# The Pyrenees: glacial landforms from the Bølling-Allerød Interstadial (14.6–12.9 ka)

Magali Delmas, Marc Oliva, Yanni Gunnell, Marcelo Fernandes, Théo

Reixach, José Fernández-Fernández, Marc Calvet

► To cite this version:

Magali Delmas, Marc Oliva, Yanni Gunnell, Marcelo Fernandes, Théo Reixach, et al.. The Pyrenees: glacial landforms from the Bølling-Allerød Interstadial (14.6–12.9 ka). European Glacial Landscapes, Elsevier, pp.361-368, 2023, 978-0-323-91899-2. 10.1016/B978-0-323-91899-2.00050-4. hal-03989974

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# PART IV. European glacial landforms from the Bølling-Allerød Interstadial (14.6–12.9 ka)

## SECTION 2 European regions that were not covered by the EISC

**Abstract:** The Bølling–Allerød (B-A) Interstadial (14.6–12.9 ka) was a period of intense glacier recession in the Pyrenees. All available proxies indicate that environmental conditions became substantially wetter and warmer than during the preceding post-LGM interval between 18.9 and 14.6 ka (early Last Glacial to Interglacial transition, LGIT). Most of the Pyrenean trunk glaciers vanished, being confined to the cirques and promoting the onset of paraglacial rock slope failures and the formation of rock glaciers. At lower elevations, forest expansion promoted a sharp decrease in catchment erosion rates. Braided glaciofluvial stream channels narrowed and gave way to meandering river styles. The detailed chronology of deglaciation during those 1700 years of rapid change has been documented for only a very small number of valleys, mostly relying on <sup>10</sup>Be or <sup>36</sup>Cl exposure ages obtained from glacially-polished bedrock steps on valley floors, with also a few results from ice-marginal boulders and/or rock glaciers. A period of glacier re-expansion occurred at the end of the B-A Interstadial in response to decreasing temperatures heralding the Younger Dryas.

**Key words:** Bølling-Allerød Interstadial, moraine, terrestrial cosmogenic nuclide exposure dating, Pyrenees

### 38. The Pyrenees: glacial landforms from the Bølling-Allerød Interstadial (14.6–12.9 ka)

(Magali Delmas, Marc Oliva, Yanni Gunnell, Marcelo Fernandes, Théo Reixach, José M. Fernández-Fernández and Marc Calvet)

#### 38.1. Evidence of intense and rapid warming

As elsewhere around the North Atlantic (Liu et al., 2009; Thiagarajan et al., 2014, Naughton et al., 2021) and throughout most of Europe (Palacios, 2021), the onset of the Bølling-Allerød Interstadial (hereafter B-A, i.e., GI-1 after Rasmussen et al., 2014) in the Pyrenees featured a peak of warmth, with temperatures similar to present-day values, followed by gradual cooling (Bernal-Wormull et al., 2021). These conditions favoured the retreat and collapse of some Northern Hemisphere ice sheets, particularly the Eurasian Ice Sheet (Brendryen et al., 2020), and drove accelerated deglaciation along the entire strike of the Pyrenean mountain range (Fernandes et al., 2021). All available proxies indicate that palaeoenvironmental conditions came very close to resembling those later prevalent during the Holocene.

On the western side of the range (Ostolo cave, 248 m a.s.l., Basque mountains), δ<sup>18</sup>O values from stalagmites record a temperature change of roughly 7.5 °C in less than 50 years around 14.938 ± 0.040 ka (model age based on 61 U–Th data points; Bernal-Wormull et al., 2021). This sharp change was almost synchronous with the onset of GI-1 in Greenland ice cores (14.692 ± 0.186 ka; Rasmussen et al., 2014). In the central Pyrenees, summer palaeotemperatures during the last glacial-to-interglacial transition (LGIT: 18.9–11.7 ka) have been inferred from chironomids in the lacustrine sediment sequence at Ech, a palaeolake (710 m). Results suggest summer temperatures of 16–17.5 °C during the B-A (GI-1), a rise of 3 to 7 °C compared to the early LGIT (i.e., Oldest Dryas: 18.9–14.6 ka), with three
short (~100 years) cooling spikes during the B-A (Millet et al., 2012). Warming of this magnitude is also recorded in marine core MD99-2646 (Gulf of Lion), where April-May SSTs rose from 12 to 19 °C around 15 cal ka BP (Melki et al., 2009).

