Quaternary International 233 (2011) 16e26

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Last Glacial Maximum and deglaciation of , central

David Palacios a,*, Javier de Marcos a, Lorenzo Vázquez-Selem b a Departamento de A.G.R. y Geografía Física, Universidad Complutense, 28040 , b Instituto de Geografía, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico article info abstract

Article history: The study area is located in the Sierra de Gredos, a portion of the range of central Spain, Available online 10 May 2010 specifically in the Gredos Gorge, on the north side of (2596 masl), where glacial forms are present down to an altitude of 1410 m, 10 km from the headwall. This paper presents cosmogenic 36Cl surface exposure dates of four closely spaced lateral moraine ridges within the area of maximum advance of the ancient glacier, 8 km from the headwall; and two glacially polished bedrock thresholds 3 and 5 km from the headwall. The dates are overall coherent and indicate a maximum advance at 26e24 ka. Subsequently the glacier front stabilized around its maximum position for w3 ka, with small scale fluctuations resulting in small, closely spaced moraines ridges. Glacier recession began after 21 ka, accelerating sharply by w16 ka. The glacier had probably disappeared from the Gredos Gorge by 15 ka. There are no traces of older or more recent moraines than those dated in this study. These results are in agreement with the well known late Pleistocene climatic evolution of the Northern Atlantic and with the glacial chronology of the Alps and other Mediterranean mountains. However, the Gredos Gorge has no geomorphic evidence of the Younger Dryas cold event, probably because of the low altitude and southern character of these mountains. Ó 2010 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction with earlier glaciations (Calvet, 2004; Pallàs et al., 2006), the mountains in northern Spain present well preserved frontal The Pleistocene glacier evolution of the Mediterranean moun- moraines, often in sequences of closely spaced ridges. These tains, at low latitudes and with a special sensitivity to climate features are characteristic of what is known as the Last Glacial change, is considered as a key factor in a global understanding of Maximum (LGM) in the North Atlantic glacial chronology and climate change, as this information complements and contrasts coincides with Marine Isotope Stage 2 (MIS2) (Björck et al., 1998; with data from mid and high latitudes (Hughes et al., 2006a,b; Walker et al., 1999; Johnsen et al., 2001). The earliest dating work Hughes and Woodward, 2008). Precise geochronological data on on the last expansion of the Late Pleistocene glaciers in the north of the extension of glaciers has recently been obtained for several the Iberian Peninsula yielded older values than the LGM. This mountains of the Iberian Peninsula, moving forward from a merely dating generally placed the Late Pleistocene maximum between descriptive stage (Fernández Mosquera et al., 2000; Jiménez- >50 and 28 ka (Mardones and Jalut, 1983; Vilaplana, 1983; Jalut Sánchez and Farias, 2002; Sancho et al., 2002; García-Ruiz et al., et al., 1992; Reille and Andrieu, 1995; Fernández Mosquera et al., 2003; Peña et al., 2004; González-Sampériz et al., 2006; Pallàs 2000; Jiménez-Sánchez and Farias, 2002; Sancho et al., 2002; et al., 2006; Delmas et al., 2008). However, most dates refer to García-Ruiz et al., 2003; Peña et al., 2004; González-Sampériz the and marginally to the Cantábrica, i.e. the et al., 2006) although a re-advance was often shown around northernmost mountains. 20 ka, coinciding with the LGM (García-Ruiz et al., 2003; González- The glacial chronology of the northern Iberian Peninsula is still Sampériz et al., 2006). The possible cause of earlier deglaciation in inconclusive. As in many other Mediterranean mountains (Hughes southern Europe would have been a particularly dry period during and Woodward, 2008), aside from the remains of moraine forma- the LGM (see review in Hughes et al., 2007). The starting point for tions and very altered high fluvioglacial terraces clearly associated the rapid retreat of the glaciers was placed between 16 and 15 ka (Fernández Mosquera et al., 2000). In clear contrast, recent research using cosmogenic nuclide * Corresponding author. Tel./fax: þ34 913945955. methods on moraine boulders and glacially abraded bedrock has E-mail address: [email protected] (D. Palacios). produced dates more in agreement with the North Atlantic glacial

