ARTICLE IN PRESS Quaternary International 138–139 (2005) 145–167 Regional synthesis of last glacial maximum snowlines in the tropical Andes, South America Jacqueline A. Smitha,Ã, Geoffrey O. Seltzera,y, Donald T. Rodbellb, Andrew G. Kleinc aDepartment of Earth Sciences, 204 Heroy Geology Lab, Syracuse University, Syracuse, NY 13244-1070, USA bDepartment of Geology, Union College, Schenectady, NY 12308, USA cDepartment of Geography, Texas A&M University, College Station, TX 77843, USA Available online 18 April 2005 Abstract The modern glaciers of the tropical Andes are a small remnant of the ice that occupied the mountain chain during past glacial periods. Estimates of local Last Glacial Maximum (LGM) snowline depression range from low (e.g., 200–300 m in the Junin region, Peru), through intermediate (600 m at Laguna Kollpa Kkota in Bolivia), to high (e.g., 1100–1350 m in the Cordillera Oriental, Peru). Although a considerable body of work on paleosnowlines exists for the tropical Andes, absolute dating is lacking for most sites. Moraines that have been reliably dated to 21 cal kyr BP have been identified at few locations in the tropical Andes. More commonly, but still rarely, moraines can be bracketed between about 10 14C kyr (11.5 cal kyr BP) and 30 14C kyr BP. Typically, only minimum-limiting ages for glacial retreat are available. Cosmogenic dating of erratics on moraines may be able to provide absolute dating with sufficient accuracy to identify deposits of the local LGM. Ongoing work using cosmogenic 10Be and 26Al in Peru and Bolivia suggests that the local LGM may have occurred prior to 21 cal kyr BP. r 2005 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction ages that are calibrated to calendar years before present (Stuiver and Reimer, 1993). The primary purpose of this synthesis of snowline This synthesis differs fromprevious studies of information for tropical South America is to serve as a regional snowline change (e.g., Hastenrath, 1967, 1971, resource for climate modelers seeking reliable informa- 1985; Nogami, 1976; Fox and Bloom, 1994; Klein et al., tion about equilibrium-line altitudes (ELAs) or snow- 1999, 2001; Dornbusch, 2001) in that it is a critical and lines to use for comparison with output from climate conservative examination of evidence for LGM mor- simulation of the Last Glacial Maximum (LGM). As aines. It includes an evaluation of the methods used by used here, the term ‘‘last glacial maximum’’ refers to workers to establish the age of glaciation, the proximity the maximum extent of glaciation in the Northern of the ages associated with glacial features to the LGM Hemisphere at about 18,000 14C yr BP (about 21,000 target age of 21,000 cal yr BP, and an assessment of the cal yr BP) as interpreted fromthe marine oxygen isotope methods used to determine the change in snowline or record (Imbrie et al., 1984). Throughout this manu- ELA. One criterion in the evaluation of chronological script 14C yr BP refers to radiocarbon years before data is the requirement that the feature have radiometric present (AD 1950) and cal yr BP refers to radiocarbon dating associated with it to be considered useful for comparison with climate modeling output. We have focused on the tropical Andes that border the Pacific Ocean south of the equator as a coherent ÃCorresponding author. Tel.: +1 315 443 2672; fax: +1 315 443 3363. geomorphic unit; the Andes of the circum-Caribbean E-mail address: [email protected] (J.A. Smith). region are treated separately (see Lachniet and yDeceased. Vazquez-Selem, this volume). The tropical Andes are 1040-6182/$ - see front matter r 2005 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2005.02.011 ARTICLE IN PRESS 146 J.A. Smith et al. / Quaternary International 138– 139 (2005) 145–167 characterized by prevailing easterlies that bring moisture to group the major study regions by the nations in which across the Amazon Basin from the Atlantic Ocean they are located: Ecuador, Peru, and Bolivia (Fig. 1). (Johnson, 1976). As a result, there is a general east–west Table 1 is a compilation of snowline data presented in gradient in snowlines, with the lowest snowlines on east- this synthesis. Estimates of snowline depression at the facing slopes of the Eastern Cordillera. Increasing LGM are summarized in Fig. 2. The discussion begins in aridity to the south produces a general north–south the north and proceeds southward through the region. gradient in snowlines as well, with the highest snowlines Our goal has been to include sites where the glaciation (6000 m) on the arid Altiplano, although the 01 chronology is based on radiometric dating and the isothermlies approximately 1000 mlower. In such areas method used to calculate changes in snowline at the of extreme aridity, the location of the ELA is highly LGM has been specified. These criteria set this study dependent on the availability of moisture, rather than apart fromprevious regional syntheses that made temperature (Johnson, 1976). In their respective surveys simplifying assumptions about LGM chronology and of geomorphic evidence for Pleistocene glaciations, incorporated snowline changes estimated by differing Hastenrath (1967, 1971, 1985), Nogami (1976), and methods. Klein et al. (1999) concluded that a pattern of rising snowlines fromeast to west and fromnorth to south also existed during the Pleistocene in the tropical Andes. 