FT-IR study on ambers produced from the Tohoku and Hokkaido regions in Japan

Abstract

We performed a series of Fourier Transform infrared (FT-IR) studies on Late Cretaceous Kuji and Iwaki ambers from the Tohoku region, Late Cretaceous Pombetsu and Manji ambers and Eocene Sunago ambers from Mikasa area in the Hokkaido region, and Eocene ambers from Fushun in China. Based on spectral features and principal component scores, these FT-IR spectra could be successfully classified into three groups: Kuji and Iwaki ambers, Pombetsu and Manji ambers, and Sunago and Fushun ambers, respectively. This grouping reflects the fact that amber-forming forests are the same or closely related species within the group, but different among the groups. Geological surveys on the Upper Cretaceous Yezo Group around the Mikasa area indicate that the Pombetsu ambers were reburied in terrestrial environments about 4 million years earlier than the Manji ambers buried in shallow marine deposits. The Late Cretaceous ambers from the Mikasa Formation frequently contain various concentrations of calcite, and their FT-IR spectra are quite different from the Kuji and Iwaki ambers which are free from calcite. These observations strongly suggest different amber-forming environments between the Tohoku and Hokkaido regions in the Late Cretaceous. In contrast, high similarities of FT-IR spectra between Sunago and Fushun ambers suggest the same or similar amber-forming environments in the Eocene near the eastern end of the Eurasian plate. Although the current geographical distance between Sunago and Fushun is about 1500 km, the distance was much closer in the Eocene before the establishment of the Sea of Japan.