Pushing the Limits of Geochemical Tephra Analysis from Ice Core Samples

Analyzing ice cores offers a unique insight to paleoclimate studies. When the ice core is correctly dated, paleoclimate proxies can provide invaluable information about past climate shifts, perturbations or interactions of various climate forcers. In addition to other dating methods, it is common to use volcanic sulfate signals as independent time markers to which the ice core timescale is forced. Furthermore, compared to other depositional settings, ice cores preserve very fine volcanic ejecta without post-depositional alterations. However, recent studies have shown temporal disconnects between volcanic sulfate and tephra deposition from the same volcanic eruption. In a number of ice cores, some sequences show a lack of a sulfate aerosols signal where a layer of volcanic ash is present. Because of sample preparation and mounting limitations, only the geochemical composition of ash particles >25 microns were used in source volcano identification in the past. It resulted in very limited identification of particles from large, climatically significant but remote volcanic eruptions. In order to improve our understanding of paleoclimate records, the refinement of the current Antarctica tephrochronology framework (AntT) is necessary. By improving existing sample preparation methods with new techniques, we effectively captured over 99% of suspended particles >2 microns. The new mounting technique also ensures flexibility for analysis, using scanning electron microscopy - electron dispersive spectroscopy (SEM-EDS) or electron microprobe analysis (EMPA). Using this new methodology, guided by developed volcano-chemical time series, ultra fine volcanic ash samples in different ice cores were prepared and analyzed. We found that in addition to major tropical eruptions, small, high-latitude eruptions could be an additional source of volcanic products in Antarctic ice cores. For example, in the interval of 1963 - 1965 C.E., (Mt. Agung eruption in Bali, Indonesia) several cryptotephra (<5 microns) particles were found and geochemically fingerprinted to additional source volcanoes. Preliminary geochemical identification points to the eruptions of Raoul, New Zealand (1964) and Hudson, Chile (1971). These early findings clearly demonstrate complexity of volcanic signal in Antarctic ice cores.