Geomorphology 139-140 (2012) 67–78

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Geomorphology

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Glacial landforms and their paleoclimatic significance in Sierra de Guadarrama,Central Iberian Peninsula

David Palacios a,⁎, Nuria de Andrés b, Javier de Marcos a, Lorenzo Vázquez-Selem c

a Departamento de A.G.R. y Geografía Física. Universidad Complutense. 28040 Madrid, Spainb Museo Nacional de Ciencias Naturales, Centro Superior de Investigaciones Científicas, 28006, Madrid, Spainc Instituto de Geografía, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico

⁎ Corresponding author.E-mail address: davidp@ghis.uccm.es (D. Palacios).

0169-555X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.geomorph.2011.10.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 May 2011Received in revised form 3 October 2011Accepted 4 October 2011Available online 20 October 2011

Keywords:Glacial landformsLast Glacial MaximumDeglaciationCosmogenic datingSierra de Guadarrama

This paper investigates the glacial evolution on the east face of Pico de Peñalara (40°51′N, 3°57′W; 2428 m),the highest elevation of Sierra de Guadarrama in the center of the Iberian Peninsula. The geomorphologic sta-bility of glacial landforms and permanence of snow cover were examined, and 18 samples were selected fordating with cosmogenic isotope 36Cl from moraine formations in four well-differentiated phases, as well asfrom a pre-glacial periglacial blockfield and a glacial threshold near the headwall. The results show the syn-chrony between the Last Glacial Maximum (LGM) advance and most results obtained recently by cosmogenicdating in southern Europe, within the MIS (Marine Isotope Stage) 2 stadial but earlier than the global LGM,i.e. between 25 and 19 ka BP. There was slow glacial retreat with minor re-advances between 19 and 16 kaand rapid deglaciation from 16 ka. This paper presents initial results, still pending confirmation, for the min-imum age of the periglacial summit block formation around 80 ka for the existence of moraine formationsolder than the last advance and for the existence of glaciers during the Younger Dryas stadial. To explainthe synchrony between two moraine formations at different altitudes, this paper formulates the hypothesisof the ‘secondary moraine’ evidenced in currently existing glaciers similar to those which existed on Peñalara.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Our understanding of the glacial advances and deglaciation of Med-iterranean mountains during the Late Pleistocene has greatly improvedover the last decade, as seen in recently published syntheses (Hughes etal., 2006a, 2006b, 2007; Hughes and Woodward, 2008; García-Ruiz etal., 2010). This is certainly the case in the Iberian Peninsula, wherenew dates have been established for the last glacial advances, especiallyin the northernmountains. However, there are still important problemsto be solved, such as the existence of landforms from several glaciations— a problem which has existed in the Pyrenees for over a century(Penck, 1883; Llopis-Lladó, 1947; Barrère, 1966). The existence of re-mains of glacial landforms which pre-date the last glaciation seems ev-ident in several valleys in the Spanish central Pyrenees (Martí-Bono,1973, 1996; Serrano, 1992, 1998; Calvet, 2004; Peña et al., 2004;Lewis et al., 2009). Remains of glacial landforms earlier than the last gla-ciation have been dated in the NW of the Peninsula, in the Serra doGerêz and Sierra de Queixa (Fernández-Mosquera et al., 2000; Vidal-Romaní and Fernández-Mosquera, 2005), aswell as in otherMediterra-nean areas includingmountains in Greece (Hughes et al., 2006a, 2006b,2007), Montenegro (Hughes et al., 2010) and the Apennines in Italy

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(Kotarba et al., 2001). However, traces of earlier glaciations have notbeen identified in other sectors of the Pyrenees, although they mayhave disappeared due to erosion (Pallàs et al., 2006; Delmas et al.,2008). In the Sierra de Gredos, in the center of the Peninsula, none hasbeen found, nor does it seem possible that they ever existed, since adeepmantle of weathered granite has developed outside the area occu-pied by glaciers of the last glaciation (Palacios et al., 2011).

Another important unsolved problem in the Iberian Peninsula isthe apparent asynchrony between moraines related to the maximumextent of the last glaciation, even within the same mountain range.This occurs in the Pyrenees, where a number of studies place themaximum advance of the last glaciation before 30 ka BP (Mardonèsand Jalut, 1983; Bordonau, 1992; Jalut et al., 1992; Montserrat,1992; Reille and Andrieu, 1995; García-Ruiz et al., 2003, 2010; Sanchoet al., 2003; Peña et al., 2004; González-Sampériz et al., 2006). Lewiset al. (2009) recently established the maximum advance in the Pyre-nees between 68 and 61 ka, i.e. considerably earlier than the Last Gla-cial Maximum (LGM) in the North Atlantic glacial chronology, whichfalls within Marine Isotope Stage 2 (MIS 2) (Björck et al., 1998; Walkeret al., 1999; Johnsen et al., 2001). Advances younger than themaximum,although clearly of less importance, seem to coincide with the LGM(García-Ruiz et al., 2003, 2010; González-Sampériz et al., 2006; Sanchoet al., 2008). However, it must be taken into account that all these dateswere obtained through 14C and Optically Stimulated Luminescence(OSL) techniques from fluvioglacial or glaciolacustrine deposits, not

68 D. Palacios et al. / Geomorphology 139-140 (2012) 67–78

directly from the glacial landforms. For the mountains in the NW of theIberian Peninsula, the published data also seem to confirm the existenceof a maximum advance earlier than the LGM, using cosmogenic 21Ne(Fernández-Mosquera et al., 2000; Jiménez-Sánchez and Farias, 2002)and U/Th dating (Moreno et al., 2010a).

