A multiple dating-method approach applied to the Sanabria Lake moraine complex (NW Iberian Peninsula, SW Europe)

Quaternary Science Reviews 83 (2014) 1e10

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A multiple dating-method approach applied to the Sanabria Lake moraine complex (NW Iberian Peninsula, SW Europe) Laura Rodríguez-Rodríguez a, *, Montserrat Jiménez-Sánchez a, María José Domínguez-Cuesta a, Vincent Rinterknecht b, Raimon Pallàs c, Didier Bourlès d, Blas Valero-Garcés e a

Departamento de Geología, Universidad de Oviedo, Arias de Velasco s/n, 33005 Oviedo, Spain Department of Earth and Environmental Sciences, University of St Andrews, Fife KY16 9AL, UK Departament de Geodinàmica i Geofísica, Universitat de Barcelona, 08028 Barcelona, Spain d Aix Marseille Université, CNRS-IRD-Collège de France, UM34 CEREGE, 13545 Aix-en-Provence, France e IPE-CSIC, Avda. Montañana 1005, 50059 Zaragoza, Spain b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 May 2013 Received in revised form 15 October 2013 Accepted 17 October 2013 Available online 16 November 2013

New evidence in the NW region of the Iberian Peninsula (w42
N 6
W) of a glacial advance coeval with the global Last Glacial Maximum (LGM) of the Marine Isotope Stage 2 has been identified through a dataset of exposure ages based on 23 10Be concentration measurements carried out on boulder samples taken from a set of latero-frontal moraines. Results span the interval 19.2e15.4 10Be ka, matching the last deglaciation period when Iberia experienced the coldest and driest conditions of the last 25 ka, and are consistent with Lateglacial chronologies established in other mountain regions from SW Europe. The extent of the LGM stade identified in this work is similar to the local maximum ice extent stade recorded and dated as prior to 33 ka using radiocarbon and optically stimulated luminescence. This work showcases how multiple-dating approaches and detailed geomorphological mapping are required to reconstruct realistic palaeoglacier evolution models. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Cosmogenic dating Glacial geomorphology Ice cap Last Glacial Maximum Lateglacial Sanabria Iberian Peninsula

1. Introduction Evidence for asynchronism between the maximum advances of mountain glaciers and continental ice sheets is reported worldwide, suggesting that the dynamics of both ice systems responded differently to rapid changes in temperature and/or moisture supply (e.g. Gillespie and Molnar, 1995; Florineth and Schlüchter, 2000; Zreda et al., 2011). Ice sheets grew to their maximum position between 33 and 26.5 ka in response to climatic forcing from decreases in summer insolation, tropical Pacific sea surface temperatures and atmospheric CO2 levels, and nearly all were at their Last Glacial Maximum (LGM) positions from 26.5 to 19e20 ka corresponding to minima in these forcings (Clark et al., 2009). The latter implies a longer time interval for the LGM episode than previous estimations based on the marine isotope and global sea

* Corresponding author. Tel.: þ34 985102936. E-mail address: [email protected] (L. Rodríguez-Rodríguez). 0277-3791/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.10.019

level records (18 14C ka BP or 21 ka cal BP; Ehlers and Gibbard, 2007). Regional differences in maximum ice extent (Würmian MIE) and timing between mountain regions and the asynchrony with the global LGM hold significant information on cryosphere dynamics and palaeoclimatic evolution during the Last Glacial cycle. In the mountain regions of southern Europe, two chronological scenarios were proposed (Hughes and Woodward, 2008): (1) a local glacial maximum several thousands of years earlier than the LGM of Marine Isotope Stage 2 (MIS 2) based on evidence from northern Iberian Peninsula, Italian Apennines and Greece dated with radiocarbon, Uranium series, and optically stimulated luminescence (OSL) techniques, and (2) a local glacial maximum close or coeval with the global LGM based on evidence from central Iberian Peninsula, Pyrenees, Maritime Alps and Turkey dated through terrestrial cosmogenic nuclides (TCN). Such contrasting scenarios could be related to regional climate variability, but also to limitations or biases of the applied dating methods, as TCN methods consistently provided the youngest ages and radiocarbon and OSL the oldest ones (Hughes and Woodward, 2008).

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In the Iberian Peninsula mountains (Fig. 1), the current knowledge about the extent, timing and number of glacial stades during the Last Glacial cycle (ca last 120 ka) has been recently reviewed (Calvet et al., 2011; Jiménez-Sánchez et al., 2013) and is summarized as follows: (1) in the Pyrenees the Würmian MIE occurred between 97 and 36 ka depending on the valleys considered (Lewis et al., 2009; Pallàs et al., 2010; García-Ruiz et al., 2013), in the Cantabrian Mountains it was prior to 38 ka (Jiménez-Sánchez and Farias, 2002; Jalut et al., 2010; Moreno et al., 2010; Serrano et al., 2012, 2013), and in the Sistema Central occurred between 33 and 26 ka (Vieira, 2008; Palacios et al., 2010, 2012; Carrasco et al., 2013); (2) a new glacial advance took place during the LGM recording a glacial advance similar in extent to previous local MIE in the eastern

