Cosmogenic dating of Late Pleistocene glaciation, southern tropical Andes, Peru

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1 JOURNAL OF QUATERNARY SCIENCE (2015) 30(8) ISSN DOI: /jqs.2822 Cosmogenic dating of Late Pleistocene glaciation, southern tropical Andes, Peru JEREMY D. SHAKUN, 1 * PETER U. CLARK, 2 SHAUN A. MARCOTT, 3 EDWARD J. BROOK, 2 NATHANIEL A. LIFTON, 4,5 MARC CAFFEE 4,5 and WILLIAM R. SHAKUN 6 1 Department of Earth Environmental Sciences, Boston College, Chestnut Hill, MA, USA 2 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA 3 Department of Geoscience, University of Wisconsin Madison, WI, USA 4 Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA 5 Department Physics and Astronomy, Purdue University, West Lafayette, IN, USA 6 Shakun, Solomons, & Bray Dental, Binghamton, NY, USA Received 11 May 2015; Revised 23 September 2015; Accepted 28 October 2015 ABSTRACT: We present Be dates on three moraines formed by small glaciers in the southern tropical Andes of Peru to determine the timing of the local Last Glacial Maximum (LGM) and subsequent deglaciation. Two terminal moraines indicate that the local LGM ended at and ka, or well before the end of the global LGM (20 19 ka). A recessional moraine perched at the base of a steep valley wall has a 10 Be age of ka, suggesting that glaciers had largely retreated from the area during the earliest stages of the last termination and before the onset of the deglacial CO 2 rise and increase in tropical sea surface temperatures. These ages generally agree with other tropical glacial chronologies and reflect a broad synchroneity between low- and high-latitude deglaciation at the LGM, but the trigger for early deglaciation of the tropics remains unclear. Copyright # 2015 John Wiley & Sons, Ltd. KEYWORDS: 10 Be surface exposure dating; cosmogenic dating; Last Glacial Maximum; Peruvian Andes; tropical glaciation. Introduction Quaternary climate change on glacial interglacial timescales exhibits a marked global coherency, despite being ultimately caused by latitudinal and seasonal variations of insolation that, when summed across the planet, have no global expression (Huybers, 2009). This disparity suggests that some areas of the planet are more sensitive to orbital forcing and may in turn induce globally coherent climate change through strong feedbacks and teleconnections to other regions (Imbrie et al., 1993; Broecker, 2013). The northern high latitudes are often viewed as this trigger region given the former presence of dynamic ice sheets there (Milankovitch, 1941; Ruddiman, 2006). The high-latitude trigger hypothesis is buttressed by the observation that widely distributed components of the climate system track boreal summer insolation over the last several glacial cycles (Imbrie et al., 1993; Lea et al., 2000; Roe, 2006; Kawamura et al., 2007). One possible scenario for a northern control suggests that rising boreal summer insolation initiated ice-sheet melt, the associated freshwater discharge weakened Atlantic overturning causing a bipolar seesaw warming in the Southern Ocean and outgassing of CO 2 that spurred global-scale warming and deglaciation (Denton et al., 2010; He et al., 2013). Glacial cycles are also correlated with a family of insolation curves representing different latitudes and seasons (Huybers, 2009), however, so they could be equally well understood as tracking various southern high-latitude insolation quantities (Huybers, 2011; WAIS Divide Project Members, 2013). Moreover, the tropics are the heat engine and the largest source of short-term variability of the present global climate system; it is therefore reasonable to posit them as a possible driver of glacial cycles (Clement and Cane, 1999; Lea et al., 2000). Differentiating among these possibilities and identifying the propagation of deglacial signals around the world requires robust, precisely dated proxy records capable of identifying Correspondence to: J. D. Shakun, as above. jeremy.shakun@bc.edu leads and lags within the climate system. Cosmogenic nuclide dating of tropical glacier moraines can address these questions, particularly as these glaciers are sensitive responders to local climate and provide an important record of high-altitude paleoclimate in the tropics (Kaser and Osmaston, 2007). Previous work on dating low-latitude moraines concluded that the local Last Glacial Maximum (LGM) in the tropics was earlier than (Smith et al., 2005; Smith and Rodbell, 2010) or synchronous with (Farber et al., 2005; Blard et al., 2007; Bromley et al., 2009) the high-latitude LGM, while some results suggest it was regionally diachronous (Zech et al., 2008). We point to several possible issues that can complicate such interpretations: (i) the incomplete and discontinuous nature of the moraine record (Gibbons et al., 1984) can lead to ambiguity in constraining regional variability; (ii) it is often unclear if the age of retreat from a local LGM moraine represents the onset of full deglaciation or only a minor recessional episode associated with unforced glacier variability (Roe and O Neal, 2009); (iii) glaciers in adjacent valleys can exhibit behavior unrelated to climatic forcing (Young et al., 2011); (iv) several tropical