ABSTRACT

The Middle Triassic has been interpreted as the epoch of the biotic recovery phase from the end-Permian mass extinction. In particular, the early Middle Triassic (Anisian) oceanic anoxic events are thought to be important as the final ones before the rebuilding of marine ecosystems with complex trophic levels such as modern. However, details of global environmental and biotic changes during the Anisian oceanic anoxic events remain unclear. In this study, we applied multivariate analysis to extract geochemical components controlling chemical compositional changes on the high resolution geochemical stratigraphy of the Middle Triassic (Anisian) bedded chert sequence in central Japan, which deposited in a pelagic deep seafloor in a low- to mid-latitude zone of the Panthalassic Ocean. Two geochemical components were extracted from the major elemental compositions. The first component explained the chemical weathering intensity, whose statistics indicate that the chemical weathering intensity increased before and decreased within or after the anoxic conditions. The second component represented the biogenic apatite accumulation, whose statistics suggest that the higher biogenic apatite accumulation during the oxic conditions and the minor variations in this dataset. Integrated stratigraphic data suggest that the chemical weathering intensity controlled the timings and the durations of the middle Anisian oceanic anoxic events and these situations affected the biogenic apatite accumulation.