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Authors personal copyChapter 11         Recent Advances in Research on Quaternary Glaciations in the Pyrenees             ...
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Authors personal copyFIGURE 11.4 Pleistocene glaciation in the Ariege catchment. 1: Razorback and hogback scarps in Mesozo...
Authors personal copy132                                                                              Quaternary Glaciatio...
Authors personal copyChapter 11         Recent Advances in Research on Quaternary Glaciations in the Pyrenees             ...
Authors personal copy134                                                                              Quaternary Glaciatio...
Authors personal copyChapter 11      Recent Advances in Research on Quaternary Glaciations in the Pyrenees                ...
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Authors personal copyChapter 11      Recent Advances in Research on Quaternary Glaciations in the Pyrenees                ...
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Authors personal copyChapter 11         Recent Advances in Research on Quaternary Glaciations in the Pyrenees             ...
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Recent Advances in Research on Quaternary Glaciations in the Pyrenees

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In the past decade, particularly since the last summary of the
subject (Calvet, 2004), the Quaternary glaciation of the Pyrenees
has been the focus of new research. Unequal progress
has been achieved on three aspects: mapping the extent of
the Pyrenean ice field, quantifying the geomorphological
impact of glaciation on the preglacial landscape and refining
the chronology of the glacial fluctuations.

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Transcript of "Recent Advances in Research on Quaternary Glaciations in the Pyrenees"

  1. 1. Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use.This chapter was originally published in the book Developments in QuaternaryScience, Vol.15, published by Elsevier, and the attached copy is provided by Elsevierfor the authors benefit and for the benefit of the authors institution, for non-commercial research and educational use including without limitation use ininstruction at your institution, sending it to specific colleagues who know you, andproviding a copy to your institution’s administrator.All other uses, reproduction and distribution, including without limitation commercialreprints, selling or licensing copies or access, or posting on open internet sites, yourpersonal or institution’s website or repository, are prohibited. For exceptions,permission may be sought for such use through Elseviers permissions site at: http://www.elsevier.com/locate/permissionusematerial From: Marc Calvet, Magali Delmas, Yanni Gunnell, Régis Braucher and DidierBourlès, Recent Advances in Research on Quaternary Glaciations in the Pyrenees. In J. Ehlers, P.L. Gibbard and P.D. Hughes, editors: Developments in Quaternary Science, Vol. 15, Amsterdam, The Netherlands, 2011, pp. 127-139. ISBN: 978-0-444-53447-7 © Copyright 2011 Elsevier B.V. Elsevier.
  2. 2. Authors personal copy Chapter 11Recent Advances in Research onQuaternary Glaciations in the PyreneesMarc Calvet1,*, Magali Delmas1, Yanni Gunnell2, Regis Braucher3 and Didier Bourles3 ´ `1 ´di-Terra, Universite de Perpignan-Via Domitia, 52 Avenue Paul Alduy, F 66860 Perpignan cedex, France JE 2522 Me ´2 ´ UMR 5600 CNRS, Universite Lumie `re-Lyon 2, 5 avenue Pierre Mende `s-France, 69676 Bron cedex, France3 ´, UMR 6635 CNRS, Aix Marseille Universite BP 80, 13545 Aix-en-Provence cedex 04, France*Correspondence and requests for materials should be addressed to Marc Calvet. E-mail: calvet@univ-perp.fr11.1. INTRODUCTION of ice extent is only available for the eastern part of the ` range, the Ariege catchment included. The synthesis byIn the past decade, particularly since the last summary of the Calvet (2004), who following the minimalist views of Tail-subject (Calvet, 2004), the Quaternary glaciation of the Pyr- lefer (1985), at the time underestimated the extent of theenees has been the focus of new research. Unequal progress ¨ Wurmian ice field, has been significantly revisedhas been achieved on three aspects: mapping the extent of (Fig. 11.1). Further west, uncertainty remains concerningthe Pyrenean ice field, quantifying the geomorphological ¨ the termini of Wurmian glaciers in the Salat and Gaveimpact of glaciation on the preglacial landscape and refin- d’Oloron catchments; for the last valley, the outermost gla-ing the chronology of the glacial fluctuations. cial deposit has been described at Escot, but there is also a Mapping of ice extent has benefited from only minor presumption that ice advance might have extended someupdates. A relatively accurate 1:100,000-scale illustration 10 km further out onto the piedmont (Gangloff et al.,FIGURE 11.1 The Pleistocene glaciation of the Pyrenees: a cartographic synthesis. 1: Glacierised areas, a: during the Wurmian; b: during the Middle ¨Pleistocene. 2: Main supraglacial mountain ridges and ice-catchment limits; main ice routes. 3: Possible extent of Pleistocene valley glaciers. 4: Maintransfluence cols. 5: Currently glacierised massifs containing small, residual cirque glaciers. The map is modified and updated after Calvet (2004). Notethe location of other figures mentioned in the text.Developments in Quaternary Science. Vol. 15, doi: 10.1016/B978-0-444-53447-7.00011-8ISSN: 1571-0866, # 2011 Elsevier B.V. All rights reserved. 127