1040-6182/$ e see front matter Ó 2010 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2010.04.029 D. Palacios et al. / Quaternary International 233 (2011) 16e26 17 chronology, including a later onset of deglaciation and considering dating on whole rock samples (Phillips, 1995; Gosse and Phillips, the pleniglacial and LGM stages as belonging to the same period. On 2001) of moraine boulders and glacially polished bedrock provide the southern side of the central Pyrenees, using 10Be dating, Pallàs the basis of the chronology. 36Cl has been shown to yield ages et al. (2006) found a maximum advance after 25 ka. In the eastern comparable to those of 10Be in areas of western North America Spanish Pyrenees, also with 10Be, Delmas et al. (2008) found where rock type and latitude are similar to those of Gredos (e.g. a maximum advance coinciding with the LGM, with slight oscilla- Phillips et al., 1997; Brugger, 2007). Initial results of this research tions between 25 and 20 ka. The difference between these two were presented in Palacios et al. (2007). dates and the earlier ones may be due to the fact that all the earlier samples were fluvioglacial or lacustrine deposits highly succeptible 2. Study area to contamination and dated by radiocarbon or OSL, and some have already been drastically corrected to lower values. The radiocarbon The study area is located on the north side of Pico Almanzor dates may have been contaminated with graphite from the Palae- (4014048” N; 517052” W; 2596 masl), the maximum altitude of ozoic schists or reworked organic material from earlier deposits Sistema Central and watershed divide between the Tormes and Tajo (Pallàs et al., 2006; Delmas et al., 2008). Notwithstanding, Delmas river basins (Figs. 1, 2 and 3). et al. (2008) see considerable homogeneity in the dating obtained The origin of the Sierra de Gredos is a great cortical thin-skin in the more westerly sectors. They interpret the lack of synchrony thrust, but it is delimited by normal E-W faults derived from a pop- between these sectors and the eastern Pyrenean mountains as up structure. This tectonic arrangement causes a marked dissym- a result of the more temperate and drier character of the latter, with metry in the slopes. The south facing slope of the Sierra is steep smaller marginal glaciers protected on the mountain slopes. with an average gradient of 15%, while the north side is gentler with In these circumstances, precise dating must be obtained for the an average gradient of 8% (gradient measured 15 km to the south more southerly mountain systems in the Iberian Peninsula, to and north of the divide). The lithology of the whole Sierra is compare with results from the northern ranges. The Sistema uniform and consists of (biotitic and granodiorites) from Central was chosen for this reason. This mountain range runs intrusions during the Variscan orogeny. These granites are heavily SW-NE across the centre of the Iberian Peninsula, has a medium tectonized, particularly the valleys transversal to the Sierra, altitude and clear glacial formations. For the Sistema Central there generally running in an NNE-SSW tectonic direction. is only preliminary data on the dating of fluvioglacial deposits by There are very few meteorological stations in the Sierra, and the thermoluminescence in the most westerly sector, the Sierra de la existing ones are located at the base, which limits understanding of Estrella, dated between 16 and 10 ka (Vieira et al., 2001). its climate. The climate of these mountains is determined by the The aim of this paper is to establish the precise chronology of frequent arrival of Atlantic depressions from the SW during the maximum glacial advances in the centre of the Iberian Penin- the autumn, winter and spring and by the predominant Azores sula, where no glaciers have existed in the historic period. The anticyclone causing very dry summers (only 10% of the annual study area, Sierra de Gredos, is part of the highest sierras of precipitation). On the north slope, mean annual precipitation is the Sistema Central, where there is evidence of the existence of the 554 mm at 1007 masl, 1704 mm at 1200 masl, and is estimated at largest glaciers in the cordillera (Fig. 1). The specific area selected 2000 mm at 2000 masl, of which 77% falls as snow. The mean was the Gredos Gorge (Garganta de Gredos) next to Pico Almazor annual temperature at 1800 masl (possible altitude of the (2596 m), the highest peak in the Sistema Central. A very detailed Equilibrium Line Altitude, or ELA, of the glaciers during the Last geomorphological study was the basis for sample collection and for Glacial Maximum) is nowadays w6 C; and 3.5 C and 2 Cat2000 validation of cosmogenic dating. Cosmogenic 36Cl surface exposure and 2500 masl, repectively. The modern 0 C isotherm is estimated

Fig. 1. Location of the study area. 18 D. Palacios et al. / Quaternary International 233 (2011) 16e26

Fig. 3. Garganta de Gredos and Gredos lake from Almanzor Summit.

