DUNBAR, Nelia W., New Mexico Bureau of Geology, New Mexico Tech, 801 Leroy Pl, Socorro 87801, nelia@nmt.edu

Volcanic ashes, or tephra, are present at a number of depths in the West Antarctic Siple Dome Ice Core. These layers, generally too diffuse to be located visually, may yield as many as several hundred, or as few as 10, individual 5-15 micron glass shards. Following specialized sample preparation techniques that allow the samples to be polished for microbeam analysis, the geochemical fingerprint of the ashes was determined using electron analysis, with supplementary ion microprobe analysis in some cases. A concentration of volcanic ash in an ice core is considered to be a significant time-stratigraphic marker, or a deposit related to a distinct eruptive event, if the layer contains at least 4-6 glass shards of similar, or geochemically-related, composition. Random glass shards are also found at many depths in the core. These are not considered to be stratigraphically meaningful, but rather a component of windblown dust, similar to the quartz, feldspar, mica and clay minerals also found in many samples.

We have geochemically tied many of the 40 tephra layers found in the Siple Dome ice core to local volcanic sources using techniques described in Perkins et al. (1995). These include the West Antarctic volcanoes Mt. Berlin and Mt. Takahe, as well as East Antarctic volcanoes Mt. Melbourne and the Pleiades. Eighteen of the Siple Dome tephra layers were erupted from Mt. Berlin volcano, located 500 km from the Siple Dome drill site. Of these, the layers that correlate to the best-dated eruptions of Mt. Berlin include a layer at 550 meters which probably correlates to the last activity of Mt. Berlin at 10.3?2.7 ka, a layer at 703 m depth which correlates to an eruption at 14.5?3.8 ka, and layers at 758 and 760 m which correlate to an eruption at 27?2.3 ka. Another robust time-stratigraphic marker is provided by a tephra layer found at 503-504 m depth, which correlates an 8.2?2.7 ka eruption from another West Antarctic volcano, Mt. Takahe, located ~1100 km from the Siple Dome drill site. This tephra layer was also recognized in the Byrd Ice Core (Wilch et al., 1999). Although few global, non-Antarctic tephra layers have been found in the Siple Dome ice core, the abundant tephra layers derived from Antarctic volcanoes are likely to be found in future Antarctic cores, and will provide time-lines and cross-correlations with existing Antarctic ice cores.