Advances in understanding melt-affected firn and ice cores from the polar regions

Rising temperatures are an increasing issue for ice core science around the world, even in the polar regions where near-surface melt and rain-on-snow events are occurring more frequently, more extensively, and adversely affect the preservation of numerous ice-core climate proxies. The age scale development for a new firn core from Young Island exemplifies the challenges and potential of records impacted by melt. In this context, my PhD research contributes to a more comprehensive understanding and new avenues of studying melt-affected firn and ice cores by employing advanced 3D X-ray scans, in-situ experiments, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). In this thesis, I first give an introduction to the formation, structural characteristics, and chemical interpretation potential of refrozen melt sections based on an extensive literature review. Against this backdrop, my core-scale microfocus 3D X-ray scans of melt-affected firn cores from four (sub-)Antarctic locations show a diversity of melt features, which differ between study sites. This suggests that melt microstructure can serve as a climate proxy reflecting cumulative melt conditions at the ice–air interface. These observations are in agreement with in-situ percolation tracer experiments that I conducted near Ny-Ålesund, Svalbard, in March 2023. They reveal heterogeneous features resulting from meltwater infiltration, including unprecedented field observations of internal layering within melt lenses. Percolation-induced stable water isotope changes are confined to melt structures, so that information could be retrieved from unaffected profile parts. Complementing these experiments, I for the first time applied LA-ICP-MS to measure soluble impurities in several melt-affected firn sections. This analysis reveals that melt-related elution of chemical signals is traceable at the microscale both within distinct and coarse-grained melt structures. Furthermore, the variability of sodium levels within melt sections points to the importance of the degree of melt–ice interaction for the preservation of primary chemistry signatures. The integrated results underscore the complexity of melt-related processes in firn and highlight the need for further in-depth research to fully understand their effects on ice-core climate reconstructions. Combined structural and chemical analyses in field and laboratory, as done in this PhD thesis, are crucial to this aim.

This PhD research project was financially supported by the British Antarctic Survey (BAS) and the Natural Environmental Research Council (NERC) Cambridge Climate, Life and Earth (C-CLEAR) Doctoral Training Partnership (grant no. NE/S007164/1). The project "Wet Fingerprints" (Chapter 5) was conducted thanks to the financial support of an Arctic Field Grant (RiS ID 12132, grant no. 342165) by the Research Council of Norway and the in-kind support of the NERC Arctic Station.