  3. 3. Authors personal copy128 Quaternary Glaciations-Extent and Chronology 1991). Both for small, transverse valleys and for the main 11.2. MIDDLE PLEISTOCENE GLACIATIONS valleys, such as the Noguera Pallaresa or the Valira, the ter- minal position of the glacier systems is also uncertain in ¨ Glaciations pre-dating the Wurmian Stage remain poorly Spain. Work by Serrat et al. (1994) and Turu i Michels documented. Neither their number nor their spatial extent ˜ ´ and Pena Monne (2006) has claimed that these glacial ter- and precise time of formation are well established. This mini extended ca. 10 km further than thus far accepted. This is partially ascribable to the fact that these older glaciers is in spite of the fact that the valley cross-sections in these did not advance much further out of the crest zone than areas are V-shaped and sometimes exhibit entrenched ¨ those during the subsequent Wurmian. It is possible that meanders. Further, glacial landforms are either scarce or ¨ they exceeded the Wurmian maximum by 1.5 km in the absent, and the interpretation of existing Pleistocene Carol (or Querol) valley and by 5–10 km in the case of deposits is inconclusive. ` the Ariege glacier. Further, the last glaciation has often bur- The impact of glacial erosion on the landscape has been ied, destroyed or reworked pre-existing older deposits. measured through a combination of approaches: (i) qualita- tive approaches have endeavoured to list and map all pre- and interglacial landforms and deposits. These features 11.2.1. On the South Side of the Range are so well preserved in some places that they necessarily The Carol catchment exhibits among the best-preserved imply extremely limited depths of Pleistocene erosion by glacial deposit sequences of the Pyrenees (Fig. 11.2). It con- glaciers. Relevant markers of limited glacial erosion are tains three generations of frontal and lateral moraines, each thick weathered mantles preserved beneath glacial till or grading to the tread of a glaciofluvial terrace. Clast assem- ice-marginal deposits, and relict erosion surfaces the Neo- blages in each terrace show distinctive weathering charac- gene age of which have been confirmed by apatite fission- teristics. Although relative dating of these moraines would track and (U–Th)/He dating (Gunnell et al., 2009). (ii) have suggested an age sequence bridging the Middle and Quantitative approaches have produced mass-balance stud- Late Pleistocene, 10Be dating has provided ages of 65 ka ies that exploit the well-preserved glacial sediment reposi- or less, suggesting that all three moraine units formed dur- tory situated on the southern side of the Carlit massif. Based ¨ ing the Wurmian Stage (Delmas, 2009). Such a young age on research by Delmas et al. (2009), catchment-scale glacial for deeply weathered glacial and glaciofluvial materials is erosion depths have been estimated to have not exceeded deceptive. It suggests that the erratic boulders, sampled 5 m during the last glacial cycle. Cirque morphometry in for CRN dating, have lost significant mass through post- Aragon (n ¼ 206; Garcıa-Ruiz et al., 2000) and in the Carlit ´ depositional rind weathering and grus formation, hence pro- and Ariege catchments (n ¼ 1066; Delmas, 2009) has also ` viding younger than expected exposure ages. Alternatively, provided some insight into spatial variations in erosional it implies that those same erratic boulders were embedded intensity. However, the overwhelming conclusion is that in the moraine and have undergone post-depositional exhu- bedrock lithology and structure are the strongest controls mation. In either case, this could explain the relatively uni- on cirque morphology: neither climatic criteria nor the form age cluster obtained for the Carol moraines. duration of glacier presence (estimated on the basis of cir- ´ The Gallego catchment (Fig. 11.3) has provided a puz- que floor elevation) have provided clear-cut regional pat- ˜ zling series of OSL ages (Pena et al., 2004; Lewis et al., terns. Overall, even where glacial landforms pervade the ¨ 2009). The large frontal moraine at Senegue was until crest zone and convey a distinctly alpine character to the ¨ now held to represent the Wurmian glacial maximum, but mountain scenery (Calvet et al., 2008), evidence points to moraines at Aurin, situated 2 km further down-valley, have a relatively limited morphological imprint of glaciation yielded contrasting ages of 38 and 85 ka. They have there- on the pre-Quaternary Pyrenean landscape. ¨ fore also been attributed to the Wurmian. Meanwhile, the Most progress devoted to understanding Pyrenean uppermost glaciofluvial terrace deposits, Qt 5, at Sabina- ˜ Quaternary landscape evolution has been achieved through nigo, which contains glacially striated pebbles and corre- constraining the chronology of glaciation, and particularly lates stratigraphically with terrace T2 deposits in other ¨ the last Pleistocene glacial cycle (Wurmian Stage). This is valleys in the eastern Pyrenees (Calvet, 1996), have pro- due to the surge of in situ-produced cosmogenic radio- vided not only two ages, 155 and 156 ka, which are attrib- nuclide (CRN) dating of glacial landforms, together with utable to marine isotope stage 6 (MIS 6; i.e. late Rissian), optically stimulated luminescence (OSL) dating of glacio- but also two much younger ages (99 and 84 ka). fluvial outwash deposits, and of the cross-correlation of ˜ The Valira catchment (Turu i Michels and Pena Monne, ´ these new data with existing radiocarbon ages of ice- 2006; Turu i Michels et al., 2007; Turu i Michels, 2009) is marginal sediments. It is supported by palynological inves- reported to contain an Eemian-age sediment fill, discovered tigations and by morphostratigraphical studies of glacial during drilling in the central valley of Andorra. Here, deeply deposit sequences. These aspects are the main focus of this weathered moraine deposits, attributable to MIS 6, are also review. exposed at Sant Julia de Loria and connect down-valley with