end of the southern Pyrenees (23e21 ka) (Pallàs et al., 2006, 2010; Delmas et al., 2008) and in the Sistema Central (22e19 ka) (Palacios et al., 2010, 2012; Carrasco et al., 2013) while shorter glacial advances were recorded in the northern Pyrenees (20e18 ka) (Delmas et al., 2011), the western end of the southern Pyrenees (García-Ruiz et al., 2003; Lewis et al., 2009), and the Cantabrian Mountains (almost ice-free conditions in some valleys by 20 ka) (JiménezSánchez and Farias, 2002); (3) frontal moraines coeval with the Oldest Dryas have been dated in the eastern Pyrenees and in the Sistema Central (Pallàs et al., 2006, 2010; Delmas et al., 2009, 2011; Palacios et al., 2010, 2012). In spite of all these new datasets, it remains unclear to what extent the differences in magnitude between local Würmian MIE and LGM stades might result from biases

Fig. 1. (a) Setting (1-Pyrenees, 2-Cantabrian Mts., 3-Sistema Central, 4-Trevinca Massif). (b) Geomorphological map of the Trevinca Massif, including published ages: radiocarbon data (1e5) (see also Table 1) and optically stimulated luminescence data (OSL) from Pias site (6e8; Pérez-Alberti et al., 2011). The rectangle indicates Fig. 2 location. (c) Ice cap reconstruction for the local maximum ice extent (MIE).

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introduced by regional climate patterns, the dating methods, or even from uncertainties in the interpretation of the feature being dated. The later can be particularly significant for glacial sequences composed of till deposits arranged as sets of frontal moraines, since depending on their preservation they can be interpreted as resulting from: (i) a single glaciation or episode of glacial advance and retreat with deposition of recessional moraines close in age, or (ii) more than one glaciation with superimposed glacial records. In the last case the moraines forming the frontal moraine complex would be very different in age. The Sanabria Lake moraine complex provides a unique glacier setting in the Iberian Peninsula to test these hypotheses. The occurrence of a well-preserved glacial sequence that includes a set of recessional moraines with related glaciolacustrine successions, allowed the combination of geomorphological techniques with several dating methods to establish the significance of the whole moraine complex. The aims of this paper are 1) to constrain the timing and extent of glaciers during the Last Glacial cycle by combining new moraine 10Be surface exposure ages with the preexisting 14C and OSL datasets, 2) to discuss the relevance of the new local chronology, by comparing it with other palaeoclimate records of Iberia and SW Europe. 2. Regional setting The Sanabria Lake area in NW Iberian Peninsula is located in the east side of the Trevinca Massif, a mid-latitude mountain range which is free of ice at present (Fig. 1). The massif highlands are characterized by a smooth topographic plateau reaching an altitude of 2128 m. The northern rim of the plateau is cut by north-facing

3

glacial cirques, which connect with Alpine glacial valleys up to 7 km-long. To the south the plateau decreases in height and slope progressively to c. 1600 m, where two main troughs, the Bibei and Tera valleys are incised. These glacial valleys are 26 km and 23.5 km-long, and drain W and E respectively. Additionally, the plateau area is drained by minor glacial valleys arranged in a radial pattern (Fig. 1). An ice cap covered the plateau during the local MIE, lowering the equilibrium line altitude to 1687 m in the Tera outlet (Cowton et al., 2009). Moraine deposits can be divided into two groups according to their distribution: (i) moraine complexes below 1600 m marking the terminal zones of the glacial valleys, and (ii) cirque moraines at altitudes above 1700 m. We focus on an area of 45 km2 in the terminal zone of the former Tera glacial outlet, where a moraine complex and ice-related deposits are particularly wellpreserved around the Sanabria Lake (Figs. 1 and 2). It includes: (i) a system of lateral moraines longer than 6 km that connects to the front with undifferentiated tills delineating the local MIE and (ii) remains of at least nine frontal moraines spreading over a distance of 2 km. Directly up-valley from this moraine complex, a 9 m-long sediment core retrieved from the deepest part of the Sanabria Lake shows a 1.8 m-thick basal unit with massive sands to banded silts and clays dated by 14C AMS between 25.6  0.4 and 14.5  0.3 cal yr BP (Rodríguez-Rodríguez et al., 2011). Considering a proglacial origin for these facies, the frontal recessional moraine enclosing the lake must be older than the basal age of the lake and consequently, some of the moraines could represent different positions of the glacier front during the post-MIE glacial retreat prior to 25.6 ka. A > 12 m-long core from the San Martín ice-dammed deposit located out with the outermost left lateral moraine and

Fig. 2. Geomorphological map of the Sanabria Lake area showing the location of the sampled boulders and the minimum exposure ages (10Be) reported in this work, expressed in ka with the analytical (1s) and external uncertainty for age comparisons with other datasets. Dates used to calculate the minimum ages for each moraine are represented in bold.

