Palaeoenvironmental and palaeoseismic implications of a 3700-year sedimentary record from proglacial Lake Barrancs (Maladeta Massif, Central Pyrenees, Spain)

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Palaeoenvironmental and palaeoseismic implications of a 3700-year sedimentary record from proglacial Lake Barrancs (Maladeta Massif, Central Pyrenees, Spain) Juan C. Larrasoaña a,⁎, María Ortuño b, Hilary H. Birks c,d, Blas Valero-Garcés e, Josep M. Parés f,1, Ramon Copons g, Lluís Camarero h, Jaume Bordonau b a

Institut de Ciències de la Terra “Jaume Almera”, CSIC, Solé i Sabarís s/n, 08028 Barcelona, Spain RISKNAT Group, Departament de Geodinàmica i Geofisica, Universitat de Barcelona, Martí i Franqués s/n, 08028 Barcelona, Spain Department of Biology, University of Bergen, Allégaten 41, N-5007 Bergen, Norway d Bierknes Centre for Climate Change, Allégaten 41, N-5007 Bergen, Norway e Instituto Pirenaico de Ecología, CSIC, Aptdo. 13034, 50080 Zaragoza, Spain f Department of Geological Sciences, University of Michigan, 2534 CC. Little Building, Ann Arbor, MI 48109-1063, USA g GEORISC S.L., Vidal i Quadras 44, 08017 Barcelona, Spain h Centre d'Estudis Avançats de Blanes, CSIC, Camino de Santa Barbara, 17300 Blanes, Spain b c

a r t i c l e

i n f o

Article history: Received 18 June 2008 Received in revised form 19 March 2009 Accepted 1 April 2009 Available online 9 April 2009 Keywords: Iberian Peninsula Proglacial lakes Environmental magnetism Plant macrofossils Glacier fluctuations Palaeoseismicity

a b s t r a c t A multidisciplinary study including sedimentological, mineral magnetic, and palaeobotanical techniques applied to a sediment core recovered from proglacial Lake Barrancs in the seismically active Maladeta Massif has provided the basis for documenting environmental changes and palaeoseismic activity in the Central Pyrenees for the last ca. 3700 yr. Lake Barrancs is located downstream of the Tempestats and Barrancs cirque glaciers and sedimentation is dominated by clastic input corresponding to seasonal changes in sediment supply. Slow fine particle settling during the winter and sediment-loaded homopycnal flows during the warm season, triggered by snow-melting and glacier outwash, have resulted in deposition of rhythmites composed of clays, silts, and sands. The predominance of finer-grained sediments and the low concentration of relatively finer magnetite grains suggest that glacier activity was very small, if not absent, before ca. A.D. 350. Their replacement by coarser-grained sediments and the overall increased (but highly oscillating) concentrations of relatively coarser magnetite grains in the uppermost 4.3 m of the record suggest the onset of glacial activity and enhanced snow-melting in the catchment of Lake Barrancs after A.D. 350. We suggest that this onset of glacial and enhanced snow-melt activity was driven by a complex balance between winter precipitation and annual mean temperatures, among other climatic variables. Peat layers suggest two dramatic lake-level drops at A.D. 300 and A.D. 450, when Lake Barrancs was drained. The mechanisms for such extreme hydrological events are not clear. Changes in the precipitation/evaporation ratio cannot account for such desiccation events. Dam failure is unlikely since there are no geomorphological evidence of breaching processes. Geomorphological and structural evidence demonstrates active faulting since formation of Lake Barrancs and reactivation during earthquake shaking. Based on this, we propose an alternative explanation for the desiccation events that involves the draining of the lake through pre-existing fractures opened by earthquakes. Further studies in Lake Barrancs and other lakes from the Maladeta massif are necessary to validate the hypotheses presented here concerning the response of glacial and snow-melt activity to climate variability and the palaeoseismic record of the Central Pyrenees. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Lake sediments are important archives of geological processes, palaeoenvironmental variations, and past human activity because they often have high accumulation rates that enable the study of these

⁎ Corresponding author. Now at Area de Cambio Global, Instituto Geológico y Minero de España, Oficina de Proyectos de Zaragoza, C/ Manuel Lasala 44, 9°B, 50006 Zaragoza, Spain. Tel.: +34 976 555 153; Fax: +34 976 555 582. E-mail address: [email protected] (J.C. Larrasoaña). 1 Now at: CENIEH, 09004 Burgos, Spain. 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.04.003

processes at resolutions down to centennial and even decadal timescales (see De Batist and Chapron, 2008). High-altitude lake records are the focus of an increasing number of studies because mountains are very sensitive to recent environmental changes (Battarbee et al., 2002; Pla and Catalan, 2005). This is especially evident for proglacial lakes, which have been shown to provide not only continuous, but also high-resolution records of glacier dynamics in locations where its geomorphological expression is very discontinuous both in time and space (Matthews and Karlén, 1992; Leemann and Niessen, 1994; Leonard and Reasoner, 1999; Dahl et al., 2003; Lie et al., 2004; Nesje et al., 2006; Chapron et al., 2007). The study of

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glacier history from proglacial lake sediments is based on the assumption that glacier size controls sediment particulate production and hence sediment transport into the lake (Lie et al., 2004; Nesje et al., 2006; Chapron et al., 2007). However, other processes such as fluvial reworking of glacial-sourced sediment load (e.g. the ‘paraglacial’ processes of Church and Ryder, 1972) might overprint the glacial signal (Lie et al., 2004). Lake sediments have been also shown to provide long and welldated records of seismic activity (Monecke et al., 2004; Becker et al.,

2005; Carrillo et al., 2008; Wagner et al., 2008), which is important because other terrestrial palaeoseismic indicators provide only discontinuous records of seismic activity that are difficult to date (Becker et al., 2005; De Batist and Chapron, 2008). In the recent years, a growing number of studies have shown the combined effects of seismicity, climate, and environmental changes on lacustrine sedimentation (Bertrand et al., 2008; Carrillo et al., 2008; Fanetti et al., 2008; Wagner et al., 2008). Thus, especial care has to be taken when studying lacustrine sedimentary records to disentangle signals of