  4. 4. Authors personal copyChapter 11 Recent Advances in Research on Quaternary Glaciations in the Pyrenees 129FIGURE 11.2 Terminal sequence of glacierdeposits in the Carol catchment: age of frontal ¨and lateral Wurmian moraines. 1: Pliocene orearly Pleistocene alluvium capping erosional sur-faces. 2: Proglacial alluvial sheet (T4), deeplyweathered. 3: Alluvial sheet (T3). 4: Proglacialalluvial sheet (T2), weathered. 5: Proglacial allu-vial sheet (T1), poorly weathered or unweathered. ¨6: Ice-marginal sediments. 7: Wurmian till withcrested moraines; materials poorly weathered. 8:Middle Pleistocene (MIS 6) till with physicallydegraded moraines, connected to T2; materialsweathered. 9: Till with highly degraded moraines,connected to T4 (perhaps MIS 12 or MIS 16);materials deeply weathered. 10 : 10Be-dated sites;note age dispersion on the more weatheredmoraines. The map is after Calvet (1996, 2004),with the geochronological data taken fromDelmas (2009).a proglacial alluvial sheet which has yielded an OSL date of young CRN ages obtained from erratic boulders preserved on125 ka. A much older ice front position is also reported ca. the valley slopes high above the poorly weathered and well-10 km further down-valley at Calvinya. This gives an age ¨ preserved lateral moraines of Wurmian age (Delmas, 2009;that is older than 350 ka (the OSL detection limit). However, Delmas et al., 2011). As in the Carol valley, the boulders alsoinspection of exposed sections reveals only very rare striated show signs of post-depositional weathering, of displacementcobbles, and sedimentological features were indicative of along fractures and even of taffoni development. Followingfluvial rather than of glacial deposition. the methodological recommendations of Putkonen and Swanson (2003), out of the sampled population, the oldest 10 Be age was considered to be the most accurate, providing11.2.2. On the North Side of the Range: a resulting age of 122 ka for the deposits near Caraybat `Focus on the Ariege Catchment (Fig. 11.4). `The Ariege moraine sequence (Fig. 11.4) has raised the same Further insight into the Middle Pleistocene glaciationsissues as those in the Carol catchment, namely anomalously has been gained by examining the relative altitudes of glacial
  5. 5. Authors personal copy130 Quaternary Glaciations-Extent and Chronology FIGURE 11.3 Age-bracketed terminal sequence ´ of glacier deposits in the Gallego catchment. 1: High-relief landforms, razorback and hogback ˆ ridges. 2: aretes. 3: Tread of highest alluvial deposits (coronas). 4: Proglacial alluvial sheets and mantled wash pediments (weathered and rubi- fied: Qt 5, alias T2). 5: Proglacial alluvial sheets ¨ (poorly weathered: Qt 6-7-8, alias T1). 6: Wurmian ice-contact sedimentary units (lacustrine or flu- vial). 7: Moraine ridges and till deposits. 8: Scoured rock surfaces on bedrock steps. 9: Successive ¨ Wurmian ice front positions. 10: Middle Pleisto- cene (MIS 6) ice fronts. 11: OSL and 14C-dated sites. After the geomorphological map by Barrere ` ˜ (1966) and data by Pena et al. (2004), Lewis et al. (2009) and Jalut et al. (1992). deposits, the advanced weathering state of which indicates even of different glacial cycles: (i) in the Tarascon basin ¨ that they are older than Wurmian. This approach has allowed ` and in the higher Ariege catchment, glacial evidence from differentiation between generations of deposits that are leg- MIS 6 occurs as trails of weathered till deposits and erratic acies of different stadial events within MIS 6, or perhaps boulders situated ca. 50–100 m higher than Wurmian ¨
  6. 6. Authors personal copyFIGURE 11.4 Pleistocene glaciation in the Ariege catchment. 1: Razorback and hogback scarps in Mesozoic or Cenozoic limestone, sandstone or con- `glomerates of the outer Pyrenean fold belt. 2: Relict Pliocene to early Pleistocene alluvial fan deposits (formation du Lannemezan). 3: Higher alluvialterrace T4, severely eroded. 4: Intermediate terrace, intensely weathered and rubified (Haute Boulbonne). 5: Lower terraces (generation T2), weathered ¨and rubified (Basse Boulbonne, Vernajoul and ice-marginal terraces of Antras and Foix–Cadirac). 6: Lowermost terrace T1, of Wurmian age, unweatheredand capped by brown soils. 7: Middle Pleistocene ice boundaries; a: probable MIS 6 ice limits, grading to the T2 terraces; b: erratic boulders or residual ¨morainic deposits dating to MIS 6 or older. 8: Wurmian maximum ice extent; the corresponding population of unweathered moraines grades to the T1 `alluvial sheets. 9: MIS 2 ice boundaries, that is, Garrabet stadial. 10: Recessional moraines at Bompas–Arignac and Berniere (19–20 ka). 11: Recessional `moraines (Petches and Freychinede) dating to the Oldest Dryas. 12: Position of dated sites. Distribution of glaciofluvial deposits in the piedmont zone,based on Hubschman (1975, 1984) and on the geological map of the Pyrenees Quaternary by Barrere et al. (2009). The ice limits and 10Be data are after `Delmas (2009) and the 14C ages after Jalut et al. (1982, 1992).
  7. 7. Authors personal copy132 Quaternary Glaciations-Extent and Chronology maximum glacial moraines. In the Foix–Montgaillard basin, 11.3. THE LAST PLEISTOCENE GLACIAL the Antras ice-contact depositional sequence and the ice- ¨ CYCLE (WURMIAN STAGE) ` marginal sediment body at Becq-en-Barguillere (Taillefer, 1973) represent two additional sites to which the MIS 6 max- The chronology of the Pyrenean last glacial cycle, corre- imum ice extent can be tethered. Given these new interpre- ¨ lated with the Wurmian Stage of the Alps by most Pyrenean tations, the ice-marginal meltwater terrace sequence at Foix– authors, has been mostly based on a large set of 14C ages Cadirac was ascribed by Hubschman (1975, 1984) to the Ris- produced over the past 30 years, supplemented in the last sian. This was because of the advanced state of weathering of five mostly by new 10Be and OSL ages. The image of the its debris. However, it occurs at a lower elevation than the ¨ Wurmian chronology provided by this body of evidence Becq and other MIS 6 benchmarks and could instead repre- is still incomplete, with contradictions between the methods sent the legacy of a deglacial event that occurred toward the employed giving sometimes puzzling contrasts between ` end of the MIS 6 cycle. Further down the Ariege valley, the adjacent valleys. Radiocarbon dating is usually indirect in Foix–Cadirac terrace grades to the Basse Boulbonne (T2) that it provides an age for proglacial or ice-marginal organic ` alluvial sheet. T2 (mapped by Barrere et al., 2009) is capped sediments whose relation to adjacent moraines is not always by rubified soils displaying a depleted upper horizon. Char- straightforward. Even though a few terminal moraines have acteristically evolved soils, such as this, suggest an episode been dated directly, most OSL dating raises similar prob- of weathering and eluviation during the Eemian Stage lems. This