Heavy sulfur-isotope enrichment in Middle Miocene onnagawa mudstones in the Green Tuff Region, Northeastern Japan

Sulfur isotope ratios are analyzed for a series of mudstones in a successive section in the Taiheizan area in the central Green Tuff region, northeast Japan. The δ³⁴S values for the Uyashinai Mudstone Member (Nishikurosawa stage), the Onnagawa Formation and the Funakawa Formation range from -45 to -14 ‰, -20 to +4 ‰ and -30 to -10 ‰, respectively. There is a marked positive shift in δ³⁴S values at the Nishikurosawa/Onnagawa boundary and an upward δ³⁴S-decreasing trend in the Onnagawa and Funakawa Formations. The present data provides evidence for environmental change in the form of stratification of the seawater, initiated at the Nishikurosawa/Onnagawa boundary and persisting during deposition of the Onnagawa Formation followed by gradual oxygenation during deposition of the Funakawa Formation.