Dating and interpreting a firn core from the East Antarctic Plateau

Abstract

Detecting and understanding potential changes in annual mean temperature and accumulation rate at the East Antarctic Plateau is crucial to assess the sensitivity and future response of the Antarctic ice sheet to global warming. Due to the very low accumulation rate and its spatial variability, the interpretation of climate proxies from shallow firn cores with centennial to decadal time resolution is challenging. A major limitation is the available time resolution obtained by available dating approaches and a reliable assessment of its uncertainty. In this study, a 204 m long firn core, B56, drilled in 2017 on the East Antarctic Plateau, is analyzed. The major goal of this study is dating the firn core by combining different dating methods based on available density data, dielectric properties, and ion chromatography (of non-sea-salt sulfate) data. To utilize density, the Herron-Langway model is used for determining the depth-age relation. In this model, temperature and snow accumulation are assumed to be constant and the relationship between the snow density and the depth below the snow surface does not change over time. Depending on the accumulation rate used, the resulting age at 200 m depth varies between 6000 to 7200 years. Secondly, the dielectric profile and non-sea-salt sulfate concentration (nssSO42-) data are used for constructing another depth-age model, thereby matching prominent data peaks to known volcanic eruptions that have been recorded in the past. Here, the resulting age at 200 m depth is determined to be about 6200 years, which compares well to published age models of firn cores from the East Antarctic Plateau. In comparison, it is recognized that all age models are consistent within the upper 40 m (stretching over the past 1000 years), but the Herron-Langway model tends to overestimate the age at greater depths. It is proposed that the findings indicate changes of the accumulation rate in the past, leading to the offset in the Herron-Langway model (which uses a constant accumulation rate). However, by combining the dating methods, it is possible not only to provide a reasonable dating over the full firn core, but also to improve the time resolution of the derived age model. This will improve the interpretation of the climate proxies extracted from this firn core and may serve as a role model for other shallow firn cores from the East Antarctic Plateau.