Carandell, 1916). The knowledge and cartography of these glacial advances was subsequently expanded to other sectors near the Gredos central massif (Carandell, 1924; Vidal Box, 1932, 1934, 1936, 1948; Hernández-Pacheco, 1933). Later, the glacier areas previously defined were described in much greater detail, and with consid- erably improved cartography (Martínez de Pisón and Muñoz, 1972; Arenillas and Martínez de Pisón, 1976; Sanz Donaire, 1979, 1981; Acaso, 1983; Rubio, 1990). More specific studies dealt with the extent of ancient glaciers and their connection with current processes (Muñoz et al., 1995; Palacios et al., 1998), and docu- mented the existence of glaciers on the southern side of the Sierra (Marcos and Palacios, 1995). Syntheses of the state of knowledge of glaciation in Sierra de Gredos were published by Pedraza and López (1980) and by Martínez de Pisón and Palacios (1997). Most of the above mentioned studies suggest a possible chronology of glacial landforms in the area. Most recognize that these landforms were produced by a single, recent (Würm) glaciation, given their homogeneity and the good preservation of moraines. However, none provided numerical dating.

3. Methods

Based on a preliminary geomorphological analysis of the study area, one of the most representative glacial valleys, the Gredos Fig. 2. AlmanzorPeak and Garganta de Gredos with the area glaciated during Last Gorge, was selected for research. Geomorphological mapping of Glacial Maximum. this valley was then produced by photointerpretation of two photo collections at 1:18,000 and 1:30,000 and by field surveys. The at 2800 masl. Between 2000 and 2300 masl, the snow lies on the glacial landforms were mapped using the 20 m resolution digital ground for 180e300 days per year (Muñoz et al., 1995). There is base provided by the local mapping authority. The resulting map evidence of the existence of permanent snowfields during the Little was used to select optimal sampling sites for 36Cl dating. Ice Age, on ledges sheltered by north-facing walls at altitudes Samples for 36Cl dating were taken from moraine boulders >1m between 2300 and 2400 m (Sancho et al., 2000). high. The initial criterion used to select them was their geomorphic In a recent study, the climatic situation of Sierra de Gredos stability, so sites on gentle slopes were preferred. The setting and during the LGM was modelled in relation to current climatic values size of the selected boulders likely minimize the chances of and the ELA depression. Based on a comparison between the previous burial by soil or sediment as well as shielding by snow modern and LGM ELAs, a cooling of 14.2 C is inferred at the LGM cover. Due to budgetary constraints only one sample of each (Allen et al., 2007). moraine was collected. Samples were also taken from well Investigations on the glacial landscape in the Sierra de Gredos preserved glacially abraded bedrock thresholds which met the began with the preliminary observations of Albert Penck (1884) following conditions: they are protrusive, to reduce the possibility on Iberian glaciation. The first publications showing a clear of a sediment cover and to minimize the effect of snow cover; they delimitation of former glaciers in the Sierra de Gredos date from the have clear glacial polish, to ensure that the surface has not been early 20th century; they identify moraine ridges and glacial abra- eroded; and they come from a site where the former glacier sion forms mainly in the central massif around Pico Almanzor was thick and erosion of the pre-glacial surface was strong, thus (Schmieder, 1915; Huget del Villar, 1915, 1917; Obermaier and minimizing the chances of 36Cl inheritance. D. Palacios et al. / Quaternary International 233 (2011) 16e26 19

Samples were collected using hammer and chisel from four boulders of different moraines, and from two glacially abraded bedrock surfaces. The laboratory procedures for whole rock summarized by Zreda et al. (1999) and Phillips (2003) were fol- lowed. Whole rock samples were crushed and ground using a roller grinder. Ground rock was dissolved in a hot mixture of hydrofluoric and nitric acids, and Cl precipitated as AgCl. A spike of isotopically enriched 35Cl was added during the dissolution, whereby Cl content could be determined during the accelerator mass spectrometry (AMS) analysis by means of the isotope dilution mass spectrometry method. The 36Cl/Cl and 37Cl/35Cl ratios were measured on AgCl targets by AMS at PRIME Laboratory (PurdueUniversity). Aliquots of rocks were powdered and analysed for major elements, U and Th by XRF spectrometry at New Mexico Tech (Bureau of Geology); and for B and Gd by neutron activation prompt gamma emission spectrometry at Activation Laboratories, Canada. Exposure ages were calculated using the program CHLOE (Phillips and Plummer, 1996, version 3 e 2003), using thermal and epithermal neutron distribution equations and 36Cl production parameters by Phillips et al. (2001); production of 36Cl by muons according to Stone et al. (1998); and latitude and elevation scaling of production rates by Lal (1991). Exposure ages were calculated assuming erosion rates of the rock surface of 0 (no erosion), 3 and 5 mm/ky.