A possible reason for this asynchronous evidence of an earlier degla-ciation in the Mediterranean area would be a particularly dry periodduring the LGM (Hughes and Woodward, 2008). But this situation isnot common to all Mediterranean mountains. In the Kaçkar mountains(3932 m asl) 10Be dating indicates a Last Glacial Maximum at least from26 ka to 18 ka (Akçar et al., 2007, 2008) and 20.3 ka (Zahno et al., 2010).On Mount Sandlas (2295 m asl) in Turkey, the LGM has been dated by36Cl to 20.4 ka (Sarıkaya et al., 2008). In the SE Italian Alps, the LGM isdated by 14C between 26.5 and 23 (Monegato et al., 2007), and in theApennines the LGM is 21.5 ka (Giraudi, 2004). In fact, all theseMediter-ranean dates for the LGM agree with the results obtained in the Alps,which show the LGM between 24 and 21 ka (Ivy-Ochs et al., 2004,2006, 2008; Preusser, 2004; Kerschner and Ivy-Ochs, 2008).

Data from the cosmogenic dating of moraine boulders seems to sup-port the conformity between global LGM and Iberian LGM. In the south-ern Central Pyrenees, using 10Be dating, Pallàs et al. (2006) found amaximum advance after 25 ka. In the eastern Spanish Pyrenees, alsowith 10Be, Delmas et al. (2008) found a maximum advance coincidingwith the LGM, with slight oscillations between 25 and 20 ka. Cowtonet al. (2009) found evidence in the Lago de Sanabria area in NW Spainthat the maximum extent of this ice cap occurred during MIS 2 fromusing radiocarbon dating. Based on 36Cl dating, Palacios et al. (2011)found a maximum advance between 26 and 21 ka in a lateral morainecomplex in the Gredos Range, Central Spain. In opposition to all thesedata, recently published 10Be dates from small valleys of the SE Pyre-nees suggest an age between 76.5±7.1 ka and 49.2±4.5 ka for a

Fig. 1. Study are

maximum advance, and before or ca. 23.9±2.5 ka for a younger ad-vance (Pallàs et al., 2010).

In this context of equivocal dating evidence, the aim of this paperis to study an area of the Iberian Peninsula, marginal to the effect ofthe glaciations, on account of its low altitude and its southern andcontinental character, but with a wide variety of moraine formswhich may indicate contrasting climatic variations. The study at-tempts to obtain absolute ages of these formations by means of cos-mogenic 36Cl dating and to explore their paleoclimatic significance.

2. Study area

The study area includes the glacial cirques on the eastern slope ofPico de Peñalara (40°51′N, 3°57′W; 2428 m), the highest elevation ofthe Sierra de Guadarrama, 60 km north of Madrid in the center of theIberian Peninsula (Figs. 1, 2). This massif is composed of glandularorthogneiss with flat summits, which are remnants of an ancient pene-plain formed during the late Paleozoic and uplifted during the Alpineorogeny. The summits are covered with a thick weathering mantle.The glacial cirques are located only on the eastern slope of Peñalara,and form two complexes: Laguna (south) and Pepe Hernando (north).There have been no glaciers during historic times and, at present, themaximum permanence of the snow patches is around 220 days/year(Palacios et al., 2003). The weather station at Puerto de Navacerrada(40°46′N, 4°19′W; 1860 m), 8 km southwest of Peñalara, provided theonly available meteorological long time-series data for the area. Meanannual temperature is 6.1 °C and mean annual precipitation is1400 mm, occurring mostly between early October and late May.

The glacial origin of these cirques was postulated by Penck (1894)and Fernández-Navarro (1915a,b). In a detailed study, Obermaier andCarandell (1917) suggested that there were moraines from two glacial

a location.

Fig. 2. Peñalara Peak Eastern slope and Laguna y Pepe Hernando Cirques (photo: Carlosde Andrés).

69D. Palacios et al. / Geomorphology 139-140 (2012) 67–78

periods, contemporary with the Riss and Würm alpine glaciations. Re-garding the origin of the glacial cirques, Hernández-Pacheco (1930)insisted on the important effect of the prevailing westerly winds,which swept the snow from the extensive flat summit surface towardthe cirques on the lee side. Alía Medina et al. (1957) and Fränzle(1959) confirmed this hypothesis. Subsequent studies examined sever-al geomorphologic matters of the area (Ontañón and Asensio-Amor,1973; Sanz Herráiz, 1977; Ontañón, 1985; Palacios and García, 1997;Palacios and Andrés, 2000; Palacios et al., 2003). Sanz Herráiz (1988)compiled all the previous studies on the glacial origin of the cirquesand suggested that the moraine ridges correspond only to the last gla-cial period, contemporary with the Würmian Stage.

3. Methods

First, a detailed geomorphologic study was carried out on the gla-cial landforms of the two cirques and their degree of preservation,using techniques from previous studies (Palacios and García, 1997;Palacios and Andrés, 2000; Palacios et al., 2003). Mapping focusedon identifying stable glacial landforms, i.e. those which are slightlyor not at all altered by mechanical erosion.

Similarly, and also following on from earlier studies (Palacios andGarcía, 1997; Palacios and Andrés, 2000; Palacios et al., 2003), obliqueaerial views of sectors with visible snow cover changes were takenduring the period 1994–2009 on a weekly basis. A snow distributionmap was obtained from each weekly photograph. Other maps weregenerated from a large collection of aerial photos from the Comuni-dad de Madrid, taken between 1957 and 2008. ArcGis software wasused to produce a final map of snow cover days, in order to selectonly the areas with least snow permanence for sampling.

The resulting geomorphological and snow cover permanence mapswere used to select optimal sampling sites for 36Cl dating. We selectedthis cosmogenic nuclide because our research group is working on theglacial chronology of different mountains, some of which do not havesuitable rock types for other nuclides (e.g. 10Be or 26Al), so we preferto use the same cosmogenic nuclide to facilitate the comparison of re-sults. In addition, 36Cl has yielded results that are consistent withthose of 10Be in areas where conditions are similar to those of ourstudy area (e.g. Phillips et al., 1997; Brugger, 2007).

