ENGLACIAL TEPHROCHRONOLOGY IN WEST ANTARCTICA: CONSTRAINTS ON ICE SHEET HISTORY, AND POTENTIAL LONGER CLIMATE RECORDS.
N.W. Dunbar1, W.C. McIntosh1, R.P.Esser1, T.I. Wilch,2, and G.A. Zielinski3
1. Department of Earth and Environmental Science and New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech,
Socorro, NM, 87801
2. Department of Geological Science, Albion College, Albion, MI, 49224
3. Climate Change Research Center, University of New Hampshire, Durham, NH, 03824
FAX (575) 835-6333 firstname.lastname@example.org
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 in which the tephra layers, and presumably the interlayered ice, are unfolded. 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, most of the Moulton tephra layers are thought to be derived from Mt. Berlin, a still-thernally-active volcano 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. Although not part of the West Antarctic ice sheet, the Mount Moulton site certainly holds some of the oldest ice West Antarctica, and should contain a long and detailed climate record. Ice at this site will be sampled in detail during the 1999/2000 field season.
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. 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. Several new, locally-derived englacial tephra layers were found at Mt. Takahe during the 1998/99 field season. Although displaying boudin-shaped features, the thicker tephra layers 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. The correlative layers have ages of between 15 and 27 ka, 27±2 ka, and between 106 and 119 ka, consistent with apparent stratigraphic order. A single tephra layer at Mt. Waesche has been directly dated using 40Ar/39Ar, and yields an apparent age of 117±7 ka. Although there are tephra layers that seem stratigraphically below the 117±7 ka layer, they appear to represent 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.