T. I. Wilch, Department of Geological Sciences, Albion College, Albion,
Michigan 49224
W. C. McIntosh, Department of Earth and Environmental Science, New
Mexico Institute of Mining
and Technology, and New Mexico Bureau of Geology and Mineral Resources,
Socorro, New Mexico
87801
N. W. Dunbar, New Mexico Bureau of Geology and Mineral Resources, Socorro,
New Mexico
87801
ABSTRACT
Late Quaternary volcanic activity at three major alkaline composite
volcanoes in Marie Byrd Land,
West Antarctica, is dominated by explosive eruptions, many capable
of depositing ash layers as
regional time-stratigraphic horizons in the West Antarctic Ice Sheet
and in Southern Ocean marine
sediments. A total of 20 eruptions at Mount Berlin, Mount Takahe, and
Mount Siple are recorded in
lava and welded and nonwelded pyroclastic fall deposits, mostly peralkaline
trachyte in composition.
The eruptions, dated by the 40Ar/39Ar laser-fusion and furnace step-heating
methods, range in age
from 571 to 8.2 ka.
Tephra from these 40Ar/39Ar-dated Marie Byrd Land eruptions are identified
by geochemical
fingerprinting in the 1968 Byrd Station ice core. The 74 ka ice-core
record contained abundant
coarse ash layers, with model ice-flow ages ranging from 7.5 to 40
ka, all of which were previously
geochemically correlated to the Mount Takahe volcano. We identify a
one-to-one geochemical and
age correlation of the youngest (ca. 7.5 ka) tephra layer in the Byrd
ice core to an 8.2 ± 5.4 ka
(2sigma uncertainty) pyroclastic deposit at Mount Takahe. We infer
that the 20-30 ka tephra layers
in the Byrd ice core actually were erupted from Mount Berlin, on the
basis of age and geochemical
similarities. If products of these youngest, as well as the older 40Ar/39Ar-dated
eruptions are identified
by geochemical fingerprinting in future ice and marine cores, they
will provide the cores with
independently dated time horizons.
More than 12 40Ar/39Ar-dated tephra layers, exposed in bare ice on the
summit ice cap of Mount
Moulton, 30 km from their inferred source at Mount Berlin, range in
age from 492 to 15 ka. These
englacial tephra layers provide a minimum age of 492 ka for the oldest
isotopically dated ice in West
Antarctica. This well-dated section of locally derived glacial ice
contains a potential "horizontal ice
core" record of paleoclimate that extends back through several glacial-interglacial
cycles. The coarse
grain size and density of the englacial tephra (mean diameters 17-18
mm, densities 540-780 kg/m3),
combined with their distance from source, indicate derivation from
highly explosive Plinian eruptions
of Mount Berlin.