3. Ecuador In a study limited to Peru, Fox and Bloom(1994) concluded that Pleistocene snowlines rose fromeast to The Ecuadorian Andes consist of two north–south- west in the Peruvian Andes. trending, near-parallel chains, the Eastern Cordillera The tropical Andes are currently dominated by valley (Cordillera Oriental) and the Western Cordillera glaciers located above altitudes of 5 km; only a few ice (Cordillera Occidental), which are separated by high- caps remain (e.g., Quelccaya Ice Cap in Peru). During altitude basins. The highest peaks in the Ecuadorean the LGM in the region, glacial ice expanded and Andes are volcanic and include ice-covered Volca´ n descended up to 1000 mor morefromcurrent terminal Chimborazo (6310 m), the highest point in Ecuador, and positions (e.g., Klein et al., 1999), ice caps were more Cotopaxi (5900 m), the world’s highest active volcano common, and many areas now devoid of ice were then (Johnson, 1976). South of about 21S the altitude of the occupied by both valley glaciers and piedmont glaciers highest peaks in the Ecuadorian Andes falls below (e.g., Clapperton, 1993). The mass-balance regime of 5000 mand modernglacial ice is absent. Peak altitudes glaciers in tropical climates with distinct wet and dry continue to decrease toward the southern end of seasons differs fromthose in high latitudes. On tropical Ecuador and the northern end of Peru, where peak glaciers, ablation typically occurs year round, altitudes are typically in the range of 3000–3500 m whereas accumulation occurs during the wet season, (Instituto Geogra´ fico Militar, 1991). which is generally the austral summer in tropical South No LGM moraines have been definitively identified America (Benn et al., this volume; Kaser and Osmaston, and dated in the Ecuadorian Andes. Near Papallacta 2002; Johnson, 1976). This mass-balance regime com- Pass on the Potrerillos Plateau, moraines have been plicates the determination of snowlines on tropical loosely constrained to a period that includes the LGM. glaciers. Clapperton et al. (1997) have bracketed the Sucus Porter (2001) outlined five techniques commonly used advance between 13 14C kyr BP (15.5 cal kyr BP) and to estimate snowlines: cirque-floor altitude, upvalley 30 14C kyr BP, but background information on the limits of lateral moraines, glaciation threshold, altitude older age is incomplete. ratios (including terminus-to-headwall-altitude-ratio, or THAR), and accumulation-area ratio (AAR). In his 3.1. Volca´n Pichincha, Western Cordillera, 01 12.50 S, comparison of the five methods, Porter referred to the 781 350 W(Fig. 1, Site 1) conclusions of Meierding (1982) that THAR and AAR methods produced the most consistent results. Benn Heine (1995a) identified seven Quaternary moraine et al. (this volume) discuss the assumptions and sets in the Ecuadorean Andes: M I–M VII (M I is requirements associated with specific techniques used oldest). He concluded that the M IV moraines were to estimate snowlines. deposited at the LGM. On Volca´ n Pichincha (4784 m; Fig. 1, Site 1) in the Western Cordillera, the M IV moraines commonly consist of narrow lateral and 2. Major study regions terminal moraines enclosing irregular, hummocky tills, which Heine inferred to be remnants of ice-cored In tropical circum-Pacific South America, national moraines. Heine reported that the M IV moraines boundaries broadly coincide with changes in the descend to altitudes of 3900–4000 mand that deposition structure and climate of the Andes. It is therefore useful of these moraines ‘‘gave evidence of a depression of the ARTICLE IN PRESS J.A. Smith et al. / Quaternary International 138– 139 (2005) 145–167 147 Fig. 1. Location of sites in the tropical Andes of South America that are discussed in the text. Thin gray lines are 3000-m contour lines; thick black lines mark political boundaries. lower limit of ice-cored moraines of about 800–1000 m the Potrerillos Plateau, with peak altitudes of less than compared with the recent occurrence of similar forms on 4400 m, is currently free of glacial ice. Clapperton et al. Mt. Chimborazo.’’ He did not provide explicit ELA (1997) estimated the modern ELA at Volca´ n Antisana at estimates for the M IV moraines. 4970750 mon the northern and western sides, where Chronological control for the M IV moraines at modern glaciers terminate at 4600 m. They did not Volca´ n Pichincha is minimal (Fig.