Characteristics of Fluorescent Organic Matter Entrapped in Silica Sinter from the Western Shore of Lake Shikaribetsu, Hokkaido, Japan

Small streams flowing into the western shore of Lake Shikaribetsu in Hokkaido are surrounded by extensive volcanic ejecta from the Shikaribetsu volcanic group, where layered silica sinters are exposed. These sinters exhibit strong fluorescence. In this study, the fluorescent silica sinters were classified into three types based on their fluorescence colors: orange, purple, and yellow. The objective of this research was to identify the organic compounds responsible for the fluorescence observed in each category. Particular focus was placed on analyzing hydrocarbon and polycyclic aromatic hydrocarbon (PAH) fractions. The PAHs in the orange fluorescent sinters were dominated by alkylated PAHs, suggesting a petrogenic origin. In contrast, PAHs in the yellow fluorescent sinters consisted mainly of unsubstituted (naked) PAHs, including 7-ring PAHs, indicating a possible pyrogenic (combustion-derived) origin. However, the maximum thermal history of the organic matter, estimated from maturity indices, does not exceed 150˚C—a temperature insufficient to account for the high-temperature conditions typically required for the formation of pyrogenic PAHs. Additionally, the fluorescence colors of the sinters do not correspond to the fluorescence characteristics of the identified PAHs. It is therefore likely that the compounds responsible for the observed fluorescence are contained within other fractions not analyzed in this study, such as ketones, esters, or alcohols.