Morteratschgletscher drains the eastern catchment of the Bernina Massif, with elevations up to 4048 m of Piz Bernina (Fig. 1). During the yearly survey of the ice margin position in autumn of 2014 the first log, about 30 cm long, was reported. In contrast to observations on e.g. Unteraar-, Mont Miné-, or Steingletscher, this was a late discovery. However, this new occurrence has yielded some basic insights into the glacier–climate system. Since 2015 the glacier has melted back into a basin, leaving the dominant cross-valley bedrock ridge ice-free. This bedrock ridge acted as the mechanical obstacle to ice flow, generating a compressive flow regime, where most particles carried in the basal part of the glacier would be lodged. Substantial glacial morphological features are present, such as roches moutonnées, boulder alignments, fluted till surfaces, glacial striae, and irregular till patches smoothing the complex bedrock topography. These till patches contain organic clasts on their surface and at depth. A description is given here of one such prominent clast (Fig. 2a). A tree trunk of about 60 cm visible length is embedded in till and sheared off at the top. The roches moutonnées marking the skyline in the back is the obstacle controlling compressive flow (Fig. 2b). During the first visit on 24 June 2025 and some exploratory digging it was clear that this trunk still had roots attached (Fig. 2b). The surface of the trunk was not as smooth as is usually the case when exhumation (melting out) is carried out some weeks or months before subaerial discovery. However, the surface of this tree has the typical compressive-pressure-induced drying cracks and is very hard. The visible roots in Fig. 2b are part of the trunk. The roots on the backside, which was severely lodged into the till, were deformed and partially sheared off and displaced by the shearing drag of the glacier.

Figure 1(a) The Morteratschgletscher in the Engadin (Switzerland) is (b) framed from the Bernina Massif. (c) Positions of the glacial ice in summer 2025 (and 2022) and of collected wood are visible (photo: UAV imagery, Lander Van Tricht. Bottom left corner coordinates (LV95): 2791565.1, 1143651.4; top right corner coordinates: 2791923.2, 1143902.8).

Figure 2The truncated tree (MORT–25-01 = H2) was found at 2156 m altitude (a) in the till in front of the Morteratschgletscher in summer 2025. It was half-deracinated and tilted around 45° in the flow direction of the glacier. (b) Roots are still attached.

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The preservation of the excavated tree trunk with roots attached (Fig. 2b) shows that it has not been dragged by the moving glacier for a long distance. The dislocation, if at all, may be on the order of a few metres to a few tens of metres only, as indicated by the preservation of the roots. It is also possible that it was run over and lodged at the site of growth and later on sheared to the place of excavation. The chaotic sediments with weathering around the roots make this latter scenario possible.

The coordinates of the excavated trunk are 2791652.51, 1143796.71. The detailed topographic map (Fig. 1c; see also Fig. 3) shows the proximity to the 2025 glacier terminus and the distribution of subfossil wood specimens on the surface. The sample MORT–25-01 (= H2 in Fig. 2) is Larix decidua Mill. The distance between sample MORT–25-01 (H2) and the ice margin was < 20 m at the time of excavation. Dendrochronological measurements provided 118 tree rings, with an estimated pith offset of 10 years.

Figure 3Extent of the Morteratschgletscher in summer 2025 with the locality of the discovered tree at 2156 m altitude (photo: Kai Zorn). (The arrow points to the sample site.)

The uncalibrated radiocarbon age of sample MORT–25-01 (= ETH-153201) is 9126 ± 29 years BP. The calibrated date corresponds to two intervals, 8454–8448 and 8431–8274 BCE (95.4 %), and a first dendrochronological analysis has established the end year for the tree at 8350 BCE, with 20 to 25 years uncertainty (Nicolussi et al., 2026). The year of 8350 BCE (or ca. 10 300 BP) must be considered to be the youngest possible end year (Fig. 4). A sample found earlier in the forefield of Morteratschgletscher (MORT–19-06) is ca. 200 years older. However, the sensitive palaeoglaciological information from tree MORT–25-01 (H2) is its lodging or growing close to or at its excavation position (Fig. 2; cf. Patzelt, 2019, pp. 182–183).

Figure 4Calibrated radiocarbon age (cal BP) of combined ¹⁴C ages for wood samples H1, H2, and H4 plotted together with the calibrated age of the oldest sample (H5).

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MORT–25-01(H2) is the most prominent among a number of wood clasts at the sediment surface or slightly embedded in the till. A selected number of them were collected in early June during a survey of the ice front. These samples have been radiocarbon-dated with the following results.

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  MORT–25-H1 = ETH-152759: 9248 ± 28 years BP

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  MORT–25-H2 = ETH-152760: 9194 ± 28 years BP
  (= second sample of tree MORT–25-01)

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  MORT–25-H3 = ETH-152761: 120 ± 21 years BP

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  MORT–25-H4 = ETH-152762: 9206 ± 28 years BP

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  MORT–25-H5 = ETH-152763: 9437 ± 28 years BP

The results of these additional samples from the vicinity of tree MORT–25-01 (H2) demonstrate a consistent age group with a ¹⁴C mean value of three samples (H1, H2, and H4) of 9216 ± 17 years BP (χ² test; df-2 T-2.1 (5 % 6.0)). However, MORT–25-H5 is even 200 radiocarbon years older than the other samples and possibly a piece of the oldest Preboreal tree at Morteratsch (and possibly the Alps in terms of oldest radiocarbon age so far), with calibration between 8792 and 8627 BCE (10 741–10 576 cal BP) (Fig. 4).

No data sets were used in this article.

CS designed the project and is responsible for the integrity of the study. LH undertook the first sampling. CS and CK conducted the main sampling in the field. IH generated the ¹⁴C age determination, and KN established the dendrochronology. CS mainly wrote the manuscript, with contributions by CK, IH, KN, and LH.

The contact author has declared that none of the authors has any competing interests.

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This paper was edited by Ingmar Unkel and reviewed by one anonymous referee.