JOURNAL OF QUATERNARY SCIENCE (2016) ISSN 0267-8179. DOI: 10.1002/jqs.2847 Latest Pleistocene and Holocene glacial events in the Colonia valley, Northern Patagonia Icefield, southern Chile DAVID A. NIMICK,1* DANIEL MCGRATH,2 SHANNON A. MAHAN,3 BEVERLY A. FRIESEN3 and JONATHAN LEIDICH4 1U.S. Geological Survey, 3162 Bozeman Avenue, Helena, MT 59601, USA 2U.S. Geological Survey, Anchorage, AK, USA; and Geosciences Department, Colorado State University, Fort Collins, CO, USA 3U.S. Geological Survey, Lakewood, CO, USA 4Patagonia Adventure Expeditions, Cochrane, XI Region, Chile Received 15 November 2014; Revised 25 January 2016; Accepted 13 February 2016 ABSTRACT: The Northern Patagonia Icefield (NPI) is the primary glaciated terrain worldwide at its latitude (46.5–47.5˚S), and constraining its glacial history provides unique information for reconstructing Southern Hemisphere paleoclimate. The Colonia Glacier is the largest outlet glacier draining the eastern NPI. Ages were determined using dendrochronology, lichenometry, radiocarbon, cosmogenic 10Be and optically stimulated luminescence. Dated moraines in the Colonia valley defined advances at 13.2 Æ 0.95, 11.0 Æ 0.47 and 4.96 Æ 0.21 ka, with the last being the first constraint on the onset of Neoglaciation for the eastern NPI from a directly dated landform. Dating in the tributary Cachet valley, which contains an ice-dammed lake during periods of Colonia Glacier expansion, defined an advance at ca. 2.95 Æ 0.21 ka, periods of advancement at 810 Æ 49 cal a BP and 245 Æ 13 cal a BP, and retreat during the intervening periods. Recent Colonia Glacier thinning, which began in the late 1800s, opened a lower-elevation outlet channel for Lago Cachet Dos in ca. 1960. Our data provide the most comprehensive set of Latest Pleistocene and Holocene ages for a single NPI outlet glacier and expand previously developed NPI glacial chronologies. Copyright # 2016 John Wiley & Sons, Ltd. KEYWORDS: Colonia Glacier; cosmogenic nuclide; glacial lake outburst flood; radiocarbon; Lago Cachet Dos. Introduction Hemisphere glacial history and our ability to corroborate reconstructed temperature records. The Northern and Southern Patagonia Icefields (Fig. 1) form The Colonia valley (Figs 1 and 2) on the eastern side of the the largest continental ice mass in the world outside of NPI contains glacial features that date back ca. 13 ka and Antarctica and Greenland (Loriaux and Casassa, 2013) and thus provide an important constraint on Patagonia Icefield occupy the only terrain (except for very southern New glacial history for the period between the LGM and the late Zealand) glaciated at their latitude during the Late Pleistocene 1800s. The Colonia Glacier, with an ice area of 288 km2 in and Holocene. Changes in the volume and extent of these 2001, is the fifth largest of 24 main outlet glaciers (Fig. 1) icefields are tied closely to changes in climate (Glasser et al., draining the NPI (Rivera et al., 2007). Because it is the largest 2004), and thus mapping and dating these fluctuations offers outlet glacier on the east side of the NPI and drains the important and unique temporal information on past climatic central part of the icefield, the Colonia Glacier is probably a changes in the Southern Hemisphere. sensitive proxy of icefield changes. Glacial landforms in the Most research on the glacial geology of the Northern valley have been mapped in the field (Tanaka, 1980; Harrison Patagonia Icefield (NPI) has focused on the modern ca. and Winchester, 2000) and from visible satellite imagery 150-year period. This research has inventoried and dated the (Glasser et al., 2009, 2012), but dating has been restricted substantial reductions in volume and areal extent of ice as the to landforms younger than ca. 150 years (Harrison and NPI and its outlet glaciers retreated from late-1800s maxi- Winchester, 2000). mum positions (e.g. Harrison et al., 2007; Rivera et al., 2007; The rapid retreat of NPI outlet glaciers during the past Davies and Glasser, 2012; Loriaux and Casassa, 2013). Other century (Harrison et al., 2007; Davies and Glasser, 2012) has studies have examined the eastward extension of the icefield increased the area of proglacial lakes surrounding the NPI by almost 200 km into Argentina during the Last Glacial (Loriaux and Casassa, 2013) and, more importantly, the Maximum (LGM) (e.g. Kaplan et al., 2004; Hein et al., 2010), frequency of catastrophic glacial lake outburst floods (GLOFs) a regional advance of NPI outlet glaciers during the Latest (Harrison et al., 2006; Dussaillant et al., 2010). Most of the Pleistocene and early Holocene (Glasser et al., 2006, 2012; known GLOFs originating from the NPI during the past Harrison et al., 2012), and Neoglacial advances during the century have occurred in the Colonia valley (Tanaka, 1980; mid to late Holocene (summarized by Aniya, 2013). How- Friesen et al., 2015). GLOFs result from the sudden and