Pollen records also confirm a significant rise in temperature and precipitation in the Pyrenees at the beginning of the B-A, with an expansion of *Betula* and *Juniperus* and a decline of *Chenopodiaceae* from 15-14.5 cal ka BP (synthesis in Jalut et al., 1992; Fletcher et al., 2010; Moreno et al., 2012, 2014). Steppe taxa nonetheless remained abundant (*Artemisia*, *Apiaceae*, *Poaceae*, *Rumex*), particularly under Mediterranean influence where xerophytic plants were most conspicuous. In the wake of *Betula*, the ingress of *Pinus* was progressively followed by *Quercus*, with *Juniperus* yielding to the advance of forest vegetation up to altitudes of ~1700 m on slopes of the northern massifs (Reille and Andrieu, 1993). As a result of these land-cover changes, catchment erosion rates decreased. On valley floors, stream channels narrowed and braided patterns gave way to single-channel meandering river styles (Stange et al., 2013, 2014; Delmas et al., 2015, 2018; Nivière et al., 2016; Mouchené et al., 2017; synthesis in Delmas, 2019).

Although the precise footprint of glaciation during the B-A in the Pyrenees has only been established for a few valleys, all existing evidence points to glacier recession already under way at the beginning of the B-A (Fig. 38.1 and Table 38.1), fully in step with climate warming trends recorded regionally and globally at that time. Evidence is mostly based on ¹⁰Be and ³⁶Cl exposure ages obtained from glacially-polished bedrock steps on valley floors, with also a few exposure ages from moraines and rock glaciers (Pallàs et al., 2006, 2010; Delmas et al., 2008, 2011; Palacios et al., 2015a, 2015b, 2017; Andrés et al., 2018; Oliva et al., 2021; Reixach et al., 2021; Fernandes et al., 2021).

<figure><img src="image_2.png" /><figcaption>**Figure 38.1.** Spatial distribution of published research on the Bølling-Allerød Interstadial (14.6-12.9 ka) in the Pyrenees. 1- Grave/upper Têt valley (Delmas et al., 2008; Reixach et al., 2021). 2- SE Carlit/upper Têt valley (Delmas et al., 2008). 3- Orri/upper Querol valley (Pallàs et al., 2010; Reixach et al., 2021). 4- Malniu (Pallàs et al., 2010; Andrés et al., 2018; Reixach et al., 2021). 5- Arànser valley (Palacios et al., 2015a; Andrés et al., 2018; Reixach et al., 2021). 6- Médecourbe/upper Vicdessos valley; Jomelli et al., 2020; Reixach et al., 2021). 7- Bacivèr/Val d'Aran, i.e., upper Garonne valley (Oliva et al., 2021). 8- Ruda/Val d'Aran, i.e., upper Garonne valley (Fernandes et al., 2021). 9-</figcaption></figure>
Noguera de Tor (Copons and Bordonau, 1996). 10- Noguera Ribagorçana (Pallàs et al., 2006). 11- Ésera (Crest et al., 2017). 12- Gállego (Palacios et al., 2015b, 2017).

## 38.2. Almost full glacier extinction in the eastern Pyrenees

In the eastern massifs of the Pyrenees such as the southern flanks of the Tossa Plana and Campcardós, and likewise the SE Carlit, exposure ages from ice-marginal deposits have shown that the landscape had been entirely deglaciated at the beginning of the B-A, thereby promoting the onset of paraglacial rock slope failures and a proliferation of rock glaciers and, more locally, small debris-covered glaciers (Fig. 38.2).