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interpreted as synchronous or subsequent to the local MIE, gives a minimum age of 21.8  0.4 cal yr BP (Rodríguez-Rodríguez et al., 2011). Published radiocarbon ages obtained from other cores from small ponds in the eastern side of the Trevinca Massif also constrain minimum ages for local glacier retreat (Fig. 2): (i) 15.7  0.4 cal yr BP at Laguna de La Roya (Allen et al., 1996; MuñozSobrino et al., 2013); (ii) 18.1  0.4 cal yr BP at Laguna de las Sanguijuelas (Muñoz-Sobrino et al., 2004) and (iii) 14.2  0.3 cal yr BP at Lleguna (Allen et al., 1996). Additional time constraints is provided by Pias site, in the western side of the Trevinca Massif, where a sedimentary sequence composed of fine-grained lacustrine deposits lying on poorly sorted sandy gravels interstratified with massive diamicton layers is well-preserved at the junction between the Bibei and Barxacoba glacial valleys. Three quartz samples retrieved from glacio-fluvial and glacio-lacustrine sand units less than 1-m thick were analyzed with OSL and yielded minimum ages of 27  2 ka, 31  3 ka, and 33  3 ka for the regional MIE (PérezAlberti et al., 2011). Based on this information, we identify two groups of chronological data post-dating the local MIE in the Trevinca Massif: (i) previous to the LGM (OSL dates) and (ii) synchronous or younger than the LGM (radiocarbon dates, Table 1). 3. Methodology A geomorphological map (at a 1:50.000 scale) was produced through photointerpretation and was used to reconstruct the MIE of local glaciers in the whole massif by using an ArcGIS database (v. 9.2) and the spreadsheet Profiler v.2 (Benn and Hulton, 2010). Ice profiles were numerically modeled along 31 profiles disposed radially from the ice cap margin to its source area along its outlet glaciers. The TCN analysis was conducted in the Sanabria Lake moraine complex, where the outermost lateral moraines were presumably formed during the local Würmian MIE stade, while the inset ones recorded subsequent post-MIE ice front locations (Figs. 1 and 2). We selected 23 samples to investigate the minimum surface exposure age for the end of the Würmian MIE stade and for three other subsequent glacial fronts. The sampling selection was designed considering the relative chronology of the moraine complex to cover as complete as possible the history of the Pleistocene glaciations in the Tera Valley (eastern side of the Trevinca Massif). A total number of five moraines were sampled, two of them (TER and MAR) corresponding to the left lateral moraine which marks the local MIE (a total of eight boulders) and the others (SAN, TET and SAU) corresponding to three subsequent glacial fronts (five boulders each one). The shielding effect of till or snow cover, the underexposure of moraine boulders and the tilting of boulder surfaces was minimized by choosing only those boulders lying at the top of the crest and showing the largest size (volume greater than a cubic

meter or >1 m3), a well anchored base (but protruding at least 1 m from the moraine surface), and a flat surface (table-type). The crystalline character of the bedrock in the Tera catchment, which is mainly composed of augen gneiss with quartz-rich streaks and granodiorites with quartz-rich veins up to 6 cm-thick, allowed getting samples very riched in quartz directly in the field. Shallow samples were taken manually with hammer and chisel. Angular elevation of the horizon was measured at each sampling site with a clinometer and the topographic shielding factors was calculated using the CRONUS-Earth calculator (version 1.1.) following Balco et al. (2008). The sample treatment was performed in the Laboratori de Cosmonúclids Terrestres de la Universitat de Barcelona (Spain). Samples were crushed and sieved to obtain the 0.25e1 mm grain fraction. Other mineral phases except quartz were removed from the samples by magnetic separation (dark minerals), froth floatation (mainly feldspars), and repeated leaching (Kohl and Nishiizumi, 1992). Aluminum content was analyzed through ICP-OES on sample aliquots at the Centre Científic i Tecnològic de la Universitat de Barcelona (CCiTUB). The aluminum content in all samples is <250 ppm to ensure an efficient separation of Be and Al during column chromatography. Muscovite grains were removed through sample shaking with clean paper sheets and 3 extra acid-batches in order to reduce the Al-content in samples SAU01, MAR03 and SAN05. For each sample between 14 and 23 g of clean quartz grains were spiked with 200 mg of 9Be carrier and completely digested in 48% HF. The carrier used was a commercial standard solution 1000 mg/l of beryllium oxide in hydrochloric acid 2% from the Scharlau Company (1.02 g/cm3). Beryllium fraction of each sample was extracted through column chromatography (Ditchburn and Whitehead, 1994). All the 10Be concentration measurements were performed at the ASTER AMS facility in Aix-en-Provence, France. The measured 10 Be/9Be ratios were corrected for lab procedural blanks and calibrated with the NIST_27900 beryllium standard (Reference Material 4325, assigned value of 2.79  0.03  1011) and using a 10Be half-life of 1.36  0.07  106 years (Nishiizumi et al., 2007). Analytical uncertainties are reported as 1s and include uncertainties associated with AMS counting statistics, standard uncertainty (certification), and chemical blank measurements. 10Be concentration in quartz samples (Tables 2e4) was calculated following Balco (2006), while the surface exposure ages were calculated with the Cronus online calculator v. 2.2.1 assuming no erosion (Balco et al., 2008; Balco, 2010). Maximum erosion rates were estimated by calculating the ratio between the height of protruding minerals from boulder surfaces and the corresponding exposure ages deduced for no erosion. Correcting exposure ages using the maximum erosion rate of 6.66  105 cm yr1 yielded