Fig. 1. A) Sketch map showing historical large earthquakes (I N VIII) in the Pyrenean region (IGN, 2006). The studied area, indicated by a black square, includes two of the greater earthquakes that occurred in the Pyrenees. B) Ortophoto of the studied area, with indication of the boundary between the Maladeta granitoid and the Palaeozoic country rocks and location of two historical earthquakes (small triangle: I = V, 2.12.1919; large triangle: Ribagorza earthquake, I = VIII–IX, 3.3.1373). NMF stands for the North Maladeta Fault located b 3 km east of the studied area. C) Geology and geomorphology of the Maladeta Massif (after Moya and Vilaplana, 1992; Copons and Bordonau, 1994, 1996; Chueca Cía et al., 2005), with location of Lake Barrancs and Core B5. The dashed line indicates the lake catchment. D) Aerial picture showing the main faults around Lake Barrancs, the bathymetry of the lake (isobaths are 2 m), and location of Core B5.

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seismo-tectonic activity from those generated by climatic variability (De Batist and Chapron, 2008). This is especially relevant for highaltitude lakes because they are located in young and active mountain belts that are very sensitive to climatic variations and can also be affected by intense seismic activity. Here we present the study of a 6.85 m long sedimentary sequence recovered from proglacial Lake Barrancs, which is located at an altitude of 2360 m a.s.l. in the Maladeta Massif (Central Pyrenees, Spain). Lake Barrancs is one of the few Pyrenean lakes located just downstream (b1.5 km) of active cirque glaciers. Moreover, the Maladeta Massif is one of the most seismically active regions within the Pyrenees (Souriau and Pauchet, 1998). The sedimentary sequence recovered from Lake Barrancs might therefore constitute a unique record of recent glacier and seismic activity in the Pyrenees. We use a combination of sedimentologic, environmental magnetic, and palaeobotanic techniques to characterize the depositional evolution of Lake Barrancs. The changes in sedimentary dynamics provide the basis for reconstructing palaeoenvironmental variations and palaeoseismic activity in the Central Pyrenees for the last ca. 3700 yr. 2. Geological setting Lake Barrancs is located in the Axial Zone of the central Pyrenees, on the northern slope of the Maladeta Massif (Fig. 1). This massif hosts

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the highest Pyrenean peak (Aneto Peak, 3404 m a.s.l.) and the largest cirque glaciers still preserved in the Pyrenean mountain belt (Fig. 2). The Maladeta Massif is composed of medium to coarse-grained granitoids emplaced within Palaeozoic sediments during the late stages of the Variscan orogeny (Leblanc et al., 1994; Evans et al., 1998), and is affected by three sets of NNW–SSE, N–S, and NE–SW oriented subvertical fractures. Near Lake Barrancs, these fractures are represented by several major, NNW–SSE oriented fault scarps with rectilinear traces and by minor, NE–SW oriented faults. The NNW– SSE oriented faults delineate an elongated ridge that separates two depressions with steep margins, the eastern one occupied by Lake Barrancs and the western one by periglacial talus deposits (Figs. 1, 2). These faults have a maximum length of 1.4 km, show normal displacements of b40 m that offset polished glacial surfaces originated during the Last Pyrenean Pleniglacial, and present glacial striae (Fig. 2C) (Moya and Vilaplana, 1992; Ortuño, 2008). The NE–SW faults have a maximum length of 200 m, show minor displacements of b10 m that offset the major NNW–SSE oriented faults, and do not bear glacial striae on their surfaces (Moya and Vilaplana, 1992). Lake Barrancs is located at b3 km NE of the Coronas fault and b7 km SW of the North Maladeta fault (Fig. 1B). These normal faults, between 12 and 18 km long, are among the few seismogenic faults with geomorphological expression recognized in the Central Pyrenees, and have been identified as the most likely source of historical earthquakes in the area, such as the MW = 5.3 Vielha (19.11.1923) and

Fig. 2. A) Lake Barrancs and its deltaic plain (DP) viewed from the south, with location of Core B5. B) Lake Barrancs viewed from the north. The Tempestats glacier (TG), the Little Ice Age (LIA) and the Holocene (H) moraines are clearly visible. The white rectangle marks the location of the fault shown in Fig. 2C. C) Detail of one of the faults located near the SW shore of Lake Barrancs. The fault displaces a polished glacial surface and shows a colour banding (dark grey, beige and orange from top to bottom) that mimics the shape of the displaced blocks. Arrows indicate the slip along the fault surface. D) Detail of the drilling camp and platform set up over the frozen surface of the lake.

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Table 1 Radiocarbon data from Core B5. Lab. ref.

Depth (cblf)

Material

14C age (yr B.P.)

Corrected 14C age (yr B.P.)

Cal. age (2σ)

Probability distribution

δ13C (‰)

Unit

BETA-122150 BETA-122151

170 286.5

Bulk sediment Bulk sediment

1240 ± 40 1900 ± 50

630 ± 40 1290 ± 50

3 3

338 426 567

Plant material Bulk sediment Plant material

1540 ± 30 2300 ± 40 2560 ± 40

1540 ± 30 1690 ± 40 2560 ± 40

1 0.952 0.048 1 1 0.501 0.157 0.342

− 26.3 − 26.7

BETA-122153 BETA-122148 BETA-122149

AD 1285–1401 AD 652–829 AD 837–867 AD 430–590 AD 249–426 BC 809–729 BC 692–659 BC 652–543

− 27.2 − 26.5 − 26.4

3 2 1

Bold numbers indicate radiocarbon ages chosen on the basis of their probability distribution.

