INVESTIGADORES
VILANOVA Isabel
congresos y reuniones científicas
Título:
Glacial fluctuations in the Coyhaique-Balmaceda sector of Central Patagonia (45°-46° S) during the Last Glacial Termination
Autor/es:
MIRANDA C.G.; MORENO P.I.; VILANOVA I.; VILLA MARTÍNEZ R.P
Lugar:
Viña del Mar
Reunión:
Congreso; International Geological Congress on the Southern Hemisphere; 2013
Institución organizadora:
Universidad Andrés Bello
Resumen:
The southern Andes contain glacial deposits and landforms that allow reconstruction of the regional glacial history which, in turn, can be used to test hypotheses of paleoclimate change at regional, hemispheric and global scales. Here we present a study that combines glacial geomorphology, stratigraphy and radiocarbon chronologies from the Coyhaique ? Balmaceda area (45° - 46°S), Central Patagonia, to decipher the temporal and spatial structure of glacier fluctuations during the culmination of the last glacial maximum (LGM) and the last glacial termination.  Geomorphic and stratigraphic evidence indicates that the Coyhaique and surrounding areas were covered by piedmont glacier lobes that originated from an expanded Patagonian Ice Sheet flowing to the east during the LGM (Arqueros, 2004; Mardones et al., 2011). We carried out glacial geomorphologic mapping of areas formerly occupied by the glacier lobe using aerial pictures (1:70,000), and whenever possible and pertinent we analyzed stratigraphic sections to identify processes and depositional environments, and obtained sediment cores from lakes located on meaningful sites.  We identify at least three moraines sets east of Coyhaique in the Pampa del Diablo and Coyhaique Alto sectors (see figure 1). The elevation of these moraines ranges between ~867 in the easternmost and ~750 m.a.s.l. in the westernmost moraines. An extensive outwash plain originates from the Pampa del Diablo moraines, at 750 m.a.s.l., sloping northeastward. We interpret these landforms as periods of advance or stillstands of the Coyhaique ice lobe. The lowermost organic layer from Lago Tranquilo, a site located at 953 m.a.s.l. near of Pampa del Diablo, yielded an AMS age of 21.9 ka (ka=1000 cal yr BP) which provides a maximum age for Pampa del Diablo moraines considering that the sector where this site is located was ice free during this stage. The lowermost radiocarbon date of Lago Mellizas (751 m.a.s.l.), a site formerly ice-covered during this stage and located 9 km upstream from Pampa del Diablo, yielded an age of 17.9 ka, which provides a minimum age for ice-free environment in this site and, therefore, a minimum age for Pampa del Diablo moraines.  In contrast to the area described above, where the landscape is dominated by depositional processes, the central part of Río Coyhaique and Lago Pollux shows an absence of ice-marginal depositional landforms and predominance of glacially scoured bedrock promontories, aspects that we interpret as a period of rapid ice recession. Radiocarbon dating from the top of a glaciolacustrine unit in sediment cores of Lago El Toro (674 m.a.s.l.) and Lago Élida (570 m.a.s.l.) indicates the development of an ice-dammed proglacial lake, with minimum ages of 17.8 and 16.7 ka for the glaciolacustrine regression phase in Lago El Toro and Lago Élida, respectively. This ice-dammed proglacial lake reached elevations up to 610 m.a.s.l. in the Río Huemules valley, interpretation based on the presence of lake shorelines in the Balmaceda sector and the aforementioned glaciolacustrine units. Deformation and glaciotectonic features found in glaciolacustrine deposits on the distal side of the Lago Baguales moraine (656 m.a.s.l.) suggest a readvance of a tributary ice lobe over proglacial lake sediments during this stage. Radiocarbon dating of the lowermost organic level from a lake sediment core retrieved from Lago Baguales, a lake impounded by a prominent frontal moraine, yielded an AMS age of 17 ka and provides a minimum age for the glacial readvance.  We identify a moraine complex south of Coyhaique, in low-lying areas of the Río Simpson, with an elevation range of ~475 and ~315 m.a.s.l. from east to west. Due to its location, we decided to name this moraine set as Río Simpson moraine system. El Blanco stratigraphic section shows a sequence of deformed glaciolacustrine clays and glaciofluvial gravels which we interpreted as intervals of retreat and readvance at the glacier snout. The distal slopes of the innermost moraines grade into outwash plains, which indicates that the ice-dammed proglacial lake had already drained from those elevations at the time of emplacement of the innermost moraines. An AMS radiocarbon date obtained from the lowermost organic level from Lago Espejo (310 m.a.s.l.), located on the proximal side of the Río Simpson moraine system, yielded an age of 15.8 ka and provide a minimum age for those moraines and ice-dammed proglacial lake regression from this elevation.  Our results from the Coyhaique area show a rapid recession of the glacial lobe from Pampa del Diablo moraines, the development of an ice-dammed proglacial lake before 17.8 ka in the Río Coyhaique valley and before of 16.7 ka in the Río Simpson- Huemules valley. During this stage, a readvance of Baguales ice lobe took place before of 17 ka. The deglaciation of the Coyhaique area was punctuated a period of glacial stabilization between 16.7 and 15.8 ka in Río Simpson-Huemules valley. At 15.8 ka sectors below 310 m.a.s.l. in Río Simpson were ice-free and the proglacial lake was drained at that elevation.  Based on these results we conclude that the Coyhaique ice lobe underwent a series of fluctuations similar to those described in Chilean Lake District (Denton et al., 1999), Lago General Carrera-Buenos Aires (Kaplan et al., 2004; Douglass et al., 2006) and Magallanes/Tierra del Fuego area (McCulloch et al., 2005), namely, an onset for the last glacial termination at ~17.8 ka and a stepwise pattern of deglaciation. Comparing our results with the chronologies from neighboring regions, the rapid ice recession and the subsequent development of an ice-dammed proglacial lake in Río Coyhaique valley prior to 17.8 ka occurred ~two thousand years before than in Lago Buenos Aires and Lago Pueyrredón area, where those fluctuations occurred at 16-15 ka (Turner et al., 2005). In the Strait of Magallanes and Bahía Inútil area, the development of a proglacial lake, prior to 17.5-16.6 (McCulloch et al., 2005) may have been about the same time. In Baguales valley the readvance at >17 ka may be comparable with the readvance which deposited the Fenix II moraine (17.6 ka) (Douglass et al., 2006) in Lago Buenos Aires and the readvance which deposited the innermost Río Blanco moraine (~18-17 ka) in Lago Pueyrredón valley (Hein et al., 2010). Our results from the Coyhaique area confirm the patagonian pattern of glacial recession during the last glacial termination, suggesting a common cause likely related to the atmospheric warming as a climate forcing mechanism which synchronizes the ice age climate and subsequent deglaciation on a global scale. Acknowledgments: FONDECYT 1121141, ICM P05-002 and PFB-23.