Late Quaternary volcanic activity in Marie Byrd Land: Potential Ar-40/Ar-39-dated time horizons in West Antarctic ice and marine cores

Late Quaternary volcanic activity at three major alkaline composite volcano es in Marie Byrd Land, West Antarctica, is dominated by ex:plosive eruption s, many capable of depositing ash layers as regional time-stratigraphic hor izons in the West Antarctic Ice Sheet and in Southern Ocean marine sediment s. A total of 20 eruptions at Mount Berlin, Mount Takahe, and Mount Siple a re recorded in lava and welded and nonwelded pyroclastic fall deposits, mos tly peralkaline trachyte in composition. The eruptions, dated by the Ar-40/ Ar-39 laser-fusion and furnace step-heating methods, range in ape from 571 to 8.2 ka. Tephra from these Ar-40/Ar-39-dated Marie Byrd Land eruptions are identifie d by geochemical fingerprinting in the 1968 Byrd Station ice core. The 74 k a 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 geochemic al and age correlation of the youngest (ca. 7.5 ka) tephra layer in the Byr d ice core to an 8.2 +/- 5.4 ka (2 sigma uncertainty) pyroclastic deposit a t Mount Takahe, We infer that the 20-30 ka tephra layers in the Byrd ice co re actually were erupted from Mount Berlin, on the basis of age and geochem ical similarities. If products of these youngest, as wed as the older Ar-40 /Ar-39-dated eruptions are identified by geochemical fingerprinting in futu re ice and marine cores, they will provide the cores with independently dat ed time horizons. More than 12 Ar-40/Ar-39-dated tephra layers, exposed in bare ice on the su mmit ice cap of Mount Moulton, 30 km from their inferred source at Mount Be rlin, 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 Anta rctica This well-dated section of locally derived glacial ice contains a po tential "horizontal ice core" record of paleoclimate that extends back thro ugh several glacial-interglacial cycles. The coarse grain size and density of the englacial tephra (mean diameters 17-18 mm, densities 530-780 kg/m(3) ), combined with their distance from source, indicate derivation from highl y explosive Plinian eruptions of Mount Berlin.