Abstract
The Central Pyrenees hosted a large ice cap during the Late Pleistocene. The cirques under relatively low-altitude peaks (2200–2800 m) include the greatest variety of glacial landforms (moraines, fossil debris-covered glaciers and rock glaciers), but their age and formation process are poorly known. Here, we focus on the headwaters of the Garonne River, namely on the low-altitude Bacivèr Cirque (highest peaks at ~2600 m), with widespread erosive and depositional glacial and periglacial landforms. We reconstruct the pattern of deglaciation from geomorphological observations and a 17-sample dataset of 10Be Cosmic-Ray Exposure (CRE) ages. Ice thickness in the Bacivèr Cirque must have reached ~200 m during the maximum ice extent of the last glacial cycle, when it flowed down towards the Garonne paleoglacier. However, by ~15 ka, during the Bølling-Allerød (B-A) Interstadial, the mouth of the cirque was deglaciated as the tributary glacier shrank and disconnected from the Garonne paleoglacier. Glacial retreat was rapid, and the whole cirque was likely to have been deglaciated in only a few centuries, while paraglacial processes accelerated, leading to the transformation of debris-free glaciers into debris-covered and rock glaciers in their final stages. Climate conditions prevailing at the transition between the B-A and the Younger Dryas (YD) favored glacial growth and the likely development of small moraines within the slopes of the cirque walls by ~12.9 ka, but the dating uncertainties make it impossible to state whether these moraines formed during the B-A or the YD. The melting of these glaciers favored paraglacial dynamics, which promoted the development of rock glaciers as well as debris-covered glaciers. These remained active throughout the Early Holocene until at least ~7 ka. Since then, the landscape of the Bacivèr Cirque has seen a period of relative stability. A similar chronological sequence of deglaciation has been also detected in other cirques of the Pyrenees below 3000 m. As in other mid-latitude mountain regions, the B-A triggered the complete deglaciation of the Garonne paleoglacier and promoted the development of the wide variety of glacial and periglacial landforms existing in the Bacivèr cirque.
Highlights
Termination-1 (T-1), the period spanning from the end of the Last Glacial Maximum (LGM, 19-20 ka; Clark et al, 2009) to the onset of the Holocene (11.7 ka; Denton et al, 2014), saw a massive world-wide glacial retreat that favored a large-scale reorganisation of oceanic and atmospheric circulation patterns, global sea level rise, redefinition of coastlines, shifts in land cover and ecosystems, and changes in greenhouse gas concentrations
This complex cirque can be divided in three large geomorphological units (Fig. 2): (i) the peaks and walls that form the head of the amphitheatre, (ii) the set of glacial, periglacial and paraglacial landforms distributed at the foot of the walls, and (iii) the large flat floor, composed of polished bedrock surfaces with small depressions and scattered erratic boulders
We collected two samples from a polished bedrock surface (A-16, A-17) that is indicative of the onset of deglaciation of the bottom of the Bacivèr valley
Summary
Termination-1 (T-1), the period spanning from the end of the Last Glacial Maximum (LGM, 19-20 ka; Clark et al, 2009) to the onset of the Holocene (11.7 ka; Denton et al, 2014), saw a massive world-wide glacial retreat that favored a large-scale reorganisation of oceanic and atmospheric circulation patterns, global sea level rise, redefinition of coastlines, shifts in land cover and ecosystems, and changes in greenhouse gas concentrations. Whereas the long-term, global-scale glacial retreat that occurred during T-1 was favored by greenhouse gas increases, regional glacial advances and retreats followed forcings at smaller scales (Denton et al, 2014). In mid-latitude regions, such as the Iberian Peninsula, glaciers shrank in response to the long-term warming that was recorded during T-1, colder millennial-scale phases favored re-expansion and warmer periods triggered accelerated shrinking (Oliva et al, 2019b). While local maximum ice extents during the last glacial cycle occurred asynchronously in different mountain ranges in Iberia (Oliva et al, 2019b), glacial oscillations during T-1 followed very similar patterns in response to changing climate in the North Atlantic region (Buizert et al, 2018; Rea et al, 2020). For areas where chronological data are available, such as the Central Range (Carrasco et al, 2015; Palacios et al, 2012), Pyrenees (Andrés et al, 2018; Palacios et al, 2017b, 2017a, 2015b), Cantabrian Mountains (Rodríguez-Rodríguez et al, 2017), and Sierra Nevada (Gómez-Ortiz et al, 2012; Palacios et al, 2016), glaciers generally advanced during the OD and YD, and retreated during the B-A and after the YD during the Early Holocene
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