the MW = 6.2 Ribagorza (3.3.1373) earthquakes (Fig. 1A) (Ortuño, 2008; Ortuño et al., 2008). During the maximum extent of the glaciers in the central Pyrenees, dated at 60–70 ka in the Cinca valley by OSL (Sancho et al., 2002) and at 20–25 in the Noguera Ribagorzana valley by means of 10Be exposure ages (Pallàs et al., 2006), the Maladeta Massif was covered by a 36 km long valley glacier that flowed down the Esera valley. This glacier overexcavated several basins, among them the one that contains Lake Barrancs. Lake Barrancs probably formed during the final stages of the Pyrenean deglaciation, when the Esera glacier was fragmented into several cirque glaciers with small ice tongues. A moraine located upstream of Lake Barrancs, at about 2400 m a.s.l. (Figs. 1C, 2B) (Moya and Vilaplana, 1992; Copons and Bordonau, 1996), formed during a transient stabilization period dated at ca. 10 ka (e.g. Early Holocene) on the basis of 10Be exposure ages of moraines on the SE slope of the Maladeta Massif (Pallàs et al., 2006). In addition, historical glacial phases have been documented in the Maladeta Massif. A prominent (tens of metres high) moraine ridge, disconnected from the present-day glacier fronts, attests for glacier advances during the Little Ice Age (LIA, 18th– 19th centuries) (Figs. 1C, 2B) (Copons and Bordonau, 1994, 1996; Chueca Cía et al., 2005). Lake Barrancs is an elongated lake (470 m long, 100 m wide) located downstream of the Barrancs and Tempestats (0.11 and 0.14 km2, respectively) cirque glaciers (Figs. 1, 2) (Copons and Bordonau, 1994, 1996; Chueca Cía et al., 2005). The lake has very steep slopes, especially on its western shore, but has a rather flat bottom composed of two small sub-basins with maximum depths of ca. 13.5 and 12.5 m (Fig. 1D). The bathymetry of the lake reflects the tectonic setting in which it is located. Thus, the steeper slope of the western shore reflects the fault scarp that bounds the Lake Barrancs basin to the west, whereas the small bathymetric high that separates the two sub-basins attests the occurrence of minor NE–SW oriented faults (Fig. 1C, D). The lake catchment (b4 km2) is composed entirely of granitoids and includes some periglacial talus deposits and glacial tills from the Holocene and LIA moraines. The lake catchment has very strong topographic gradients (N1000 m in 2 km) and is largely snowcovered from November to May, when large snow avalanches can transport coarse sediments to the frozen surface of the lake. Snowmelting in the catchment occurs typically between May and June. Melt waters have eroded the Early Holocene and LIA till sediments. As a result, a proglacial cone has formed downstream of the Barrancs glacier and a delta has occupied nearly half the depression where Lake Barrancs is located (Figs. 1C, 2A). The eastern shore of Lake Barrancs is covered by screes generated by rockfalls from the steep slopes.

(Wright, 1980). The longest core (B5) was drilled at 11 m water depth without reaching the substratum, and comprises a nearly continuous composite sequence 6.85 m thick. In the laboratory, core B5 was split in two halves, logged, described, and sampled. Sedimentological descriptions include lithology, colour, grain size, and sediment textures and structures. Total organic carbon contents of some representative samples were determined using a LECO elemental analyzer. Three bulk organic matter samples and two samples of terrestrial plant macrofossils were AMS 14C dated at the Beta Analytic laboratory (Miami, USA). The AMS dates were calibrated with CALIB v.5.0.2 (Stuiver and Reimer, 1986; Stuiver et al., 2005). Plant macrofossils were extracted from two samples (8 cm3) from an

3. Materials and methods Drilling in Lake Barrancs was carried out from the frozen surface of the lake in March 1998 (Fig. 2D). Five cores, 6 cm in diameter, were recovered from the small bathymetric high located in the central part of the lake (Figs. 1D, 2A) using a stationary piston corer (Montserrat, 1992) designed after the modified Livingstone coring apparatus

Fig. 3. A) Plot of depth versus 14C ages for Core B5. The solid line represents the fit of the two samples on plant remains to a line that intercepts the top of the core. The dashed line represents the fit of the three samples on bulk sediment (open symbols) to a line, with a similar slope, that intercepts the core top at ca. 610 yr BP. B) Depth-age model for Core B5 constructed after correcting the bulk sediment samples for a reservoir age of 610 yr. Horizontal bars represent the error at the 2σ interval.

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organic-rich layer located in the middle part of Core B5. The samples were disaggregated in water and sieved through a 125 μm mesh. The plant remains were systematically picked out at 12× magnification under a stereo-microscope and identified (Birks, 2001). Sampling for environmental magnetic measurements was done by pushing 2×2×2 cm standard plastic boxes into the working half of core B5. Sampling was performed continuously through the sedimentary section. Magnetic properties were measured at the Palaeomagnetic Laboratory of the Ludwig Maximilians Universität (Munich, Germany), and include: 1) the low field magnetic susceptibility (χ); 2) an anhysteretic remanent magnetization (ARM), applied in a dc bias field of 0.05 mT parallel to an axially-oriented peak alternating field (AF) of 100 mT; and 3) two isothermal remanent magnetizations applied at 0.2 T ([email protected] T) and 1.5 T (SIRM). χ was measured with a KLY-2 magnetic susceptibility bridge using a field of 0.1 mT at a frequency of 470 Hz. ARM was produced using a 2G Enterprises AF demagnetizer, and was measured with a 2G Enterprises three-axis cryogenic magnetometer (noise level of b7×10− 6 A/m). Both the [email protected] T and SIRM were produced using a home-made pulse magnetizer and were measured with the same cryogenic magnetometer. All magnetic properties were normalized by the dry weight of the samples. We have used different magnetic properties and interparametric ratios to determine downcore relative variations in the type, concentration, and grain size of magnetic minerals (Thompson

