discusssion

Abstract : The paper by Bourgois et al. (2016) presents new cosmogenic isotope concentrations data that permit them to propose a scenario for the post-glacial evolution of the Lago General Carrera/Buenos Aires (LGCBA). In this comment, we do not discuss the validity of the proposed ages, although ages deduced from cosmogenic isotope concentrations may be affected by complex history, particularly when they are obtained in erratic boulders or dropstones (Putkonen & Swanson, 2003; Delmas et al., 2011). We simply note here basic geomorphological evidence challenging the complex scenario proposed by Bourgois et al. (2016), and showing that the LGCBA has never been endorheic following the retreat of the main glacial tongue. This observation has significant implications on the post-glacial climatic evolution of that area. As observed by Bourgois et al. (2016) and several previous authors (e.g., Caldenius, 1932; Turner et al., 2005; Hein et al., 2010), shorelines around the lake evidence periods of higher water levels. In the Rio de Las Dunas and Rio Los Maitenes area, Bourgois et al. (2016) note four major terrace levels corresponding to fan deltas whose elevation vary from 500 meters above sea level (m-asl) for the highest T4 terrace, to ~300 m-asl for the lower terrace, the present-day elevation of the lake being 201 m-asl. They propose that the four fan deltas formed in a short time period between 13.7 +/-0.8 ka and 10.9 +/-1.3 ka, i.e. in less than 1000 years per fan delta. In the " discussion " section of their paper, Bourgois et al. (2016) propose that the three higher terraces (T2-T4) accumulated in ice-walled lake environment, while a major ice-tongue still existed along the lake. They note that the T1 shoreline is largely preserved all around the lake, and they propose that this terrace formed during an endorheic period. The fan delta corresponding to the present-day elevation of the lake (T0 according to the terminology of Bourgois et al. (2016)) would be much more recent, less than 6.7 ka-old. Between the appearance of the T1 and T0 fan deltas, Bourgois et al. (2016) propose that a glacial advance occurred, followed by a tremendous rise of the level of the lake that would have reached elevations larger than 500 m in the Rio de Las Dunas area, i.e. above T4. At that time, according to Bourgois et al. (2016), despite this elevation is now higher than the present-day elevation of the Perito Moreno outlet towards the Atlantic Ocean, the lake would still have been endorheic. Then, Bourgois et al. (2016) conclude that the posterior isostatic rebound uplifted the Rio de Las Dunas area with respect to Perito Moreno from more than 135 m.
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Joseph Martinod, B. Pouyaud, S. Carretier, B. Guillaume, G. Hérail. Geomorphic Records along the General Carrera (Chile)–Buenos Aires (Argentina) Glacial Lake (46°–48°S), Climate Inferences, and Glacial Rebound for the Past 7–9 ka : A discusssion. Journal of Geology, University of Chicago Press, 2016, 124 (5), pp.631 - 635. ⟨10.1086/687550⟩. ⟨insu-01713360⟩

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