Tephra layers in blue ice, Allan Hills, Antarctic: A new source of glacial tephrochronological data.

N W Dunbar, P R Kyle, W C McIntosh, and R P Esser (Geoscience and Bureau of Mines and Mineral Res., New Mexico Tech, Socorro, NM, 87801; (575) 835-5783; nelia@nmt.edu)

Blue ice at Allan Hills, Antarctica, contains abundant tephra layers as well as terrestrial windblown (?) dust layers, and mixed tephra and dust layers. The layers range from thin, faint laminae to distinct bands as thick as 50 cm. They dip from near-horizontal to near-vertical, depending on the geometry of local ice flow. Detailed GPS mapping reveals that individual ash and dust bands can be traced for up to 10 km, and that the same sequence of tephra and dust bands can be recognized throughout a single geographic area, irrespective of local ice flow conditions. The tephra and dust layers appear to have been deposited as stratigraphic layers rather having been emplaced by shear at the base of the ice sheet, based on their consistent orientation, coherent stratigraphy and the nature of their contacts with adjacent ice. They have sharp lower contacts interpreted as depositional surfaces, and diffuse upper contacts where mixing occurred with later snow. No significant shearing or brittle deformation the dust and tephra section was observed.
Petrographic observations show that many of the layers are primary tephra, ranging from basaltic glass fragments to trachytic pumice and shards. Tephra range in size from <2 to 250 m, and appear fresh and unabraded in many layers. Potential source volcanoes are up to 500 km away for the coarser tephras. Because of their ease of access and sampling, blue ice areas offer an alternative to deep ice cores in the reconstruction of regional and possible global volcanic records. 40Ar/39Ar dating of large tephra samples, which can be readily collected from blue ice areas, offers a means of establishing a chronology which may extend back to 300 ka or more. Furthermore, dated layers found in blue ice areas may be geochemically correlated with tephra in deep ice cores.

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