A preliminary investigation of the timing of the local ...

Quaternary International 449 (2017) 149e160

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A preliminary investigation of the timing of the local last glacial maximum and deglaciation on HualcaHualca volcano - Patapampa Altiplano (arid Central Andes, Peru) -Reygosa a, *, David Palacios b, Lorenzo Va zquez-Selem c Jesús Alcala noma de M Facultad de Filosofía y Letras, Universidad Nacional Auto exico, Ciudad Universitaria, 04510, Ciudad de M exico, Mexico Departamento de Geografía, Universidad Complutense, C/ Profesor Aranguren S/N, 28040, Madrid, Spain c noma de M Instituto de Geografía, Universidad Nacional Auto exico, Ciudad Universitaria, 04510, Ciudad de M exico, Mexico a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 February 2016 Received in revised form 23 June 2017 Accepted 26 July 2017 Available online 17 August 2017

We studied the timing and area of the late Pleistocene maximum glacier extent, the readvance or stillstand phases and the deglaciation processes in the HualcaHualca volcano and Patapampa Altiplano, located in the southern arid Peruvian Andes. We used geomorphological mapping and 9 36Cl cosmogenic surface exposure dating of moraine boulders as well as polished and striated bedrock surfaces. The cosmogenic exposure ages indicates that the local Last Glacial Maximum occurred at 17e16 ka on the HualcaHualca volcano. Moreover, the Patapampa Altiplano ice cap had already disappeared by ~12 ka while glaciers on the HualcaHualca also experienced a marked retreat at that time. However, the low number of samples in the study area and the different chronologies derived from the available production rates do not allow us to provide a detailed glacial reconstruction. We recommend a more intensive dating campaign and the development of a local production rate for 36Cl in the Central Andes. © 2017 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Central andes 36 Cl surface exposure dating Last glacial maximum Late glacial Tauca and coipasa paleolake cycles

1. Introduction Tropical glaciers are highly sensitive indicators of global climate change (Kaser and Osmaston, 2002). Dating glacial tropical deposits therefore provides valuable data on glacier evolution and the characteristics of past climate fluctuations. In this context, the Central Andes constitute a key area due to the existence of wellpreserved glacial sequences. In recent years, in situ cosmogenic exposure dating has provided new knowledge about the age of the maximum glacier extent (defined here as the lowest altitudinal position reached by the glaciers according to geomorphological evidence), readvance or stillstand phases and the beginning of deglaciation in the Central Andes. However, our analysis of published chronological data in this region indicates a non-homogeneous glacier response to the global Last Glacial Maximum (LGM), defined as the global maximum volume reached by ice sheets at approximately 26.5 to 19/18 ka (Clark et al., 2009), as well as various late Pleistocene cold

* Corresponding author. E-mail addresses: [email protected] (J. Alcal a-Reygosa), zquez-Selem). (D. Palacios), [email protected] (L. Va http://dx.doi.org/10.1016/j.quaint.2017.07.036 1040-6182/© 2017 Elsevier Ltd and INQUA. All rights reserved.

[email protected]

events: Heinrich 1, Younger Dryas and Antarctic Cold Reversal. Several chronologies show that the Local Last Glacial Maximum in the Central Andes coincided with or took place several thousand years prior to the LGM. This is the case of the valleys close to Junin Lake (Seltzer et al., 2002; Smith et al., 2005), Zongo valley (Cordillera Oriental; Smith et al., 2005), Rurec valley (Cordillera Blanca; Farber et al., 2005), San Francisco and Suturi valleys (Cordillera Oriental; Zech et al., 2007b), Cochabamba range (Kull et al., 2008), Coropuna volcano (Bromley et al., 2009), Huancho valley (Huayhuash range; Hall et al., 2009), Uturuncu volcano (Blard et al., 2014), the Northern Peruvian Andes near Cajamarca (Shakun et al., 2015), Cordillera Carabaya (Bromley et al., 2016) and the arid diagonal (Ward et al., 2017; Zech et al., 2017). Moreover, Mark et al. (2017) have calculated the average of published cosmogenic exposure ages available in Peru and Bolivia related to the LGM. The mean age is ~25 ka but there is an estandar deviation of 4.0 ka and an uncertainty of ~7 ka. In contrast, a number of studies suggest that the Local Last Glacial Maximum and marked readvance or stillstand glacial phases took place during the Late Glacial in phase with the Tauca and Coipasa paleolakes cycles in the following areas: Quelccaya ice cap ~o n and (Clapperton, 1993; Kelly et al., 2012), headwaters of Maran Huallaga rivers (Cordillera Oriental; Rodbell, 1993), Tunupa volcano