is because sampling strategies target alluvial (MIS 5e) climatic optimum. (ii) Erratic boulders sourced sheets that are sometimes situated several tens of kilometres by the granitic and gneissic Aston massif have been found from the nearest ice front deposits. CRN dating, in contrast, resting on the Mesozoic limestone and shale outcrops at Car- systematically allows direct dating of glacial landforms, aybat, down-valley from the Antras delta and 50 m above it. whether moraines, isolated erratic boulders or ice-scoured This boulder trail was produced by one or several ancient gla- bedrock steps. However, CRN exposure ages are, by defi- ciers of corresponding thickness and has been tentatively cor- nition, minimum ages, with the precautions this limitation related with elevated moraine deposits on the outer flanks of inevitably brings upon interpretation of the data. ´ ´ the Arize massif and in the Tarascon basin at Quemenailles ` and Menties. In their lower reaches, these ice masses may have extended as far as Pech de Varilles, at the proximal edge ¨ 11.3.1. The Age of Wurmian Maximum Ice of the Pamiers plain. At Pech de Varilles, Faucher (1937) Extent: In or Out of Phase with the Global Last reported erratic boulders of gneiss exceeding 6 m in diame- Glacial Maximum? ter, sourced from the Aston massif. He hypothesised that they were of glacial origin. However, other geologists later either The mid-latitude Pyrenean mountain range is under direct argued that the boulders had been supplied from the under- influence of the North Atlantic weather systems. This lying Eocene conglomerate bedrock or else ascribed them to makes the Pyrenees an ideal setting for testing whether or unconformable Pliocene fan deposits that excavated the not mid-latitude mountain glaciation and global climatic Eocene conglomerate and locally contain greater than 1 m- fluctuations were synchronous (Gillespie and Molnar, sized quartzite boulders. Nevertheless, the boulders 1995; Hughes and Woodward, 2008; Thackray et al., described by Faucher are conspicuous by their petrography ¨ 2008). The now classic view that the Wurmian Pyrenean and provenance, by their large size, and by their extremely maximum ice extent and the global last glacial maximum limited weathering state. They occur ca. 460 m a.s.l., that (LGM-defined as the time period between 23 and 19 ka is, 90 m above the tread of the deeply weathered Haute Boul- cal. BP, Mix et al., 2001) did not occur simultaneously bonne (T3) terrace, which occupies the opposite bank of the was presented in the 1980s on the basis of radiocarbon ages ` Ariege river in the same area (Calvet, unpublished). These obtained on ice-contact lake sediments on the north side of indications imply that this elevated erratic boulder trail is the range (Andrieu et al., 1988; Jalut et al., 1988, 1992; of early Middle Pleistocene age and correlates, as in the Andrieu, 1991; detailed synthesis in Delmas, 2009, Carol valley, with the older alluvial terrace T4 deposits. This pp. 96–109). This radiocarbon-based perspective on topographical position suggests that the erratics at Pech de ¨ Pyrenean glaciation advocated a Wurmian maximum ice Varilles are comparable to the ancient moraine deposits that extent between 70 and 50 ka. After this time, the glaciers occur much further west at Arudy. The latter also occur at stagnated slightly up-valley from their maximal advance elevations similar to those of the relict Pliocene to early positions before retreating to the uppermost reaches ca. Pleistocene piedmont deposits of the Gave d’Oloron catch- 29–25 ka, depending on sites. To explain the recessional state ´ ment (Hetu and Gangloff, 1989). CRN dating was not of the Pyrenean ice cap by the time of the global LGM (which attempted on the Pech de Varilles erratics because field evi- is broadly correlable to MIS 2), it has been argued that cli- dence indicates that they have been exhumed from a finer matic conditions were too dry to sustain extensive ice fields matrix and have been displaced by slope movements. on the Pyrenees. Although data are comparatively scarcer, a
  8. 8. Authors personal copyChapter 11 Recent Advances in Research on Quaternary Glaciations in the Pyrenees 133similar model has been proposed for the southern side of Amidst mounting uncertainty over radiocarbon ages ofthe Pyrenees. In the Noguera Ribagorcana catchment ¸ ice-marginal materials, critics have begun to question the(Fig. 11.5), the ice-marginal site of Llestui, interpreted asynchroneity of the Pyrenean glacial model. Basing their ¨as marking a Wurmian post-maximum ice position, was reassessment on biostratigraphical and geochemical evi-initially radiocarbon dated at 31–34 14C ka BP (Vilaplana, dence, palynologists in the late 1980s were first to challenge1983). However, the samples were found to have been con- the view that Pyrenean glacier fluctuations were out oftaminated by inorganic carbon and later redated at 18–21 phase with the global LGM (Turner and Hannon, 1988;14 C ka BP (Bordonau et al., 1993). For reasons of sus- Reille and Andrieu, 1995). The first CRN ages producedpected sample contamination, the 30 14C ka BP radiocar- for glacial landforms in the Noguera Ribagorcana (Pallas ¸ `bon age obtained from the Bassots palaeolake, which is et al., 2006) prompted a more systematic critique of radio-ponded behind the terminal lobe of the Ribagorcana ¸ carbon ages from the Pyrenees, most of them being deemedmoraine, has also been discounted as unreliable (Bordonau unreliable and were rejected. The competing hypothesis ofi Ibern, 1992). ¨ synchroneity between the Wurmian maximum ice extent and the global LGM was asserted. This was based on 10 Be ages of 18.1 Æ 1.9 ka obtained from a lateral moraine of the Noguera Ribagorcana trunk glacier and 21.3 Æ 4.4 ka ¸ obtained from an erratic boulder close to the terminal lobe (Fig. 11.5). These 10Be ages were revised to 19.2 Æ 2 of the Noguera Ribagorcana trunk glacier and 22.6 Æ 4.7 ka, ¸ ´ ` respectively, by Rodes (2008). Pallas et al. (2006) have nev- ertheless not rejected the hypothesis that a relatively pro- tracted ‘Pleniglacial’ might have prevailed between before 30 ka and ca. 20 