4. Geomorphological context and sample selection

In Sierra de Gredos, the Garganta de Gredos (Gredos Gorge) was selected as it is one of the largest valleys and has an excellent array of glacial landforms, including glacially polished thresholds and well preserved moraines with abundant large boulders (Figs. 4 and 5). The highest point of the headwall is Pico Almanzor (2596 m). Two sub-basins (Gargantón and Circo de la Laguna) extend to the north of this peak and join at an altitude of 1740 m. The Garganta de Gredos runs parallel to Garganta del Pinar (to the west) and Garganta de Las Pozas (to the east). Both of them were also glaciated, but their glaciers did not reach the glacier in Gredos Gorge. In Gredos Gorge the upper limit of lateral moraines is at 1870 m on the right side of the valley, and at 1670 m on the left side, i.e. >300 m above the present day river bed. The left lateral moraine consists of two ridges. Moraines are severely eroded by ravines and debris flow activity. At its maximum position, the glacier snout ended at 1410 m, where the lowermost glacial deposits are found. The left side of the glacial valley is very steep whereby moraines on this slope are severely eroded. Near the position of the maximum advance the slope angle decreases, and up to 7 moraine ridges are preserved. The valley widens and becomes less steep around the confluence of Garganta de las Pozas. There, the glacial snout spread over its right side, became thinner, and locally formed up to 5 lateral moraine ridges. These ridges are located on a gentle slope and therefore are well preserved, except around the conflu- Fig. 4. Garganta de Gredos and location of 36Cl sampling sites. The inset corresponds to ence of Garganta de las Pozas. Downstream from this area, the right the area represented in Fig. 5. side of the valley becomes very steep and moraines are absent. Four of the five right lateral moraine ridges of Garganta de (GREDOS 5) and 2.0 km (GREDOS 6) downstream from the head- Gredos were selected for 36Cl dating, just north of the confluence wall, respectively (Figs. 4 and 6). with Garganta de las Pozas, 7.5 km from the valley headwall No glacial deposits are present downstream from the point (samples GREDOS 1, 2, 3 and 4) (Figs. 4e6). The most external and considered as the maximum advance, located at 1410 m. The the most internal ridges were selected along with the two best in that area is chemically weathered at great depth. There are no preserved intermediate ridges. Moraine boulders were selected in moraines upstream from those next to the maximum position, except each of these ridges based on the criteria described above. As for small protalus ramparts formed above 2300 m near the headwall moraine ridges are practically composed of single strings of blocks, cirque, similar to those dated to the LIA by Sancho et al. (2000). there is no risk that they were formed by aggregation of sediments The ELA of Gredos Gorge during the maximum advance was of successive advances (Gibbons et al., 1984; Osborn, 1986). calculated using three different methods: the Toe to Headwall In order to estimate the timing of deglaciation, two samples of Altitude Ratio method (THAR ¼ 0.4) yielded 1820 m; the glacially polished bedrock were selected as described above, 5.3 km Accumulation Area Ratio method (AAR ¼ 0.65), 1827 m; and 20 D. Palacios et al. / Quaternary International 233 (2011) 16e26

Fig. 5. Geomorphological map of the Garganta de Gredos morainic area. Legend: 1. e Weathered mantle; 2. e Tor area; 3. e Disperse moraine boulders 4. e Moraine ridges; 5. e Glacially polished outcrops in the bottom of the valley; 6. e Glacially polished outcrops in the slopes of the valley; 7. e Debris flows; 8. e Rockfall cones; 9. e Torrent areas; 10. e Alluvial fans; 11. e Solifluction lobes; 12. e lacustrine deposits; 13. e Old fluvial terrace; 14. e Young fluvial terrace; 15. e Fluvial channel; 16. e Area of Fig. 9. the Maximum Elevation of Lateral Moraines method (MELM), insensitive to erosion of the rock surface. At the highest erosion rate 1815 m. prescribed (5 mm/ka), ages are only 2e7% younger than those assuming zero erosion. 5. Cosmogenic dating results Exposure ages calculated using production parameters of 36Cl determined by Swanson and Caffee (2001) are 34% younger than Tables 1 and 2 and Figs. 7e10 show 36Cl ages of six samples those based on the parameters by Phillips et al. (2001). The authors calculated as described above. Exposure ages are relatively favour results based on production rates by Phillips et al. (2001) for D. Palacios et al. / Quaternary International 233 (2011) 16e26 21

