Quaternary Science Reviews 28 (2009) 2514–2526 Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev Relative timing of last glacial maximum and late-glacial events in the central tropical Andes Gordon R.M. Bromley a,*, Joerg M. Schaefer b, Gisela Winckler b, Brenda L. Hall a, Claire E. Todd c, Kurt M. Rademaker a a Department of Earth Sciences and the Climate Change Institute, Edward T. Bryand Global Sciences Center, University of Maine, Orono, Maine 04469-5790, USA b Lamont-Doherty Earth Observatory, Geochemistry, Route 9W, Palisades, NY 10964, USA c Pacific Lutheran University, Tacoma, WA 98447, USA article info abstract Article history: Whether or not tropical climate fluctuated in synchrony with global events during the Late Pleistocene is Received 3 November 2008 a key problem in climate research. However, the timing of past climate changes in the tropics remains Received in revised form controversial, with a number of recent studies reporting that tropical ice age climate is out of phase with 18 May 2009 global events. Here, we present geomorphic evidence and an in-situ cosmogenic 3He surface-exposure Accepted 18 May 2009 chronology from Nevado Coropuna, southern Peru, showing that glaciers underwent at least two significant advances during the Late Pleistocene prior to Holocene warming. Comparison of our glacial- geomorphic map at Nevado Coropuna to mid-latitude reconstructions yields a striking similarity between Last Glacial Maximum (LGM) and Late-Glacial sequences in tropical and temperate regions. Exposure ages constraining the maximum and end of the older advance at Nevado Coropuna range between 24.5 and 25.3 ka, and between 16.7 and 21.1 ka, respectively, depending on the cosmogenic production rate scaling model used. Similarly, the mean age of the younger event ranges from 10 to 13 ka. This implies that (1) the LGM and the onset of deglaciation in southern Peru occurred no earlier than at higher latitudes and (2) that a significant Late-Glacial event occurred, most likely prior to the Holocene, coherent with the glacial record from mid and high latitudes. The time elapsed between the end of the LGM and the Late-Glacial event at Nevado Coropuna is independent of scaling model and matches the period between the LGM termination and Late-Glacial reversal in classic mid-latitude records, suggesting that these events in both tropical and temperate regions were in phase. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Compelling evidence now exists for major shifts in tropical temperature and precipitation during the Late Pleistocene (ice-core The role of the tropics in climate change has significant impli- data: Thompson et al., 1995, 1998; lake-core data: Baker et al., cations both for our understanding of climate change at various 2001a, b; glacial-geologic data: Smith et al., 2005a, b, 2008; Zech timescales and, in turn, for prediction of future climate scenarios. et al., 2007, 2008), challenging the traditional view of tropical Specifically, the relationships between tropical and extra-tropical climatic stability (CLIMAP, 1981). However, estimates of the timing climate bear directly on two of the foremost problems in palae- and magnitude of these events remain highly variable, as demon- oclimate research: the causes of ice ages and of abrupt climate strated in the comprehensive review of tropical glacial records by change. The structure and timing of orbital-scale climate oscilla- Smith et al. (2008). Recent glacial-chronologic research in the tions in the tropics are highly relevant for discriminating among the Andes of central Peru and northern Bolivia (Smith et al., 2005a, b, potential drivers of ice ages, including astronomical forcing, 2008) suggested that glaciers there reached their maxima as early atmospheric water vapour, and CO2. Moreover, as the energetic as 29–35 ka and were in recession during the global Last Glacial powerhouse of the earth, the tropics have huge potential to trigger Maximum (LGM; w17–25 ka), implying that the tropics were out of and/or amplify climate changes (e.g. Pierrehumbert, 1999). phase with global climate during the last glacial cycle. Based on a cosmogenic-nuclide glacial chronology from Hawaii, Blard et al. (2007) reported that the climate of the central tropical Pacific w * Corresponding author. Tel.: þ1 207 581 2190; fax: þ1 207 581 1203. remained fully glacial until 15 ka. This is consistent with the E-mail address: [email protected] (G.R.M. Bromley). Greenland ice-core temperature record (Andersen et al., 2004) but 0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.05.012 G.R.M. Bromley et al. / Quaternary Science Reviews 28 (2009) 2514–2526 2515 Fig. 1. (a). Map of Peru showing location of Nevado Coropuna (NC), Lago Junin (LJ), the Cordillera Blanca (CB), and the Cordillera Real (CR), Bolivia. (b) Locations of sites mentioned in text and lava flows (black) on Nevado Coropuna (NC). Quebrada Ullullo (QU); Quebrada Santiago (QS); Quen˜a Ranra (QR); Quebrada Sigue Chico (QSC); Lago Pallar Cocha (LPC); Quebrada Huayllaura (QH). C-I