Abstract
Newly described outcrops, excavations and sediment cores from the region of Ultima Esperanza, Magallanes, contain tephra derived from the large late-glacial explosive R1 eruption of the Reclus volcano in the Andean Austral Volcanic Zone. New radiocarbon dates associated to these deposits refine previous estimates of the age, to 14.9 cal kyrs BP (12,670±240 14C yrs BP), and volume, to >5 km3, of this tephra. The geographic and stratigraphic distribution of R1 also place constraints on the evolution of the ice-dammed proglacial lake that existed east of the cordillera in this area between the termination of the Last Glacial Maximum (LGM) and the Holocene. This proglacial lake generated wavecut terraces, and also caves, such as the Cueva de Milodon, along the highest prominent terrace. The current elevation of these terraces depends on the total amount of post-glacial isostatic rebound, which is unknown. Due to differential rebound, the highest prominent lake terraces decrease in height from west-to-east, from ~170 m a.s.l. on Peninsula Antonio Varas west of Seno Ultima Esperanza, to ~150 m a.s.l. around Lago Sofia, and down to ~125 m a.s.l. along their easternmost margin. The presence of thick deposits of R1 tephra in some of the caves around Lago Sofia implies that the proglacial lake had already dropped below its highest level prior to the time of this eruption, and, in fact, even earlier, prior to 16.1 cal kyrs BP (13,560±180 14C yrs BP), when land mammals first occupied these caves. The depositional environment of R1 in a core from Dumestre bog suggests that the lake level was in fact 70 m a.s.l. until 12.8 cal kyrs BP (10,695±40 14C yrs BP). However, a 14.2 cal kyrs BP (12,125±85 14C yrs BP) Mylodon pelvis from a nearby site, located at only ~7 m a.s.l., suggests that the lake could have emptied, for at least a brief period, to this low level at this time. This latter datum, combined with the lack of any prominent terraces between the highest ones (170-125 m a.s.l.) and much lower ones (at only 30 m a.s.l. on Peninsula Antonio Varas and 20 m a.s.l. along the coast north and south of Puerto Natales), suggests abrupt changes in the lake level after the R1 eruption. The likely mechanism for producing these changes in Ultima Esperanza was the catastrophic failure and subsequent re-sealing of an ice dam in Paso Kirke, the only below sea-level pathway west to the Pacific north of Fjordo Obstruccion. The final stage of lake drainage, from the lower terrace level (20-30 m a.s.l.) occurred at 10.3 cal kyrs BP.
Highlights
Reclús volcano, one of six volcanoes that comprise the Andean Austral Volcanic Zone (AVZ; Fig. 1; Stern et al, 1976, l984, 2007; Harambour, 1988; Stern and Kilian, 1996; Stern, 2004), produced a large (>1 km3) explosive eruption R1 at ~14.9 cal kyrs BP (12,685±260 14C yrs BP; Stern, 1990, 1992, 2008), during the transition between the Last Glacial Maximum (LGM) and the Holocene
They provide an important chronologic marker for the interpretation of the paleo-climatic changes that affected the disappearance of piedmont glaciers that filled the Estrecho de Magallanes during and after the LGM, and they constrain the evolution of the ice-dammed proglacial lakes that existed in the strait before it was open to Evolution of ice-dammed proglacial lakes in Última Esperanza, Chile
This paper summarizes the environments within which the tephra derived from the R1 eruption of the Reclús volcano were deposited in the region of Última Esperanza, their ages, and the implications for the evolution of the proglacial lake in this region during the late-glacial transition to the Holocene
Summary
One of six volcanoes that comprise the Andean Austral Volcanic Zone (AVZ; Fig. 1; Stern et al, 1976, l984, 2007; Harambour, 1988; Stern and Kilian, 1996; Stern, 2004), produced a large (>1 km3) explosive eruption R1 at ~14.9 cal kyrs BP (12,685±260 14C yrs BP; Stern, 1990, 1992, 2008), during the transition between the Last Glacial Maximum (LGM) and the Holocene. More proximal and significantly thicker tephra deposits from the same R1 eruption are observed in outcrops, excavations and bog and lake sediment cores in Última Esperanza, Chile (Fig. 2; Cárdenas, 2006; Sagredo, 2007; Stern, 2008). These deposits occur both along the shores and inside the area occupied by a large ice-dammed proglacial lake, along with lobes of the generally receding but episodically advancing glaciers, that existed east of the cordillera where Golfo Almirante Montt, Seno Última Esperanza, Fiordo Obstrucción and Lago Sofía are located. This paper summarizes the environments within which the tephra derived from the R1 eruption of the Reclús volcano were deposited in the region of Última Esperanza, their ages, and the implications for the evolution of the proglacial lake in this region during the late-glacial transition to the Holocene
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