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and Oldfield, 1986; Verosub and Roberts,1995; Peters and Dekkers, 2003). χ has been used as a first order indicator for the concentration of magnetic (s.l.) minerals. SIRM/χ and the S-ratio (operationally defined as [email protected] T/SIRM; Bloemendal et al., 1992) have been used to detect changes in magnetic mineralogy (Verosub and Roberts, 1995; Peters and Dekkers, 2003). Then, the ARM and the “hard” IRM (HIRM, operationally defined as [email protected] T; Thompson and Oldfield, 1986) have been used as a proxy for the concentration of relatively low- and high-coercivity minerals, respectively (Verosub and Roberts, 1995). Finally, the ratios between SIRM and χ and between SIRM and χARM (i.e. the anhysteretic susceptibility; King et al., 1982), have been used to detect downcore changes in magnetic grain size, provided that they correspond to a single magnetic mineral (Thompson and Oldfield, 1986; Verosub and Roberts, 1995; Peters and Dekkers, 2003). All results from core B5 are referred to centimetres below the lake floor (sediment surface) (cblf). 4. Results 4.1. Chronology The chronological model for the Lake Barrancs sequence is based on five AMS radiocarbon dates ranging from 2560 ± 40 14C yr BP at 567 cblf to 1240± 40 14C yr BP at 170 cblf (Table 1). The 14C age–depth

Fig. 4. Lithostratigraphy, sedimentary units and facies associations (A), and selected magnetic properties (B) of Core B5.

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relationship shows a reversal between two samples at 286.5 (bulk sediment) and 338 (plant remains) cblf, which suggests that the age of the bulk sediment samples could be too old due to some reservoir effect affecting bulk organic matter (Fig. 3A). The most likely explanations for such old ages in bulk organic matter samples are the presence of old carbon from the catchment as constituent of the fine-grained sedimentary load and the uptake of old carbon (leached from the bedrock catchment) by phytoplankton (e.g. Lowe and Walker, 1997). The two samples dated on plant remains gave 14C ages that conform to a straight line that intercepts the top of the core at ca. 30 yr BP (Fig. 3A). On the contrary, the three samples dated on bulk sediment gave 14C ages that fit (R = 0.98) to a line that intercepts the core top at ca. 610 14C yr BP (Fig. 3A). The slope of this line is very similar to that of the samples on plant remains and the top of the core. This suggests that all the three bulk sediment samples, not only the one at 286.5 cblf, are affected by a more or less constant reservoir age of ca. 610 yr. Since old carbon eroded from the catchment would presumably have variable ages, the most likely explanation for such constant ageing in lacustrine-derived organic matter is a hard-water effect. It is comparable to the ca. 940 yr discrepancy reported for 14C ages in the neighbouring Lake Redon by Camarero et al. (1998), who consider it to be a hard-water reservoir effect. After correcting the bulk sediment radiocarbon ages for the 610 yr reservoir age, the Barrancs age model was constructed using calibrated ages chosen on the basis of their maximum probability distribution within the 2σ interval (Table 1). The corrected age model is based on linear interpolation between calibrated radiocarbon dates, and indicates that the sediment sequence of Lake Barrancs recovered in core B5 spans the last 3700 yr (Fig. 3B). The average accumulation rate is about 1.85 mm/yr, lower but comparable to a previous report from Lake Barrancs in which no reservoir effect was considered (i.e. 2.6 mm/yr; Copons et al.,1997). In any case, sedimentation rates in Lake Barrancs are several times larger than in other high-altitude Pyrenean lakes such as Lake Tramacastilla (ca. 0.4 mm/yr; García-Ruiz et al., 2003) and Lake Redon (ca. 0.055 mm/yr; Pla and Catalan, 2005), suggesting high sediment delivery to the lake during the last millennia. 4.2. Lithostratigraphy Core B5 includes a nearly continuous sequence of proglacial sediments composed of fine sands, silts, and clays organized in three

facies associations (Fig. 4). Facies association F1 is composed of light brown to grey clays, silts, and sandy silts with occasional, discrete, mm-thick intervals of organic debris. These sediments have a poorly developed horizontal lamination that is marked by faint colour and textural variations. Facies association F2 is composed of light brown to grey clays and silts that contain dispersed organic debris and include abundant layers of sandy silts and fine sands. The sandy silts and fine sands occasionally contain distinctive whitish layers, which can be up to 1 cm thick and often show a turbidite-like fining-upward grain-size gradation. Facies association F2 sediments have a well-developed millimetre to centimetre horizontal lamination that is marked by textural and colour variations. Facies association F2 is scarce in the lowermost 3 m of the record but dominates between 430 and 70 cblf. Facies associations F1 and F2 have low TOC contents (b0.4% in weight). It is worth mentioning that, although no dropstone has been observed throughout the studied sequence of core B5, the presence of dropstones in Lake Barrancs sediments is evidenced by the difficulties faced when drilling other sites, which had to be abandoned when encountering large blocks that prevented further drilling. Distinctive organic-rich dark layers (facies association F3) occur between 344 and 363 cblf, and 424–427 cblf. The upper one is intercalated within a thick (70 cm) facies association F2 interval (Fig. 4). This 19-cm thick layer is composed of silts and sands, which include large quartz, feldspar and biotite grains of up to 3 mm at the base of the layer. The distinctive dark colour is caused by disseminated organic matter and mm-scale vegetal remains, which give a mean TOC content of nearly 3% by weight. At the top of the layer, vertical root structures penetrate 10 cm into the layer. Plant macrofossils associations are composed of Calluna vulgaris, Rhododendruon ferrugineum, Selaginella selaginoides, Juniperus communis, Salix sp., Betula (peduncula/pubescens), Ranunculus sp., and Carex sp. (Table 2). An associated fauna of oribatids, trichopterans, chironomids, and fragments of other insects also occurs. This assemblage is typical of Pyrenean heathlands located around mountain streams up to 2200 m a.s.l. (Villar et al., 1997). The lower dark layer (424–427 cblf) appears at the base of a thin (20 cm) facies association F2 interval (Fig. 4). This layer is also composed of silts and sands that contain organic debris (including root fragments) and large quartz, feldspar, and biotite grains (b2 mm) in its lower part.