Table 1 Published radiocarbon data for the Sanabria Lake region; calibrations were done using the CalPal Software and the CapPal 2007 Hulu Curve (Weninger et al., 2007; Weninger and Jöris, 2008). Site

Latitude DD Longitude DD Elevation (m) Lab reference Material

Depth (cm) Dating method

1) Laguna La Roya 2) Lleguna

42.1

6.46

1608

12618

Bryophytes

770e774

42.1

6.42

1050

GrN-22750

3) Laguna 42.1 Sanguijuelas 4) Sanabria 42.1 Lake 42.1

6.41

1080

GrN-22759

6.42

1000

Poz-20095

Bulk sediment 767.5e770 Decay (peat) counting Bulk sediment 245e250 Decay (gyttja) counting 14 C AMS Plant remains 728

6.42

1000

Poz-12367

Bulk sediment 891

14

C AMS

5) San Martín

6.43

1203

Poz-30103

Peat

14

C AMS

42.1

1097

14

C AMS

14

C age yr BP Reference

12940  60

Allen et al., 1996

12130  130 Muñoz-Sobrino et al., 2004 14780  190 Muñoz-Sobrino et al., 2004 12330  60 Rodríguez-Rodríguez et al., 2011 21460  140 Rodríguez-Rodríguez et al., 2011 18090  90 Rodríguez-Rodríguez et al., 2011

14

C cal age (1s) yr BP

15740  400 14170  285 18060  360 14495  345 25585  375 21835  360

Table 2 Field and laboratory data used to calculate the

10

Be surface exposure age in the Sanabria Sector, eastern side of the Trevinca Massif.

Rock type

Cover Erosion rate [Be-10]d Latitudea (DD) Longitudea (DD) Elevationb (m) Elev/Pressure Thickness Density Shielding þ/ (atoms g1) Name of Be-10 flag (cm) (g cm3) correctionc correction (cm yr1) standarization (atoms g1)

TER01 TER02 TER03 TER04 TER05 TET01 TET02 TET03 TET04 TET05 SAU01 SAU02 SAU03 SAU04 SAU05 SAN01 SAN02 SAN03 SAN04 SAN05 MAR01 MAR02 MAR03

Quartz vein Quartz vein Quartz vein Quartz vein Quartz vein Quartz vein Quartz vein Quartz vein Quartz vein Quartz vein Quartz vein Granodiorite Granodiorite Granodiorite Granodiorite Augen gneiss Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite

42.15304 42.1537 42.15448 42.15538 42.15906 42.15962 42.1598 42.15983 42.16129 42.16193 42.12219 42.12333 42.12333 42.12333 42.12372 42.12142 42.12142 42.12113 42.12075 42.12075 42.13399 42.13382 42.13357

a

Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine Moraine

boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder boulder

6.74708 6.7471 6.74726 6.74741 6.74002 6.74601 6.74568 6.74397 6.73841 6.73798 6.,69877 6.69843 6.69843 6.69843 6.69843 6.68438 6.68438 6.68449 6.68442 6.68442 6.7345 6.73386 6.73337

1653 1661 1666 1681 1731 1673 1676 1693 1698 1691 1006 1008 1008 1008 1009 1002 1002 1002 1001 1001 1389 1384 1382

Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std Std

5 2.8 2.5 3 1.8 2.6 3 3 4 3 3 4 1.7 3.6 4 3.5 1.5 2.3 2.2 1.7 4 2 2

2.65 2.65 2.65 2.65 2.65 2.65 2.65 2.65 2.65 2.65 2.65 2.67 2.7 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67

1 1 1 1 1 0.99757 0.99750 0.99869 1 1 0.99946 0.99946 0.99946 0.99946 0.99946 0.99959 0.9959 0.99959 0.99959 0.99959 0.99974 0.99974 0.99974

None None None None None None None None None None None None None None None None None None None None None None None

0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666 0.0000666

261406.1 310390.5 296851.1 299379.7 326600.1 286473.0 275653. 2 280007.2 273782.0 249016.1 156669.1 153478.8 150479.5 151435.6 146346.7 166827.7 173119.3 154821.1 178669.2 172478.2 218458.1 233471.1 254194.0

9972.2 12543.1 10096.3 9868.1 10739.9 11192.0 9380.7 9196.8 9717.8 8599.6 6065.7 5823.6 5390.6 5829.9 5625.2 6191.5 6904.4 9494.3 6395.7 7462.1 8685.6 11816.8 11722.5

NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900 NIST_27900

With reference to WGS84 datum. Uncertainty 5 m. c Assuming exponent x ¼ 2.3 in the cosmic ray flux equation: cos^x (zenith angle). d Blank values used (ratio10Be/9Be in at/at): 3.52546E-15 for samples TER01, TER03, TET01, TET02, TET05, SAU02, SAU03, SAU04, SAU05, SAN01, SAN03, and SAN04; 4.88089E-16 for samples TER02, TER04, TER05, TET03, TET04, SAU01, SAN02, SAN05, MAR01, MAR02, and MAR03.

