On the timing of the opening of Western Pacific's back-arc seas: Data from Sr, O and C isotopes in Miocene molluscs of Sakhalin and Hokkaido

Abstract

This study presents Sr-isotope data to determine the stage affiliation of different parts of the Miocene Sertunaiskaya and Kurasiyskaya formations in Sakhalin, as well as the upper part of the Togeshita Formation in Hokkaido. Additionally, it confirms a diachronous boundary between the Sertunaiskaya and Kurasiyskaya formations. The data received will be useful for globally correlating the Miocene formations of the Sakhalin-Hokkaido area. The study indicates that the Japan and Kuril back-arc basins in the Western Pacific underwent their final opening stages during the Middle Miocene (18.1–17.7 and 15.9–14.2 Ma, respectively), as determined by the SIS method. This process was likely influenced by both global and local factors. One of the local influences on the formation of back-arc basins in the Western Pacific appears to be the influx of cool boreal waters, as indicated by the δ18O signals. The Miocene data from the Sakhalin-Hokkaido area, including evidence from Sr, O, and C isotopes, combined with corresponding paleontological, stratigraphic, palaeomagnetic, and palaeobiogeographic data from the Paleogene and Neogene, provide a four-stage chronological sequence of events related to the opening of the Western Pacific's back-arc seas. This paper presents original information on the Miocene C-isotope excursion in the Western Pacific, which are correlated with the Monterey Event. Recent data on this topic may have enhanced our comprehension of the conditions required for the formation of hydrocarbons, which are found in substantial amounts in the Miocene of northern Sakhalin and Hokkaido.