Table 2 Macrofossil plants and invertebrate remains in two selected samples from the organic-rich layer between 344 and 363 cblf. Sample

Depth (cm)

Macrofossil plants Plant type

Plant part

Abundance

Invertebrate remains

Abundance

4.18

345

Seeds Flowers Leaves, stems, twigs Leaves Bark fragments Bud scales Leaf glands Seeds Leaf fragments Achene (small) Nutlets

f vr p

362

1 3 f 4 r 3 oc 2 oc 1 1 oc 1 oc 2 oc oc

Oribatid mites Chironomids Insect fragments

4.26

Calluna vulgaris Calluna vulgaris Calluna vulgaris Juniperus communis Salix sp. Betula (peduncula/pubescens) Rhododendron ferrugineum Rhododendron ferrugineum Rhododendron cf. ferrugineum Ranunculus sp. Carex sp. (tristigmata) Roots Leaf and twig fragments Calluna vulgaris Selaginella selaginoides Twigs Leaf fragments

Chironomids Trichoptera Insect fragments

r vr p

f = frequent. oc = occasional. r = rare. vr = very rare. p = present.

Other constituents

Twigs Megaspores

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Facies associations F1 to F3 define two stratigraphic units (Fig. 4). The uppermost unit (Unit 2) is dominated by facies association F2, which is progressively replaced by facies association F1 towards the top of the unit. The presence of the two organic-rich layers in the lower part of Unit 2 enables identification of two deepening sequences that range from subaerial conditions (indicated by facies association F3) to relatively deeper lacustrine sedimentation (indicated by the increased abundance of facies association F1 towards the top). Unit 1 is dominated by facies association F1, and appears also to form a deepening sequence (Fig. 4). 4.3. Environmental magnetism Magnetic susceptibility values oscillate around 8 × 10− 8 m3/kg throughout the record except in the two organic-rich layers at around 350 and 430 cblf and at a facies association F1 interval located around 650 cblf, where magnetic susceptibility values drop sharply well below 7 × 10− 8 m3/kg (Fig. 4). These values are similar to those of samples from the Maladeta granitoid, which typically range between 4 and 12 × 10− 8 m3/kg (i.e. 10–30 × 10− 5 SI; Leblanc et al., 1994). This is consistent with the fact that the granitoid rocks that surround Lake Barrancs are dominantly paramagnetic (Leblanc et al., 1994) and are the main source for sediments accumulated in Lake Barrancs. Low magnetic susceptibility values such as those in the two organic-rich layers might indicate a different concentration of detrital minerals or, alternatively, a post-depositional change in the primary magnetic signal. Sediments from facies associations F1 and F2 are characterized by S-ratios ranging between 0.7 and 0.9, which contrast with the distinctively low (down to 0.6) S-ratios in the two organic-rich layers (facies F3) (Fig. 4). SIRM/χ values oscillate between 0.5 and 3.5 kA/m except in the uppermost organic-rich layer, which shows distinctively higher SIRM/χ values of up to 9 kA/m. Similarly, SIRM/χARM ratios range between 1 and 3 kA/m except in the uppermost organic-rich layer, which shows significantly higher ratios of up to 4 kA/m. High S-ratios combined with low SIRM/χ values indicate that the main magnetic mineral characterizing facies associations F1 and F2 is magnetite (Snowball, 1993; Verosub and Roberts, 1995; Peters and Dekkers, 2003). This magnetite is mainly derived from glacial abrasion of the granitoid catchment rock and its subsequent transport to the lake mainly by snow-melt waters and glacier outwash. Downcore variations in ARM intensity reveal a rather low (b2 × 10− 6 Am2/kg) and constant concentration of magnetite in Unit 1, which contrasts with oscillating values between 2 × 10− 6 and 10 × 10− 6 Am2/kg in Unit 2. Concerning parameters indicative of grainsize variations, SIRM/χ values show rather constant values (around 1 kA/m) in Unit 1 and highly oscillating values (1–4 kA/m, excluding F3 intervals) in Unit 2. In contrast, SIRM/χARM ratios show rather constant values of around 1.8 kA/m throughout the core, with only subtle variations in Unit 2 (facies F3 excluded). The combination of such SIRM/ χARM and SIRM/χ ratios indicates that magnetite grains in facies associations F1 and F2 range between 0.01 and 0.03 µm in size (Peters and Dekkers, 2003). The high variability of SIRM/χARM ratios within this grain-size range (Peters and Dekkers, 2003), coupled with their rather constant values throughout units 1 and 2, prevents the use of SIRM/χARM ratios as a proxy for variations in magnetite grain size. On the contrary, SIRM/χ ratios, which tend to increase for coarser magnetite within the 0.01–0.03 µm grain-size range (Peters and Dekkers, 2003), show a distinctively different behaviour between units 1 and 2. Thus, SIRM/χ ratios of up to 4 kA/m in facies association F2 sediments suggest that they have coarser magnetite grains compared with facies association F1 intervals (around 1 kA/m), which is consistent with the sedimentological description of both facies types. In any case, it should be stressed that these grain-size variations are very subtle, probably in response to the original grain-size distribution of magnetite in the parent Maladeta granitoid.