The samples for 36Cl dating were taken from moraine boulders>1 m high, located on prominent moraine ridges and with gentleslopes to minimize the chances of previous burial by soil or sedimentand shielding by snow cover. Samples were collected from successivemoraine ridges in order to test the internal consistency of the results.One sample was collected from a boulder in the periglacial blockfieldoutside the cirques, on the NW slope of the massif, to evaluate the re-lationship between the periglacial and glacial landforms.

Samples for 36Cl datingwere collected using hammer and chisel fromboulders of different moraines, and from two glacially abraded bedrocksurfaces. The laboratory procedures for whole rock summarized byZreda et al. (1999) and Phillips (2003) were followed. Whole rock sam-ples were crushed and ground using a roller grinder. The ground rock

was dissolved in a hot mixture of hydrofluoric and nitric acids, and Clprecipitated as AgCl. A spike of isotopically enriched 35Cl was added dur-ing the dissolution, whereby Cl content could be determined during theaccelerator mass spectrometry (AMS) analysis by means of the isotopedilution mass spectrometry method. The 36Cl/Cl and 37Cl/35Cl ratioswere measured on AgCl targets by AMS at PRIME Laboratory (PurdueUniversity). Aliquots of rocks were powdered and analyzed as follows.(a) Samples PEÑALARA 1; LAGUNA 3.1 and 3.2; and LAGUNA 4 and 5:major elements, U and Th by XRF spectrometry at NewMexico Tech (Bu-reau of Geology); and B and Gd by neutron activation prompt gammaemission spectrometry (PGNAA) at Activation Laboratories, Canada. (b)The rest of the samples: major elements by fusion Inductively CoupledPlasma, trace elements by mass spectrometry, and Boron by PGNAA, atActivation Laboratories. Exposure ages were calculated in two differentways: (i) CHLOE program (Phillips and Plummer, 1996, version 3 –

2003), using thermal and epithermal neutron distribution equationsand 36Cl production parameters by Phillips et al. (2001); production of36Cl by muons according to Stone et al. (1998); and (ii) latitude and ele-vation scaling of production rates by Lal (1991). Exposure ages were cal-culated assuming erosion rates of the rock surface of 0 (no erosion), 3and 5 mm/ka.

4. Results

The easthern slope of Pico Peñalara consists of two glacial cirqueswith different characteristics (Fig. 3). The La Laguna cirque, maximumwidth 1.6 km and 1.7 km long, ends in a complicated moraine complex.The lowermostmoraine formations are outside the cirque, at aminimumaltitude of ~1850 m asl. In the northern sector an eroded moraine arcforms a >40m-thick bulge (Phase Laguna 1). This moraine is cut bythe stream of Arroyo de la Laguna. South of the stream there are twosmall parallel moraine arcs, each one around 500 m long and only20 m wide, composed of large boulders (Phase Laguna 2). The ridgeshave a maximum thickness of 6 m and are very well preserved, withpractically no signs of erosion, except at the end where the stream hascut through. The northern arc and two southern arcs are considered byvarious authors to be coeval, either from the Rissian Stage (Obermaierand Carandell, 1917) or from the maximum advance of the WürmianStage (Sanz Herráiz, 1988), but in this paper the authors prefer to differ-entiate them into two stadials around the Würm maximum, given thesignificant difference in their preservation, composition and size.

The moraine arcs described above are the lowermost, but certainlynot the most prominent. At a minimum altitude of ~1950 m asl, thereis a large moraine accumulation which completely closes off the cir-que and whose front is still weakly eroded by the Arroyo de la Lagunastream (Phase Laguna 3) (Fig. 4). The frontal sector of this moraineconsists of at least 4 different ridges, with some ephemeral lakes inbetween them. The lateral moraine ridge which closes off the cirqueto the north is especially well preserved, and is only altered on itsnorthernmost side by some nivation hollows. The one which closesit to the south was destroyed by an old ski station installation.

Further toward the headwall, various moraine ridges indicate thatthe glacier split at the end of its life cycle in two small glacier tongues,separated by a rocky threshold. The southern one left a well pre-served moraine which probably was subject to flow, adopting a rockglacier morphology (Phase Laguna 4). It stands on a rock platformand has not been affected by the post-glacial rockfalls and block slideswhich formed on the cirque headwall.

The northern cirque (Pepe Hernando) is 1.7 km long with a maxi-mum width of 1.3 km. Only one moraine arc is preserved, although it isvery large, as in other cirques of Sierra de Guadarrama. The cirque isshorter on the north (only 700 m), where the moraine encloses Lagunade Claveles lake.

At present snow cover shows amarked contrast within the cirques(Fig. 5). Some sectors have prolonged snow cover, with more than220 days per year, mainly on the headwalls of the cirques and

Fig. 3. Geomorphologic map of Peñalara Peak Eastern slope and cosmogenic samples location.

70 D. Palacios et al. / Geomorphology 139-140 (2012) 67–78

northeast slopes of the moraine ridges; other sectors have almostnone, with less than 60 days/year, especially the summits of the mo-raine ridges and their southwest slopes.

This snow cover distribution is closely related to the stability of themoraines. Theirwest-oriented slopes arewell preserved, except those af-fected by anthropogenic erosion. In contrast, their northeast-orientedslopes are carved out by many nivation hollows which lead to theirdisintegration, forming solifluction lobes at the foot. All the sampleswere taken from areas with less than 100 snow-covered days per year.

The rocky outcrops are barely altered by post-glacial erosion in bothcirques, except for the occasional incision by torrential streams and the

action of avalanches and rock falls in some channels. However, therocky thresholds,with their summits polishedby ice andwithprominentprojections where the snow accumulates briefly, were very near the sur-face – not more than 20 m below the ice mass, as shown by themorainetopography (Fig. 6). This makes these thresholds unsuitable as samplingpoints, since presumably there has been very little erosion by the glacierand so there is excessive cosmogenic inheritance. The rocky headwall ofthe two cirques, where there was more glacial erosion, is not consideredan ideal sampling point either, since the post-glacial and even currenterosion and the snow cover are too intense to obtain reliable dating.Only one protruding threshold is considered reliable, on the headwall

Fig. 4. Laguna Cirque frontal moraine: Phase Laguna 2 (photo: David Palacios). Fig. 6. Lateral moraine (North) of Pepe Hernando Cirque (photo: David Palacios).