moraines have few or widely spaced dates, which can yield large age uncertainties; and (v) recent improvements in production rate calibrations indicate that previously reported tropical ages are too young by 11 15% (Blard et al., 2013; Kelly et al., 2013; Martin et al., 2015), suggesting that a re-evaluation of proposed triggering mechanisms for tropical deglaciation may be required (Schaefer et al., 2006). To help address these issues, we dated several moraines recording the termination of the local LGM in the northern Peruvian Andes. Study location Our study site lies in the presently unglaciated Cajamarca region of northern Peru (Fig. 1). Temperatures are relatively constant throughout the year and are only slightly below freezing during austral winter in the highlands. Precipitation is derived from the Atlantic Ocean and Amazon Basin to the east and exhibits marked seasonality associated with the Copyright # 2015 John Wiley & Sons, Ltd.

2 842 JOURNAL OF QUATERNARY SCIENCE Fig. 1. Location of sites discussed in the text, including our field site in Cajamarca, Peru (yellow star), other published Peru moraine 10 Be records (gray dots), precipitation records (blue dots), and tropical Atlantic (green dots) and Pacific (red dots) SST records. annual migration of the intertropical convergence zone (Johnson, 1976; Hastenrath, 1991). Maximum rainfall occurs during austral summer (Oct Apr) while almost no precipitation falls during austral winter (Jun Aug). Snowfall occurs only sporadically due to the relatively high temperatures and lack of precipitation during winter. We sampled large quartzite boulders on moraines deposited by three former glaciers. Moraines in this area are 3900 m above sea level, only a few hundred meters below the highest summits, and were deposited by small glaciers (0.5 4 km long). We sampled nine boulders from the North Camp left and right lateral moraines, which demarcate the maximum extent of a 2.5-km-long valley glacier (Fig. 2). The North Camp moraines appear to represent the local LGM, as suggested by the absence of glacial features further down valley. In an immediately adjacent valley, we sampled nine boulders from the Galeno moraine, a multi-crested moraine extending < 0.5 km down a steep quartzite dipslope into a larger U-shaped valley that was presumably ice-filled at the LGM (Fig. 2). The position of the Galeno moraine suggests that it represents final deglaciation in this area. Finally, we sampled 11 boulders and one polished bedrock outcrop from the San Cirilo moraine complex (Fig. 3), 30 km to the west of the North Camp and Galeno moraines. This is a large, hummocky moraine complex composed of dozens of moraine ridges and kettles distributed over a broad 6-km 2 upland of <100-m relief. This moraine complex appears to represent the maximum extent of local ice (i.e. the local LGM) and its hummocky topography represents stagnant ice wastage following the rise of the equilibrium-line altitude (ELA) above this flat upland (Fig. 3). We include the bedrock sample with the boulder samples in computing a mean age for the San Cirilo moraine. Because the ELAs of these former three glaciers are within 100 m of each other (Fig. 4), ages on the associated moraines provide a close age constraint on the termination of the regional LGM. Fig. 2. Topographic relief map showing boulder 10 Be ages (white dots) on the North Camp (blue lines) and Galeno (red lines) moraines. Outliers are italicized. Note that the North Camp lateral moraines mark the local maximum ice extent, and the Galeno moraine is a series of concentric ridges perched on the side of a larger glacial valley and thus interpreted to post-date the local LGM. Base map from Google Maps. isolated quartz from the rock samples following the procedures of Kohl and Nishiizumi (1992) as modified by Licciardi (2000) and Rinterknecht (2003) for the facilities at Oregon State University. Beryllium extraction from quartz followed the methods of Licciardi (2000) and BeO targets were prepared for analysis at PRIME Lab at Purdue University following standard procedures. Ages were calculated using a sea-level high-latitude production rate of at. g 1 a 1 (Shakun et al., 2015) and LSD scaling (Lifton et al., 2014); results using the St and Lm scaling schemes (Balco et al., 2008) are reported for comparison. This production rate value is based on 10 recent high-quality, globally distributed studies and a whole-sky cutoff rigidity approximation as described by Methods Cretaceous quartzite underlies the moraines (Longo, 2005). We used a hammer and chisel to sample near-horizontal, unweathered surfaces from relatively tall ( m) boulders on or generally within a meter or two downslope of moraine crests to reduce the risk of past exhumation and rolling. We Fig. 3. Topographic relief map showing boulder 10 Be ages (white dots) from the San Cirilo moraine complex. Outliers are italicized and a bedrock age is underlined. Maximum ice extent is shown by the thick blue line. The complex is composed of dozens of moraine ridges (thin blue lines) and kettle lakes (light blue) distributed across a broad, flat upland of < 100 m relief, and is interpreted to reflect stagnant ice wastage. Base map from Google Maps.

