Early Alpine glaciations at ca. 2 Ma revealed by 26Al–10Be burial dating of endokarst sediments in the western–central European Alps

The onset and potentially time-transgressive latitudinal development of Northern Hemisphere glaciations during the Plio-Quaternary represents a key component of global late Cenozoic climate dynamics. The timing of the earliest Alpine glaciations has been debated since the pioneering work of A. Penck and E. Brückner at the beginning of the 20^th century. Over the past three decades, cosmogenic nuclide burial dating has provided absolute ages on glacial and fluvioglacial sedimentary deposits in the Alpine forelands, progressively refining the chronology of early glaciations as methodological advances have emerged and as the number of analyzed samples has increased. Recent syntheses from the northern Alpine foreland (Deckenschotter) and the Ivrea amphitheater indicate that piedmont glacier lobes developed between 1.2 and 0.8 Ma, suggesting that the first extensive Alpine-wide glaciations occurred during the Mid-Pleistocene Transition. However, reconstructions based solely on surface deposits are strongly affected by preservation biases, as repeated glaciations may have eroded older sedimentary archives. As a result, Early Pleistocene surface records may have been largely removed by subsequent, more extensive glaciations.

Karst systems provide an alternative archive for early glaciations. During glacial periods, glaciers in contact with karst conduits inject detrital material (allochthonous or autochthonous) into subsurface voids, where sediments can be preserved for several million years. These buried deposits can be dated using in situ cosmogenic nuclides such as ²⁶Al–¹⁰Be in quartz. Prior to this study, very few geochronological and sedimentological data existed for such ancient glacio-karst deposits (i.e. Early Pleistocene). New ²⁶Al–¹⁰Be burial ages obtained from 20 detrital endokarst sediment samples (sands and pebbles) in the western to central Alps (Vercors, Chartreuse, Haut-Giffre and Bernese Alps), together with a synthesis of existing dating in the central Alps, allow the first spatio-temporal reconstruction of glacial sediment injections into the Alpine karst. Consistent with surface records, the burial ages reveal major glacial sediment injections into the karst around 0.8 Ma, at the end of the Mid-Pleistocene Transition. However, the data also point to a much earlier phase of widespread injections around 2 Ma. These early injections occurred both in high-elevation (>2500 m) headwater karst systems and in peripheral karst networks bordering the major Alpine valleys along the mountain front, demonstrating that widespread early glaciations affected the entire Alpine chain around 2 Ma. Because Alpine valleys were less deeply incised during the Early Pleistocene, glacier geometries differed significantly from those of Middle and Late Pleistocene glaciations, with thinner and potentially less extensive ice bodies. These earliest Alpine glaciations are contemporaneous with major advances of the Eurasian and North American ice sheets, consistent with extensive Northern Hemisphere glaciations at that time, predating the intensification of glaciations initiated at the Mid-Pleistocene Transition. Ultimately, this study highlights the potential of mountain karst systems as long-term archives for reconstructing Quaternary climate transitions.