ka. This means that glacier snouts may have been stationed far down the valleys not only dur- ing the global LGM but also for quite some time before it. The body of geochronological information available today, however, provides evidence of a somewhat less static history of glacier fluctuation, and consequently, a less seri- ous antagonism between the in-phase and out-phase schools of Pyrenean glaciation. The asynchroneity of maximum advances of mountain and continental glaciers needs no ¨ longer to be challenged. The Wurmian maximum ice extent now appears to have occurred everywhere quite early dur- ¨ ing the Wurmian Stage, irrespective of the dating methods used to test this hypothesis. More controversial is the relative position of ice fronts during the MIS 2 or more accurately during the global LGM, either because of insuf- ficient data or because of genuine variability between valleys along the mountain range. ¨ 11.3.2. Dating of Pyrenean Wurmian Terminal Moraine Occurrences On the Spanish side, the most abundant evidence has been ´ obtained from the Gallego catchment (Fig. 11.3). The ter- ¨ minal moraine at Senegue has yielded two OSL ages of 36 Æ 3 and 36 Æ 2 ka, respectively (Pena et al., 2004; Lewis ˜ et al., 2009). In the western part of the catchment, elevationsFIGURE 11.5 Terminal sequence of glacier deposits in the Noguera do not exceed 2.7 km. As the radiocarbon ages of 30–25 14C ¨Ribagorcana catchment, age of the Wurmian moraines. 1: Mountain ridge. ¸ ka BP obtained from proglacial lake sediments around Tra- ¨ ¨2: Unweathered Wurmian till or moraine. 3: Wurmian fluvio-lacustrine ¨ice-contact deposit. 4: Wurmian ice front limits and typical U-shaped val- ´ macastilla and Pourtalet demonstrate (Garcıa-Ruiz et al.,ley cross-profiles; 14C- and 10Be-dated sites. The map and chronological 2003; Gonza ´ lez-Samperiz et al., 2006) this area was already ´ ` ´data are after Bordonau et al. (1993), Pallas et al. (2006) and Rodes (2008). partially deglaciated in MIS 2 and cut off from transfluent
  9. 9. Authors personal copy134 Quaternary Glaciations-Extent and Chronology sources of northern ice spilling over the Col du Pourtalet. A data are still unpublished but they clearly confirm that core obtained from a cirque located 2 km to the east of the ¨ the Wurmian maximum ice extent occurred much earlier Col du Pourtalet indicates a possible readvance between than the global LGM. The ice-marginal lake sediment 24.17 and 19.25 14C ka BP. This advance would have sequence at La Massana, which formed where the Valira occurred during MIS 2 even though ice had vacated the del Nord and Valira d’Orient glaciers separated, has been topography at elevations situated above that site since subjected to detailed sedimentological and stratigraphical before 19.25 14C ka BP. The position of the Gallego ice ´ analysis of its progradational deltas. AMS radiocarbon front in MIS 2 is difficult to establish, but the 20.8 14C dates have returned ages of 25.63, 21.51 and 17.43 14C ka BP (i.e. 24.217 ka cal. BP) radiocarbon age obtained ka BP (29031 to 27820, 24150 to 23310, 21097 to 20271 from the base of the Bubal lake deposits (Jalut et al., ka cal. BP) and indirectly confirm the presence of this lake 1992) suggests that the LGM glacier front was stationed since $ 41 ka on the basis of sedimentation rate calcula- as much as 15 km up-valley from Senegue. ¨ tions. The lake sediments subsequently recorded a succes- In the Cinca catchment, the maximum extent of the Ara sion of three glacier advances of diminishing intensity, all glacier is constrained by ice-contact lake deposits at Linas ´ falling chronostratigraphically within MIS 2 (Turu i de Broto. A 50-m drillhole here produced a mid-core AMS Michels, 2002). The maximum ice extent may have been 14 C age of 30.38 14C ka BP (Martı-Bono et al., 2002), indicat- ´ captured by a 59 Æ 1.18 21Ne ka age obtained from an ¨ ing an early age of Wurmian maximum ice extent and possi- ice-scoured bedrock surface that was sampled on the upper ble maintenance of this advanced position until MIS 2 and flank of the glacial trough above the village of Canillo global LGM time. In the Cinca valley, till at Salinas de Sin (Turu i Michels et al., 2004). is associated with glaciofluvial deposits and has yielded In the eastern Pyrenees, 10Be exposure ages show that OSL ages of 46 Æ 4, 63 Æ 6 and 71 Æ 15 ka. The Salinas expo- the MIS 2 maximum ice advance reached positions within sure has been subjected to glaciotectonic deformation, proof ¨ a few hundred metres of the Wurmian maximum ice extent. that it has also been overridden by the glacier since 46 ka ˆ The frontal moraine of the Tet glacier at Mont Louis (Lewis et al., 2009). (Fig. 11.6) has provided an age of 21.4 Æ 3.7 ka (Delmas On the Noguera Ribagorcana (Fig. 11.5), MIS 2 ages ¸ et al., 2008), since then recalibrated as 22 Æ 3.6 ka (Delmas, have been obtained by 10Be dating of deposits situated in 2009). However, the dated boulders belong to the innermost the vicinity of the presumed glacier terminus (Pallas ` ˆ ¨ lobe crest of the Tet Wurmian terminal moraine sequence. et al., 2006). The terminal lobes, however, have been signif- The two outermost lobes have so far not been dated but icantly eroded, making it difficult to draw any firm conclu- could correspond to the sites of maximum ice extent sions from this valley about the age of the Wurmian ¨ ¨ reached earlier during the Wurmian stage. In the Carol maximum ice extent. In the Valira catchment, most of the catchment (Fig. 11.2), the right flank lateral moraine has FIGURE 11.6 Terminal sequence of Wurmian ¨ ˆ glacier deposits in the Tet catchment: age of fron- tal and lateral moraines. 1: Quaternary incision of valleys into the Cenozoic erosion surface of the Col de La Perche. 2: Alluvial sheet T2, weathered (MIS 6). 3: Proglacial alluvial sheet T1, unweath- ¨ ered and connected to the Wurmian moraines. 4: ¨ Wurmian unweathered till and moraines, with the position 10Be exposure-dated sites. 5: Succes- ¨ ¨ sive Wurmian ice front positions. 6: Wurmian ice- contact sedimentary units (lacustrine or fluvial). The mapping and geochronological data are after Delmas et al. (2008) and Delmas (2009).