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and Cerro Azanaques (Clayton and Clapperton, 1997; Blard et al., 2013), Laguna Baja valley (Cordillera Blanca; Farber et al., 2005), Milluni valley (Cordillera Apolobamba; Smith et al., 2005), valleys located to the east of Junin lake (Smith et al., 2005), Cordon de Santa Rosa (Cordillera Oriental; Zech et al., 2007a), Huara Loma valley (Cordillera Oriental; Zech et al., 2007b; May et al., 2011), San Francisco and Suturi valleys (Cordillera Oriental; Zech et al., 2007b; May et al., 2011), Cochabamba range (Kull et al., 2008), Juellesh and Tuco valleys (Cordillera Blanca; Glasser et al., 2009), Sajama volcano (Smith et al., 2009), Jahuacocha, Mitococha and Carhuacocha valleys (Cordillera Huayhuash; Hall et al., 2009), Coropuna volcano (Bromley et al., 2009, 2011), Nevado Illimani (Smith et al., 2011), Nevado Huaguruncho (Cordillera Blanca; Stansell et al., 2015) and Cordillera Carabaya (Bromley et al., 2016). After this period, deglaciation was relatively quick although minor readvances and/ or stillstands interrupted the glacier retreat in the early Holocene (Mark et al., 2017). The analysis of several proxy data series from the Central Andes suggests a cold and relatively humid LGM (Thompson et al., 1995, 1998; Baker et al., 2001; Fornace et al., 2014). Thompson et al. (1995) estimated a temperature decrease of 8e12 C during this period, supported by analysis of ice cores from Nevado Huascar an (Peru). Meanwhile, the Heinrich 1 and Younger Dryas events were characterized by a substantial increase in precipitation as reflected in the Tauca and Coipasa paleolake cycles (Blard et al., 2011; Placzek et al., 2013). According to Hastenrath (1971), Klein et al. (1999), Amman et al. (2001) and Sagredo and Lowell (2012), ice masses located in arid regions react to changes in precipitation while glaciers in areas with high precipitation levels are more sensitive to temperature variations. Based on the above criteria, we expect to find Late Glacial deposits in the arid western Central Andes due to wetter conditions on the Altiplano during the Late Glacial time than the LGM. To

assess this hypothesis we selected HualcaHualca volcano and the Patapampa Altiplano, both located in the arid western Peruvian Andes, where no previous research has been conducted on the glacial evolution. This study is based on a detailed geomorphological analysis and 9 36Cl surface exposure ages of moraine boulders and polished and striated bedrock outcrops, providing an initial chronological setting of maximum extent and deglaciation of the HualcaHualca - Patapampa glacier system. 2. Regional setting HualcaHualca (6025 masl) is the northernmost stratovolcano of the Ampato volcanic complex (15 24 - 15 510 S/71 510 - 73 W; 6288 masl), located in the western Central Andes (Fig. 1). This complex is located 70 km northwest of Arequipa city and covers 630 km2. To the north, the HualcaHualca borders the Colca canyon where fluvial erosion has generated a relief of 3500 m. To the east and west, HualcaHualca rises 2000 m above the surrounding Altiplano (4000 masl) while to the south it lies adjacent to Sabancaya, the youngest active volcanic structure of the Ampato complex. The construction of the HualcaHualca volcano began during the late Miocene or the early Pleistocene as a result of an increase in the subduction angle between the Nazca and South America plates (Stern, 2004). The mountain consists of andesitic rocks and presents a horseshoe-shaped caldera due to the collapse of the northern flank. HualcaHualca is characterized by intense glacial processes (Fig. 2). The lack of significant volcanic activity has allowed the preservation of slopes affected by glacial abrasion and overdeepening, moraines, and polished and striated rock surfaces. The four valleys selected to reconstruct the glacial evolution in this study (Huayuray, Pujro Huayjo, Mollebaya and Mucurca) include the most complete and best preserved sequence of glacial deposits of

Fig. 1. Location of HualcaHualca volcano and Patapampa Altiplano in the context of the Central Andes.


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Fig. 2. Eastern flank of HualcaHualca. A well preserved moraine of the Maximum Glacier Entent (foreground) and the position of the modern glacier front (background) are visible.