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The distinctively low S-ratios that characterize the two organicrich layers indicate that the main magnetic mineral present in facies association F3 is not magnetite, but rather a relatively-higher coercivity mineral phase (Verosub and Roberts, 1995). Diagenetic reactions in sediments are mainly driven by the metabolic activity of microbes, which consume oxygen (under oxic conditions), nitrate, manganese and iron oxides (under suboxic conditions), and sulphate (under anoxic conditions) to degrade buried organic matter (Froelich et al., 1979). Microbially-mediated reduction of sulphate during earliest diagenesis releases sulphide, which reacts with iron-bearing minerals (including magnetic grains) and dissolved iron to form iron sulphides. Significant accumulations of organic matter in lake sediments easily drives diagenetic reactions to the point where detrital magnetic grains are dissolved and the magnetic iron sulphide greigite is formed (Snowball, 1991). This process is favoured when sulphide is produced in low amounts (e.g. Roberts and Weaver, 2005; Larrasoaña et al., 2007), which typically happens in small lakes due to the low availability of dissolved sulphate. Given the high organic content of the two organic-rich layers, and keeping in mind that hematite and goethite are unstable under reducing conditions (Canfield et al., 1992), we interpret that the most likely magnetic mineral in F3 sediments is greigite because it better explains the combined lower S-ratios and the distinctively higher SIRM/χ and SIRM/χARM values (Snowball, 1991, 1993; Roberts, 1995) of the two organic-rich layers. Authigenic growth of greigite and reductive dissolution of magnetite accounts for the low χ values of the organic-rich layers, because magnetite has a larger specific magnetic susceptibility compared to greigite (Roberts, 1995). HIRM values give an indication of the concentration of the relativelyhigher coercivity minerals, and indicate that large amounts of greigite were formed in the upper organic-rich layer, but not in the lower one. 5. Discussion 5.1. Sedimentary processes in Lake Barrancs Facies associations F1 and F2 are typically deposited in proglacial lakes in which rhythmic sedimentation is controlled by the seasonal alternation of intense snow and glacier melting during the late spring and summer with freezing conditions over the winter and early spring (Ashley, 1995; Chapron et al., 2007). Between May and June, flood events triggered by snow-melting cause cold and sediment-loaded runoff waters to flow into the lake. They form an underflow current that is gradually mixed with the lake water. Over the course of the summer, glacier outwash takes over as the main source of underflow currents. As a result of these floods, also called homopycnal flows (Bates, 1954), the sedimentary load moves by advection throughout the water column until it eventually settles down draping the lake bottom (Ashley, 1995, Chapron et al., 2007). Homopycnal flows are an effective mechanism for separating coarse- and fine-grained sediments due to their differential settling times (Ashley, 1995), which accounts for the alternation of clays, silts, and sands that characterizes facies associations F1 and F2. During winter and early spring, when the surface of the lake is frozen, sedimentation is restricted to finer particles settling out of suspension, which accounts for the presence of thin clay layers within facies associations F1 and F2 sediments. The higher abundance of sandy silts and fine sands in facies association F2 is interpreted as an increased intensity and/or frequency of homopycnal flows (Chapron et al., 2007), which might also explain the better-developed laminations in these sediments. Snow and glacier melting throughout the spring and summer, coupled with the availability of easily erodable moraine material in the catchment, might explain the high accumulation rates (1.85 mm/yr) that characterize the sediment core from Lake Barrancs. In this regard, it is worth mentioning that a substantial part of the sedimentary load produced by the Barrancs and Tempestats glaciers is deposited in the proglacial cone formed downstream of the Barrancs glacier and in the

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delta that occupies the proximal part of Lake Barrancs (Figs. 1, 2A). Despite this, intense glacial grinding, together with meltwater runoff and erosion of glacial deposits, provides a very effective mechanism for transferring sedimentary load from the glaciated areas of the catchment into the lake. Facies association F3 sediments were formed in subaerial conditions, as shown by the plant remains, the high TOC, and the in situ roots. The presence of aquatic invertebrates and Selaginella might indicate sporadic (e.g. seasonal) water-logged conditions which would allow the preservation of organic remains as a peaty deposit. At least for the organic-rich layer at around 350 cblf, the plant macrofossil assemblage indicates that subaerial conditions lasted long enough to enable the development of a dwarf-shrub community and the accumulation of 19 cm of peat. The tree-birch remains probably originated from trees growing on the valley sides. It might be argued that these two organic-rich layers correspond to reworked peat and palaeosoil fragments that were transported by snow avalanches or slid from the deltaic plain located upstream. We discard these possibilities and propose that these organic-rich layers are palaeosoils formed in situ during low-lake-level conditions because: 1) the presence of roots in a vertical position and the absence for internal deformation is not consistent with a slump (i.e. folded) geometry; and 2) the basal parts of the organic-rich layers contain the coarsest sand grains reported in the section (Fig. 4), which is compatible with sudden base level falls and the concomitant arrival of coarser detrital material.

5.2. Palaeoenvironmental implications Unit 2 is characterized by the predominance of coarser-grained sediments (facies association F2) and by overall increased (but highly oscillating) concentrations of relatively coarser magnetite grains. This indicates that periods of enhanced detrital supply into Lake Barrancs have been common between ca. A.D. 350 and the present. Conversely, Unit 1 is dominated by finer-grained sediments (facies association F1) and by constantly low concentrations of relatively finer magnetite grains, which indicates lower detrital supply into the lake before A.D. 350 and back to, at least, 1700 B.C., which is the lower boundary of our record. Present-day detrital supply into Lake Barrancs is governed by homopycnal flows triggered by snow-melt and glacier outwash. Enhanced glacier and snow-melt activity are typically manifested by coarser grain sizes (Lie et al., 2004; Nesje et al., 2006; Paasche et al., 2007; Chapron et al., 2007). Based on the sedimentary and magnetic record of Lake Barrancs, we conclude that glacier activity was very small (if not absent) and snow-melting was largely reduced before A.D. 350, but that they have been active in the catchment of the lake since then. No obvious link is seen between sedimentary facies and magnetic parameters from the Lake Barrancs sequence with Late Holocene global climatic variations recognized at different locations throughout the Iberian Peninsula (Gutiérrez-Elorza and Peña-Monné, 1998; Desprat et al., 2003; Riera et al., 2004; Gil-García et al., 2007) (Fig. 5). Thus,

Fig. 5. Age variations of selected magnetic properties from Core B5 compared with a regional climatic record of winter mean temperatures (Lake Redon, Pla and Catalan, 2005) and the sequence of climate periods recorded in the Iberian Peninsula (Gutiérrez-Elorza and Peña-Monné, 1998; Desprat et al., 2003; Riera et al., 2004; Gil-García et al., 2007 ). The left column indicates the timing of historical earthquakes recorded in the area and the timing of the two possible earthquakes inferred from the occurrence of facies F3 sediments. ARM and SIRM/χ data for facies association F3 are not shown because they do not indicate variations in magnetite concentration and grain size.