(w6 ky) five moraine ridges formed at short distance from each other, indicating a slow thinning and narrowing of the glacier after the maximum advance. The sample from the outer moraine (GREDOS-1) yields an age of 24.2 0.9 ka, i.e. w1 ka older than that of the second moraine (GREDOS-2), but the error bars associated to the AMS analytical uncertainty of both samples largely overlap. Thus the formation of the first (outer) and second moraines prob- ably occurred between 26 and 24 ka. The exposure age decreases from 25.2 1.2 ka (second moraine, GREDOS-2) to 22.8 1.2 ka (third moraine, GREDOS-3), then to 21.0 0.5 ka (fifth and inner- most moraine, GREDOS-4). Glacial polish at 1650 masl (GREDOS-5) indicates that by 15.7 0.7 ka the glacier front had receded 2.3 km horizontally and 200 m vertically, with respect to its position at w21 ka. Another sample of glacially polished bedrock (GREDOS-6), collected at 2000 masl next to Laguna Grande, 3.3 km upstream from GREDOS-5, yields 16.2 1.5 ka. The distance between these sites (3.3 km), the absence of recessional moraines in between them, and the similar exposure ages of the two samples suggest rapid glacier recession some time between w17.5 and w15 ka. There is a clear difference between the 36Cl ages from moraine boulders (26e20 ka) and those from glacially polished bedrock sites (w17.5e15 ka), which separates the phase of moraine formation at and near the maximum position, from the phase of glacier recession. In summary, the results suggest: (1) a glacial advance peaking at 26e24 ka, i.e. short before the Last Glacial Maximum (LGM); (2) stagnation/slow recession around the peak of the LGM; (3) onset of deglaciation after 21 ka; and (4) major deglaciation taking place at a fast pace around 16 ka. This is a preliminary interpretation, based only on 6 exposure ages. However, the excellent internal agreement of exposure ages of samples located at very short distances from each other supports this interpretation.

6. Discussion

The results obtained in Gredos coincide fully with recent research on glaciation of the Spanish Pyrenees involving cosmo- 36 Fig. 6. Cl dating sampling sites. Gredos-1, -2, -3 and -4 are moraine boulders; genic nuclide dating techniques (10Be). According to Pallàs et al. Gredos-6 is polished bedrock. (2006) the maximum advance for the Noguera Robagorçana valley (situated between 3240 and 900 masl on the south slope of the following reasons: first, the latitude of the study site is similar the central Pyrenees) took place at <25 ka. The stabilization of the to the latitude of most calibration sites used by Phillips et al. (1997, glacier front lasted at least until 21.4 ka. The problem in this valley 2001); second, the same laboratory protocols were used; and third, is that terminal moraines are not well preserved and the dating was exposures ages based on Phillips et al. (2001) are more consistent carried out on erratic blocks which do not necessarily indicate the with the current knowledge on the timing of the maximum maximum advance. As in Gredos Gorge, there is clear evidence of advance and deglaciation in western European mountains, as a subsequent rapid deglaciation which led to the disappearance of discussed below. the valley glacier around 15 ka. By that time the glaciers occupied Overall the six 36Cl exposure ages reported here agree well with only headwall cirques. the morphostratigraphy (Table 2 and Figs. 7e10). Considering the These results are also similar to those of Delmas et al. (2008) in AMS analytical uncertainty, the whole series of moraine ridges the Têt valley, in the southeastern Spanish Pyrenees, to the east of probably developed between 26 and 20 ka. During this period the Pico Carlit (2921 masl). In this case the terminal moraines of the

Table 1 Chlorine-36 ratio and chemical data for Garganta de Gredos samples.

36 15 Sample ID Cl/(10 Cl) Cl Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 B Gd U Th (ppm) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (ppm) (ppm) (ppm) (ppm) GREDOS-1 676 23 45.1 2.99 1.02 13.53 73.20 0.12 3.36 0.93 0.66 0.03 3.77 5.5 0.6 2 6 GREDOS-2 1065 47 39.2 3.09 0.79 15.32 70.54 0.09 5.50 1.13 0.46 0.03 2.51 16.3 0.8 2 7 GREDOS-3 812 42 40.3 3.32 0.68 15.31 71.39 0.09 4.71 1.20 0.35 0.03 2.32 20.0 0.8 2 5 GREDOS-4 558 17 57.3 3.32 0.64 15.31 72.22 0.10 4.67 1.19 0.33 0.03 2.24 27.6 0.8 2 5 GREDOS-5 392 16 80.9 3.10 0.56 14.83 71.92 0.11 5.21 0.76 0.44 0.03 2.49 8.9 1.7 3 16 GREDOS-6 345 20 91.9 3.42 1.23 15.89 68.50 0.09 3.72 1.67 0.68 0.05 4.07 22.9 1.8 3 12 22 D. Palacios et al. / Quaternary International 233 (2011) 16e26

Table 2 36 Cl ages and sample data for Garganta de Gredos moraine boulders and glacially polished surfaces; ST is the topographic shielding factor; Af,e is the effective attenuation length for fast neutrons.

