High degree of soil development and large carbon stocks in soils above the last glacial maximum trimline in the North-Western Italian Alps

Most of the Alpine range has been influenced by glacial or erosional processes during Pleistocene glacial periods, which erased most pre-existing soils and landforms. Consequently, most soils in the Alps began forming after the end of the Last Glacial Maximum (LGM), or after the Younger Dryas (Egesen stadial, ca. 11,700 years BP). However, surfaces above the trimline (the upper limit reached by valley and cirque glaciers) still retain “old” morphologies and can be considered paleosurfaces, often covered by fossil or active periglacial features. This study aimed to assess soil development and carbon stocks in soils located above and below the LGM trimline in the North-Western Italian Alps, using a modified Profile Development Index (PDI) coupled with physical and chemical analyses. To date, no studies have explicitly compared soils above and below the LGM trimline in the Alps. We explored relict surfaces above the LGM trimline in the North-Western Italian Alps, characterized by gentle slopes and periglacial features such as blockfields and blockstreams, but lacking glacial traces such as erratics, roches moutonnées and striations. We excavated soil profiles in stable sites and compared them with nearby similar glaciated areas to assess soil development relative to trimline position. In most cases, we found strongly developed Podzol above the trimline, with thick E horizons, or Umbrisols with A-Bh horizons over 1 m thick, often richer in organic carbon than soils on glaciated terrains. Despite similar elevation, vegetation, and lithology, soils below the trimline were thinner and less developed. This study provides new insights into the Quaternary landscape evolution of the Alps through a detailed assessment of soil development and suggests that PDI may be used in future regional-scale studies as a proxy to reconstruct former glacier extent and trimline position, thus improving our understanding of glacial dynamics and landscape evolution in the Alps.