HualcaHualca volcano. Modern glaciers show a marked deglaciation in recent decades; for instance in the Huayuray valley has a calculated retreat of 50% between 1955 (2.2 km2) and 2000 (1.2 km2) , 2015). The other site studied, Patapampa (15 430 40- 15 460 (Alcala S/71430 30- 71360 W; 4940 masl), is an Altiplano located 17 km southeast of HualcaHualca volcano (Figs. 1 and 3). We chose this Altiplano because it is not an active volcanic area, allowing us to compare the glacial dates obtained in HualcaHualca volcano. The climate of the study area is modulated by seasonal changes in the Intertropical Convergence Zone, which controls convective zone flow across the Amazon Basin and the South American summer monsoon. During the wet season, from December to March, the Intertropical Convergence Zone shifts southwards producing 70e90% of the annual precipitation (800e1000 mm) (Dornbusch, 1998; Herreros et al., 2009). Humid air masses rarely arrive from the Pacific Ocean due to the persistent temperature inversion layer at 800 masl, associated with the large scale atmospheric subsidence conditions and the Humboldt Current (Rutllant and Ulriksen, 1979; Garreaud et al., 2003). Temperatures do not change significantly during the year. Measurements of mean annual air temperature in Chachani volcanic complex (16º03- 16º200 S/71º39- 71º250 W; 6057 masl) based on Hobo Pendant thermometers, located 65 km SE of the study area, show intra-annual variations of ~3.38 C at 4850 masl,

s et al., 2011). ~3.36 C at 4976 masl and ~5.97 C at 5331 masl (Andre 3. Methods 3.1. Geomorphology Based on the analysis of vertical aerial photographs (scale fico Nacional de Peru, 1955), a detailed 1:35,000, Instituto Geogra geomorphological map at scale 1:20,000 was produced to repre-Reygosa sent glacial landforms of the HualcaHualca volcano (Alcala et al., 2016). The map was verified in the field, and facilitated the selection of sites in the Huayuray, Pujro Huayjo, Mollebaya and Mucurca valleys that exhibits a well-preserved glacial record for cosmogenic 36Cl surface exposure dating. Lateral and terminal moraines corresponding to the maximum glacier extent and readvance phases were identified on the map and correlated using position and morphological characteristics. This information was supplemented with fieldwork focusing on glacially-abraded bedrock and moraine boulders. 3.2. In situ cosmogenic

36

Cl sampling strategy

To determine the age of glacial features, we collected samples

Fig. 3. Panoramic view of the Patapampa Altiplano. The Ampato volcanic complex is located in the background.

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Fig. 4. Location of sampling sites for cosmogenic

36

Cl surface exposure dating and geographic context of the study area.

for 36Cl surfaces exposure dating (Fig. 4) with a hammer and chisel on stable andesite boulders >1 m high, located on the crests of moraines. We also collected samples from polished and striated bedrock outcrops close to (and inside) the outer moraines in order to determine the timing of deglaciation from the maximum glacier extent. One sample of glacial polish was also collected from the Patapampa Altiplano to constrain the timing of deglaciation on the Altiplano. We minimized possible previous shielding by snow, tephras associated with volcanic activity and soils taking the samples on the top of the surfaces (~2e5 cm). The use of 36Cl is justified as the rocks in the study area are intermediate and mafic volcanic.

3.3. Lab protocol We followed the whole-rock protocol of preparation of samples for 36Cl analysis by Zreda et al. (1999) and Phillips (2003). Lichens, mosses and other organic material were removed from the samples

with a brush. Samples were crushed using a roller grinder and sieved to retrieve the sand size fraction. The chemical preparation was carried out at PRIME Laboratory (Purdue University). There, the sand fraction size was leached in deionized water and HNO3 to remove atmospheric Cl and then dissolved in a mixture of HNO3 and HF acids. A spike of isotopically enriched 35Cl was added during the dissolution process. The isotope dilution method allowed the 36 Cl and total Cl to be measured simultaneously (Desilets et al., 2006). The ratios 36Cl/Cl and 37Cl/35Cl were determined by Accelerator Mass Spectrometry (AMS) analysis at PRIME Laboratory. Aliquots of bulk rock and target fraction (elements for 36Cl production by spallation and muon capture: Ca, K, Ti, Fe) were analyzed at Activation Laboratories (Ancaster, Canada) to measure: (i) major elements, by fusion inductively coupled plasma optical emission spectrometry (ICP-OES); (ii) trace elements, by inductively coupled plasma mass spectrometry (ICP-MS); and (iii) boron, by prompt-gamma neutron activation analysis (PGNAA). The field and analytical data are presented in Table 1.

