2018 Volume 72 Issue 4 Pages 195-211
The dark-gray mudstone of the Araya Formation distributed in the Tochio area of Nagaoka City, Niigata Prefecture yields various calcareous nodules, such as cylindrical, lenticular, spheroidal and irregularshaped. Some of the cylindrical nodules contain glendonite as the nucleus, which is surrounded by a marly mantle and a rim forming a concentric structure. The carbon and oxygen isotope ratios of the nodules were measured and the genetic link between glendonite and calcareous nodules was studied. The nodules are essentially classified into two types: the glendonite-bearing type and the glendonite-free type. The authigenic mineral composition of the glendonite-bearing type is single-phase calcite, whereas the glendonite-free type is comprised of single-phase calcite and a calcite-siderite mixed phase. There is also a difference in the carbon and oxygen isotopic ratios between the two types. The δ13C values increase in the following order: calcite of glendonite-bearing type, calcite of glendonite-free type, calcite of two-phase nodules, siderite of two-phase nodules. In contrast, the δ18O values of calcite decrease in the same order as above. The isotopic ratios and chemical composition of the nodules represent an almost continuous process of sequeling from sulphate reduction to methane fermentation, following the decomposition of burrowing animals.
