The chronology of the Last Glacial Maximum and deglacial events in central Argentine Patagonia

Abstract

This paper evaluates the chronology of the last glacial cycle and deglaciation in the Lago Pueyrredón valley of central Patagonia, 47.5º S, Argentina. The valley was a major outlet of the former Patagonian Ice Sheet and the moraines that record its fluctuations are an important proxy record of climate change in southern South America. Such moraines are well-preserved in the Lago Pueyrredón valley owing in part to the semi-arid environment east of the mountain front. Here, we provide the first direct chronology for the age of the "Rio Blanco" moraine system by utilizing cosmogenic-nuclide surface exposure ages. Boulders on the moraines give 10 Be exposure ages that indicate the Last Glacial Maximum (LGM) maximum extent occurred by 27-25 ka. Subsequent advances occurred at ca. 23 ka, ca. 19 ka, and ca. 18-17 ka. Initial deglaciation began after 18-17 ka and was interrupted as evidenced by the Lago Columna moraines upvalley. Subsequently the outlet glaciers occupying both the Lago Pueyrredón basin (Chilean name: Lago Cochrane) and the Lago Buenos Aires basin (Chilean name: Lago General Carrera) to the north, rapidly retreated more than 80 kilometers at around 16.5-15 ka. The timing of the LGM maximum extent and the onset of deglaciation occurred broadly synchronously throughout Patagonia. Deglaciation resulted in a series of interconnected glacierdammed lakes in the region that initially drained toward the Atlantic Ocean and later drained to the Pacific Ocean as a consequence of disintegrating ice in the Andes.

A glacial geomorphological map of the southernmost ice lobes of Patagonia: the Bahía Inútil – San Sebastián, Magellan, Otway, Skyring and Río Gallegos lobes

“…Recent cosmogenic dating of outwash depth-profiles at Lago Pueyrredón, east of the North Patagonian icefield, further supports these old ages (Hein, Dunai, Hulton, & Xu, 2011). The inner nested limits have been attributed to advances during intervening glaciations prior to the LGM, largely using cosmogenic dating (Hein et al, 2009;Hein et al, 2010;Hein et al, 2011;Glasser et al, 2011;Kaplan, Douglass, Singer, & Caffee, 2005;Singer et al, 2004a;Singer et al, 2004b;Wenzens, 2006).…”

The timing and cause of glacial advances in the southern mid-latitudes during the last glacial cycle based on a synthesis of exposure ages from Patagonia and New Zealand

“…(Kaplan et al, 2004Douglass et al, 2005Douglass et al, , 2006Glasser et al, 2012) Río Bayo valley -47 -73 3 (3) (Glasser et al, 2006) Nef valley -47 -73 6 (5) (Glasser et al, 2012) Pueyrredón -47/-48 -71/-73 19 (16) (Hein et al, , 2010(Hein et al, , 2011Glasser et al, 2012) San Martín valley -49 -72/-73 10 (10) (Glasser et al, 2011) Río Guanaco -50 -73 21 (21) (Murray et al, 2012) Lago Argentino -50 -73 30 (27) (Ackert et al, 2008;Kaplan et al, 2011) Torres del Paine -51 -73/-74 54 (45) (Fogwill, 2003;Moreno et al, 2009;García et al, 2012) Río Gallegos -51/-52 -71/-72 7 (6) (Kaplan et al, 2007;Evenson et al, 2009;Sagredo et al, 2011) Magellan -52/-53 -69/-71 17 (10) Kaplan et al, 2008Kaplan et al, , 2007 BI-SSb -53/-54 -68/-70 46 (34) Kaplan et al, 2007Kaplan et al, , 2008Evenson et al, 2009;Darvill et al, 2015) Total within Last Glacial Cycle 289 (241)…”

The glacial geomorphology of the Lago Buenos Aires and Lago Pueyrredón ice lobes of central Patagonia

“…Alternatively, they could represent degraded moraines, or moraines that have been dissected by meltwater channels. Continuous hummocky ridges are exclusive to the southern LC-P margin, where they have previously been depicted as discrete, unbroken moraine ridges Hein et al, 2010).…”

“…In addition, Hein et al (2009Hein et al ( , 2010, Hein, Dunai, Hulton, and Xu (2011) re-mapped the outer moraine systems of LC-P and identified a series of well-preserved moraine ridges, outwash terraces, palaeo-shorelines and landslide deposits. Hein et al (2010) also noted some morphological differences between local LGM moraine sets, which included hummocky and sharpcrested forms.…”

“…At LGC-BA a further six terminal moraine sequences were deposited between ∼1016 and ∼760 ka, with another six formed between ∼760 and ∼109 ka ). The chronology of major ice lobe fluctuations has since been refined through direct dating of glacial deposits, including cosmogenic nuclide exposure dating of moraine boulders (Boex et al, 2013;Douglass et al, 2006;Hein et al, 2010;Kaplan et al, 2004Kaplan et al, , 2011 and outwash gravels (Hein et al, , 2011, and luminescence dating of glaciofluvial outwash sediments Harrison, Glasser, Duller, & Jansson, 2012;Smedley, Glasser, & Duller, 2016 Ar ages of earlier investigations, and provided evidence for additional ice lobe advances during MIS 2, 3, 6 and 8.…”