Sr isotope ratios and diatom ages of the Neogene carbonate concretions in Akita and northern Yamagata areas, Northeast Japan

Abstract

Strontium isotope ratios and diatom fossil ages were determined for carbonate concretions developed in Neogene sedimentary rocks in the Akita and northern Yamagata areas, northeastern Japan. The diatom biozones of five carbonate concretions were assigned to Denticulopsis dimorpha (North Pacific Diatom biohorizon 5D), Rouxia californica (7A), and lower Neodenticula kamtschatica (7Ba) zones. ⁸⁷Sr/⁸⁶Sr isotope ratios of carbonate concretions were distributed between the values of paleo-seawater and those of igneous rocks in the study area, ranging from 0.709013 to 0.706749. The younger the age of a concretion, the smaller the deviation of its ⁸⁷Sr/⁸⁶Sr ratio from that of paleo-seawater. This indicates that the carbonate concretions were generated from pore water that had been contaminated by igneous material associated with volcanic activity in the area during the middle to late Miocene．

Fossil shells occurring with concretions in the Tentokuji and Sasaoka Formations have Sr isotope ratios similar to that of paleo-seawater, leading to a hypothesis that the measured Sr isotope ratios of both fossil shells and concretions are primary and have remained unchanged throughout the burial process. This is supported by a significant difference in Sr isotope ratios between a given concretion and an included molluscan fossil with a preserved aragonite shell. Similarly, carbonate concretions in the horizons lower than the Funakawa Formation may also have inherited relatively low Sr isotope ratios from pore water during the early stages of diagenesis.