The layering of polar firn - investigations of the climatic impact on polar firn structure using high resolution density measurements and 3D-X-ray-microfocus-computer-tomography

Abstract

The polar firn structure is investigated in terms of layering using high-resolution density measurements of 19 firn cores and X-ray-microfocus-computer-tomography image analysis of 6 surface firn cores. The impact of local climate conditions such as mean annual temperature and accumulation rate on the generation, structure and evolution of the layering is studied. We find at all sites a high variability in density and microstructure due to the layered character of the firn. The standard deviation of the measured physical properties is used as a proxy to parameterize the layering and to compare the degree of layering at different sites. The results indicate, that the seasonal stratigraphy in ice core records, which is assumed to be formed by a continuous deposition at the surface and yields the basis of paleo-climate studies, is not necessarily formed at the surface by depositional mechanisms, but by densification and snow metamorphism deeper down in the firn column, influenced by impurities. The layering is also displayed in the microstructure, but shows a very complex pattern as the result of the combined effect of initial layering, sintering and short-term changes in local climate conditions. Nevertheless the microstructure from very different sites and very diverse metamorphic states shows some surprisingly well defined relationships: the specific surface area of the firn samples, which can be determined by several methods, can be described bijective from the measured chord length (average intersection of the ice phase with a line). This strengthens the assumption, that optical properties of snow and firn can be described by an effective radius of spheres, showing the same specific surface area as the measured sample. For any applications where the effective ice-air-interface of the firn is important, (air-exchange, chemical interactions, interactions with optical properties, microwaves), the easy-to-measure-specific surface area is sufficient to obtain the effective radius of the firn structure. The results of this study point out, that the stratigraphy, i.e. the amplitude and the frequency of the density variability, is not a constant property of firn, which is continuously buried in the firn column with depth and time. The original surface density and its variability, the original surface grain size and its variability and incorporated impurities as well as the interaction of the frequencies of the density variability and impurity concentration variability configure the dynamic evolution of the layering of the firn with depth. The layering of polar firn shows a random generation at the surface and a very dynamic evolution with depth, which is influenced not only by the microstructure and the local climate conditions but also by impurities incorporated into the firn at the surface.