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Sample Sample type name

b

5

6

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Table 3 10 Be exposure ages for moraine boulders from the Tera catchment for constant production rate model. Output from the CRONUS-Earth online calculator (Balco et al., 2008). Sample name

Exposure ages e constant production rate model:

Results not dependent on spallogenic production rate model:

Scaling scheme for spallation: Lal (1991)/Stone (2000)

TER01 TER02 TER03 TER04 TER05 TET01 TET02 TET03 TET04 TET05 SAU01 SAU02 SAU03 SAU04 SAU05 SAN01 SAN02 SAN03 SAN04 SAN05 MAR01 MAR02 MAR03

Thickness scaling factor

Shielding factor

Production rate (muons) (atoms/g/yr)

Internal uncertainty (yr)

Exposure age (yr)

External uncertainty (yr)

Production rate (spallation) (atoms/g/yr)

0.9597 0.9772 0.9796 0.9756 0.9852 0.9788 0.9756 0.9756 0.9676 0.9756 0.9756 0.9674 0.9859 0.9706 0.9674 0.9714 0.9876 0.9811 0.9819 0.9859 0.9674 0.9835 0.9835

1 1 1 1 1 0.9976 0.9975 0.9987 1 1 0.9995 0.9995 0.9995 0.9995 0.9995 0.9996 0.9996 0.9996 0.9996 0.9996 0.9997 0.9997 0.9997

0.307 0.309 0.31 0.311 0.317 0.311 0.311 0.312 0.312 0.312 0.25 0.25 0.251 0.25 0.25 0.25 0.251 0.25 0.25 0.251 0.283 0.284 0.284

635 781 624 606 630 690 579 560 594 523 620 599 544 597 578 637 699 967 652 758 667 897 891

16566 19224 18270 18300 19071 17594 16946 16983 16657 15098 15940 15719 15125 15459 14974 17102 17462 15712 18143 17440 16705 17635 19237

1578 1851 1712 1708 1780 1683 1588 1584 1569 1417 1522 1496 1426 1474 1428 1622 1676 1677 1712 1700 1602 1781 1901

15.54 15.91 16.01 16.12 16.89 16.04 16.03 16.25 16.19 16.24 9.62 9.55 9.74 9.58 9.56 9.55 9.71 9.64 9.64 9.68 12.85 13.01 12.99

Table 4 10 Be exposure ages for moraine boulders from the Tera catchment for time-varying production models. Output from the CRONUS-Earth online calculator (Balco et al., 2008). Sample name

Exposure ages e time-varying production models Scaling scheme for spallation: Lifton and others e 2005

Time-dependent

Exposure age (yr)

External uncertainty (yr)

Dunai e 2001 Exposure age (yr)

External uncertainty (yr)

Exposure age (yr)

External uncertainty (yr)

Exposure age (yr)

External uncertainty (yr)

17,016 19,610 18,671 18,687 19,393 18,002 17,367 17,389 17,067 15,552 16,878 16,654 16,053 16,391 15,900 18,055 18,421 16,651 19,111 18,399 17,364 18,290 19,883

2118 2456 2302 2299 2385 2246 2141 2138 2111 1918 2103 2071 1986 2041 1980 2242 2304 2222 2366 2322 2170 2358 2530

17,260 19,832 18,903 18,919 19,619 18,240 17,610 17,632 17,313 15,812 17,087 16,865 16,268 16,603 16,116 18,259 18,623 16,861 19,309 18,601 17,592 18,512 20,093

2140 2474 2321 2318 2403 2267 2162 2159 2132 1942 2121 2089 2005 2060 1999 2258 2319 2242 2381 2338 2190 2377 2547

16,709 19,236 18,320 18,333 19,016 17,668 17,049 17,069 16,754 15,278 16,644 16,424 15,836 16,167 15,685 17,797 18,154 16,422 18,831 18,133 17,076 17,980 19,537

1773 2059 1919 1914 1985 1881 1785 1781 1763 1602 1770 1742 1667 1718 1666 1883 1939 1914 1984 1961 1823 2001 2136

16,439 18,996 18,075 18,102 18,839 17,424 16,801 16,835 16,522 15,021 15,907 15,692 15,113 15,439 14,966 17,035 17,385 15,686 18,046 17,364 16,602 17,499 19,049

1527 1784 1650 1646 1712 1626 1534 1529 1517 1373 1481 1456 1389 1436 1392 1576 1628 1641 1660 1652 1554 1728 1839

