Evolution of Glacial Lake Cochrane During the Last Glacial Termination, Central Chilean Patagonia (∼47°S)

Abstract

Large ice-dammed lakes developed along the eastern margin of the Patagonian Ice Sheet (PIS) during the Last Glacial Termination (T1). Their spatial/temporal evolution, however, remains poorly constrained despite their importance for deciphering fluctuations of the shrinking PIS, isostatic adjustments, and climate forcing. Here we examine the distribution and age of shoreline features deposited or sculpted by Glacial Lake Cochrane (GLC) in the Lago Cochrane/Pueyrredón (LCP) basin, Central Patagonia, following recession of the LCP glacier lobe from its final Last Glacial Maximum (LGM) moraines. GLC drained initially toward the Atlantic Ocean and continuing ice shrinking opened new drainage routes allowing the discharge toward the Pacific Ocean. We identify five clusters of lake terraces, shorelines, and deltas between elevations ∼600–500 (N5), ∼470–400 (N4), ∼360–300 (N3), ∼230–220 (N2), and ∼180–170 masl (N1) throughout the LCP basin. The distribution of these clusters and associated glaciolacustrine deposits provide constraints for the evolving position of the damming glacier bodies. Elevation gradients within the landform clusters reveal glacio-isostatic adjustments that enable us to quantify the magnitude of deglacial rebound and construct isostatically corrected surfaces for the different phases in the evolution of GLC. Our chronology, based principally on radiocarbon dates from lake sediment cores and new OSL dating, suggests that these phases developed between ∼20.7–19.3 ka (N5), ∼19.3–14.8 ka (N4), ∼14.8–11.3 ka (N3), and shortly thereafter (N2 and N1). The N3 landforms are the most ubiquitous, well-preserved, and voluminous, attributes that resulted from a ∼3,500-year long period of glacial stability, enhanced sediment supply by peak precipitation regime, and profuse snow and ice melting during the most recent half of T1. This scenario differs from the cold and dry conditions that prevailed during the brief N5 phase and the moderate amount of precipitation during the N4 phase. We interpret the limited development of the N2 and N1 landforms as ephemeral stabilization events following the final and irreversible disappearance of GLC after N3. This event commenced shortly after the onset of an early Holocene westerly minimum at pan-Patagonian scale at ∼11.7 ka, contemporaneous with peak atmospheric and oceanic temperatures in the middle and high latitudes of the Southern Hemisphere.

Rapid terrace incision and Quaternary landscape evolution in central Patagonia

“…2b) by abandonment of the T22 level. T22 is the youngest mapped terrace (+3 m) and is therefore closest in timing to the river capture event, dated to ~15 ka (Mercer, 1976;Turner et al, 2005;Hein et al, 2010;Thorndycraft et al, 2019a;Vásquez et al, 2022), that created the wind gap (Fig. 3d).…”

“…This timing of lake level fall was corroborated by a weighted mean optically stimulated luminescence (OSL) age of 14. 8 ± 1.1 ka from a prograding delta forming in the lowered lake level (Vásquez et al, 2022). The cause of the drainage reversal was ice‐sheet thinning and recession that opened a Pacific drainage route via the lower Baker valley (Hein et al, 2010; Thorndycraft et al, 2019a).…”

Chronology of the Bayo River valley deglaciation and implications for the Late Pleistocene Atlantic-to-Pacific drainage reversal of the General Carrera-Buenos Aires palaeolake, Patagonia-Chile

“…These paleoshorelines indicate that the level of the lake stabilized at an intermediate elevation, before dropping to its current level. Paleoshorelines are also visible at similar elevations around the present-day Cochrane-Pueyrredon Lake to the South, suggesting that the ancient lake was unifying the present-day GCBAL and Cochrane-Pueyrredon Lake (Turner et al, 2005;Vásquez et al, 2022). This unified lake has been named Chelenko Lake by Davies et al (2018) and Thorndycraft et al (2019).…”

“…(Deseado level) to 360-300 m a.s.l. (Chelenko level) around 14.8 ka according to radiocarbon dates from lake sediment cores and new OSL dating (Vásquez et al, 2022).…”

“…These lakes are the precursors of the current GCBAL (e.g., Caldenius, 1932;Turner et al, 2005;Thorndycraft et al, 2019). Many geomorphological studies focused on the description and dating of the glacial deposits (e.g., Douglass et al, 2006;Kaplan et al, 2004), pro-glacial and lacustrine deposits (e.g., Bendle et al, 2017a;Turner et al, 2005;Vásquez et al, 2022) associated with the LLGM and deglaciation that border the great lakes of the eastern Patagonian Cordillera foothills (e.g., GCBAL). For example, Bendle et al (2017b) show detailed geomorphological maps of these landforms.…”