An oxygen isotope record from the Foscagno rock-glacier ice core, Upper Valtellina, Italian Central Alps

Abstract

New high-resolution isotopic records (δ18O, δD and deuterium excess), from an ice core drilled in the Foscagno rock glacier (Italian Central Alps), are presented. The δ18O data suggest a clear division between an upper part (2.5 and 4 m), showing relatively homogeneous values, and a middle part (4—7.65 m), showing seasonal variations of this parameter. The isotopic analyses confirm previous results (crystallographic and chemical analyses) suggesting a division of this relict glacier ice body into an upper part, between 2.5 and 4 m, where melting and refreezing processes occur, and a middle part, between 4 and 7.65 m, where the isotopic signal is preserved. Larger deuterium excess variations ( d = δD−8*δ 18O) are found in the massive ice (below 4 m depth) rather than in the overlying ice. These are in antiphase with the δ18O but without any clear correspondence with the presence of the debris layers. Postdepositional processes could have affected, at least partially, the isotopic content of the original precipitation. The radiocarbon dating of a leaf ( Salix spp.) found in the massive ice from another nearby borehole in the same rock glacier gave a calendar age ranging between AD 765 and 1260. The expected δ18O values of the present-day precipitation in the Foscagno valley are of the same order as those found in the massive ice (−12.4‰). This similarity would suggest climate conditions not very different from present day, in good agreement with other available palaeoclimate reconstructions for this period. However, only more abundant precipitation would make the existence of a glacier possible in a climate not very different from that of the present.