ever, the history of NPI advance and retreat between LGM catastrophic drainage of supraglacial or proglacial lakes deglaciation and the late-1800s maximum generally is poorly and present a significant hazard to human populations and known (Glasser et al., 2004; Masiokas et al., 2009; Aniya, infrastructure (Richardson and Reynolds, 2000). Thus, under- 2013). This lack of knowledge limits studies which compare standing the history and mechanisms of the GLOFs in the either different parts of the NPI and its neighboring glaciers Colonia valley has both local and global implications. (e.g. Douglass et al., 2005) or the NPI with the better studied A better resolved chronology of Colonia Glacier advance and Southern Patagonia Icefield (SPI) (e.g. Strelin et al., 2014). retreat is essential to future studies of Colonia valley GLOFs. More generally, this void limits our understanding of Southern Studies in the Colonia valley were undertaken to identify glacial landforms and deposits and to collect data and ÃCorrespondence to: D. A. Nimick, as above. samples suitable for improved constraint on the post-LGM E-mail: [email protected] glacial history of the NPI. Field studies focused on moraines Copyright # 2016 John Wiley & Sons, Ltd. JOURNAL OF QUATERNARY SCIENCE Figure 1. Satellite image show- ing Northern Patagonia Icefield, primary outlet glaciers, location of the Colonia (Fig. 2) and Cachet (Fig. 3) valleys, and cos- mogenic 10Be sampling sites (tri- angles) and site names from Glasser et al. (2012). NPI outline and mask are from the Global Land Ice Measurements from Space (GLIMS) glacier database (Davies, 2012). Base is from ESRI online World Imagery map ser- vice (www.arcgis.com/home/ item.html?id=10df2279f9684e4a 9f6a7f08febac2a9, accessed 16 October 2014) and is a compila- tion of satellite images taken during 1999–2011 by various sources. in the Colonia valley proper (Fig. 2) and deposits and terminus (elevation of ca. 200 m) to the base of an icefall landforms in the tributary Cachet valley containing Lago (elevation of ca. 950 m), where this outlet glacier flows from Cachet Dos (Fig. 3), a lake currently dammed by the Colonia the NPI. Glacier. The Colonia Glacier today and in the past created ice- dammed lakes in two valleys tributary to the Colonia valley. Description of study area Both lakes have been intermittent through time depending on the position of the Colonia Glacier and its effectiveness as a The Colonia valley extends east and north-east from the NPI dam. GLOFs are (or were) a feature of both ice-dammed lakes to the Rı´o Baker (Fig. 1). The eastern 19-km reach of the (Tanaka, 1980; Dussaillant et al., 2010). One of these ice- valley is drained by the Rı´o de la Colonia (henceforth dammed lakes is the current Lago Cachet Dos in the Cachet abbreviated to Rı´o Colonia), a braided glacial outwash river valley (Fig. 2). The second lake formed in the Arco valley on with a wide (3 km), largely unvegetated floodplain com- the south side of the Colonia valley (Fig. 5G) when the posed of fluvio-glacial sediment. The center reach of the Colonia Glacier abutted the north flank of Cerro Colonia. valley contains Lago Colonia, an 8-km-long moraine- This paleo Lago Arco existed during much of the 20th century dammed lake (Fig. 4A). The 5-km reach upstream of the lake and was larger than the present-day moraine-dammed Lago contains a sparsely vegetated outwash plain with the eroded Arco shown in Fig. 2 (Tanaka, 1980). GLOFs from paleo Lago remains of moraines (Fig. 4B) formed during the past ca. Arco occurred from before 1930 to 1968 (Tanaka, 1980), 150 years (Harrison and Winchester, 2000) and a proglacial while GLOFs from Lago Cachet Dos started in 2008 (Dussail- lake at the terminus of the Colonia Glacier (Fig. 5G). lant et al., 2010). Our field studies included the Cachet valley The Colonia Glacier extends north-west 18 km from its because determining previous fluctuations in the size and Copyright # 2016 John Wiley & Sons, Ltd. J. Quaternary Sci. (2016) GLACIAL EVENTS IN THE NORTHERN PATAGONIA ICEFIELD Figure 2. Satellite image of Col- onia valley showing sampling sites and glacial landforms and deposits. Image produced from Landsat8 data collected on 18 January 2014. Topographic con- tour lines were derived from ASTER Global Digital Elevation Model (GDEM) v2. Elevations are referenced to the 1984 World Geodetic System (WGS84)/1996 Earth Gravitational Model (EGM96) geoid. Lacustrine trim- line and 2007 Lago Cachet Dos boundary are from Friesen et al. (2015). ARCO, CLARO and LCM are Cerro Colonia lateral moraine, Rı´o Claro lateral mo- raine and Lago Colonia terminal moraine, respectively. Locations of sampled boulders are within the circle marking the location of each cosmogenic sampling site. À extent of Lago Cachet Dos provides one way to constrain the to be 11.7 cm a 1 and 26 a, respectively, based on previous chronology of Colonia Glacier advance and retreat.