-Reygosa et al. / Quaternary International 449 (2017) 149e160 J. Alcala Table 1 Field and analytical data for

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Cl samples from HualcaHualca volcano and Patapampa Altiplano.

Sample ID Latitude Longitude Elevation Sample thickness Shielding factor incorporating all efects Snow shielding factor Scaling factor for nucleonic production Scaling factor for muonic production Effective fast neutron attenuation length Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 Cl B Sm Gd U Th Sample mass Mass of 35Cl spike solution Concentration Spike solution Analytical stable isotope ratio Analytical 36Cl/Cl ratio Corrected 36Cl concentration

153

Hualca 1

Hualca 2

Hualca 3

Hualca 4

Pujro Huayjo Pujro Huayjo Patapampa 3 1 2

Patapampa 4 Mucurca 1

( S) ( W) (masl) (cm) (unitless)

15.74 71.76 4444 3.0 0.97

15.67 71.85 4408 0.8 0.99

15.68 71.85 4512 1.5 0.99

15.64 71.85 4144 2.0 0.97

15.82 71.95 4521 3.0 0.99

15.82 71.96 4450 3.0 0.99

15.76 71.63 4671 2.0 0.998

15.74 71.64 4886 1.0 1.0

15.75 71.98 4460 3.0 0.99

(unitless) (unitless)

1.0 10.63

1.0 10.45

1.0 10.93

1.0 9.24

1.0 11.0

1.0 10.66

1.0 11.76

1.0 12.9

1.0 10.7

(unitless)

3.84

3.8

3.92

3.48

3.93

3.85

4.13

4.4

3.86

(g cm2)

160.0

160.0

160.0

160.0

160.0

160.0

160.0

160.0

160.0

(wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (g) (mg)

3.76 2.45 15.52 61.50 0.39 3.19 4.66 0.96 0.08 5.86 525.1 11.9 6.2 4.5 1.5 8.8 30.15 1.01

3.72 2.46 15.33 62.32 0.56 2.84 4.57 0.99 0.08 6.31 81.1 9.8 5.8 3.8 1.5 8.0 30.19 1.01

3.92 1.79 15.38 62.62 0.39 3.67 3.85 0.83 0.06 5.12 228.2 21.0 5.4 3.9 2.4 13.1 30.18 1.01

4.37 1.45 16.16 63.90 0.40 3.99 3.39 0.72 0.05 4.38 129.7 19.8 5.2 3.4 2.9 16.5 30.10 0.988

4.22 2.11 15.67 62.45 0.10 3.24 4.66 0.96 0.07 5.86 59.4 13.0 6.0 3.8 2.2 10.6 30.10 1.04

3.49 5.76 16.49 53.49 0.07 1.59 6.37 1.18 0.11 9.88 101.1 4.3 4.8 3.6 0.7 3.0 30.57 1.04

4.01 1.14 18.09 62.94 0.15 3.30 4.51 0.83 0.05 4.25 273.34 28.0 4.7 3.3 3.3 14.2 30.15 1.14

3.87 1.15 17.03 62.37 0.24 3.49 4.66 0.96 0.06 5.86 42.2 22.1 6.4 4.6 3.2 16.8 30.16 1.003

4.26 2.07 15.55 62.68 0.04 3.20 4.66 0.96 0.07 5.86 27.8 17.1 6.4 3.9 2.1 10.4 30.23 1.023

(g g1)

1.0

1.0

1.0

1.00

1.00

1.00

1.00

1.0

1.00

3.39 ± 0.01 4.78 ± 0.03 3.729 ± 0.02 4.153 ± 0.02 5.476 ± 0.38 4.49 ± 0.3 3.68 ± 0.04 6.21 ± 0.04 7.88 ± 0.47 (35Cl/ (35Cl þ 37Cl)) 36 15 ( Cl/10 Cl) 301.2 ± 7.47 726.7 ± 16.11 357.6 ± 6.28 543.6 ± 11.58 504.4 ± 18.57 425.8 ± 18.25 310.53 ± 15.80 906.2 ± 27.72 554.2 ± 24.58 atoms per 2856217.0 1429192.0 1589940.0 1507711.00 806626.00 975356.00 1662912.91 1182168.0 581522.00 gram of rock

3.4. In-situ cosmogenic

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Cl exposure ages calculations