<figure><img src="image_3.png" /></figure>

A

<figure><img src="image_4.png" /></figure>

B

<caption>**Figure 38.2.** Rock glacier development at the time of the Bølling-Allerød Interstadial (14.6–12.9 ka).</caption>

A. Transitional landforms, including a debris-covered glacier (lower part) and a rock glacier (upper part), in the north-facing cirque of Bacivèr Peak (Noguera Pallaresa watershed). B. Moraine and rock glacier in an east-facing cirque of Pic Carlit (Têt watershed).

a mis en forme : Police :Gras, Italique

In the Campcardós massif, the last presence of cirque glaciers dates back to the early LGIT (GS-2.1a; see Chapter 21). Glacier extinction occurred there during the B-A (Pallàs et al., 2010, Andrés et al., 2018; Reixach et al. 2021). Given the elevation of those south-facing cirque floors (2420 to 2550 m), it can be inferred that the ELA was locally at least 2500 m. Among south-facing cirques of the neighbouring Carlit massif, exposure ages of 16.1 ± 2.0 ka, 16.9 ± 1.9 ka and 16.6 ± 2.4 ka were obtained from glacially-polished bedrock steps situated at 2350 and 2410 m (sites J and K, respectively, see Fig. 38.3 for exposure ages without snow correction). In the south-facing upper
Arànser valley (Tossa Plana massif), exposure ages of 13.7 ± 1.1 ka (PIR 11-12), 14.7 ± 1.4 ka (PIR 11-08), and 12.6 ± 1.1 ka (PIR 11-09) were obtained from rock glacier boulders situated at elevations ranging between 2150 and 2450 m (Palacios et al., 2015a; Andrés et al., 2018; Reixach et al., 2021).

The only evidence of surviving cirque glaciers is restricted to suitably snow-fed areas at high elevations. This also includes some S- or SE-facing cirques exposed to moist airflow from the northwest locally spilling over ridgetops. An example is the Grave (upper Têt valley), where a cirque glacier appears to have subsisted during the B-A (weighted mean ¹⁰Be exposure ages of 13.8 ± 0.6 ka and 13.1 ± 0.6 ka obtained from two frontal moraines situated at elevations of 2150 and 2160 m; samples at sites M, N and O, Fig. 38.3; Delmas et al., 2008); and where the inferred ELA was ~2450 m (Reixach et al., 2021). Such was also the case in the upper Querol valley (Col de Puymorens), where NE-facing Orri Cirque hosted a valley glacier 3.8 km long, descending to an elevation of 2100 m, and with a local ELA around 2420 m (weighted mean age of 14.5 ± 0.6 ka based on exposure ages obtained from the corresponding lateral moraine: 14.4 ± 1.4 ka (LOR01), 15.0 ± 1.5 ka (LOR03), 13.7 ± 1.0 ka (LOR04), and 15.4 ± 1.9 ka (LOR05); Fig. 38.3; Pallàs et al., 2010; Reixach et al., 2021). Lastly, Médecourbe cirque (upper Soulcem valley, Ariège) hosted a 1.5 km long glacier with an ELA around 2525 m (weighted mean age of 13.2 ± 0.9 ka). Those results are based on four ¹⁰Be exposure ages from a frontal moraine located at 2230 m (Fig. 38.3; Jomelli et al., 2020; Reixach et al., 2021).
<figure><img src="image_5.png" /><figcaption>**Figure 38.3.** East Pyrenean ice extent at the time of the Bølling-Allerød Interstadial (14.6–12.9 ka).</figcaption></figure>

A. Ice extent in Médecourbe cirque at the time of the Bølling-Allerød interstadial or Younger Dryas, depending of whether or not a snow correction factor is applied. B. Ice extent in Médecourbe cirque at the time of the Bølling-Allerød Interstadial and/or Younger Dryas. C. Location map. Red stars locate $^{10}$Be and $^{36}$Cl sampling sites: 1- Grave (upper Têt watershed), frontal moraine at 2150 m, site M (Delmas et al., 2008; ages recalculated after Reixach et al., 2021). 2- Grave (upper Têt valley), frontal
moraine at 2160 m, sites N and O (Delmas et al., 2008; ages remodelled after Reixach et al., 2021). 3-SE Carlit (upper Têt watershed), bedrock step at 2350 m (site J; Delmas et al., 2008; ages remodelled for this review). 4-SE Carlit (upper Têt watershed), bedrock step at 2410 m, site K (Delmas et al., 2008), ages remodelled for this review). 5-Orri (upper Querol watershed), lateral moraine, site LOR (Pallàs et al., 2010; ages remodelled after Reixach et al., 2021). 6-Malniu (Pallàs et al., 2010; Andrés et al., 2018; ages remodelled after Reixach et al., 2021). 7-Arànser, rock glacier, site PIR11 (Palacios et al., 2015a; Andrés et al., 2018; ages remodelled after Reixach et al., 2021). 8-Médecourbe (upper Vicdessos valley), frontal moraines, site 'Med' (Jomelli et al., 2020; ages remodelled after Reixach et al., 2021). Exposure ages with and without a snow correction factor (SCF values indicated in brackets) are presented in separate boxes, with weighted mean ages displayed below the curly bracket. For site K, exposure ages are reported without (top) and with (bottom) a snow correction factor (SCF value indicated in brackets).

