Is temperate ice a good paleoclimatic archive? Insights from the Adamello temperate ice core (Italian Alps)

Abstract

Glaciers are well known for providing valuable climatice and environmental information which are made available through the retrieval of ice cores. Not all glaciers are equal in this respect, however. The best sites to drill ice cores for paleoclimatic purposes are the cold portions of accumulation basins. The term cold, when referred to ice, indicates ice whose temperature is constantly below the pressure melting point. The importance of cold sites for ice core science is related to the fact that under cold conditions, the stratigraphic signals used for paleoclimatic reconstructions are best preserved because of the absence of meltwater. Because of climate change, cold portions of mountain glaciers are rapidly changing. The rise of atmospheric temperature is impacting the thermal properties of ice and firn, leading to their warming. As a consequence of this, many cold accumulation basins of high-altitude glaciers are turning to temperate and their mass balances are approaching negative values. This is posing issues on the ability of glaciers to preserve climatic and environmental signals. This is related to two distinct processes. At first, temperate ice, by definition contains a fraction of liquid water which can interfere with the preservation of chemical and physical signals. Secondarily, negative mass balances related to increased melt rates, imply the loss of upper ice layers, obliterating the most recent stratigraphic signals normally used for calibration with instrumental data. The possibility to retrieve reliable paleoclimatic records from mountain glaciers in the future, is thus questionable. This will only be possible if the ice core science community develops new methods and competencies to extract information from temperate ice addressing meltwater disturbances. To this aim, a 223 m long ice core was drilled in 2021 at the Adamello glacier, in the Italian Alps. At the drilling site (3100 m a.s.l.) the glacier has a negative mass balance and a temperate regime. Thus, the site is ideal to test to what extent temperate ice can be used as a paleoclimatic archive. To this aim, a set of paleoclimatic proxies has been investigated in the upper part of the ice core. We present here preliminary results. They show that while most of the analytes are significantly affected by meltwater percolation and regelation, some of them, in particular the less soluble ones, still exhibit a detectable seasonality. This has allowed to develop a chronology, estimate the age of surface ice and identify what proxies are best preserved in temperate ice.