In-situ cosmogenic 36Cl exposure ages were calculated using the spreadsheet developed by Schimmelpfennig (2009) and Schimmelpfennig et al. (2009). We used several production rates of

cosmogenic 36Cl from the spallation of Ca: 48.8 ± 3.4 atoms 36Cl (g Ca) 1 a 1 (Stone et al., 1996), 42.2 ± 4.8 atoms 36Cl (g Ca)1 a1 (Schimmelpfennig et al., 2011), 56.27 36Cl (g Ca)1 a1 (Borchers et al., 2016) and 56.0 ± 4.1 (g Ca)1 a1 (Marrero et al., 2016). We used 36Cl production rates from spallation of K of 148.1 ± 7.8

Table 2 Cosmogenic 36Cl surface exposure ages from HualcaHualca volcano and Patapampa Altiplano. Results for the available 36Cl production rates. (1) Spallation of Ca: 56.27 36Cl (g Ca)1 a1 (Borchers et al., 2016); Spallation of K: 156.09 atoms 36Cl (g K)1; Production rate of epithermal neutrons from fast neutrons in the atmosphere at the land/atmosphere interface: 626 ± 46 neutrons (g air) 1 a 1 (Phillips et al., 2001). (2) Spallation of Ca: 56.0 ± 4.1 (g Ca)1 a1 (Marrero et al., 2016); 155 ± 11 atoms 36Cl (g K) 1 (Marrero et al., 2016); Production rate of epithermal neutrons from fast neutrons in the atmosphere at the land/atmosphere interface: 759 ± 180 neutrons (g air) 1 a -1 and 696 ± 185 neutrons (g air) 1 a 1 (Marrero et al., 2016). (3). Spallation of Ca: 42.2 ± 4.8 atoms 36Cl (g Ca)1 a1 (Schimmelpfennig et al., 2011); Spallation of K: 148.1 ± 7.8 atoms 36 Cl (g K) 1 a 1 (Schimmelpfennig et al., 2014); Production rate of epithermal neutrons from fast neutrons in the atmosphere at the land/atmosphere interface: 626 ± 46 neutrons (g air) 1 a 1 (Phillips et al., 2001). (4) Spallation of Ca: 48.8 ± 3.4 atoms 36Cl (g Ca) 1 a 1 (Stone et al., 1996); Spallation of K: 148.1 ± 7.8 atoms 36Cl (g K) 1 a 1 (Schimmelpfennig et al., 2014); Production rate of epithermal neutrons from fast neutrons in the atmosphere at the land/atmosphere interface: 626 ± 46 neutrons (g air) 1 a 1 (Phillips et al., 2001). Sample

Exposure Age (1)

Exposure Age (2)

Exposure Age (3)

Exposure Age (4)

Hualca 1 Hualca 2 Hualca 3 Hualca 4 Pujro Huayjo 1 Pujro Huayjo 2 Patapampa 3 Patapampa 4 Mucurca 1

13.4 ± 2.3 17.8 ± 1.6 12.0 ± 1.5 16.6 ± 1.6 9.3 ± 1.3 12.0 10.5 ± 1.3 12.0 ± 0.9 8.0 ± 0.8

11.7 ± 2.1/12.5 ± 2.2 16.9 ± 1.7/17.3 ± 1.7 11.0 ± 1.5/11.5 ± 1.5 15.5 ± 1.8/16.0 ± 1.8 9.0 ± 1.3/9.2 ± 1.3 11.2 ± 2.3/11.5 ± 2.4 9.5 ± 1.3/10.0 ± 1.3 11.8 ± 1.05/11.9 ± 1.0 7.8 ± 0.8/7.9 ± 0.8

13.9 ± 2.5 19.4 ± 1.9 12.7 ± 1.6 17.6 ± 1.9 10.2 ± 1.5 13.3 ± 2.7 11.1 ± 1.5 13.3 ± 1.1 8.8 ± 0.9

13.7 ± 2.4 18.8 ± 1.8 12.5 ± 1.6 17.4 ± 1.8 9.9 ± 1.4 12.7 ± 2.6 10.9 ± 1.45 12.9 ± 1.1 8.5 ± 0.9

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Fig. 5. Geomorphological map of the Huayuray valley (northern side of HualcaHualca volcano), location and characteristics of the sampling sites for

36

Cl surface exposure dating.

Fig. 6. Geomorphological map of the Pujro Huayjo valley (southwestern side of HualcaHualca volcano), location and characteristics of the sampling sites for dating.

