A Maturing Tephra Record in the West Antarctic Ice Sheet

Tephra layers found in many Antarctic ice cores range from sub-centimeter thick, visible layers to cryptotephra consisting of sparse, fine-grained (<10 micron) glass particles. Location of tephra particles has improved with advances in methods of detecting cryptotephra in ice cores. Identification of tephra layers traditionally relied on visible detection or association with sulfate aerosols, but is now supplemented by downhole optical logging (Bay et al., 2001). Improved analytical techniques for glass characterization, such as high quality quantitative electron microprobe analysis, and more complete information on source eruptions has facilitated identification of tephra horizons in ice cores. Two deep ice cores drilled in West Antarctica (Siple Dome and WAIS Divide) contain rich tephra records, with the former containing 37 tephra layers and the latter containing several hundred distinct, visible layers, many of which are likely to be tephra. Most of the tephra layers with strong correlations to sources are derived from Antarctic volcanoes, many from two large West Antarctic stratovolcanoes Mt. Berlin and Mt. Takahe, tephra from which have also been recognized in the marine record (Hillenbrand et al., 1988). A well-defined ash layer is found at a depth of between 190.37-190.39 m depth in the WAIS Divide core, containing 20 um ash shards that are chemically correlated to the the Pleaides volcanoes, in northern Victoria Land. This tephra layer correlates to one found in a Siple Dome (B) ice core (97.2 to 97.7 m depth) and in the Taylor Dome ice core (79.2 m depth). Deeper parts of the WAIS Divide ice core correspond to a time interval of abundant regional volcanism, represented by the large number of visible dust bands and cloudy layers in the core (A. Orsi, pers. comm., 2010). A distinct "visible brown layer" at a depth of 1586.363 m. (8.279 Ky BP preliminary age) is very likely to be from a major eruption of the West Antarctic volcano Mt. Takahe (8.2±5.4 , Wilch et al., 1999). This layer is found at a depth of 503.58-503.87 m in the Siple Dome A core (SMDA) corresponding to 8.167-8.181 Ky before 1950, and almost certainly to a visible layer identified and analyzed in the Byrd ice core at 788 m (Palais et al., 1988). A visible double layer at 1741.246 m (9.57 KyBP preliminary age) may correspond to a very distinct tephra layer in the SDMA core at a depth of around 550 m (corresponding to an age of around 9.7 Ky before 1950). This layer is derived from the West Antarctic stratovolcano, Mt. Berlin. In the segment of WAIS Divide ice core between 2251 and 2557 m depth (15.2 to 20.6 preliminary age), numerous dust bands and cloudy layers are reported in the ice. This corresponds to the age of ice in the Byrd Core that contained many volcanic layers (Gow and Williamson, 1971), and also an interval in the SDMA where numerous distinct tephra layers associated with highly explosive eruptions of Mt. Berlin were found. Detailed quantitative chemical analyses will allow one-to-one correlations between tephra layers in the WAIS Divide and SDMA cores, but once made, will allow one additional tool to tie the records in these two cores together.