Desilets and others

TER01 TER02 TER03 TER04 TER05 TET01 TET02 TET03 TET04 TET05 SAU01 SAU02 SAU03 SAU04 SAU05 SAN01 SAN02 SAN03 SAN04 SAN05 MAR01 MAR02 MAR03

exposure ages older by only w 200 years. Boulder ages discussed in this work were not corrected for erosion nor snow/vegetation cover to ensure that the reported ages are treated as minimum exposure ages. Moraine ages were obtained as the error-weighted mean of the exposure ages calculated for its boulders in the constant production rate model. Only the oldest boulders of each moraine, whose exposure ages are overlapping at 1s, were used to derive the error-weighted mean moraine ages (Fig. 3): (i) TER-02, TER-05, MAR-03 for the TER-MAR moraine; (ii) SAN-02, SAN-04, SAN-05 for

the SAN moraine; (iii) TET-01, TET-02, TET-03 for the TET moraine; and (iv) SAU-01, SAU-02 SAU- 04 for the SAU moraine. This TCN chronology is compared with pre-existing OSL and 14C datasets in the discussion. 4. Results The geomorphological map (Fig. 1) summarizes the main glacial evidence on the Trevinca Massif and its relative chronology,

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Fig. 3. 10Be exposure ages calculated with online CRONUS calculator (Balco et al., 2008; v. 2.2.1 updated in Balco, 2010). Samples are arranged from left to right according to distance from cirque headwall or ice dome (case of TET moraine). Black dots indicate which samples were used to derive moraine minimum exposure ages, while error bars indicate the analytical uncertainty (1s) for each sample. Boulder ages are tightly grouped within each moraine.

allowing the reconstruction of the ice cap dimensions. Ice fronts reached altitudes between 1580 and 950 m, 1510e1110 m, 1290e 980 m, and 1170e1060 m in the East, North, West and South sides of the massif, respectively. The lowest altitudes correspond to the

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Tera and Bibei valleys, where the glacial front fluctuations left remarkable sets of moraines. According to the ice surface model, glaciers extended 475 km2 over the massif during the local MIE, reaching up to 200 m of ice thickness on the plateau and up to 450 m in the glacial valleys. The Sanabria Lake moraine complex indicates that the glacier outlet that flowed along the Tera valley was 23.5 km long during the local MIE and recorded a total length reduction of 3 km by the time of the SAU moraine deposition. The width of this glacier tongue, measured at the eastern part of the Sanabria Lake basin, changed from 3.7 km to 1.7 km between the local MIE and the SAU stade, while the ice thickness reduced ca 130 m according to ice surface estimations made in the Tera Valley using the spreadsheet Profiler v.2 (Benn and Hulton, 2010). A minimum age of 19.2  1.8 10Be ka (n ¼ 3) can be established for the gently sloped left lateral moraine (TER-MAR) (Figs. 2 to 4; Tables 3 and 4). The SAN frontal moraine marks a subsequent glacial still stand or minor readvance no later than 17.7  1.7 10Be ka (n ¼ 3). The minimum age for the TET moraine, located in a small tributary of the main Tera Valley, suggests the lack of connection between glacier streams of both valleys by 17.2  1.6 10Be ka ago (n ¼ 3). Meanwhile, the overall retreating Tera ice front located downwards from the Sanabria over-deepened depression (today occupied by the Sanabria Lake) deposited another five frontal moraines. We report a minimum age of 15.7  1.5 10Be ka (n ¼ 3) for the SAU moraine, which dams modern day Sanabria Lake.

Fig. 4. Examples of moraines sampled for the TCN analysis: (a) panorama of the TER moraine looking northwards, with TER01 in the foreground and the Tera valley in the background; (b) panorama of the MAR sample sites including one of the ice-dammed deposits formed behind the Sanabria Lake north lateral moraine; and (c) TET moraine panorama looking northeastward; including in the foreground the boulder where sample TET04 was collected. Boulder age uncertainties include only the analytical uncertainty.