Sample Unit Zero erosion 1 mm/kyr 3 mm/kyr 5 mm/kyr Latitude Longitude Elevation ST Af,e age (ka) age (ka) age (ka) age (ka) (N) (W) (m) (unitless) (g/cm2) GREDOS-1 Moraine 1 24.2 0.9 23.7 0.8 23.0 0.8 22.6 0.8 40.3002 5.2609 1650 0.986 162 GREDOS-2 Moraine 2 25.2 1.2 25.1 1.2 25.1 1.2 25.3 1.2 40.3009 5.2621 1640 0.988 162 GREDOS-3 Moraine 3 22.8 1.2 22.6 1.2 22.3 1.2 22.3 1.2 40.3023 5.2647 1575 0.989 171 GREDOS-4 Moraine 5 21.0 0.5 20.8 0.5 20.4 0.5 20.2 0.5 40.3036 5.2668 1550 0.993 171 GREDOS-5 Glacial polish 15.7 0.7 15.5 0.7 15.1 0.6 14.8 0.6 40.2860 5.2727 1650 0.974 172 GREDOS-6 Glacial polish 16.2 1.5 15.9 1.4 15.3 1.4 15.0 1.3 40.2568 5.2777 2000 0.903 178 maximum advance are clearly defined and appear as a sequence of Dryas, around 17 ka. Evidence of a similar re-advance has not been multiple closely spaced arcs, as in Gredos Gorge. The dates obtained found in Gredos. Indeed, the phase of rapid recession identified in for these moraines of maximum advance are very similar to those Gredos around 16 ka falls within the Oldest Dryas. of Gredos Gorge, ranging from 24 to 21 ka. Subsequent deglaciation This study and those by Pallàs et al. (2006) and Delmas et al. was very fast and occurred over less than 4 thousand years. (2008) overall coincide in placing the maximum glacier advances Younger recessional moraines suggested to Delmas et al. (2008) the in Gredos and the Pyrenees within MIS2. This clearly contradicts hypothesis of a fast but ephemeral re-advance during the Oldest previous research done in the Pyrenees and the NW of the Iberian

Fig. 7. Location of 36Cl samples in the Garganta de Gredos context. D. Palacios et al. / Quaternary International 233 (2011) 16e26 23

Fig. 8. Orthophoto (1998) of the area of the samples GREDOS-1, -2, -3 and -4 in the lateral morainic complex of Garganta de Gredos. The stream visible corresponds to Garganta de Las Pozas near the confluence with Garganta de Gredos. See location in Fig. 5.