36

Cl surface exposure


atoms 36Cl (g K) 1 a 1 by Schimmelpfennig et al. (2014), 156.09 atoms 36Cl (g K) 1 by Borchers et al. (2016) and 155 ± 11 atoms 36Cl (g K) 1 by Marrero et al. (2016); from spallation of Ti of 13 ± 3 atoms 36Cl (g Ti) 1 a 1 by Fink et al. (2000); and from spallation of Fe of 1.9 atoms 36Cl (g Fe) 1 a 1 by Stone et al. (2005). We applied the production rate of epithermal neutrons from fast neutrons in the atmosphere at the land/atmosphere interface (626 ± 46 neutrons (g air) 1 a 1; 696 ± 185 neutrons (g air) 1 a 1; 759 ± 180) proposed by Phillips et al. (2001) and Marrero et al. (2016). We selected the ages derived from the production rates of Marrero et al. (2016) because they were obtained at similar latitude and elevation (Huancane, Peru) than our study area. However, we present the result of the available production rates to show the differences (Table 2). The elevation/latitude scaling factors for nucleonic and muonic production were established using CosmoCalc (Vermeesch, 2007), which is based on the Stone (2000) scaling model. The shielding factor was estimated using the Topographic Shielding Calculator v1.0 provided by CRONUS-Earth Project (2014). 4. Results 4.1. Huayuray valley Huayuray is located on the north flank of HualcaHualca volcano. The bottom of the upper part of the valley is characterized by an alternation of glacial rocky steps and hollows surrounded by debris

155

sheets and scattered talus rock glaciers associated with scarp weathering. Downstream, between 3650 and 4900 masl, there is a moraine complex formed by four ridges. The outermost lateral moraine is situated between 3650 and 4400 masl and is 7000 m long with a maximum width of 420 m. Below 4000 masl this moraine tends to narrow, becoming a thin ridge on its lowest stretch and is locally affected by the formation of debris flows. Upvalley, two lateral moraines are located between 4250 and 4900 masl. At the lowest point, they are closely spaced but above 4500 masl they divide into two clearly independent ridges. Both moraines reach a height of 120 m and present an arched morphology that deflects the path of the stream along the bottom of the valley. They have well-defined crests with some boulders >1 m high. On the left margin (north) of the Huayuray valley only two lateral moraines are preserved. The outer ridge, located at 3650e4000 masl, is 1700 m long with a maximum width of 200 m. The other ridge lies between 4450 and 4800 masl, it is 1650 m long, 60 m high and is divided in 3 different elongated deposits. Based on the geomorphological characteristics described above, we decided to collect samples for 36Cl surface exposure dating from the moraine ridges located on the right margin (north) of Huayuray valley as they represent the most complete moraine sequence (Fig. 5). One sample (Hualca 4) was collected from the outer moraine, yielding an age of 16.0 ± 1.8 ka. Two more samples were taken from the lateral moraines: Hualca 2 yields an age of 17.3 ± 1.7 and Hualca 3 yields an age of 11.5 ± 1.5 ka.

Fig. 7. Geomorphological map of the Mollebaya valley (eastern side of HualcaHualca volcano), location and characteristics of the sampling sites for

36


156


4.2. Pujro Huayjo valley The Pujro Huayjo valley is located on the southwest slope of HualcaHualca volcano and is 16 km long with a maximum width of 1.5 km. The highest area is partially filled by a chaotic morainic sheet. On both sides of the valley, between 4250 and 4985 masl, a voluminous lateral moraine 12 km long with a well-defined crest is preserved, related to the maximum glacier extent. Its morphology shows evidence of post-depositional modification, caused by debris flows, rock falls and debris cones. Polished bedrock appears on a rock step at the base of the outer moraine. In addition, between 4250 and 4450 masl, there is a fine morainic ridge which is related to an advance or stabilization phase of the ice mass (Fig. 6). One sample (Pujro Huayjo 1) was collected from the outer, voluminous moraine to determine the timing of stabilization of the moraine. Another sample (Pujro Huayjo 2) was taken from polished and striated bedrock at the inner base of the moraine to estimate when deglaciation began. The results show that the age of Pujro Huayjo 1 is 9.2 ± 1.3 ka and Pujro Huayjo 2 is 11.5 ± 2.4 ka (Fig. 6).

