Deglaciation of the eastern flank of the north patagonian icefield and associated continental‐scale lake diversions

Abstract

. We examine the deglaciation of the eastern flank of the North Patagonian Icefield between latitudes 46° and 48°S in an attempt to link the chronology of the Last Glacial Maximum moraines and those close to present‐day outlet glaciers. The main features of the area are three shorelines created by ice‐dammed lakes that drained eastwards to the Atlantic. On the basis of 16 14C and exposure age dates we conclude that there was rapid glacier retreat at 15–16 ka (calendar ages) that saw glaciers retreat 90–125 km to within 20 km of their present margins. There followed a phase of glacier and lake stability at 13.6–12.8 ka. The final stage of deglaciation occurred at c. 12.8 ka, a time when the lake suddenly drained, discharging nearly 2000 km3 to the Pacific Ocean. This latter event marks the final separation of the North and South Patagonian Icefields. The timing of the onset of deglaciation and its stepped nature are similar to elsewhere in Patagonia and the northern hemisphere. However, the phase of lake stability, coinciding with the Antarctic Cold Reversal and ending during the Younger Dryas interval, mirrors climatic trends as recorded in Antarctic ice cores. The implication is that late‐glacial changes in southern Patagonia were under the influence of the Antarctic realm and out of phase with those of the northern hemisphere.

The Last Glacial Maximum and deglaciation in central Patagonia, 44°S–49°S

“…The LBA outlet lobe initially retreated to an unknown distance, leading to the formation of a proglacial lake. A subsequent readvance then deposited the Menucos moraine above varved lake sediment Kaplan et al, 2004;Turner et al, 2005). Cosmogenic 10 Be ages from moraine boulders on the Menucos moraine yielded range between 18.3 ka and 15.2 ka .…”

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

“…The most prominent lake shorelines occur east of LGC-BA and LC-P ( Figure 13) and rise to ∼300 m higher than contemporary lake levels. Previous shoreline mapping has enabled several reconstructions of proglacial lake evolution (Bourgois et al, 2016;Glasser et al, 2016;Turner et al, 2005). In comparison, we mapped a greater number of shoreline features, including very faint, closely spaced shoreline fragments located between the major wave-cut scarps.…”

“…These ice lobes advanced to the Argentine steppe (Caldenius, 1932), blocked regional river systems and caused a ∼200 km westward shift in the drainage divide towards the Patagonian cordillera, which diverted meltwater eastward to the Atlantic Ocean (Bell, 2008;Glasser et al, 2016;Turner, Fogwill, McCulloch, & Sugden, 2005). During deglaciation, large proglacial lakes developed in the basins between terminal moraines and the ice front (Bell, 2008;Turner et al, 2005).…”

“…Turner et al (2005), Bell (2008) and, more recently, Glasser et al (2016), mapped palaeoshorelines and raised lacustrine deltas formed at proglacial lake margins during glacier recession that contribute to a regional model of deglacial palaeolake development and drainage.…”

“…Flat-topped, sediment accumulations in the mouths of tributary valleys, and perched above modern lakes, represent raised lacustrine deltas (Figure 13; Bell, 2008Bell, , 2009Glasser et al, 2016;Hein et al, 2010;Turner et al, 2005). At LGC-BA, raised deltas are often flanked by beaches (Figure 13(D)).…”