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before A.D. 350, facies association F1 sediments with low concentrations of finer-grained magnetite dominate during both cold (i.e. the Subatlantic Cold Period, 975 B.C.–250 B.C.) and warm (i.e. the Subboreal Climate Optimum, b975 B.C.; the Roman Warm Period, 250 B.C.–A.D. 450) periods. Similarly, after A.D. 300, both cold (i.e. the Dark Ages, A.D. 450–A.D. 950; the Little Ice Age, A.D. 1400–A.D. 1850) and warm (i.e. the Medieval Warm Period, A.D. 950–A.D. 1400; the recent warming period, NA.D. 1850) events are characterized by the alternation of facies association F1 and F2 sediments with variable concentrations of generally coarser magnetite grains. This lack of correlation might be related to the different response of sedimentary facies dynamics in highaltitude lakes to temperature changes during the Holocene, and underlines the difficulties in linking globally-established climatic events with environmental changes at a specific mountain location with its own physiographic and environmental responses to climate change. We have also compared the Lake Barrancs record with a regional record of climate variability based on chrysophyte cysts from Lake Redon (Fig. 5) (Pla and Catalan, 2005), which is located just 8 km east of Lake Barrancs at a similar altitude (2240 m a.s.l.). Since distribution of chrysophyte cysts is related mainly to altitude, downcore variations in chrysophyte cysts have been used to estimate a local altitude anomaly that reflects changes in winter mean temperatures through time (negative and positive altitude anomalies indicate warmer and colder temperatures, respectively) (Pla and Catalan, 2005). The Lake Redon record shows rather small winter temperature anomalies (WTA) between 1700 B.C. and A.D. 300, when mean winter temperatures usually oscillated between 0.2 °C warmer and − 0.5 °C colder than present. Around A.D. 300, mean winter temperatures show a significant shift toward warmer conditions that lasted till A.D. 950, when they decreased ca. 1.4 °C in 100 yr. After A.D. 1050, when mean winter temperatures were 1.1 °C colder than today, they show a progressive warming trend that is characterized by large-amplitude oscillations of up to 1 °C/100 yr. The coincidence between the onset of glacial and enhanced snow-melt activity in the catchment of Lake Barrancs at A.D. 350 with a significant increase of winter mean temperatures seems to discard winter temperature conditions as the main mechanism leading to glacier development and enhanced snowmelt activity in the Maladeta Massif. Recent (i.e. NA.D. 1800) glacier activity in the massif has been shown to depend largely on winter precipitation, which supplies most of the snow necessary for glacier development, and annual temperatures, which influence glacier ablation not only during the summer months (Chueca Cía et al., 2005). Concerning the intensity of snow-melting activity, it is likely to depend on the balance between winter precipitation (which influences snow accumulation) and summer temperatures. We interpret that the possible response of glacier and snow-melt activity to different climatic variables might have conditioned the complexity of the Lake Barrancs magnetic record. Following the suggestion of Matthews and Karlén (1992) and Lie et al. (2004), further studies of proglacial and “control lakes” (i.e. devoid of recent glacier activity in the catchment) in the Maladeta Massif are necessary to isolate glacier activity from snow-melt dynamics and to establish the link between both processes and climate variability. 5.3. Palaeoseismic implications Keeping in mind that core B5 was drilled near the deepest part of the lake, the proposed formation of the palaeosoils at 350 and 430 cblf would imply the nearly complete desiccation of Lake Barrancs at around A.D. 450 and A.D. 300, respectively. Estimated lake-level drops are of about 13.5 and 14.25 m (i.e. 11 m of water depth plus the thickness of the overlying sediments). It might be argued that, instead of such dramatic lake-level drops, formation of the palaeosoils would respond to relatively smaller level drops (i.e. 5–6 m) imposed on a long-term deepening trend, which might explain the lake-level fall required to explain the onset of

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Unit 1. In any case, these dramatic lake-level drops could be explained by desiccation during periods of dam failure or periods of negative hydrological balance. The outlet of the lake corresponds to a bedrock barrier originating from glacial over-deepening during the Last Pyrenean Pleniglacial (Figs. 1D, 2A), which therefore eliminates the possibility of recent (b3 kyr) events of dam failure and building required to explain successive episodes of lake-level drop and its subsequent refilling. Concerning the second possibility, a comparison with the palaeoclimate proxy produced from Lake Redon reveals that the two palaeosoils formed at the beginning of the warm period starting at ca. A.D. 300 (Fig. 5). Although this warm period might have favoured an increase in evaporation, it is extremely unlikely that it modified the hydrological balance of the alpine catchment, including the complete melting of the glaciers, strongly enough to enable desiccation of the lake. In order to find an alternative explanation for the formation of the two palaeosoils, it is useful to consider the tectonic and geomorphologic setting of Lake Barrancs. The NNW–SSE faults that delineate the lake basin offset glacial polished surfaces attributed to the Last Pyrenean Pleniglacial, and their surfaces bear glacial striae generated during the Younger Dryas (Ortuño, 2008). In at least one of the fault surfaces, three bands of different bedrock colour mimic the shape of the displaced blocks (Fig. 2C), which indicates different weathering stages linked to exhumation of the fault in three distinctive events. Moreover, the NNW– SSE oriented faults are displaced by minor NE–SW oriented faults that do not display glacial striae at their surfaces. In addition, the surface of the fault located near the Aneto glacier (Fig. 1C) does not have glacial striae despite of being located upwards from the LIA moraine and very close to the present-day glacier front. All these observations, together with the important historical and instrumental seismicity in the area, indicate that faulting around Lake Barrancs has been active since the Last Pyrenean Pleniglacial (20–25 ka. Pallàs et al., 2006) and up to the present (Moya and Vilaplana, 1992). Regardless of their timing, the length of all these faults is always b1.4 km, well below the lower boundary for rupture length of seismogenic faults (~3.8 km for normal faults, Wells and Coppersmith, 1994). The location of the faults near the bottom of the valley makes a gravitational origin also unlikely (Moya and Vilaplana, 1992). The most plausible explanation for these faults is their activation as secondary faults during seismic shaking along the seismogenic Coronas fault, which is located just 4 km SW of Lake Barrancs and has historical and instrumental seismicity (Ortuño, 2008). In addition to seismic shaking, displacement along these faults might have been aggravated by uplift of the valley bottom in response to vertical unloading following over-deepening and glacier melting, a process that has been studied in similar alpine settings (Ustaszewski et al., 2008). Considering the widespread evidence for neotectonic activity in the studied area, we propose an alternative explanation for the desiccation of Lake Barrancs that involves the drainage of the lake through pre-existing fractures opened by earthquakes. In this regard, the faults that delineate the western shore of the lake and its central bathymetric high (Fig. 1D) clearly stand out as the putative fractures causing drainage of the lake. The rapid transition from the palaeosoils back to facies F2 sediments indicates that lacustrine conditions were rapidly re-established, which is consistent with a scenario of rapid sealing of the fractures acting as a subaquatic outlet by sediments dragged by the outflowing water. If our interpretation is correct, then the Lake Barrancs record shows evidence for the occurrence of two seismic events along the Coronas Fault in the short time interval between A.D. 300 and A.D. 450. To explain the origin of the lowermost deepening sequence, other earthquakes that occurred before B.C. 1700 might also need to be considered. Since these seismic events are detected by lake-drainage and the formation of palaeosoils, we consider that no estimate of the intensity of the putative earthquakes can be made. It should be kept in mind that the surface expression of an earthquake

