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.
Highlights
The Last Glacial Termination (T1: ∼18–11 ka, ka 103 calibrated years before present) is the most recent transition from glacial maximum to interglacial conditions and constitutes a key target for understanding the functioning of the global climate system through Quaternary ice ages
Landforms grouped in N5 are present on the northern and southern valley walls surrounding the Lago Cochrane/Pueyrredón (LCP) basin (Figures 2, 3), with its westernmost position found at the Dos Arroyos sector
This distribution suggests that the LCP glacier lobe receded ∼95 km from the Río Blanco moraines, stabilized and established an effective dam for the development of Glacial Lake Cochrane (GLC) west of Dos Arroyos (Figure 11A)
Summary
The Last Glacial Termination (T1: ∼18–11 ka, ka 103 calibrated years before present) is the most recent transition from glacial maximum to interglacial conditions and constitutes a key target for understanding the functioning of the global climate system through Quaternary ice ages. The western portion of Patagonia (40°–55°S), in southern South America, is a key region to monitor T1 and associated SWW swings because it is the sole continuous landmass that extends south into the subantarctic realm, intersecting a large portion of the SWW belt It harbored the largest ice mass in the Southern Hemisphere outside Antarctica during the LGM, the PIS, which allows the study of past changes in the cryosphere, atmosphere, hydrosphere, and biosphere, and their reciprocal relationships. The majority of models do not consider the glacio-isostasy effects, which caused an important uplift on the western sector The latter factor was discussed by Thorndycraft et al (2019), who produced isostatically corrected lake surfaces. Their approach is based on digital elevation models (DEM) with coarse vertical (∼20 m) and horizontal (∼30 m) resolution along with selected geomorphologic, stratigraphic, and chronologic constraints
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