Direct Dating and Geochemical Correlations of Englacial Tephra Layers at two Sites in West Antarctica.

Dunbar, N.W., McIntosh, W.C., Esser, R.P., Wilch, T.I., Zielinski, G.A.2 Department of Earth and Environmental Science and New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM; 2. Climate Change Research Center, University of New Hampshire, Durham, NH

Direct dating and geochemical correlation of englacial tephra at two blue ice sites in West Antarctica, Mt. Moulton and Mt. Waesche, reveal that ice in the high summit caldera of Mt. Moulton is close to 500,000 yrs old, whereas the site in the West Antarctic ice sheet reveals no ages older than 117,000 yrs. The stratigraphy and morphology of many of the englacial tephra layers suggests that the layers were deposited on snow at the time of the volcanic eruption, and were incorporated into the ice with little reworking or mixing, indicating that the age of the tephra layer is truly representative of the age of the surrounding ice.

The Mt. Moulton site, located at an elevation of 2800 meters in an ice-filled summit crater, displays a simple stratigraphy (Figure 1). Many of the tephra layers at Mt. Moulton are thick (up to 10cm) and coarse (pumice up to 3 cm). Based on chronological and geochemical correlations, many of the Moulton tephra layers are thought to be derived from Mt. Berlin, located approximately 30 km away. The ice hosting the tephra is likely to have been thinned, based on the boudinaged appearance of the thicker tephra layers. Seven of the tephra layers at the Mt. Moulton site have been directly dated using 40Ar/39Ar geochronology to (in stratigraphic order from top to base) 15±2; 25±4; 92±2; 106±2;119±2; 142±7; and 492±10 ka (Fig. 1). Although not part of the West Antarctic ice sheet, the Mount Moulton site may be the location of some of the oldest ice in West Antarctica.

In contrast to Mt. Moulton, the Mt. Waesche site, located at between 1900-2000 meters elevation in a blue ice field on the south side of the volcano, displays a complex, deformed stratigraphy (Figure 2). The tephra layers at Mt. Waesche are dominantly either coarse and basanitic, probably of local derivation, or fine and trachytic, mainly derived from Mt. Berlin or Mt. Takahe. Although displaying boudin-shaped features, the thicker tephra layers, such as the "Great Wall" (up to 4 m thick) at Mt. Waesche appear relatively undeformed, whereas some of the thinner, intercalated layers are strongly isoclinally folded. Three tephra layers at Mt. Waesche have been geochemically correlated with layers found at Mt. Moulton. Based on age constraints on the correlative layers at Mt. Moulton, ages for the Waesche layers are around 20, 50 and 75 ka, respectively, consistent with apparent stratigraphic order (Fig. 2). A tephra layer adjacent to, and west of, the "Great Wall" (Fig. 2) has been directly dated using 40Ar/39Ar, and yields an apparent age of 117±7 ka. The easternmost part of the section (east of the "Great Wall"), appears, based on geochemical correlations, to be an overturned repetition of some part of the upper section. Hence, there is no evidence for ice significantly older than 117 ka at Mt. Waesche.