  10. 10. Authors personal copyChapter 11 Recent Advances in Research on Quaternary Glaciations in the Pyrenees 135been dated with great precision by three samples: as early as 29–25 14C ka BP, and no post-maximum ice front22.9 Æ 2.7, 22.1 Æ 3.4 and 21.7 Æ 2.8 ka (Delmas, 2009). position corresponding to MIS 2, for example, similar to the ˆHowever, as in the case of the Tet moraine sequence, the Garrabet frontal system, has been clearly identified. Thedated lateral moraine connects with a frontal lobe that is sit- interpretations of Jalut et al. (1992) and Reille and Andrieuuated ca. 0.6 km up-valley from the Vinyola terminal lobe. (1995), respectively, have diverged on these issues.The dated lateral moraine is therefore not the oldest in the Accordingly, a discussion of their respective merits in the ¨Wurmian sequence. Finally, the small Malniu glacier, context of this review seems timely. Assuming first thatwhich remained disconnected from the Carol trunk valley the 38.4 Æ 2 14C ka BP age obtained at the Biscaye sitethroughout the Wurmian stage, has yielded 10Be ages of ¨ by Mardonnes and Jalut (1983) is invalid and should be76.5, 42.3 and 40.6 ka. The post-maximum Malniu frontal replaced by the AMS 14C ages obtained by Reille andlobe, which is situated 0.5 km further up-valley, provided Andrieu (1995) from glaciolacustrine clays at the Lourdesan age of 21.3 ka, which accords with the 23.9 ka age of a ¨ 1 terminal moraine site, a Wurmian maximum ice extentcoeval lateral moraine preserved on the valley side (Pallas ` older than 20.025 14C ka BP (22486 to 21562 ka cal. BP)et al., 2010). appears correct, with vacation of the piedmont zone Among the north-facing glacial catchments, only the between 16.67 and 14.46 14C ka BP. New 10Be exposure `Ariege basin has recently delivered new data (Delmas, data from the Gave de Pau catchment have been obtained2009; Delmas et al., 2011). Results, which largely rely on from morainic boulders situated at the Ossen diffluence site,10 Be exposure ages, confirm the classic view of asynchro- immediately above the town of Lourdes. However, withneity between mountain glaciation and the continental ice- five Lateglacial and Holocene ages and another ofsheet record and are broadly compatible with existing 53.5 Æ 38 ka, results remain inconclusive (Rodes, 2008). ` ¨results from the Spanish catchments. The Wurmian maxi- Likewise, the moraine sequence at Aucun has yielded four 10mum ice extent (Fig. 11.4) is constrained by two CRN ages Be ages ranging between 13 and 10 ka. All these ages,that suggest two glacier advances of comparable extent: a however, are suspiciously young for this small glacier,81.4 Æ 14.6 ka age obtained from an erratic boulder in the which at that time was disconnected from the Gave devicinity of the terminal moraine and a younger age of Pau trunk valley glacier. At the time, as shown by the posi-34.9 Æ 8.5 ka obtained on the mid-valley lateral moraine ` tion of the lateral moraines at Argeles-Gazost (Barrere ` `at Larcat. The Ariege glacier advanced to within 2 km of et al., 1980), this larger glacier still descended to elevationsthe town of Foix. The older age is in good agreement with below 400 m a.s.l. and to positions less than 10 km from thethe 91.4 Æ 2.4 ka U–Th age obtained for the stalagmite floor ¨ outermost Wurmian ice front at Lourdes. Assuming now,in the prehistoric cave at Niaux, which occurs 8 km up-val- instead, that the chronology advocated for the Wurmian ¨ley and is just slightly older than the Niaux glacial deposits maximum ice extent by Mardonnes and Jalut (1983) is(Sorriaux, 1981, 1982; Bakalowicz et al., 1984). The MIS 2 ` valid, it is feasible that the Argeles-Gazost and Aucunmaximum extent ice front has not been directly dated, but moraine ridges could define the post-maximum glacierits position almost certainly coincides with the Garrabet position during MIS 2 peak.frontal moraine, which is located 8 km up-valley from the If these local insights are assembled into a more coher- ¨Wurmian terminal moraine. It has two recessional fronts ent Pyrenean-scale picture, it appears that the Wurmian ¨ `at Bompas–Arignac and Berniere, respectively. This chro- maximum ice extent occurred somewhat earlier than thenology was established indirectly by a surface exposure age global LGM, perhaps with glaciers advancing out of thewith a strong component of nuclide inheritance (31.2 ka) on crest zone to similar positions repeatedly during MIS 5d,the bedrock step of the Tarascon basin and by the age of an 5b and 4. By definition, all 10Be ages are minimal exposureadjacent erratic boulder resting directly on the dated bed- ages and thus cannot precisely date the moraine ridges thatrock-step surface (16.7 ka). Such a conjunction suggests have been sampled. Caution, therefore, must be exercisedso far unreported ice recessions and readvances, that is sta- when determining the exact number and position of glacierdials, within the Pyrenean last glacial cycle. Other bedrock advances and retreats. For example, the MIS 2 ice frontssteps have yielded exposure ages of 19 ka, for example, on a have only been positively identified in the eastern, morescoured rock exposure situated 2.5 km up-valley from Gar- Mediterranean part of the mountain range: for example,rabet. These ages equate to a $ 19 ka age for the Berniere ` ˆ ` in the Tet, Carol, Malniu, Ariege, Valira and Ribagorcana ¸frontal lobe and provide the timing of the deglaciation valleys. It further appears that, in this region, MIS 2 iceincrement that followed the Garrabet stage. ¨ fronts also nearly reached the Wurmian maximum glacier So far, no new data have been produced that might help positions. The MIS 2 ice fronts reached those positions atto update the radiocarbon-dated terminal lobes of the the time of, or just before, the global LGM (i.e. betweenGaronne, Gave de Pau and Gave d’Ossau catchments 23–19 ka cal. BP, Mix et al., 2001). In the western catch-(Fig. 11.1). In these western tracts of the Pyrenean range, ments (Garonne, gave de Pau, Gave d’Ossau, Gallego),the glaciers are thought to have receded to the upper valleys which are under greater direct influence from Atlantic