71D. Palacios et al. / Geomorphology 139-140 (2012) 67–78

of the La Laguna cirque, since it clearly preserves glacial abrasion andthus it is certain that there has been no post-glacial erosion.

Above the headwalls, on the northwest slope, there is a large peri-glacial blockfield. The blockfield is very stable and practically flat in

Fig. 5. Snow cover map from 1994/2009 s

the highest area near the summit of Pico Peñalara. Because of thewind force, very little snow accumulates on this sector.

Based on the study of the geomorphology and snow cover, suit-able locations for cosmogenic 36Cl dating were identified (highly

eries of Peñalara Peak Eastern slope.

Table 1Field and analytical data for 36Cl samples from Peñalara Peak Eastern slope.

Sample ID PEÑALARA 1 LAGUNA 1 LAGUNA 2.1 LAGUNA 2.2 LAGUNA 2.3 LAGUNA 2.4 LAGUNA 2.5 LAGUNA 3.1 LAGUNA 3.2

Latitude (°N) 40.85000 40.84022 40.83525 40.83508 40.83000 40.83575 40.83550 40.85000 40.85000Longitude (°W) 3.96000 3.94577 3.94618 3.94602 3.93000 3.94707 3.94707 3.96000 3.96000Elevation (m) 2385 1876 1885 1886 1875 1888 1889 1950 1920Sample thickness (cm) 2.5 1.5 2.0 4.0 4.0 4.5 3.0 2.0 4.0Shielding factor (Unitless) 0.966 0.986 0.996 0.996 0.995 0.937 0.939 0.997 0.995Effective fast neutron (g cm−2) 173 168 168 170 168 166 174 169 168Na2O (wt.%) 1.62 2.51 2.37 2.60 2.32 2.75 2.81 2.65 2.58MgO (wt.%) 2.06 0.54 1.63 1.18 1.17 0.69 0.72 0.86 0.78Al2O3 (wt.%) 17.05 13.97 15.15 14.33 14.83 13.70 13.28 13.97 13.82SiO2 (wt.%) 65.13 73.05 69.05 59.21 59.74 72.57 72.19 72.57 73.21P2O5 (wt.%) 0.06 0.10 0.06 0.10 0.08 0.09 0.06 0.08 0.08K2O (wt.%) 4.45 4.98 3.48 4.77 4.32 4.90 4.38 4.52 5.11CaO (wt.%) 0.58 0.77 1.01 1.05 0.84 0.94 1.10 1.08 0.88TiO2 (wt.%) 0.61 0.33 0.69 0.62 0.49 0.36 0.37 0.50 0.42MnO (wt.%) 0.07 0.03 0.05 0.04 0.05 0.03 0.04 0.03 0.04Fe2O3 (wt.%) 6.22 2.55 5.12 4.15 3.58 2.79 2.94 3.36 3.37Cl (ppm) 58.8 41.8 181.8 73.7 52.7 164.7 190.1 103.3 218.2B (ppm) 18.7 19.8 65.1 9.8 29.4 10.5 14.7 6.8 67.0Sm (ppm) 3.5 4.1 2.0 8.3 4.0 4.0 3.9 2.4 1.5Gd (ppm) 3.5 4.0 2.0 7.5 3.4 3.8 3.5 2.4 1.5U (ppm) 2.0 1.5 2.0 2.9 1.9 4.1 4.0 2.0 3.0Th (ppm) 8.0 9.3 7.0 22.6 10.7 11.7 12.0 12.0 7.036Cl/Cl ratio(de-spiked)

(36Cl/1015 Cl) 3145.1 1357.3 280.9 614.2 677.3 310.3 265.0 417.2 241.7

36Cl/Cl 1σ uncertainty(de-spiked)

(36Cl/1015 Cl) 59.6 34.8 9.5 21.7 23.1 9.7 9.5 17.8 10.1

Sample mass (g) 60.40 30.54 31.95 31.01 30.57 30.25 30.55 50.50 20.50Mass of 35Cl spikesolution

(g) 3.04 1.04 1.06 1.01 1.04 1.07 1.05 4.01 3.01

Concentrationspike solution

(g g−1) 0.99 1.00 1.00 1.00 1.00 1.00 1.00 0.99 0.99

Analytical stableisotope ratio

(35Cl/(35Cl+37Cl))

6.576±0.015

6.450±0.080

3.870±0.060

4.931±0.008

5.744±0.058

3.998±0.043

3.862±0.010

6.235±0.009

5.850±0.013

Analytical36Cl/Cl ratio

(36Cl/1015 Cl) 1690.0±33.0

742.0±19.0 237.0±8.0 424.0±15.0 410.0±14.0 255.0±8.0 224.0±8.0 235.0±10.0 144.0±5.9

Sample ID LAGUNA 3.3 LAGUNA 3.4 LAGUNA 3.5 LAGUNA 3.6 LAGUNA 3.7 PEPE 1 PEPE 2 LAGUNA 4 LAGUNA 5

Latitude (°N) 40.83000 40.83755 40.83000 40.84190 40.84180 40.84993 40.85213 40.83585 40.84181Longitude (°W) 3.95000 3.95360 3.95000 3.95200 3.95200 3.94373 3.94453 3.96087 3.95848Elevation (m) 1967 1957 1958 2071 2077 2113 2125 2130 2190Sample thickness (cm) 4.0 4.5 3.0 3.0 2.0 1.2 4.0 2.5 2.5Shielding factor (unitless) 0.997 0.988 0.990 0.990 0.990 0.966 0.929 0.990 0.961Effective fast neutronattenuation length