moraines are shown as black lines and C-II moraines as red lines. Dashed box represents area shown in Fig. 2. several millennia after the last glacial termination as expressed in four currently accepted scaling protocols (Balco et al., 2008) and mid-latitude glacier records (w18 ka; Schaefer et al., 2006) and in a global compilation of 3He production-rate calibration data the Antarctic temperature and CO2 records (w17–18 ka; Monnin (Goehring et al., in preparation; Table 2, Appendix Tables A2–5). et al., 2001). Correlating their Hawaiian data with the Greenland This approach demonstrates the ambiguity currently inherent to ice-core record, where significant deglacial warming did not occur surface-exposure dating at high elevations in the tropics and, in the until the Bølling-Allerød (14.6 ka), Blard et al. (2007) concluded absence of local production-rate calibration sites, yields the range that the climate systems of the North Atlantic and central tropical of possible glacial chronologies at Nevado Coropuna. We then Pacific are linked atmospherically during glacial periods. Together, compare our preliminary 3He chronology from southern Peru to these two tropical glacier studies indicate that the LGM and the last published 10Be chronologies from central Peru (Smith et al., 2005a, termination might not have been regionally or globally synchro- b, 2008) and the Cordillera Blanca (Farber et al., 2005). To be nous during the Late Pleistocene. In contrast, a high-resolution internally consistent, we recalculated these datasets using the glacier record from the Eastern Cordillera of Bolivia (Zech et al., CRONUS online calculator and production rates given by Balco et al. 2007, 2008), using 10Be surface-exposure dating, suggests that (2008). glaciers in the tropical Andes did follow a pattern similar to glaciers at higher latitude during the Late Pleistocene, substantiating the model of global LGM climate synchrony proposed by Denton et al. 2. Geologic setting (1999) and Schaefer et al. (2006). To address this important conflict, we are reconstructing Late Nevado Coropuna (6425 m; 15 330S, 72 930W), located Pleistocene glacier fluctuations on Nevado Coropuna in southern 150 km northwest of the city of Arequipa in the Cordillera Ampato Peru (Fig. 1a). As sensitive indicators of climate change (Oerle- (Fig. 1a), is the largest, highest volcano in Peru and dates back to mans, 1994, 2001; Anderson and Mackintosh, 2006), glaciers the Miocene (Venturelli et al., 1978). The mountain is formed advance and retreat in response to fluctuations of temperature primarily of andesite lavas characterised by phenocrysts of and precipitation. Through careful reconstruction of former glacier pyroxene, plagioclase, titanomagnetite, amphibole and biotite set extent and chronologic constraint of moraine records, we can in a light brown hyalopilitic groundmass (plagioclase and determine both the timing and magnitude of glacier events, and pyroxene microlites: Venturelli et al., 1978). The last period of therefore climate change, in glaciated and formerly glaciated volcanic activity on Nevado Coropuna produced three prominent regions. The recent development of cosmogenic-nuclide surface- andesite flows, shown in Fig. 1b. Although the lavas have not been exposure dating as a glacial-chronologic tool is resulting in dated, Venturelli et al. (1978) suggested they were formed during constraint of glacier records in tropical, high-mountain regions the Early Holocene. The surrounding landscape comprises an (e.g. Farber et al., 2005; Smith et al., 2005a, b, 2008; Zech et al., undulating plateau dissected by deep canyons – including the 2007, 2008) where radiocarbon dating typically is problematic due world’s deepest – and topped with high, isolated volcanic peaks. to a lack of organic material. However, cosmogenic-nuclide Despite its proximity (w100 km) to the Pacific coast, the prevailing production rates in low latitudes/high altitudes remain poorly easterly airflow maintains a semi-arid climate at Nevado Coro- constrained due to the lack of local production-rate calibration puna, with most annual precipitation (800–1000 mm: Dornbusch, sites. The necessary scaling of the production rates from existing 1998) arriving during the short summer wet season (December– calibration sites at higher latitudes and lower altitudes, including March). Today, the mountain supports an ice cap of approximately the correction for temporal changes in the geomagnetic field 60 km2, which is restricted to elevations >5500 m on the north (which are highest in the tropics), remains a considerable source side and >5000 m on the south side. An abundance of large, well- of uncertainty (Balco et al., 2008). preserved moraines (as much as 100 m high) and other glacial We present new glacial-geomorphic maps of Nevado Coropuna, landforms attest to multiple periods when the ice cap was greatly 3 as well as He surface-exposure data from glacial landforms, such expanded and glaciers existed at far lower elevations than they do as moraines and drift edges.