3 COSMOGENIC DATING OF LATE PLEISTOCENE GLACIATION, PERU 843 Fig. 4. Valley profiles from moraine terminus to valley head for each site (solid lines), and estimated equilibrium line altitudes using a toe headwall altitude ratio of 0.4 (dashed lines). Shakun et al. (2015) (corresponding St and Lm global production rate values are also at. g 1 a 1 ). We favor using a well-constrained global production rate over locally derived rates because the global rate takes into account potentially unrecognized inter-site geologic variability that might not be adequately characterized in a single local calibration site. Another reason we avoid local calibrations is that the geographic and/or temporal extents over which a given local production rate calibration is valid are not well defined. We prefer ages derived using the LSD model over the earlier models because it is the least biased model presently available (based on a formulation that accurately reproduces measured atmospheric cosmic ray fluxes globally) (Lifton et al., 2014). Relevant data are given in Table 1 and photographs of each boulder are provided in the Supplementary Material (Figs S1 S3). Results and discussion Each moraine has one to three boulder ages that are >10 ka older than the rest of the population, which we exclude as clear outliers reflecting prior exposure up valley (Fig. 5). To provide a consistent and objective basis for evaluating whether any of the remaining boulder ages from each moraine may be statistical outliers we used Chauvenet s criterion, which assesses the likelihood that a particular sample would be obtained from a population given the observed mean and standard deviation. As Chauvenet s criterion assumes a normal distribution, we first confirmed that the assumption of normality for each population cannot be rejected at the P ¼ 0.05 level using a Wilk-Shapiro test. Chauvenet s criterion excludes the oldest remaining age from each of the San Cirlo and Galeno moraines. We then computed an error-weighted mean age and standard error from the remaining samples for each moraine. Our data indicate that the local LGM occurred during Marine Isotope Stage 2 (MIS 2), with ages on the LGM moraines of Be ka at North Camp and Be ka at San Cirilo (Fig. 5). Other published 10 Be chronologies from the tropical Andes (Farber et al., 2005; Smith et al., 2005; Smith and Rodbell, 2010; Wesnousky et al., 2012) and magnetic susceptibility records from Lakes Junin and Titicaca (Seltzer et al., 2002) also indicate that glaciers throughout the region were at their maximum extents during MIS 2 (Fig. 6g), coincident with maximum cooling of the oceans adjacent to tropical South America (R uhlemann et al., 1999; Lea et al., 2000, 2006; Schmidt et al., 2004; Benway et al., 2006; Weldeab et al., 2006; Jaeschke et al., 2007; Leduc et al., 2007) (Fig. 6b,c) and peak moisture as reflected in nearby speleothem and lake records (Baker et al., 2001; Kanner et al., 2012; Cheng et al., 2013) (Fig. 6a). There is considerable variability in the timing of the local LGM in the tropical Andes, however, with moraine ages ranging from 30 to 20 ka (Fig. 6g). This variability during the LGM may reflect millennial-scale precipitation variability, perhaps related to Heinrich events (Baker et al., 2001; Smith and Rodbell, 2010; Kanner et al., 2012), or unforced glacier variability (Roe and O Neal, 2009). The broad distribution of boulder ages on many of the moraines suggests that post-depositional geomorphic processes and inheritance may also contribute to this spread (Applegate et al., 2010). Cosmogenic ages of the local LGM elsewhere in the tropics (Shanahan and Zreda, 2000; Barrows et al., 2011; Kelly et al., 2014) are similar to those from the tropical Andes (Fig. 6g). The broad spatial scale of this signal implies that temperature was the major driver of the tropical LGM, given that temperature anomalies exhibit less spatial variability than precipitation (Oerlemans, 2005), although precipitation may have modulated the magnitude or timing of glacier maxima regionally. These tropical LGM ages are also similar to the timing of maximum extents of mid-latitude glaciers (Shakun et al., 2015) as well as the high-latitude ice sheets (Clark et al., 2009) and the LGM sea-level lowstand (Fig. 6d) (Lambeck et al., 2014), reflecting a general synchroneity between high- and low-latitude glaciation. We use dated moraines up-valley from the LGM moraines to constrain the minimum age of the onset of deglaciation. In this regard, the Galeno moraine suggests that deglaciation of the Cajamarca region was essentially complete by Be ka (Fig. 5), probably because only a small rise in ELA (100 m) is required for this response (Fig. 4). Several other 10 Be-dated moraines up-valley from local LGM positions as well as lake cores in the Peruvian Andes, however, also indicate that regional deglaciation was underway by 20 ka (Fig. 6g). What triggered tropical deglaciation at this time? Mechanisms commonly associated with deglacial climate change suggest that deglaciation would have occurred after 20 ka (Denton et al., 2010; Shakun et al., 2012). For example, greenhouse gas concentrations did not begin rising until 18 ka (Fig. 6e) (Marcott et al., 2014). Ice-sheet retreat and sea-level rise could induce tropical deglaciation through (i) albedo-driven warming of the high latitudes causing a weakening of tropical winds and upwelling or exchanging warmer waters with the tropics (Bush and Philander, 1998); (ii) flooding the Sunda Shelf leading to tropical atmospheric circulation-related warming (Bush and Fairbanks, 2003); and (iii) lifting snowlines in proportion to rising sea level (Hanson and Hooke, 2011). Significant ice-sheet retreat and sea-level rise, however, did not begin until after 20 ka (Clark et al., 2009; Lambeck et al., 2014). Freshwater forcing of the Atlantic Meridional Overturning Circulation (AMOC), also related to ice-sheet retreat, might affect tropical temperature and hydrology, at least in the Atlantic basin (Zhang and Delworth, 2005), but the AMOC did not begin to decrease until after 19 ka (Clark et al., 2012). While orbital cycles could impact tropical glaciation, most likely through changes in mean annual insolation as tropical glaciers ablate throughout the year, obliquity variations decreased mean annual insolation in the low latitudes throughout the LGM and deglaciation, presumably favoring ice growth rather than retreat (Fig. 6f). Dust forcing of deglacial climate is poorly

4 844 JOURNAL OF QUATERNARY SCIENCE Table 1. Cajamarca, Peru 10 Be ages. Sample Lat. ( ) Long. ( ) Elev. (m) Thickness (cm) Shielding 10 Be (10 5 at g 1 ) 1s (10 5 at g 1 ) Standard LSD age (ka 1s) St age (ka 1s) Lm age (ka 1s) Galeno moraine MC-G KNSTD MC-G KNSTD MC-G KNSTD MC-G KNSTD MC-G KNSTD MC-G KNSTD MC-G KNSTD MC-G KNSTD MC-G KNSTD Moraine: San Cirilo moraine SC KNSTD SC KNSTD KNSTD SC KNSTD SC KNSTD SC KNSTD SC KNSTD SC KNSTD SC KNSTD SC KNSTD SC KNSTD SC KNSTD Moraine: SC-8^ North Camp moraine MC-NC KNSTD MC-NC KNSTD MC-NC KNSTD MC-NC KNSTD MC-NC KNSTD MC-NC KNSTD MC-NC KNSTD MC-NC KNSTD MC-NC KNSTD Moraine: ^Bedrock sample. Outlier not included in moraine age. Ages were calculated using LSD (Lifton et al., 2014), as well as St and Lm (Balco et al., 2008) scaling models, and a sea-level high-latitude production rate of at g 1 a 1 (Shakun et al., 2015) (appropriate for each model considered). Blanks run alongside samples ranged from 3.21 to 7.84 E Be/Be total, or % of the measured sample ratios. No blank corrections were made. KNSTD and 07KNSTD standards as reported in Nishiizumi (2004) and Nishiizumi et al. (2007). Measurement uncertainties reflect analytical error only. Errors on individual boulders give external (combined measurement and production rate) uncertainties, and moraine ages are reported as the error-weighted mean and standard error of boulder ages.