(g cm−2) 170 171 171 167 170 153 174 165 170

Na2O (wt.%) 2.10 2.99 2.88 2.24 2.78 3.07 2.04 2.46 2.46MgO (wt.%) 0.93 0.52 0.53 1.56 0.92 1.05 1.53 0.92 0.92Al2O3 (wt.%) 13.57 13.97 13.42 16.14 14.42 15.39 15.08 14.93 14.93SiO2 (wt%) 72.41 74.29 74.07 68.97 72.18 70.79 69.68 72.76 72.76P2O5 (wt.%) 0.10 0.04 0.09 0.07 0.12 0.09 0.09 0.05 0.05K2O (wt.%) 4.63 4.77 4.52 3.28 4.48 3.24 4.73 3.74 3.74CaO (wt.%) 0.72 0.97 0.86 0.73 0.68 1.49 0.87 0.64 0.64TiO2 (wt.%) 0.46 0.27 0.30 0.65 0.43 0.46 0.52 0.32 0.32MnO (wt.%) 0.04 0.03 0.03 0.05 0.03 0.03 0.04 0.03 0.03Fe2O3 (wt.%) 3.21 2.11 2.39 4.57 3.23 3.35 4.05 2.83 2.83Cl (ppm) 63.1 134.8 109.8 79.4 80.3 25.8 53.7 84.4 57.6B (ppm) 15.3 41.0 57.3 57.7 36.2 27.1 22.6 20.9 20.9Sm (ppm) 5.9 3.4 3.4 5.3 3.8 4.0 5.3 0.7 0.7Gd (ppm) 5.4 3.1 3.1 4.2 3.4 3.2 4.5 0.7 0.7U (ppm) 2.3 3.4 4.1 2.0 2.8 1.8 4.5 2.0 2.0Th (ppm) 15.4 9.5 9.6 11.3 8.8 9.5 13.7 6.0 6.036Cl/Cl ratio(de-spiked)

(36Cl/1015 Cl) 533.8 332.6 374.0 552.1 529.8 1117.5 767.2 348.4 406.4

36Cl/Cl 1σuncertainty(de-spiked)

(36Cl/1015 Cl) 24.6 10.0 10.5 14.3 19.9 67.1 28.0 12.0 14.1

Sample mass (g) 30.34 30.11 30.61 30.35 30.48 30.56 30.48 50.00 70.00Mass of 35Clspike solution

(g) 1.02 1.02 1.05 1.03 1.02 1.02 1.04 3.00 3.04

Concentrationspike solution

(g g−1) 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.99 0.99

Analytical stableisotope ratio

(35Cl/(35Cl+37Cl))

5.290±0.009

4.153±0.004

4.401±0.013

4.872±0.020

4.833±0.049

8.360±0.024

5.722±0.017

6.000±0.900

6.172±0.052

Analytical36Cl/Cl ratio

(36Cl/1015 Cl) 347.0±16.0 265.0±8.0 284.0±8.0 385.0±10.0 372.0±14.0 483.0±29.0 466.0±17.0 203.0±7.0 231.0±8.0

Notes: Water content of 0.005% and density of 2.5 g cm−3 were assumed for all samples. Snow shielding was not included in shielding corrections.

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stable sites, without excessive snow cover). These include the mo-raine crests of the lower arcs (Phases Laguna 1 and 2), the crests ofthe northern frontal and lateral moraines of the main ridge in this cir-que (Phase Laguna 3) and the crest of the rocky glacier-moraine(Phase Laguna 4) in La Laguna cirque. The crests of the Pepe Her-nando cirque moraines were also considered ideal, especially thosein the northern sector, because they are very large and stable.

Notwithstanding the widespread presence of glacial polish, therocky surfaces above the moraines in general are not considered suit-able as sampling points because of potential problems of cosmogenicnuclide inheritance associated with a thin glacier. The cirque headwallswhere also considered unsuitable due to intense post-glacial and evenmodern erosion and to the thick snow cover. Only one protrudingthreshold was sampled on the headwall of the La Laguna cirque.

Finally, the blockfield which covers the summit of the Pico Peña-lara was sampled because it is stable and representative of this typeof formation in the Sierra de Guadarrama.

Samples were collected from selected geomorphologic units basedon the criteria explained in the two previous sections (Table 1, Fig. 7).A sample was taken from the largest andmost stable block in the block-field on the summit of Pico Peñalara (PEÑALARA 1). In the Laguna cir-que, samples were taken from the different moraine complexes. Onthe most eroded ridge, Phase Laguna 1, a sample was taken only fromthe largest andmost stable boulder (LAGUNA1). Five sampleswere col-lected from the two lowestmoraine ridges in the southern sector, PhaseLaguna 2, as the blocks are very large and very stable: on the outermostridge, one from the interior (LAGUNA 2.1) and two from the exteriorslope (LAGUNA 2.2 and 2.3); on the innermost ridge, one from theinner slope (LAGUNA 2.5) and one from its summit (LAGUNA 2.4).Seven samples were collected in connection with the most prominentadvance in Laguna cirque (Phase Laguna 3): three from the outermostridge of the central sector (LAGUNA 3.1, 3.2 and 3.3); two from the in-nermost ridges (LAGUNA 3.4 and 3.5); and two from the northern later-al moraine, one on the exterior (LAGUNA 3.6) and the other on theinterior (LAGUNA 3.7). On the highest moraine complex (Phase Laguna4) a sample was taken from the most stable and prominent boulder

Fig. 7. Moraine boulders where samples have b

(LAGUNA 4). Finally a samplewas taken from the thresholdwith glacialabrasion below Pico Peñalara (LAGUNA 5). In the Pepe Hernando cir-que, two samples were collected from the summit of the northernmostlateral moraine ridge (PEPE 1 and PEPE 2).

