Geochemical detection of alkaline cryptotephra from Ulleung Island in Lake Biwa and Suigetsu cores

Abstract

This is the first report of the supersensitive and effective detection of alkaline cryptotephras by instrumental neutron activation analysis (INAA). Cryptotephra are tephra horizons that are invisible to the naked eye. The Ta/Sc elemental ratio of glass shard in a tephra known to be from an alkaline volcano is reported as ca. 15, whereas the average Ta/Sc value in typical sediments is below 0.1. We detect two unknown alkaline cryptotephras, which were not detected by susceptibility measurements, from down-core profiles of the Ta/Sc ratio in a piston core from Lake Biwa in Japan. The Th/Sc ratio is a useful indicator for verifying the absence of interference by Ta-rich heavy mineral. Two or three alkaline cryptotephras are also found from annually laminated sediment in a drilled core from Lake Suigetsu in Japan, and are correlated with those from Lake Biwa.