  11. 11. Authors personal copy136 Quaternary Glaciations-Extent and Chronology weather conditions, MIS 2 appears to have coincided 19.25 14C ka BP (i.e. 22–23 ka cal. BP; Gonzalez-Samperiz ´ ´ instead with a recessional period. It is too soon to decide et al., 2006); at the Col de Lhers (Fig. 11.4), it occurred whether such contrasts should be ascribed to methodologi- ca. 22–20 14C ka BP (Jalut et al., 1982, 1992) and probably cal artefacts inherent in the dating techniques used, or likewise at the Col de Puymorens. whether the observed differences in glacier behaviour along After those events, glacier ice rapidly vacated most Pyr- the strike of the mountain range truly reflect palaeoclimatic enean valleys, although it did not necessarily undergo a differences. Far from being implausible, such differences ˆ steady or uniform decline. In the Tet catchment, for exam- could indeed have been controlled over the western Medi- ple, four moraine systems, both lateral and frontal, form a terranean by the Balearic low atmospheric pressure centre, regular sequence receding back into the crest zone. Degla- which may have been more active than Atlantic weather ciation also affected cirques, at least in the south-eastwards systems during MIS 2 because of the southerly position ˆ facing Tet catchment. For example, fossil peat exposed at ¨ of the Polar Front at the time (Florineth and Schluchter, the Grave-Amont site (elevation: 2150 m) has yielded three 14 2000; Hughes and Woodward, 2008). C ages that cluster around 20 ka cal. BP (from top to bot- tom: 20.25–19.87, 19.204–18.666 and 20.26–19.37 ka cal. BP; Delmas, 2005; Delmas et al., 2008). The reliability of 11.3.3. Evidence of Rapid Deglaciation from these ages is reinforced by the fact that they were obtained from well-preserved Sphagnum. This material was free the end of the global LGM from graphite contamination from the Palaeozoic schist The stages of ice recession are only known for certain val- outcrops present in the catchment (this was verified by leys, and it is not yet possible to produce a coherent synthe- X-ray diffraction analysis of the matrix) and with no possi- sis for the entire Pyrenean range. Evidence of a first ˆ ble hard-water effect. In the Tet catchment, the Llat reces- recessional stage is provided by frontal moraines situated sional moraine (2200 m a.s.l.) has yielded a 10Be age of only a short distance behind the MIS 2 maximum ice 18.4 Æ 2.3 ka (Delmas et al., 2008; Delmas, 2009). Like- advance fronts, with additional evidence of significantly wise, the small Malniu catchment exhibits a recessional thinner glaciers merely lining the valley floors (Delmas, moraine, with a 10Be age of 18.2 Æ 0.5 ka, situated on the ` 2009). In the Ariege catchment (Fig. 11.4), the ice field ` 2200 m a.s.l. contour (Pallas et al., 2010). Further west, was dismembered into disjunct trunk and tributary valley in the Valira catchment, valley glaciers and the Massana ` glaciers, with, for example, the Courbiere, Vicdessos and moraine-dammed lake seem to have persisted until 17.5 14 Lauze glaciers forming disconnected, self-contained sys- C ka BP (Turu i Michels, 2002), and the lower lateral tems. In the Cabanes basin, the glacier rapidly melted to moraine at Engolaster has returned a 10Be age of half of its thickness. During this time, a number of new 18.077 Æ 1.31 ka (Turu i Michels et al., 2004; Turu i ice-contact lakes and deltas also formed, such as at Michels, 2009). Along the Noguera Ribagorcana, the snout ¸ Ascou–Goulours and Axiat–Senconac on the trunk valley of an 8-km-long valley glacier reaching an elevation of right flank, and at Niaux and Surba on the trunk valley left 1560 m a.s.l. formed the Santet moraines, with a mean flank. The frontal moraines of this period are situated at 10 Be age of 13.7 Æ 0.9 ka (the oldest boulder being Bompas–Arignac (19.1 Æ 3.8 ka, a 10Be exposure age 16.1 Æ 2.8 ka: Pallas et al., 2006). Rodes (2008) has recali- ` ´ obtained indirectly from a ice-scoured bedrock step just brated the Santet ages to 14.6 Æ 1.1 and 17 Æ 0.7 ka, respec- down-valley from the Bompas-Arignac ice front) and at tively. Meanwhile, in the upper subcatchment of Berniere (18.8 Æ 1.3 ka, 10Be age obtained on an erratic ` the Noguera de Tor, which is the main tributary to the boulder). The corresponding exposure ages are in remark- ¨ Noguera Ribagorcana river, the Redo d’Ayguestortes lake, ¸ able agreement with the 19.1 Æ 1.6 ka U–Th age from the dammed by a frontal moraine at an elevation of 2110 m a.s. younger stalagmite floor that seals the glacial deposits l. was ice free by 13.47 Æ 0.06 14C ka BP (Copons and Bor- trapped in the Niaux cave (Sorriaux, 1981, 1982; Bakalo- donau, 1996). wicz et al., 1984). This evidence also heralds the end of gla- In the north-facing catchments of the Pyrenees, valley cial sedimentation at this site, with the cave becoming glaciers several kilometres long appear to have persisted abandoned above the level of glacier activity. until, or even readvanced during, the Oldest Dryas Sta- ˆ ˆ A similar story emerges from the Tet ice field, with the Tet ` dial. This is well observed in the Ariege catchment at glacier snout receding ca. 2.5 km from the Mont Louis termi- Freychine ` des, where a 4- to 6-km-long glacier tongue nal lobe to the site of the Borde frontal moraine (Fig. 11.6). persisted until 13 14C ka BP ($16 ka cal. BP) alongside Three 10Be exposure ages from the Borde moraine have indi- an ice-marginal lake (Jalut et al., 1982, 1992). This has cated that recession occurred ca. 18.1 Æ 3.9, 19.4 Æ 2.5 or since been confirmed by CRN dating, with a bedrock- 21.1 Æ 3.1 ka (Delmas et al., 2008; Delmas, 2009). This step 10Be exposure age of 14.4 Æ 1.1 ka (Delmas, 2009; period also coincided with the disappearance of major trans- ` Delmas et al., 2011). In the upper Ariege catchment, fluence cols: at the Col du Pourtalet, this occurred before the Petches recessional moraine sequence (Fig. 11.4)