5 COSMOGENIC DATING OF LATE PLEISTOCENE GLACIATION, PERU 845 Fig Be dates from Cajamarca, Peru, moraines with 1s external uncertainties. The error-weighted mean and standard error of the boulder ages are represented by the horizontal lines and surrounding gray boxes. Inset graphs display all boulder ages from each moraine with outliers shown as open symbols. Fig. 6. Paleoclimate data discussed in the text. (a) Precipitation proxy records from El Condor Cave, Peru (Cheng et al., 2013) (d 18 O, cyan), Pacupahuain Cave, Peru (Kanner et al., 2012) (d 18 O, navy), and Salar de Uyuni (Baker et al., 2001) (natural gamma radiation, purple). (b) Eastern equatorial Pacific SST records from sites TR (Lea et al., 2006) (light purple), TR (Lea et al., 2000) (magenta), ME005A-43JC (Benway et al., 2006) (red), MD (Leduc et al., 2007) (orange) and MD0005A- 24JC (Kienast et al., 2006) (yellow). (c) Tropical Atlantic SST records from sites PL07-39PC (Lea, 2003) (mint green), GeoB3910 (Jaeschke et al., 2007) (green), GeoB3129 (Weldeab et al., 2006) (dark green), M (R uhlemann et al., 1999) (blue-green) and VM (Schmidt et al., 2004) (blue). (d) Global sea level (Lambeck et al., 2014). (e) Atmospheric CO 2 (Ahn and Brook, 2008; Marcott et al., 2014). (f) Mean annual insolation at the equator (Laskar et al., 2004). (g) The error-weighted mean and standard error of 10 Be ages on local LGM moraines (blue symbols with error bars) and the first moraine suggesting significant retreat upvalley (red symbols with error bars) in equatorial Africa (squares; Kelly et al., 2014), northern Peru (diamonds; this study), and central and southern Peru (circles; Farber et al., 2005; Smith et al., 2005; Smith and Rodbell, 2010). Boulders considered outliers by the original authors were excluded. All ages were recalculated with LSD scaling (Lifton et al., 2014) and a sea-level high-latitude 10 Be production rate of at. g 1 a 1 (Shakun et al., 2015). Blue crosses show the timing of initial deglaciation inferred from Lakes Junin and Titicaca magnetic susceptibility records (Seltzer et al., 2002).

6 846 JOURNAL OF QUATERNARY SCIENCE constrained, but at least in central Peru (Thompson et al., 1995), the equatorial Pacific (Winckler et al., 2008) and Antarctica (Lambert et al., 2008), it did not begin decreasing until after 18 ka. Finally, tropical sea surface temperature (SST) records exhibit little warming before 19 ka (Fig. 6b,c) (Shakun et al., 2012), suggesting a decoupling of changes in SSTs and ELAs. An important issue is to what extent these early deglacial ages might reflect remaining uncertainties in cosmogenic nuclide dating. Systematic inheritance could explain anomalously old ages, whereas shielding or erosion of boulder surfaces would yield underestimated ages. Global 10 Be production rates do not yet incorporate glacial isostatic uplift or time-varying atmospheric thickness corrections for calibration sites, which might shift calculated ages younger, particularly at high elevations (Staiger et al., 2007). Furthermore, low-latitude, high-altitude production rates remain less well constrained by calibration sites than the mid- and high latitudes, which introduces some uncertainty into ages. Regardless, if this early timing for initial ice pullback in the tropics is robust, it poses a challenge in understanding the mechanisms that triggered tropical glacier retreat. Conclusions 10 Be dating of two terminal moraines and one recessional moraine in the northern Peruvian Andes indicates that the local LGM terminated by 20 ka, which together with glacial evidence elsewhere indicates a pattern of deglaciation in the tropics before the onset of rising CO 2 or other obvious large-scale forcings. We conclude that current understanding of the last deglaciation does not explain the trigger for tropical deglaciation or its connection to high-latitude climate change, highlighting an important challenge for future work. Supporting material Figure S1. Galeno Moraine Figure S2. 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