mapped at 4300e4700 masl, with an outer ridge more voluminous than the other two. A sample (Hualca 1) was collected from the crest of the outer moraine that yields an exposure age of 12.5 ± 2.2 ka (Fig. 7). 4.4. Mucurca valley The Mucurca valley, on the western flank of the HualcaHualca volcano, is 7.5 km long and 1.5 km wide. There are currently no glaciers in Mucurca valley but a paleoglacier formed moraines at 4350e5000 masl. The bottom of the valley above 4600 masl is characterized by the presence of a morainic sheet. The morainic sequence of Mucurca valley is complex. Several lateral moraines are observed, the outermost moraine being the largest and related to the maximum glacier extent. Upstream, a frontal moraine is located at 5000 masl. A sample (Mucurca 1) was collected on the crest of the outermost moraine, yielding an exposure age of 7.9 ± 0.8 ka (Fig. 8). 4.5. The Patapampa Altiplano

4.3. Mollebaya valley The Mollebaya valley is located on the eastern flank of HualcaHualca volcano. It is 12.5 km in length with maximum width of 1.5 km. Between 4700 and 5200 masl, the bottom of the valley is filled with a morainic sheet as a result of the recession of a debriscovered glacier. At some points, this material appears disturbed due to the effects of snow-ice melt water. Three lateral moraines were

Patapampa is a 12.5 m long, 6 km wide Altiplano located to the east of HualcaHualca volcano. It was covered by an ice cap during the last glaciation, as shown by glacially polished and striated features distributed throughout the Altiplano. Moreover, there are several moraines surrounding Patapampa associated with the former expansion of the ice cap. One sample (Patapampa 4) was taken from glacially polished

Fig. 8. Geomorphological map of the Mucurca valley (western side of HualcaHualca volcano), location and characteristics of the sampling sites for

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Fig. 9. Satellite view of Patapampa Altiplano, location and characteristics of the sampling sites for

and striated bedrock on the top of the southern edge of the Altiplano at 4886 masl, yielding an exposure age of 11.9 ± 1.0 ka. Another sample (Patapampa 3) was collected from a boulder of the outermost moraine located on the southern edge of Patapampa at 4671 masl, yielding an exposure age of 10.0 ± 1.3 ka (Fig. 9). 5. Discussion The limited number of 36Cl exposure ages does not allow us to carry out a conclusive reconstruction of the glacier evolution on the HualcaHualca volcano and Patapampa Altiplano. Three 36Cl dates (Pujro Huayjo 1, Mucurca 1 and Patapampa 3) from moraine boulders were considered outliers because they are younger than those from glacially polished bedrock which exhibit a similar and therefore consistent exposure age. Moreover, these samples were collected on moraines of the maximum glacier extent and they are much younger than the LGM and the Late Glacial time, revealing the significant effects of post-depositional processes (including possible shielding by tephra) in the underestimation of moraine ages despite the climatic aridity. However, we can deduce some significant aspects of the glacial evolution based on the available data. The 36Cl exposure ages from the outer moraines indicates that the local Last Glacial Maximum and readvances or stillstands (Hualca 1, 2, 3 and 4) at HualcaHualca volcano took place during the Late Glacial time when the deepest paleolakes (Tauca and Coipasa cycles) were formed on the Altiplano (Fig. 10) (Blard et al., 2011; Placzek et al., 2013). A similar glacier response has also been reported in both the arid and the humid central Andes (Clapperton, 1993; Rodbell, 1993; Clayton and Clapperton, 1997; Farber et al., 2005; Smith et al., 2005; Zech et al., 2007a, 2007b; Kull et al., 2008; Hall et al., 2009; Glasser et al., 2009; Smith et al., 2009, 2011; Bromley et al., 2009, 2011,

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2016; May et al., 2011; Kelly et al., 2012; Blard et al., 2013; Stansell et al., 2015). After the readvance or stillstand dated ~11.5 ka, the deglaciation was the main process on HualcaHualca volcano and Patapampa Altiplano coinciding with the end of Coipasa paleolake cycle and the onset of the Holocene. However, Mark et al. (2017) indicate minor readvances and/or stillstand in other mountains of the Central Andes during the early Holocene. In this sense, HualcaHualca volcano present glacial deposits upvalley from -Reygosa the moraines of the Last Local Glacial Maximum (Alcala et al., 2016), which could be of that age. Thus, the hypothesis of a Local Last Glacial Maximum and significant glacier readvances during the Late Pleistocene in the study area, associated to wetter conditions, seems to be confirmed according to the results of this work. Moreover, a detailed analysis of the available in situ cosmogenic data related to the Local Last Glacial Maximum from the arid Central Andes allow pointing out significant discrepancies. The age obtained on HualcaHualca volcano coincide with the chronology of Nevado Sajama (Smith et al., 2009) and Tunupa volcano (Blard et al., 2013) but is not in agreement with Coropuna volcano (Bromley et al., 2009), that preserve the major ice mass of the arid Central Andes, and Uturuncu volcano (Blard. et al., 2014) where it ocurred in phase or several thousand years prior the LGM, respectively. Moreover, Bromley et al. (2016) suggest that the local Last Glacial Maximum took place ~ 29 ka ago at Cordillera Carabaya (humid Central Andes). Bromley et al. (2016) also recalculated previous published 10Be exposure ages from Cordillera Blanca (humid Central Andes; Smith et al., 2005) and confirmed, as previously postulated Smith et al. (2005), that glaciers formed the outermost moraines ~ 30 ka. A similar glacier behavior is reported from the Arid Diagonal, where the Local Last Glacial Maximum predate the LGM (Ward et al., 2017; Zech et al., 2017). Then, the deglaciation

