The provenance of the sediment in an overdeepening and its implications for the distribution of glacier ice in the Bern area (CH)

The extent and distribution of glaciers on the Swiss Plateau during the Last Glacial Maximum (LGM) can be determined from the geological record. However, similar reconstructions for the glaciations that preceded the LGM are far more difficult to be made due to the inaccessibility of suitable sedimentary records. Here, we explored Quaternary sediments which were deposited during the MIS 8 glaciation at least 250 ka ago, and which were recovered in a drilling that was sunk into an overdeepening W of Bern (Switzerland). We analyzed the sediment-bulk chemical composition of the deposits to investigate the supply of the material to the area by either the Aare Glacier or the Valais Glacier. The potential confluence of these two glaciers in the Bern area makes this location ideal for such an analysis. We determined the sediment-bulk chemical signal of the various lithological units in the central Swiss Alps where the glaciers originated, which we used as endmembers for our provenance analysis. We then combined the results of this fingerprinting study with the existing information on the sedimentary succession and its deposition history. This sedimentary suite is composed of two sequences A (lower) and B (upper), both of which comprise a basal till that is overlain by lacustrine sediments. The till at the base of Sequence A was formed by the Aare Glacier. The overlying lacustrine deposits of an ice-contact lake were mainly supplied by the Aare Glacier. The basal till in Sequence B was also formed by the Aare Glacier. The provenance signal points towards a simultaneous material supply by both the Aare and the Valais Glaciers during the formation of the lacustrine sediments in Sequence B. We use these findings for a paleogeographic reconstruction. During the time when Sequence A and the basal till in Sequence B were deposited, the Aare Glacier dominated the area. This strongly contrasts with the situation during the LGM, when the Aare Glacier was deflected by the Valais Glacier towards the NE. Probably, the Valais Glacier was less extensive during MIS 8. However, part of the lacustrine sediments deposited within Sequence B could only have been supplied by the Valais Glacier, indicating that the glacier did not cover the study area, yet had been in close proximity to the study area. We thus postulate that during the deposition of Sequence B both the Aare Glacier and the Valais Glacier were connected to this lake that had formed at the foot of these glaciers. These glaciers potentially also dammed this lake. In conclusion, we could outline a detailed scenario of sediment supply to the investigated overdeepening during the MIS 8 glacial period based on the provenance and sedimentological data, and that glaciers were arranged in a different way than during the LGM.