Table 2 and Fig. 8 show the results of 36Cl dating. Sample PEÑA-LARA 1 (periglacial blockfield above the cirques) exhibits the oldestdate, ca. 83 ka, which most probably results from a complex exposurehistory and agrees with the overall idea that the summit areas wererelatively stable during MIS 2.

The exposure ages from the moraine complexes in general agreewith our relative chronology. The oldest date corresponds to what hasbeen called Phase Laguna 1, with an approximate age of 32 ka (SampleLAGUNA1), >10 ka older than the oldest age obtained for Phase Laguna2, (ca. 20 ka), on the most external boulders of this phase (LAGUNA 2.2and 2.3). The rest of the samples from Phase Laguna 2 are around 16 ka(LAGUNA 2.1, 2.4 and 2.5) and are highly homogeneous. However, thesamples from the central sector of Phase Laguna 3 yield ages similarto the youngest in Phase Laguna 2, i.e. ca.16 ka (Samples LAGUNA 3.1,3.2, 3.3, 3.4 and 3.5). In contrast, samples from the north lateralmoraineyield older results: ca. 25 ka for the outer boulder (LAGUNA 3.6) and ca.19 ka for the inner boulder (LAGUNA 3.7). These results agree withthose from the north lateral moraine in Pepe Hernando (PEPE 1 andPEPE 2), in a very similar geomorphologic location to sample LAGUNA3.7 and with identical dates, ca. 19 ka. The sample from Phase Laguna4 moraine (LAGUNA 4) yields an age of 16 ka. A glacially polishedthreshold (LAGUNA 5) indicates deglaciation between 12 and 11 ka.

5. Discussion

5.1. A secondary moraine hypothesis

If we compare the results obtained by cosmogenic dating with thegeomorphic setting, there is only one fact that seems to be illogical,i.e. that the most modern samples from Phase Laguna 2 have thesame age as Phase Laguna 3, although the latter is topographicallyabove the former. If the dates obtained are correct, as suggested by

een taken in the Peñalara Southeast face.

Table 236Cl exposure ages, sample type and sample location. Ages are reported for four assumed erosion rates (0, 1, 3, and 5 mm/ka). Errors correspond to the analytical uncertainty of theAMS 36Cl determination (one standard deviation).

Sample Phase Sample type Zero erosion age (ka) 1 mm/kyr age (ka) 3 mm/kyr age (ka) 5 mm/kyr age (ka)

PEÑALARA 1 ? Blockfield block 83.1±1.7 80.3±1.7 81.4±1.8 88.2±2.4LAGUNA 1 Laguna 1 Moraine boulder 31.8±0.9 31.7±0.8 31.7±0.9 32.1±0.9LAGUNA 2.1 Laguna 2 Moraine boulder 16.4±0.6 15.9±0.6 15.1±0.5 14.6±0.5LAGUNA 2.2 Moraine boulder 20.7±0.8 20.4±0.8 19.9±0.7 19.7±0.7LAGUNA 2.3 Moraine boulder 20.5±0.7 20.3±0.7 20.1±0.7 20.0±0.7LAGUNA 2.4 Moraine boulder 17.2±0.6 16.6±0.6 15.8±0.5 15.2±0.5LAGUNA 2.5 Moraine boulder 16.7±0.7 16.1±0.6 15.3±0.6 14.7±0.5LAGUNA 3.1 Laguna 3 Moraine boulder 17.0±0.8 16.6±0.7 16.0±0.7 15.6±0.7LAGUNA 3.2 Moraine boulder 17.9±0.8 17.5±0.8 17.0±0.7 16.7±0.7LAGUNA 3.3 Moraine boulder 16.2±0.8 16.0±0.8 15.7±0.7 15.5±0.7LAGUNA 3.4 Moraine boulder 17.4±0.6 17.0±0.5 16.5±0.5 16.1±0.5LAGUNA 3.5 Moraine boulder 18.8±0.6 18.5±0.5 18.1±0.5 17.9±0.5LAGUNA 3.6 Moraine boulder 26.4±0.7 26.0±0.7 25.5±0.7 25.4±0.7LAGUNA 3.7 Moraine boulder 19.6±0.8 19.3±0.8 19.0±0.7 18.8±0.7PEPE 1 Moraine boulder 19.5±1.2 19.5±1.2 19.7±1.2 19.9±1.3PEPE 2 Moraine boulder 19.9±0.8 19.7±0.8 19.5±0.7 19.9±0.2LAGUNA 4 Laguna 4 Moraine boulder 15.7±0.6 15.6±0.6 15.4±0.5 15.3±0.5LAGUNA 5 Polished bedrock 11.7±0.4 11.6±0.4 11.4±0.4 11.2±0.4

74 D. Palacios et al. / Geomorphology 139-140 (2012) 67–78

the homogeneity of the results in all cases, a geomorphic explanationmust be sought in similar current glaciers where this juxtapositioncan be observed.

An example can be found in theKebnekaisemassif (67° 54′N, 18° 31′E, 2114 m asl), in northern Sweden. The glaciers in this massif are of asimilar size to those existing in Peñalara, between 2 and 3 km long,and their formation is also closely related to the accumulation of snowon the lee of the wet westerly winds (Schytt, 1959; Karlén, 1973). Anexample of this is the Björlings glacier, situated below the east wall ofthe Kebnekaise peak, 1.8 km long and with a maximum width of1 km. The glacier runs NW–SE, veering to the SW at the end. Justwhere it changes direction, after a long stretch on a wide, almost flatplatform at an altitude of 1600 m, the glacier accumulated a large lateralmoraine during the LIA, with a steep slope toward the east. Protected bythis moraine and on its east slope there is a 200 m-long snow field(Fig. 9). The extensive, practically flat glacier surface acts as a sourceof snow to this snowfield or incipient glacier, which can be clearly iden-tified and has stable minimum limits on all the descriptive maps andphotos from 1910 to the present day. From a field survey, evidencewas observed thatmoraine blocks had slid across the snowfield and ac-cumulated at its base. The Kebnetjähka, another smaller glacier 700-mlong, starts lower down than this moraine at an altitude of 1500 m, pro-tected by another flat rocky step.

