Thursday, October 01, 2009, 4:30PM - 5:30PM
During explosive volcanic eruptions, pyroclastic material (tephra) ejected into the atmosphere is largely deposited proximal to the volcanic source. However, if eruptions are explosive enough to penetrate the stratosphere, fine tephra particles may be transported many thousands of kilometers, sometimes globally, depositing on distal lands, oceans, and ice caps. Elusive grains invisible to the naked eye (< 100 µm) termed cryptotephra can serve as key chronostratigraphic markers well beyond the range of visible tephra beds, constraining sediment ages and enhancing regional correlation.
In this pilot study, we push the limits of cryptotephra detection in distal High Arctic clastic Holocene lake sediments. Clastic sediments pose a significant problem for isolating and concentrating cryptotephra grains just tens of microns in size. Using a sequence of acid digestions, sieves, and heavy liquid separations of varying densities we successfully reduced the amount of clastic matrix material and increased the probability of finding cryptotephra. By carefully searching through thousands of minerogenic grains for glass shards using scanning electron imagery and energy-dispersive x-ray analysis (EDS), our study revealed cryptotephra in a lake basin in Spitsbergen, Svalbard, located more than 1000 km from the nearest source of Holocene tephra. The two glass shards discovered in a single sample have been analyzed by electron microprobe together with Icelandic reference samples, but the source eruption is as yet unidentified. By re-polishing and re-analyzing the particles, we increased the amount of quantitative compositional data that could be acquired from such sparse, fine glass shards. The striking morphological similarity between phytoliths (biogenic silica) and cryptotephra also adds to the challenge, but these are rapidly distinguished by EDS analysis.