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The lack of geomorphological features indicating the location of the ice margins at higher elevations than the TET moraine does not allow establishing a detailed paleogeographic model for each subsequent deglaciation stade. 5. Discussion The minimum 10Be ages cover the time interval 19.2  1.8e 15.7  1.5 10Be ka (uncertainties include those associated with the analytical procedures and the production rate) and are consistent with the relative chronology derived from the geomorphological sequence (Fig. 2). Combining the new 10Be dataset with published 14 C AMS and OSL results from the whole massif, we have identified two glacial advances very close in extent. The oldest one is recorded by glacio-fluvial sediments at Pias site (western Trevinca Massif) which yielded OSL ages w27e33 ka (Pérez-Alberti et al., 2011), suggesting the occurrence of a local MIE earlier than MIS 2. The youngest one is supported by our TCN results. According to the new set of minimum 10Be ages, the whole moraine sequence was developed between 19.2 and 15.7 ka (Fig. 2). The outermost TERMAR lateral moraine yields a minimum age of 19.2  1.8 10Be ka for the glacier retreat; the minimum age for the sedimentation onset in the San Martín valley (as a consequence of the moraine runoff blockage) was radiocarbon-dated as 21.8  0.4 cal ka BP (Rodríguez-Rodríguez et al., 2011). Both minimum ages are consistent with a second glacial advance during the LGM of MIS 2 very close in extent to the previous MIE. The SAN moraine, located in a retreated position compared with the TER-MAR moraine, was dated at 17.7  1.7 10Be ka, which is consistent with the minimum 14 C AMS age of 18.1  0.4 cal ka BP obtained from Laguna de las Sanguijuelas located between both moraines (Muñoz-Sobrino et al., 2004) (Fig. 2). The TET moraine was deposited shortly after, at a minimum age of 17.2  1.6 10Be ka, marking the separation between the main Tera glacier and its eastern tributary. Between 17.7 and 15.7 10Be ka, the retreating ice front built up five frontal moraines. The SAU moraine damming the Sanabria Lake represents the youngest frontal moraine of this sequence and yields a minimum exposure age of 15.7  1.5 10Be ka. The consistence between ages obtained from the boulders in each moraine and published radiocarbon data suggests that the 10Be results are not significantly affected by moraine post-depositional erosion in this area. The 14C AMS age of 15.7  0.4 cal yr BP obtained at the base of the lacustrine deposit known as Laguna La Roya, located at 1608 m altitude next to the eastern plateau edge, suggests that at least the southern part of the plateau could have been partly iceefree by the time of the SAU moraine deposition (Allen et al., 1996; MuñozSobrino et al., 2013; Fig. 2). This hypothesis is coherent with an ELA rising of about 100e150 m compared to the local MIE, which would have caused a drastic reduction (by more than 80%) in the extent of the accumulation zone in the southern part of the plateau. Therefore, the ice flow inputs from the SegunderaeCárdena valley would have been reduced and the ice source area would have been located in the northern part of the Tera catchment where the plateau records its highest elevation (Fig. 1). The basal 14C AMS ages obtained at the Lleguna core (14.2  0.3 cal yr BP; Allen et al., 1996) and at the top of the basal detrital unit of the Sanabria Lake core (14.5  0.3 cal yr BP; Rodríguez-Rodríguez et al., 2011) would represent the timing of glacial front retreat from the eastern part of the Sanabria Lake. We note that the new 10Be ages and the published 14C AMS dataset are generally consistent, but one question remains concerning the disparity between the minimum ages obtained for the base of the Sanabria Lake sequence (25.6 ka cal BP), the San Martín ice dammed deposit (21.8 ka cal BP) (Rodríguez-Rodríguez et al., 2011) and the TER-MAR lateral moraine (19.2 10Be ka) reported in this work. The Sanabria Lake sequence is considerably older than

the TER-MAR moraine and its associated ice dammed deposit. A possible explanation is a local LGM stade (prior to 21.8 ka) advancing over the Sanabria Lake basin and eroding the upper part of the sedimentary sequence related to the previous Würmian MIE retreat (w33 ka). Although ice-push structures (folding, faults) were not found in the basal unit of the Sanabria core, the occurrence of iron oxide-rich intervals could suggest a sedimentary unconformity, otherwise difficult to identify in the massive sandy levels. Alternatively, basal ages of the Sanabria Lake and San Martin sequences (derived from bulk sediment samples) could be older than the enclosing sediments if any reworking had happened or if considerable reservoir effects had taken place. However, the consistence between the 14C AMS data obtained from these sequences and the OSL ages obtained in the Pias site at the western side of the massif suggests that this is not likely to be the case. There is a strong coherence between the Sanabria dataset and other mountain regions in the Iberian Peninsula. The Würmian MIE stade observed in the Sanabria area (prior to 33 ka) correlates with a similar stade documented in the Sistema Central w200 km southwards (33e26 ka) (Vieira, 2008; Palacios et al., 2010, 2012; Carrasco et al., 2013; Domínguez-Villar et al., 2013), but is younger than the one recorded in the Cantabrian Mountains w100 km northwards (prior to 38 ka) (Jiménez-Sánchez and Farias, 2002; Jalut et al., 2010; Moreno et al., 2010; Serrano et al., 2012, 2013) and in the Pyrenees w450 km northeastwards (97e36 ka) (Lewis et al., 2009; Pallàs et al., 2010; García-Ruiz et al., 2013). According to multi-proxy reconstructions based on Iberian lacustrine and marine records, cold and relatively wet conditions during the MIS 5 to MIS 4 transition were responsible for the MIE of glaciers, at least, in northern Iberia; a shift towards greater aridity during MIS 4 and MIS 3 would have been responsible for subsequent deglaciation (Moreno et al., 2012). The mountain glacial advance during the global LGM has also been recorded differently across Iberia, reaching quasi-MIE positions not only in Sanabria, but also in the eastern part of the southern Pyrenees (Pallàs et al., 2006, 2010; Delmas et al., 2008) or the Sistema Central (Palacios et al., 2010, 2012; Carrasco et al., 2013). In other areas, like the Cantabrian Mountains or the northern and southwestern Pyrenees, the glacial advance was clearly less extensive (e.g. Delmas et al., 2011; Jiménez-Sánchez et al., 2013). Subsequent glacial retreat in Sanabria started no later than 19.2 10Be ka and recorded several glacial front stabilizations until 15.7 10Be ka. This glacier evolution is consistent with palaeoclimate reconstructions based on speleothem and lacustrine sequences from the Iberian Peninsula which indicate that the coldest and driest interval of the last 25 ka took place between 18.2 and 15.4 ka (Moreno et al., 2012) during the Lateglacial, coinciding with the Mystery Interval, a stadial that occurred during the earliest phase of the last deglaciation (17.5e 14.5 cal ka BP) coevally with low boreal summer and high austral summer insolation and with low temperatures in Greenland (HE1) (Denton et al., 2005, 2006). The Sanabria dataset is also coherent with glacial evidence from other mid-latitude mountains across SW Europe. In the Alps, the Late Würmian glaciation reached the Alpine lowlands between 30 ka and 21 ka and was followed by a series of glacial front stabilizations between 18 and 17 ka to 14.7 ka and 11.3 to 9.7 ka (IvyOchs et al., 2008; Darnault et al., 2012 and references therein). In the Tatra Mountains (western Carpathians), three trimlines have been identified and constrained with cosmogenic isotope 36Cl revealing that a post-LGM glacial retreat started no later than 21.5 ka and subsequent glacial advances occurred at w17 and 12 ka (Makos et al., 2013). Minimum moraine ages based on UeTh analysis, carried out in secondary calcite cements, and thermo luminescence (TL) analysis obtained from glacio-fluvial sediments in the Pindus Mountains supported an LGM advance of glaciers