  12. 12. Authors personal copyChapter 11 Recent Advances in Research on Quaternary Glaciations in the Pyrenees 137has been age-bracketed by a 10Be age from the Ax bed- ¨ the Wurmian could soon arise from systematic CRN datingrock step of 15.4 Æ 3.4 ka (Delmas, 2009; Delmas et al., of vertical profiles in exposures of glaciofluvial outwash2011). This implies the presence of a 26-km-long valley deposits and of terminal moraines. This would advanta-glacier during the Oldest Dryas Stadial. It was well sup- geously complement, for example, in the Carol and Ariege `plied by tributary valleys descending from the elevated valleys, the rather haphazard dating of scattered erraticcrest zone and feeding into the trunk valley just 5 km boulders (out of a population of heterogeneous and thusfrom its terminus. At the Puymorens col, the lateral inconclusive CRN ages, a single boulder has provided amoraine of a comparatively shorter glacier that des- suitable age of 122 ka, Delmas et al., 2011). The last glacialcended to the carol valley yielded three 10Be ages around cycle, in contrast, has been increasingly well constrained `15 ka (Pallas et al., 2010). through the use of multiple chronometers. Glaciers during ¨ the Wurmian appear to have behaved in a more erratic, non-11.3.4. The Last Glacial Bastions: Moraines in uniform manner than previously believed, particularly by ¨ ‘mono-glacialists’ according to whom the ‘Wurm’ cyclethe Crest Zone Cirques consisted of a single episode of ice advance followed byThe chronology of the cirque glaciers is quite variable and a single episode of ice retreat (Taillefer, 1985). Neverthe-depends on their altitude and their position within the less, the asynchroneity between the Pyrenean mountain gla-mountain mass. As a result, it still remains difficult to pro- cial chronology and the global LGM seems confirmed. Anduce a definitive synthesis on the latest Pleistocene cirque ¨ important interstadial within the Wurmian has beenglaciation of the Pyrenees. In the central Pyrenees, residual ` revealed in the Ariege catchment, with glaciers havingglaciers still exist on the most elevated massifs, and evi- extended repeatedly to approximately the same positionsdence that some of these grew during the Little Ice Age during stadial advances and interstadial recessions. MIShas ushered in the idea, still poorly investigated, of a ‘neo- 2, which coincided with significant post-maximum read-glacial’ resurgence during the Holocene (Gellatly et al., vances to the outermost terminal moraines in the eastern1992). Other cirques, however, were ice free by the time part of the Pyrenees, remains poorly represented in theof the Oldest Dryas as demonstrated, for example, by paly- ´ western half of the range (Gallego, Garonne, Gave denostratigraphy in the cirques of the Madres massif (Reille ` Pau). In the Ariege basin, perhaps because it is located inand Lowe, 1993). In good agreement with 10Be ages the transition zone between east and west, MIS 2 morainesobtained from bedrock steps, most cirques in the south-east- ¨ stand distinctly within the earlier Wurmian maximum ter-ern Pyrenees were almost entirely deglaciated by the mini. It remains too early to confirm whether this east-to-Allerd Interstadial (Delmas, 2005, 2009), at a time when west gradient is a genuine signal reflecting differences inthe tree-line rose rapidly to altitudes of 1800 m a.s.l. on glacier response to palaeoclimatic differences, or whethernorth-facing slopes (Reille and Andrieu, 1993). Neverthe- it is the consequence of insufficient or insufficiently preciseless, in conditions locally favoured either by snow drifts radiometric age brackets. Regional climatic differences cer- ˆor by limited insolation, such as in the Tet (i.e. upper Grave) tainly became quite sharp after the onset of the latest Pleis-valley, some small glaciers persisted or reformed during the tocene deglacial stage, and have remained so as a functionYounger Dryas Stadial, as attested by a 11.6 Æ 1.8 ka, 10Be- of topographical setting and local conditions. Finally,dated moraine (Delmas et al., 2008; Delmas, 2009). This although ice receded rapidly everywhere after the globalappears to have also occurred in the Lorri valley above Puy- LGM, the detailed chronology of cirque glaciation is stillmorens, supported by a 10Be age of 11.8 Æ 0.6 ka (Rodes, ´ difficult to unravel because of its geographical variability.2008; Pallas et al., 2010). More surprising are the 10Be ages ` More work on these aspects is expected in the coming years.of 10.4 Æ 1.2 and 10.3 Æ 1.8 ka (Pallas et al., 2006), recali- `brated to 11 Æ 1.3 and 11 Æ 1.9 ka (Rodes, 2008), that have ´ REFERENCESbeen attributed to the Mulleres frontal moraine and itsneighbouring bedrock step in the Noguera Ribagorcana val- ¸ ´ Andrieu, V., 1991. Dynamique du paleoenvironnement de la vallee mon-´ley. This would imply the presence, in the Noguera Riba- ´ ´ tagnarde de la Garonne (Pyrenees centrales, France) de la fin des temps ` ` ´ glaciaires a l’actuel. These de Doctorat de 3e cycle, Universite de Tou-gorcana catchment, of a 3.5-km-long glacier with a front ¸ louse-le-Mirail, 311 pp.stationed as low as 1700 m a.s.l. at the beginning of the ´ Andrieu, V., Hubschman, J., Jalut, G., Herail, G., 1988. Chronologie de laHolocene Period. ´ ´ ´ ´ deglaciation des Pyrenees francaises. 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