### 38.3. Extensive glacier recession in the western and central Pyrenees

¹⁰Be exposure ages from two glacial cirques in the headwaters of the upper Garonne valley (Val d'Aran) confirm the regional indices of rapid warming presented in Section 38.1. Samples obtained from polished bedrock surfaces from the lip (1950-2000 m), floor (2200-2300 m) and upper headwalls (2400-2440 m) of Bacivèr Cirque, for example, document rapid retreat between 14.8 and 14.2 ka (Oliva et al., 2021). The data also suggest full deglaciation of this west-facing cirque (local peaks around 2600-2650 m) by the end of the B-A, with local variants further addressed in Section 38.4. A similar process occurred in tributary watersheds of the Ebro River, such as in small cirques of the upper Gállego valley where valleys sub-catchments at elevations of ~2600-2700 m were apparently also glacier-free by 14 ka (Palacios et al., 2015b). Deglaciation promoted very active paraglacial dynamics inside the recently ice-free cirques, leading to the formation of permafrost-related landforms such as rock glaciers on cirque slopes (Palacios et al., 2017).

The last remaining valley glaciers likewise vanished from the Ésera and Noguera Ribagorçana valleys by the end of the B-A, confining residual glaciers thereafter to the cirques (Pallàs et al., 2006; Crest et al., 2017). A widespread population of lakes also became a feature in the most elevated cirques. Sediment cores from Lake Redó (Noguera de Tor valley), for example, have yielded a record of glacio-lacustrine sedimentation beginning around 13,470 ± 60 yr BP (16,068 to 16,481 cal yr BP) and continuing until the early Holocene, as shown by a radiocarbon age of 9,880 ± 60 yr BP (11,243 to 11,457 cal yr BP) obtained from gyttja deposits in the stratigraphy (Copons and Bordonau, 1996).

### 38.4. Local evidence of glacial oscillations in the higher central part of the mountain range

Greenland ice cores recorded several cold phases during the LGIT, such as the Older Dryas or the Intra-Allerød Cold Period, nested within the warmer B-A interval (Rasmussen et al., 2014). The regional trend of glacier recession in the Pyrenees likewise recorded short periods of centennial-scale cooling at Ech palaeolake (710 m; Millet et al., 2012), Lake Estanya (670 m; Vegas-Vilarrúbia et al., 2013), and in the Portalet sedimentary fill sequence (1802 m, uppermost Gállego watershed; González-Sampériz et al., 2006). These colder spikes may have promoted glacier expansion among north-facing cirques of the central Pyrenees in areas surrounded by high peaks (> 2800 m) and benefiting from suitably large accumulation zones above 2400-2500 m. This would have favoured
the persistence, and perhaps the readvance, of short cirque glaciers (< 1 km) in the lower elevation belts of the central and western Pyrenees, with glaciers expanding to greater lengths (< 3–5 km) in the vicinity of higher peaks. In Piniecho Cirque (upper Gállego valley), for example, moraines from the late B-A have been found extending 0.4–0.6 km from the headwall at elevations of 2300–2350 m (elevation of surrounding ridgetops: ~2700 m; Palacios et al., 2017).