Fig. 8. Results of the 36Cl analysis from Peñalara samples.

Another example can be found in the small glaciers on the Tröllas-kagi Peninsula in northern Iceland, where many similar cases to theone described above can be found. The maximum length of these gla-ciers is around 3 km and their existence is due to the windsweptsnow from the wide summit platforms accumulating at the foot of thewalls in the lee of the wet southerly winds (Caseldine and Stötter,1993). An example of this is the Hofsjökull glacier, to the east of theJökulfjall massif (65° 38′N; 18° 50′W, 1402 m asl). This glacier, 2.9 kmlong andmax. 1.8 kmwide, is coveredwith debris and its accumulationarea rests on a flat surface at an altitude of 1000 m asl. The glacier endsin a snout 60 m high, covered with a very thick layer of debris. At thefront, sheltered by this ramp and fed by the windblown snow fromthe whole glacier accumulation area, a permanent snowfield or initialglacier has formed, only 100 m long, and blocks from the main glacierslide over it and accumulate at its base (Fig. 10). The limits of this snow-field are stable in all the maps and photograph collections since 1945.

These observations have been made in current marginal glacierareas where small glaciers, sheltered from the wet winds, are formedby the windswept snow from large summit platforms, as in Peñalara.They indicate that the topography of some glaciers facilitates the forma-tion of permanent snowfields or secondary glaciers at the base of theirmoraines. As observed in the Björlings and Hofsjökull glaciers, thesnow that falls on the wide flat surfaces of their frozen masses isswept by the wind and accumulates on the leeward side of the mo-raines. This accumulation of snow then forms permanent snowfields,and even small glaciers, below the accumulation area of the main gla-cier. Boulders from the main moraine fall on these snowfields to formsecondary moraines at the base, as the lower limits of these snowfieldsare stable, as in Björlings and Hofsjökull (Fig. 11). Around 20–16 ka, theglacier in the La Laguna cirque in Peñalara had the same topographicalconditions as the modern glaciers described above.

Fig. 9. Björlings glacier and secondary moraine (photo: www.naturphoto.net).

Fig. 10. Hofsjökull glacier and secondary moraine (photo: David Palacios).

Fig. 12. Hypothesis about the glacial evolution of Laguna Cirque during Late Pleistocene:A)Advance later that 31 ka; B) Last GlacialMaximumAdvance around26 ka; C) formationof a secondary moraine between 22 and 16 ka; D) retreat of the glacier and formation of asmall rock glacier 16–15 ka; E) definitive disappearance of the glaciers around 12 ka.

75D. Palacios et al. / Geomorphology 139-140 (2012) 67–78

If this hypothesis is applied to Peñalara, deposits of Phase Laguna 2could be interpreted as a result of accumulation from a 300 m-longsnowfield formed in the shelter of the large moraine formation ofPhase Laguna 3 and, therefore, contemporary with it. In fact, the old-est blocks of the two phases (Laguna 2 and 3) are located in the distalposition of the external arc of Laguna 2, thus supporting the hypoth-esis. This hypothesis would solve three geomorphological enigmaswhen interpreting these formations, e.g.:

a) why the lower moraines (Laguna 2) are much smaller than thehigher moraines (Laguna 3);

b) why the lower moraines (Laguna 2) conserve their formationssimilar to the higher ones (Laguna 3); and

c) why there are several moraine arcs only in Laguna Cirque and notin any other cirques of Sierra de Guadarrama.

5.2. Glacial evolution of Sierra de Guadarrama

This paper presents a first approximation to the glacial evolutionof Sierra de Guadarrama (Fig. 12).

The exposure age of ca. 83 ka on the blockfield (sample PEÑALARA1) may be considered just a first approximation to date this type ofslope deposit, which is common in Sierra de Guadarrama on NW-facing slopes above 2300 m asl. The apparent age suggests that thesummit areas above the cirques remained largely free of glacial activ-ity and, therefore, represent a non-glacial surface relict (Goodfellow,2007). The exposure age is probably the result of a complex exposurehistory similar to the one reported for geomorphic surfaces locatedabove glacial trimlines (e.g. Brook et al., 1996; Stone et al., 1998).

Fig. 11. Natural conditions model observed in the formation of a secondary moraine inBjörlings and Hofsjökull glaciers, which could be the same in Laguna Cirque at the endof the Pleistocene.

Additional dating is needed to obtain a more precise idea of the ageof the blockfield.

This paper does not provide any conclusive information about theage of the oldest glacial formations in these mountains. The dateobtained for Phase Laguna 1 (LAGUNA 1) of ca. 32 ka comes from a sin-gle boulder of a poorly preserved moraine. It can be considered as aminimum age for this arc, but cannot be used to support the existenceof glacial deposits older than the last glaciation, nor can it be used as astrong support for the hypothesis of an Iberian maximum advance ear-lier thanMIS 2. The problem is difficult to solve asmost boulders on thismoraine are unstable; furthermore, our investigations have shown thatthere are no similar moraine formations remaining in other sectors ofthe Sierra. Middle Pleistocene moraines must have been overriddenby Late Pleistocene glaciers in the Sierra.

On the other hand, the results for samples of Phases Laguna 2 andLaguna 3 and from the Pepe Hernando cirque are consistent withthose obtained in the nearby Sierra de Gredos, 140 km to the SE, withinthe samemountain range of Sistema Central (Palacios et al., 2011), if weuse the ‘secondary moraine’ hypothesis. Just as in Gredos, a local maxi-mum advance can be observedwithinMIS 2, somewhat earlier than theglobal LGM, around 26 ka, followed by marked stability until 20–19 ka.This is shown by themost external samples from the lateralmoraines inPhase Laguna 3 (LAGUNA 3.6 and 3.7, PEPE 1 and PEPE 2), which werenot altered by the secondary moraine. The secondary moraine of Phase

76 D. Palacios et al. / Geomorphology 139-140 (2012) 67–78

Laguna 2 dismantled part of the central sector of moraine Laguna 3, sothat the latter does not keep such old boulders. The external bouldersof Phase Laguna 2 were deposited around 21–20 ka (samples LAGUNA2.2 and 2.3) when the main glacier was still developing.