Peninsula (Mardones and Jalut, 1983; Vilaplana, 1983; Jalut et al., 1992; Reille and Andrieu, 1995; Fernández Mosquera et al., 2000; Jiménez-Sánchez and Farias, 2002; Sancho et al., 2002; García- Ruiz et al., 2003; Peña et al., 2004; González-Sampériz et al., 2006). However, the two recent studies in the Pyrenees do admit that their results may be compatible with earlier dates of maximum advance. Pallàs et al. (2006) consider that in other valleys of the central and western Pyrenees older moraines, preserved from erosion, may have a pre-LGM age, although still within MIS2. Fig. 9. Photographs of the area of samples GREDOS-1, -2, -3 and -4 (A); and GREDOS-5 Delmas et al. (2008) consider that their study area (eastern and -6 (B). Pyrenees), compared to sites showing much earlier maximum glaciation and deglaciation in the Pyrenees and the NW Iberian Turkey, towards the coast of the Aegean Sea on Mount Sandras Peninsula, is in a far more marginal situation because of its (2295 masl), the LGM has been dated by 36Cl to 20.4 ka, with southern character and its longer distance to the moist Atlantic a sudden retreat thereafter (Sarıkaya et al., 2008). In the SE Italian winds. For this reason, glaciers in the eastern Pyrenees were always Alps, Monegato et al. (2007) estimate (by 14C) the onset of glacier very small and well protected from radiation due to their topo- advance at 30 ka and the onset of retreat at 19 ka, with maxima in graphic setting, so that they retreated only when there were drastic temperature oscillations. This case is somewhat similar to that of the small glaciers in the arid polar Urals, which have retreated only a few hundred metres since the LGM (Mangerud et al., 2008). As for northwestern Iberia, it is worth noticing that in Serra de Queixa two samples of glacial polish related to deglaciation yield exposure ages (cosmogenic 21Ne) of 22 17 and 15 7ka(Fernández Mosquera et al., 2000), thus broadly in agreement with 36Cl cosmogenic ages of deglaciation in Gredos and the Pyrenees mentioned above. In the same area, Cowton et al. (2009) reconstructed the largest LGM ice cap in Iberia outside the Pyrenees, situated in Parque Natural Lago de Sanabria. Radiocarbon ages from lake deposits inside but near the outermost moraines suggest that the maximum extent of this ice cap occurred during MIS 2 and deglaciation was initiated before 14e15 14C ka B.P. (i.e. before 17e18.5 cal ka B.P.), thus generally in accordance with the chronology of Gredos. The results are also in agreement with those from other Mediterranean mountains where cosmogenic dating has been applied. In the Kaçar mountains (3932 masl), in northern Anatolia Fig. 10. Summary of 36Cl cosmogenic exposure ages from Garganta de Gredos. Listed near the Black Sea, 10Be dating indicates a Last Glacial Maximum at ages are calculated for zero erosion (thick bars), with error bars showing AMS analytical error of the 36Cl/Cl measurement. Open triangles are ages assuming least from 26 ka to 18 ka, with a rapid and drastic retreat lasting a uniform erosion rate of 3 mm/ky (not shown for glacially polished outcrops, where until 15.4 ka, when the glaciers had already disappeared from the actual erosion is negligible). Age calculations are based on production parameters by main valleys, as in Gredos (Akçar et al., 2007, 2008). Further west in Phillips et al. (2001). 24 D. Palacios et al. / Quaternary International 233 (2011) 16e26 the area between 26.5 and 23 cal ka. In the Apennines, glaciers were less extensive. The glacier front reached its maximum around started to expand earlier than 22 ka, and began to retreat slowly 26e24 ka, then became relatively stable, with small fluctuations, around 21.5 ka and more rapidly about 16 ka (Giraudi, 2004), just like other European mountain glaciers. Marked recession simultaneously with those of Gredos. started after 21 ka and progressed at high pace around 16 ka. The On the other hand, this chronological framework for the maximum advance in Gredos Gorge generally falls within the LGM maximum advance and rapid retreat observed in Gredos, is also as defined by Mix et al. (2001), namely 23e19 ka, or by Clark et al. supported by results from the great icefield of the Alps. In this area (2009), namely 26.5e19 ka. The glacier remained largely stable the glacial snouts reached their maximum around 21 ka, then with minimum retreat throughout most of the LGM. retreated from their outer moraines, with a rapid recession Similarly the retreat process of the Gredos glaciers was largely especially around 19 ka (Ivy-Ochs et al., 2004)or17.5ka(Preusser, coeval to that of other mountains of Europe, according to the most 2004). By w18 ka, 80% of the alpine ice had already disappeared. recent dating. The retreat began soon after the peak of the LGM The deglaciation of the large valleys culminated w15.4 ka (Ivy-Ochs around 21 ka, accelerating drastically around 16 ka, when the et al., 2006, 2008). The moraine complex which Penck and glacier finally disappeared from most of the valley probably in less Brückner (1901e1909) had classified as Würm can now be than 1000 years. defined as formed between 24 and 21 ka (Preusser, 2004). Recent These results support the idea of the unified behaviour of cosmogenic dating has shown how the glacier fronts stabilized mountain glaciers in the Iberian Peninsula and the Mediterranean during that period, with only small re-advances and retreats as a whole within the