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considerably less extensive than other glacial advances associated to previous cold stages (MIS 6 and MIS 12) in northwestern Greece (Woodward et al., 1995, 2008; Hughes et al., 2006). Similar UeTh studies in the Montenegro Mountains (Dinaric Alps) also revealed a local LGM advance considerably less extensive than previous glaciations, followed by several glacial front stabilizations at 17, 13 and 12 ka (Hughes et al., 2010). Finally, cosmogenic isotope datasets based on 36Cl from different mountain areas across Turkey support an LGM stade between 26 and 20.3 ka, followed by several glacial advances during the Lateglacial (Akçar et al., 2007; Sarikaya et al., 2009; Zahno et al., 2010). In summary the available dates from the Sanabria Lake glacier record support: (i) at least two glacial advances close in extent (a double local MIE) during the Last Glacial cycle, the first one prior to 33 ka and the second one during the LGM of MIS 2; and (ii) a sequence of glacial front stabilizations between 19.2 and 15.4 10Be ka that is synchronous with the Lateglacial evolution of other mountain glaciers in southern Europe. Glacier evolution in southern latitudes is closely controlled by moisture availability and, at orbital time scales, to the latitudinal shifts of climate patterns. The North Atlantic Polar Front shift towards the South was more pronounced during MIS 2 than during MIS 4, pushing the prevailing westerly storms more to the south and reducing the precipitation in central and northern Europe accordingly (Florineth and Schlüchter, 2000). The latter would explain a shorter extent of glacial advances in northern Iberian mountain areas, particularly the Cantabrian Mountains and the northwestern Pyrenees. Nevertheless, additional chronological data are needed to test this hypothesis, especially in the Cantabrian Mountains where available chronologies for the Last Glacial cycle are currently based on 14C AMS and OSL. 6. Conclusions New chronological data based on the cosmogenic nuclide 10Be in the Sanabria area supports a sequence of moraines formed between 19.2 and 15.4 10Be ka, interpreted as the result of successive stabilizations during general retreat during the Lateglacial. This record constitutes additional evidence for a glacial episode coeval with the global LGM of MIS 2 in SW Europe. The combination of the geomorphological observations, published 14C and OSL dates, and the new TCN results, suggests a double local MIE, since the local LGM advance might be close in extent compared to the previous Würmian MIE stade (33 ka). Our chronology correlates well with: (i) published TCN chronologies established in other mountain areas of southern Europe; and (ii) palaeoclimatic reconstructions for Iberia, which indicate that the coldest and driest conditions for the last 25 ka took place during the Lateglacial, matching the Mystery Interval. This study demonstrates that the combination of geomorphological surveys and several dating techniques can help to constrain chronologies and develop more accurate palaeoglacial models, particularly in areas with complex moraine systems. Acknowledgments Research funded by Spanish CICYT projects LIMNOCALe CGL2006-13327-C04-01, GRACCIE-CSD2007-0006, TOPOIBERIACSD2006-0041 (Consolider Ingenio Program), RISKNAT2009SGR520, and Fundación Patrimonio Natural CyL. L. RodríguezRodríguez is granted by the Severo Ochoa (FICYTeAsturias) and the FPU (Spanish MECD) programs. The AMS facility ASTER (CEREGE) is supported by the INSU/CNRS, the French MESR, and the CEA institute. We are grateful to M. Arnold, G. Aumaître, and K. Keddarouche for the AMS measurements and to the Administration of the Sanabria Natural Park for the sampling permissions. We thank

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