As in the eastern Pyrenees (Section 38.2), local topoclimatic conditions must have played a prominent role in the magnitude, distribution, and chronology of glacier behaviour during the B-A, but with additional evidence of glacier response to the colder oscillations. Ages of 14.8 and 14.0 ka, for example, were obtained from polished surfaces at lower altitudes (1860–1900 m) on the floor of Ruda valley (uppermost Garonne watershed; Fernandes et al., 2021), where glacier retreat and ice thinning during the B-A were interrupted by two brief stillstands (perhaps even involving a minor readvance), with corresponding moraine ages of 13.5 and 13.0 ka (Fernandes et al., 2021). Bacivèr cirque recorded a similar trend, with moraines 0.3 km from the headwall corresponding to a stillstand around 12.9 ka (Oliva et al., 2021), and thus also heralding the onset of the Younger Dryas.

### 38.5. Conclusion

Despite the limited chronological constraints on the glacial landforms generated during the B-A Interstadial, it appears that glacier behaviour between 14.6 and 12.9 ka was relatively uniform along and across the Pyrenean range. The period recorded a major glacier recession phase as early as 15–14 ka (transition between GS-2.1a and GI-1) caused by a rise of ELAs by about 100 m with respect to their earlier LGIT positions (Table 38.1). This trend matches the Greenland ice stratigraphic record as well as all climatic and environmental proxies recorded regionally. Brief glacier stillstands and/or advances recorded at a small number of locations in the higher central ranges also occurred in step with colder oscillations of the B-A interval such as the Older Dryas and the Intra-Allerød Cold Period.

<table><caption>Table 38.1. Glaciated environments in the Pyrenees during the Bølling–Allerød Interstadial (14.6–12.9 ka): a summary</caption><thead><tr><th>Valley (massif or cirque)</th><th>Climatic conditions</th><th>Age of maximum glacier extent (ka)</th><th>Proportion of ice cover (at maximum extent) relative to LGM (%)</th><th>ELA at time of B-A (m)</th><th>Glacier-front elevation at maximum extent (m)</th><th>Main landforms</th><th>Key publications</th></tr></thead><tbody><tr><td>Arânser (Tossa Plana)</td><td>Warm and humid (close to present-day conditions)</td><td>Cirque glaciers became extinct during the B-A. Period of rock glacier development (regional ELA > 2500 m)</td><td></td><td></td><td></td><td></td><td>Palacios et al., 2015a; Andrés et al., 2018; Reixach et al., 2021</td></tr><tr><td>Malniu (Campcardós)</td><td></td><td>Cirque glaciers became extinct during the B-A (regional ELA > 2500 m)</td><td></td><td></td><td></td><td></td><td>Pallás et al., 2010; Andrés et al., 2018; Reixach et al., 2021</td></tr><tr><td>Grave (Carlit)</td><td></td><td>13.8 ± 0.6</td><td>5</td><td>2450</td><td>2150</td><td>Frontal moraine</td><td>Delmas et al., 2008; Reixach et al., 2021</td></tr></tbody></table>
<table><tr><td>Querol valley (Orri)</td><td>14.5 ± 0.6</td><td>3</td><td>2420</td><td>2100</td><td>Lateral and frontal moraines</td><td>Pallás et al., 2010; Reixach et al., 2021</td></tr><tr><td>Soulcem/Ariège (Médecourbe)</td><td>13.2 ± 0.9</td><td>3</td><td>2525</td><td>2230</td><td>Frontal moraine</td><td>Jomelli et al., 2020; Reixach et al., 2021</td></tr><tr><td>Upper Gállego</td><td colspan="6">Ice-free at ~2600–2700 m after 14 ka</td></tr><tr><td>Val d'Aran (Bacivèr)</td><td>15–14</td><td>5</td><td>-</td><td>1950</td><td>Polished surfaces</td><td>Oliva et al., 2021</td></tr><tr><td>Val d'Aran/upper Garonne (Ruda)</td><td>15–14</td><td>5</td><td>-</td><td>1860–1900</td><td>Polished surfaces</td><td>Fernandes et al., 2021</td></tr><tr><td>Val d'Aran/upper Garonne (Ruda)</td><td>13.5</td><td>4</td><td>2460</td><td>2080</td><td>Frontal moraine</td><td>Fernandes et al., 2021</td></tr></table>

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