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depends not only on its magnitude, but also on the location of the rupture within the fault and the hypocentral depth. It is thus possible, for instance, that a moderate (MW = 5–6) seismic event sufficed to open a pre-existing fracture and cause drainage of the lake, but that a larger earthquake did not result in significant fracture opening. This seems to be the case for the historical Ribagorza event (MW = 6.2, A.D. 1373), for which no surface rupture has been described in historical archives probably as a result of its large hypocentral depth (16 km, see Ortuño, 2008). Future detailed studies should be carried out in order to identify sedimentological evidence for seismic shaking in the form of widespread seismites (such as homogenites, slump deposits and/or chaotic deposits; Rodríguez-Pascua et al., 2000; Bertrand et al., 2008). Combined with very-high-resolution seismic surveys, these studies can be very helpful for testing the hypothesis presented here concerning the drainage of Lake Barrancs, and also for providing new insights into the palaeoseismic history of the Maladeta Massif. 6. Conclusions The sequence recovered from proglacial Lake Barrancs is composed of three sedimentary facies associations. Facies associations F1 and F2 are composed by clays, silts, and sands that have low (b0.4%) TOC contents, whereas facies association F3 is composed of organic-rich (TOC ~ 3%) silts and sands and occurs in two distinctively dark layers at 344–363 and 424–427 centimetres below the lake floor (cblf). Facies associations F1 and F2 respond to seasonal changes in sediment supply, which is characterized by slow particle settling during the winter and by the arrival of sediment-loaded homopycnal flows, triggered by snow-melting and glacier outwash, during the warm season. Combined low IRM/χ values and high S-ratios indicate that magnetite is the main magnetic mineral in facies associations F1 and F2. Higher contents of relatively coarser magnetite grains, indicated by high ARM and IRM/χ values, are preferentially associated with facies association F2, which is richer in coarser-grained sediments and displays better-developed horizontal laminations compared to facies association F1. The predominance of coarsergrained graded sediments (facies association F2) and of overall increased (but highly oscillating) concentrations of relatively coarser magnetite grains upwards from 430 cblf suggest the onset of glacial activity and enhanced snow-melting at ca. A.D. 350. Low concentrations of relatively finer magnetite grains and the predominance of facies association F1 sediments in the lower part of the record suggest that glacier activity was insignificant (if not absent) and snow-melting was largely reduced before that age. Plant macrofossil assemblages and the presence of roots in a vertical position indicate that facies association F3 represents in situ formation of two palaeosoils at A.D. 300 and A.D. 450. Combined high IRM/χ and low S-ratios of facies association F3 indicate that greigite likely formed authigenically during degradation of organic matter in the two palaeosoils. These in situ soils imply sudden and substantial lake-level drops lasting at least for decades. Geomorphological and palaeoclimatic evidence indicates that neither dam failure nor climate variability can account for these lake-level falls. An alternative hypothesis proposed here for these lake-level drops is that Lake Barrancs was drained through a pre-existing fracture network reactivated by earthquakes in the short time interval between A.D. 300 and A.D. 450. This scenario is consistent with the widespread evidence of historical seismic activity in the area and with structural evidence from the Maladeta Massif, which indicates that formation and subsequent evolution of the Barrancs basin is related to fault reactivation during seismic shaking along the neighbouring seismogenic Coronas fault. Our results strengthen the view that proglacial lakes constitute excellent archives of past glacier and snow-melt dynamics in alpine settings, and suggest that they might also provide a reliable record of seismic activity in young and active mountain belts.

Acknowledgements We are very grateful to Nikolai Petersen and the staff of the Palaeomagnetic Laboratory of the Ludwig Maximilians Universität (Munich, Germany), where the rock-magnetic analyses were carried out, for their hospitality, technical assistance, and discussions at the early stages of this study. This research was supported by projects AMB93-0814-C02-01 and PB96-0815, and by a MEC Ramón y Cajal contract (JCL). We are very grateful to Jordi Catalan, who kindly provided results from Lake Redon and helped to obtain the bathymetry of Lake Barrancs. We also thank Alberto Sáez and an anonymous reviewer for their useful comments, and Santiago Giralt for his helpful and efficient editorial handling.

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