global climatic evolution of the North (Kerschner and Ivy-Ochs, 2008), as this study shows for Gredos. Atlantic. New geochronology studies are taking place in other Further north in Europe very similar data have recently been put glaciated areas of the Sistema Central. They will enable testing of forward, e.g. in the Polar Urals, Russia, where the maximum these results. advance has been dated at 21.5 ka, coinciding fully with the LGM (Mangerud et al., 2008). Acknowledgments These results correlate very well with data on the evolution of sea levels, which fell sharply from w30 ka, and reached their lowest This work was supported by the Ministerio de Ciencia e Innova- levels w21 ka. During those 10,000 years sea level shows only small ción of Spain (projects CGL 2008-2324 and CGL 2009-7343), the variations, and starts to rise rapidly, especially from 19 ka, with very Research Group BSCH/UCM: 931562 Physical Geography of High high rates between 16 and 12.5 ka and between 11.5 and 8 ka Mountain Areas, Caja Madrid and benefited from the help of Prime (Lambeck and Chappell, 2001; Lambeck et al., 2002; Peltier and Lab (Purdue University). We thank Philip Hughes and an anony- Fairbanks, 2006; Allen et al., 2007). This coincidence is reinforced mous reviewer for constructive comments that improved the by recent dating work on the mountain glacial chronologies. The manuscript. start of the maximum advance of the Alpine glaciers was earlier than previously thought, and is now considered to be w30 ka References (Preusser, 2004; Ivy-Ochs et al., 2006, 2008; Monegato et al., 2007). This hypothesis is also defended in the Pyrenees (Delmas et al., Acaso, E. 1983. Estudio del Cuaternario en el Macizo Central de Gredos. Tesis Doctoral, Facultad de Ciencias. Universidad de Alcalá de Henares. (Inédita), 2008). At a broader scale, the glacial chronology of Gredos clearly 442 pp. falls into the Last Glacial Maximum (26.5 ka to 20e19 ka) as defined Akçar, N., Yavuz, V., Ivy-Ochs, S., Kubik, P.W., Vardar, M., Schlüchter, C., 2007. recently in the global review (including continental and mountain Paleoglacial records from Kavron Valley, NE Turkey: field and cosmogenic exposure dating evidence. Quaternary International 164e165, 170e183. glaciation, and relative sea level) by Clark et al. (2009). Akçar, N., Yavuz, V., Ivy-Ochs, S., Kubik, P.W., Vardar, M., Schlüchter, C., 2008. A case An extensive search in Gredos Gorge and other similar valleys of for a down wasting mountain glacier during Termination I, Vercenik Valley, NE Sierra de Gredos for older glacial deposits, e.g. as those of Serra da Turkey. Journal of Quaternary Science 23, 273e285. Gerez and Sierra de Queixa in northwest Iberia (Fernández Allen, R., Siegert, M.J., Payne, A.J., 2007. Reconstructing glacier-based climates of LGM Europe and Russia e part 2: a dataset of LGM climates derived from Mosquera et al., 2000; Vidal-Romaní and Fernández-Mosquera, degree-day modelling of palaeo glaciers. Climate of the Past Discussions 3, 2006), the Apennines in Italy (Kotarba et al., 2001) and the 1167e1198. mountains of Greece (Hughes et al., 2006c, 2007) yielded no Arenillas, M., Martínez de Pisón, E., 1976. La morfología glaciar de la Serrota (Ávila). Boletín de la Real Sociedad Geográfica 112 (1), 21e34. results. It is possible that the LGM glaciers in Sierra de Gredos were Björck, S., Walker, M.J.C., Cwynar, L.C., Johnsen, S.J., Knudsen, K.L., Lowe, J.J., larger than those of previous advances and overrode their evidence. Wolhfarth, B., INTIMATE members, 1998. An event stratigraphy for the last On the other hand, whether the glaciers remained at the headwall termination in the North Atlantic region based on the Greenland ice-core record: a proposal by the INTIMATE group. Journal of Quaternary Science 13, 283e292. of Gredos Gorge or re-advanced during the Younger Dryas, is not Brugger, K.A., 2007. Cosmogenic 10Be and 36Cl ages from late Pleistocene terminal evident in the geomorphic record and is difficult to elucidate at this moraine complexes in the Taylor river drainage basin, central Colorado, USA. point. Quaternary Science Reviews 26, 494e499. Calvet, M., 2004. The Quaternary Glaciation of the Pyrenees. In: Ehlers, J., In summary, the exposure ages dates presented in this paper are Gibbard, P.L. (Eds.), Quaternary Glaciations e Extent and Chronology. Part I: in clear agreement with those from other mountains of Europe, Europe. Elsevier B.V., Amsterdam, pp. 119e128. especially with cosmogenic surface exposure ages, and fit within Carandell, J., 1924. La topografía glaciar del macizo del Trampal-Calvitero (Béjar), vol. 45. Boletín del Instituto Geológico y Minero de España. 75e96. the growing body of evidence of a global Last Glacial Maximum at Clark, P.U., Dyke, A.S., Shakun, J.D., Carlson, A.E., Clark, J., Wohlfarth, B., 26.5e19 ka. The difference between these results and those from Mitrovica, J.X., Hostetler, S.W., McCabe, A.M., 2009. The Last Glacial Maximum. earlier studies carried out in the Pyrenees and the northeast of the Science 325, 710e714. Iberian Peninsula is probably due to problems with the dating Cowton, T., Hughes, P.D., Gibbard, P.L., 2009. Palaeoglaciation of Parque Natural Lago de Sanabria, Northwest Iberia. Geomorphology 108, 282e291. techniques or to local factors which favoured larger advances prior Delmas, M., Gunnell, Y., Braucher, R., Calvet, M., Bourlès, D., 2008. 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