Physical setting and tephrochronology of the summit caldera ice record at Mount Moulton, West Antarctica

A site on the shoulder of the ice-filled summit crater of Mount Moulton, located at an elevation of 2800 m in West Antarctica, exhibits a 400-m-long section of exposed blue ice and intercalated tephra layers. A total of 48 tephra layers are observed. Many are thick (up to 10 cm) and coarse (pumice up to 3 cm). Detailed dip measurements and global positioning system (GPS) mapping reveals a simple geometry of parallel tephra layers all exhibiting consistent dips. The simple stratigraphy of the tephra layers suggests that although the local ice is thinned, based on the boudinaged appearance of the thicker tephra layers, it is otherwise undeformed. Local measured ice motion and ablation rates are similar, averaging around 3 cm/yr for the 4-yr measurement period. Most of the Mount Moulton tephra layers are trachytic, and are derived from Mount Berlin, a still thermally active volcano located similar to 30 km away to the west. Eight of the tephra layers at the Mount Moulton site have been directly dated, using 40Ar/39Ar geochronology of potassic feldspar phenocrysts, to (in stratigraphic order from top to base of the blue ice section) 10.5 +/- 2.5; 24.7 +/- 1.5; 92.1+/- 0.9; 104.9 +/- 0.6; 118.1 +/- 1.3; 135.6 +/- 0.9; 225.7 +/- 11.6; and 495.6 +/- 9.7 ka. The depth-age curve for the Mount Moulton site, based on these radioisotopic ages, is qualitatively similar to that of the Siple Dome ice core, also in West Antarctica. Although not part of the West Antarctic ice sheet, the Mount Moulton site certainly contains some of the oldest known ice in West Antarctica, and provides a long and detailed climate record.