Evolution of ice-dammed proglacial lakes in Ultima Esperanza, Chile: implications from the late-glacial R1 eruption of Reclus volcano, Andean Austral Volcanic Zone

Abstract

Newly described outcrops, excavations and sediment cores from the region of Ultima Esperanza, Magallanes, contain tephra derived from the large late-glacial explosive RI 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 (14)C yrs BP), and volume, to >5 km(3), of this tephra. The geographic and stratigraphic distribution of RI 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 wave-cut 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 similar to 170 m a.s.l. on Peninsula Antonio Varas west of Seno Ultima Esperanza, to similar to 150 m a.s.l. around Lago Sofia, and down to similar to 125 m a.s.l. along their easternmost margin. The presence of thick deposits of RI 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 (14)C 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 <80 m a.s.l. at the time of this eruption. The original lake may have drained to this level across the low elevation pass between Fiordo Obstruccion and Seno Skyring, and subsequently into Seno Otway and the Pacific Ocean, when Canal Jeronimo opened up prior to the RI eruption. Another suite of cores, from the Eberhard site, indicate that the lake persisted at >70 m a.s.l. until 12.8 cal kyrs BP (10,695 +/- 40 (14)C yrs BP). However, a 14.2 cal kyrs BP (12,125 +/- 85 (14)C yrs BP) Mylodon pelvis from a nearby site, located at only similar to 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 RI 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.