The glacial history since the Last Glacial Maximum in the Forni Valley (Italian Central Alps). Reconstruction based on Schmidt's Hammer R-values and crystallinity ratio indices of soils

Abstract

Knowledge about deglaciation after the Last Glacial Maximum (LGM) in the mountain areas of the European Alps is still limited. In this study, we used Schmidt Hammer R-values (SH) and Crystallinity Ratio (CRF) indices of soils, together with historical data, to outline the glacial evolution of the Forni Glacier the biggest Italian glacier until some years ago, from the LGM to the present. The study area is in the Italian Central Alps, which has a well-known history after the LIA but many gaps before. By comparing results from the Forni Glacier and nearby Gavia Pass, we found that weathering rates may differ for the same lithology; therefore, SH requires a local calibration curve. A total of 6 different glacial phases were found in the study area before the LIA: 15 ka (phase I), 12.2 ka (phase II), 9.5 ka (phase III), 4.1 ka (phase IV), 3.2 ka (phase V), and 1.5 ka (phase VI). Phase I and phase II are common glacial phases in the Alps. Phase III was the biggest Holocene advance and the following were smaller than the LIA, accordingly to what happened in Triftjegletscher, a glacier on the northern side of the Alps with a similar catchment morphology. In the close Gavia Pass area, phase both phase III and phase V were larger than the LIA, suggesting that catchment morphology was more important than proximity for controlling Holocene glacial evolution. Interestingly, phase IV is rare in the Alps, but is contemporary to the 4 ka cold event. It was finally possible to identify the maximum of the LIA expansion in 1810 CE and this agree with recent studies that collocate one of the peaks of the LIA in the Alps in the first half of the 1800.