The Holocene history of Arsine Glacier (Western European Alps): a detailed 10Be record of oscillations driven by climate and modulated by rock avalanches

Lateglacial to Early Holocene (EH) glacier variations in the Alps have recently been thoroughly refined using ¹⁰Be terrestrial cosmogenic nuclide (TCN) exposure dating of moraines. In contrast, knowledge of the spatial extent reached by glaciers during the Neoglacial period (∼4.2–0.14 ka) is still fragmentary because of the scarcity of preserved moraines predating the Little Ice Age (LIA). In addition, although the climatic meaning of the geomorphological imprints left by debris-covered glaciers or rock glaciers is increasingly investigated, direct comparison with those of debris-free glaciers is often not possible. Here, we provide an unprecedentedly large TCN dataset for the region (n = 53) constraining the behaviour of Arsine Glacier, French Alps, from the end of the Lateglacial to the end of the LIA. Stadial moraines are present in both the western and eastern valleys formerly occupied by two separate branches of the ice body, up to c. 3 km from the extant glacier front. Further upslope, the forefield of Arsine Glacier is characterized by two prominent geomorphic complexes. The first is a peculiar landform with concentric ridges, previously interpreted as a moraine-derived rock glacier. It is framed by an extensive and chaotic boulder field and is overtopped – in the most glacier-proximal part – by the second outstanding feature, a large complex of unvegetated lateral and frontal composite moraines. The chronological results indicate that the most distal moraine sets testify to five oscillations of the western glacier branch between 11.88 ± 0.43 ka and 11.10 ± 0.37 ka. Robust constraints on the spatial extent of these positions show depression of the glacier equilibrium line altitude ranging from 224 m to 169 m relative to the LIA. The innermost EH complex dated here is interpreted as being related to the regional expression of the Preboreal Oscillation (PBO). The previously interpreted moraine-derived rock glacier was found to rather represent a moraine complex deposited following a rock avalanche covering the eastern branch of the ice body and the surrounding floor. This catastrophic event occurred at 10.71 ± 0.42 ka, during the first significant warming of the Preboreal period, as shown by dating of the framing boulder field deposit. Dating of the ridges of the resulting moraine complex shows a subsequent advance of c. 1250 m peaking at 10.25 ± 0.42 ka, followed by persistence of the debris-covered eastern tongue outboard the LIA extent until 9.52 ± 0.43 ka. By contrast, during this interval no prominent moraine was deposited by the western glacier branch, not affected by the rock avalanche. Like elsewhere in the Alps, no ¹⁰Be boulder ages corresponding to the Mid-Holocene (8.2–4.2 ka) were found. Dating of the glacier-proximal moraine complex allows assigning its deposition to the Neoglacial period. The emplacement of this feature had started no later than 3.74 ± 0.02 ka. A subsequent period of glacier maxima deposited a till unit dated on both sides of the glacier at 2.69 ± 0.14 ka. During the Common Era, the first LIA-like advance occurred at 1.67 ± 0.05 ka. During the LIA, an early advance (eLIA) deposited the outermost Neoglacial frontal moraine 650 ± 30 a ago. Finally, the innermost preserved lateral moraine was deposited at the end of the LIA, 170 ± 10 a ago. The Arsine Glacier record is, to our knowledge, the most detailed ¹⁰Be-based Holocene-long glacier chronology in the Alps, and agrees well with other existing glacier records in the region. We propose that, apart from the directly climate-driven glacier advances and corresponding ice-marginal moraine deposits of the EH and the Neoglacial, a large EH rock avalanche occurred following warm-period-related destabilization of the cirque headwalls, bringing large amount of debris onto one part of the glacier. This excessive supply – estimated to a volume of c. 8.4 × 10⁶ m³ – led the eastern glacier branch to evolve into a heavily debris-covered tongue that recorded climate-related oscillations between ∼10.3 and 9.5 ka outboard the LIA extent.