Annual changes in firn density and compaction under Alpine Climate conditions: first multi-year CMP radar observations from the Grosser Aletschgletscher

Abstract

Abstract. The glacier mass balance is driven by primary processes such as snowfall and surface melt. Secondary processes, like firn pack warming, percolation, and refreezing, are becoming more important even in high-elevation basins, which are triggered by warming Alpine regions. All these processes are functionally related to temporal changes in the firn pack, predominantly its density structure. Here, we provide a detailed assessment of annual changes in firn density, stratigraphy, and compaction rate at different locations of the glacier accumulation area, using multi-year common-midpoint (CMP) radar measurements, representing the first such analysis for a glacier in the European Alps. To achieve this, we combined repeat geophysical observations, predominantly ground-penetrating radar (GPR)-based common-midpoint (CMP) surveys, with direct firn-core investigations from the accumulation area of the Grosser Aletschgletscher, Switzerland. We estimated temporal changes in firn density and compaction rates within the identified 8–9 annual layers using internal reflection horizons (IRHs) from repeat CMP measurements. In addition, we analysed relationships between firn-core-derived chemical impurities and stable isotopes. Our results suggest that the annual changes in firn density decrease with depth and age, with the largest change (∼ 130 kg m−3 yr−1) occurring at the near-surface annual layers (∼ 7–8 m depth) at a low-lying accumulation area where the summer surface melt is more significant than at the higher elevations. Similarly, the estimated compaction rate (maximum ∼ 0.3 m yr−1 at ∼ 7–8 m depth) decreases with depth and age. The CMP-derived density–depth profile agrees with the firn-core results, demonstrating that CMP measurements are a valuable alternative for increasing the spatial distribution of observations and complementing invasive, labour-intensive glaciological measurements. We also estimated spatial changes in firn density and accumulation along a GPR transect and traced the spatial extent of the firn body. The secondary results, obtained by comparing GPR observations from winter 2024 and 2025, suggest that glacier dynamics may influence firn stratigraphy that requires further investigation in future studies. Our results demonstrate that the combination of multi-year GPR profiles, CMP analyses, and firn-core observations can quantify temporal changes in firn density, stratigraphy, and compaction rate, thereby contributing to the future calibration of firn-densification models and improving glacier mass-balance estimates.