Holocene History of Río Tranquilo Glacier, Monte San Lorenzo (47°S), Central Patagonia

Abstract

The causes underlying Holocene glacier fluctuations remain elusive, despite decades of research efforts. Cosmogenic nuclide dating has allowed systematic study and thus improved knowledge of glacier-climate dynamics during this time frame, in part by filling in geographical gaps in both hemispheres. Here we present a new comprehensive Holocene moraine chronology from Mt. San Lorenzo (47°S) in central Patagonia, Southern Hemisphere. Twenty-four new 10Be ages, together with three published ages, indicate that the Río Tranquilo glacier approached its Holocene maximum position sometime, or possibly on multiple occasions, between 9,860 ± 180 and 6,730 ± 130 years. This event(s) was followed by a sequence of slightly smaller advances at 5,750 ± 220, 4,290 ± 100 (?), 3,490 ± 140, 1,440 ± 60, between 670 ± 20 and 430 ± 20, and at 390 ± 10 years ago. The Tranquilo record documents centennial to millennial-scale glacier advances throughout the Holocene, and is consistent with recent glacier chronologies from central and southern Patagonia. This pattern correlates well with that of multiple moraine-building events with slightly decreasing net extent, as is observed at other sites in the Southern Hemisphere (i.e., Patagonia, New Zealand and Antarctic Peninsula) throughout the early, middle and late Holocene. This is in stark contrast to the typical Holocene mountain glacier pattern in the Northern Hemisphere, as documented in the European Alps, Scandinavia and Canada, where small glaciers in the early-to-mid Holocene gave way to more-extensive glacier advances during the late Holocene, culminating in the Little Ice Age expansion. We posit that this past asymmetry between the Southern and Northern hemisphere glacier patterns is due to natural forcing that has been recently overwhelmed by anthropogenic greenhouse gas driven warming, which is causing interhemispherically synchronized glacier retreat unprecedented during the Holocene.

Holocene environmental and climate evolution of central west Patagonia as reconstructed from lacustrine sediments of Meseta Chile Chico (46.5° S, Chile)

“…These climatic conditions were responsible for glacial advances between 6-4 ka throughout Patagonia (Davies et al, 2020;Mercer, 1976;Glasser et al, 2004;Aniya, 1995;Kaplan et al, 2016). Moraine deposits from this period are found around the North Patagonian Icefield (Fernández et al, 2012;Harrison et al, 2012;Nimick et al, 2016;Bertrand et al, 2012) and Monte San Lorenzo in central Patagonia (Sagredo et al, 2021(Sagredo et al, , 2018, around the South Patagonian Icefield (Reynhout et al, 2019;Strelin et al, 2014;Kaplan et al, 2016), as well as in the Cordillera Darwin in southern Patagonia and on the northern Antarctic Peninsula (Kaplan et al, 2020). This stage could have also triggered a series of large dam-breaching events in several lakes of the Río Baker catchment, as documented by dated glacial lake outburst flood (GLOF) deposits (Benito et al, 2021).…”

“…For the same time, moraine deposits are documented at ca. 9.5 ka for Laguna San Rafael (Harrison et al, 2012), at 9.8-6.7 ka for Monte San Lorenzo (Sagredo et al, 2021), and at 11-5.8 ka for Fachinal (Fig. 9; Davies et al, 2020;Douglass et al, 2005a).…”

“…Moraine deposits in the vicinity of the North Patagonian Icefield dated to between 10-6 ka are scarce (Douglass et al, 2005a;Harrison et al, 2007;Sagredo et al, 2021). They indicate that during this period the Patagonian landscape was mainly formed by ice recession and splitting up of this major ice sheet into several smaller glaciers covering regional topographic highs.…”

Rapid glacier recession at Monte San Lorenzo (Patagonia) in response to abrupt Southern Hemisphere warming 13.0–12.0 ka BP

“…We have no landform or geochronological data for the Early Holocene but we infer ice was stabilized, or readvanced to, the M13 moraine complex in the mid-Holocene (~5.6 ka, Fig. 6F), based on the dating evidence from the RT6 moraine in the Tranquilo valley (Sagredo et al, 2017(Sagredo et al, , 2018(Sagredo et al, , 2021. Glaciar Calluqueo remained stable at M13 during the Late Holocene or may have undergone recession/readvance during this time, based on multiple moraine crests.…”

“…Across central Patagonia there is now an improved understanding of the timing and extent of major Quaternary ice limits of the former Patagonia Ice Sheet (Davies et al , 2020). For example, mapping and dating has determined ice limits for the Great Patagonian Glaciation at 1.1 Ma (Hein et al , 2009, 2011; Tobal et al , 2021), the Last Glacial Maximum (Hein et al , 2010; Garcia et al , 2018), the Antarctic Cold Reversal (ACR) (Sagredo et al , 2018; Davies et al , 2018; Mendelova et al , 2020) and Holocene neoglacials (Nimick et al , 2016; Kaplan et al , 2016; Reynhout et al , 2019; 2020; Sagredo et al , 2021). Rates of glacier recession during the Last Glacial Interglacial Transition (LGIT), defined as 15–11.7 ka (Lowe and Hoek, 2001) can be difficult to constrain in Patagonia as most outlet glaciers terminated in large proglacial lacustrine environments or because mountain glaciers remain inaccessible, both limiting landform reconstructions.…”