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sensing techniques. For example, they suggest that a change to a warming climate may rise the Equilibrium Line Altitude of small glaciers above the maximum elevation, exposuoring the whole glacier area to ablation. By contrast, extensive glaciers have a wider altitudinal range and therefore the Equilibrium Line Altitude will be located below the maximum elevation. Therefore, the glacier of HualcaHualca volcano, less extensive and with a lower elevation than Coropuna volcano, could advance during the LGM but readvances during the Late Pleistocene, related to wetter conditions, would remove the moraines deposits associated to the LGM. By last, despite cosmogenic exposure provides new relevant chronological information, is not possible to establish a consistent age of glacial evolution since the LGM in the Central Andes due to the use of several cosmogenic production rates and scaling schemes. An example is the estandar deviation of 4.0 ka and an uncertainty of ~7 ka obtained by Mark et al. (2017) when they calculated the mean age of published data of moraines of the LGM in Peru and Bolivia. To reduce the uncertainty and compare the available cosmogenic ages, it would be necessary to provide a precise in situ cosmogenic production rate of each cosmogenic nuclide and a standard scaling scheme for the Central Andes. 6. Conclusions

Fig. 10. Correlation between the cosmogenic 36Cl surface exposure ages obtained in the study area, the Northern Hemisphere cold and warm climatic episodes since the Last Glacial Maximum and the paleolake cycles in the Andean Altiplano. The reconstruction of the paleolakes cycles is based on studies developed by Blard et al. (2011) and Placzek et al. (2013).

The 36Cl exposure ages from moraines related with the Local Last Glacial Maximum and readvances or stillstand phases at HualcaHualca volcano suggest a connection with the wetter conditions during the Late Pleistocene. Moreover, the retreat of the glaciers of HualcaHualca volcano and the disappearance of the ice cap of Patapampa took place after the termination of the Coipasa cycle and the onset of the Holocene. However, the scarcity number of samples and the different chronologies derived from the available production rates do not enable us to provide a precise glacier evolution of the study area. To determine a precise glacier reconstruction in the study area, we think is necessary to collect more samples and establish a precise production rate for 36Cl in the Central Andes. Acknowledgments

preliminary began after ~25 ka until ~16 ka when most of these areas show geomorphological evidence of glacier advance, readvance or stillstand phase in synchronicity with Tauca paleolake high level (Smith et al., 2005; Blard et al., 2013, 2014; Bromley et al., 2016; Ward et al., 2017; Zech et al., 2017). This chronology of readvance or stillstand phase at ~16 ka coincide with the Local Last Glacial Maximum on HualcaHualca volcano and, therefore, would not support the tradicional idea proposed by Hastenrath (1971), Klein et al. (1999), Amman et al. (2001) and Sagredo and Lowell (2012) that suggest that ice masses located in arid regions react to changes in precipitation while glaciers in areas with high precipitation levels are more sensitive to temperature fluctuations. To explain the apparently positive response of both the glaciers of the arid and the humid mountains of the Central Andes to a cold and relatively humid LGM (Thompson et al., 1995, 1998; Baker et al., 2001; Fornace et al., 2014) and higher moisture atmosphere concentration during Tauca and Coipasa paleolake cycles (Blard et al., 2011; Placzek et al., 2013), we propose an alternative idea. According to Ramirez et al. (2001) and Rabatel et al. (2013), the size and elevation of the glacier control the magnitude of mass balance loss or gain. Thus, an extensive ice cap, as Coropuna volcano, has a minor response time to climate changes than small glaciers as HualcaHualca volcano. This different reaction has also been reported by Racoviteanu et al. (2008) in the Cordillera Blanca where they estimate the glacier changes from 1970 to 2003 using remote

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