These dates coincide with results frommost of the cosmogenic dat-ing studies obtained in the last few years in southern Europe, includingthe Pyrenees, for moraines which show the best preserved maximumadvance of the last glaciation (Ivy-Ochs et al., 2004, 2006, 2008; Preus-ser, 2004; Kerschner and Ivy-Ochs, 2008). This period also coincideswith the last global maximum extent of the ice-sheet and mountain-glaciers (Clark et al., 2009) and it is earlier than the global sea level low-stand, which reached its lowest level ~21 ka (Lambeck and Chappell,2001; Lambeck et al., 2002; Peltier and Fairbanks, 2006; Allen et al.,2008). This has recently been confirmed in Iberian Peninsula bypaleo-biogeographical (López-García et al., 2010) and by speleothemresearch (Moreno et al., 2010a, 2010b), showing evidence of wetter cli-mate conditions previously to LGM.

A slow retreat began after 19 ka, modified by short re-advances, atthe same time depositingmost of the boulders sampled in Phase Laguna2 (LAGUNA 2.1, 2.4 and 2.5) and Laguna 3 (LAGUNA 3.1–3.5). The oldestexposure ages do not come from the outermost boulders of the centralsector of Phase Laguna 3, since this external sector is the onewhichwasdismantled by the snowfield as it formed a secondary moraine. In fact,the oldest exposure age corresponds to the innermost boulders, asthey were better protected from the action of the snowfield (LAGUNA3.5). Recent data seem to confirm that this period, centered on the glob-al LGM, coincided in the Iberian Peninsula with a cold but arid period,with minimum lake levels (Morellón et al., 2009; Vegas et al., 2010).

Just as in Sierra de Gredos, the glacier retreat process acceleratedbetween 16 and 15 ka (Palacios et al., 2011). In Peñalara, the glacierretreated sharply and its upper surface ceased to act as the snowsource feeding the secondary snowfield. The secondary snowfield dis-appeared, whereby the main moraine (Laguna 3) and the secondarymoraine (Laguna 2) stabilized at the same time.

Phase Laguna 4 is an anomaly in the cirques of the Sierra deGuadarrama and only exists in the La Laguna cirque. The results of thesample LAGUNA 4 (rock glacier) indicate that this corresponds to ashort-lived phenomenon within a continuous, rapid deglaciation pro-cess. It is probably related to rockfalls on top of remnants of a retreatingglacier, which would allow its transformation into a typical rock glaciermorphology and the survival of the ice for decades, as currently occursin the post LIA deglaciation in the Sierra Nevada mountain range inSouth Spain (Gómez et al., 2003).

The rapid retreat and disappearance of the glacial tongues between16 and 15 ka is also a constant in the results frommany othermountainsin southern Europe, including the Pyrenees (González-Sampériz et al.,2006; Pallàs et al., 2006; Delmas et al., 2008; Sancho et al., 2008; Pallàset al., 2010), Sierra de Queixa (Fernández-Mosquera et al., 2000), Lagode Sanabria (Cowton et al., 2009), the Alps (Ivy-Ochs et al., 2004, 2006,2008; Preusser, 2004; Kerschner and Ivy-Ochs, 2008) and the Kaçarmountains in the east Mediterranean (Akçar et al., 2007, 2008; Zahnoet al., 2010). It also coincideswith themaximumglacial retreat on a glob-al level (Clark et al., 2009) and the disappearance of the cold fauna of theIberian Peninsula (Álvarez-Lao and García, 2011).

Sample LAGUNA 5, from glacial polish near Peñalara cirque head-wall, indicates full deglaciation around 11.7 ka, at the beginning of theHolocene. We do not know if Laguna 5 was the result of a new glacialexpansion or if it represents the total deglaciation. Nevertheless, thefact that the possible new advance did not affect the rock glacier sup-ports the first possibility.

6. Conclusions

The Sierra de Guadarrama had glacial activity beforeMIS 2,which hasnot yet been detected in the center of the Iberian Peninsula, although theresults of this paper do not permit precise dating. As in Sierra de Gredos,

the local last maximum advance took place within the global LGM, last-ingwith considerable stability between ca. 25 and 19 ka. Between 19 and16 ka there was a slow retreat, punctuated by small re-advances, with avery rapid retreat after 16 ka and full deglaciation around 12 ka. Al-though based on a single exposure age, this paper presents evidence ofdeglaciation just at the beginning of the Holocene (and therefore of thepresence of a glacier during the Younger Dryas stadial) on the high head-walls of the cirques in the center of the Iberian Peninsula.

The particular topography of some glaciers situated in marginalareas, and which originated from windswept snow from higher flatsurfaces, may have given rise to the formation of permanent snow-fields or secondary glaciers, which in turn formed secondary mo-raines fed by the blocks from the main moraines. This is preciselythe peculiar topography of the La Laguna glacier cirque in Peñalara.The hypothesis of a secondary moraine can be applied to explainthe geomorphological contrasts between the different moraine com-plexes of Peñalara and the results obtained from dating their blocks.

Acknowledgments

Thiswork has been supported by theMinisterio de Ciencia e Innova-ción of Spain (project: CGL 2009-7343), the Research Group BSCH/UCM: 931562 GEOGRAFÍA FÍSICA DE ALTA MONTAÑA and from FUN-DACIÓN CAJAMADRID. We thank the fieldwork support from PeñalaraNatural Park authorities and rangers.

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