Late Permian to Early Triassic environmental changes in the Panthalassic Ocean: Record from the seamount-associated deep-marine siliceous rocks, central Japan
Abstract In order to infer the late Late Permian to early Early Triassic environmental changes in a pelagic realm of the Panthalassic Ocean, the stratigraphic variations of TOC (wt.%) and δ13Corg (‰) of the PTB siliceous rock section (~ 3.5 m in thickness) were examined. The study section crops out in the Mino terrane, central Japan, and consists of the upper Upper Permian chert (Changhsingian) and lower Lower Triassic (Induan) black claystone intermittently with thin chert beds. The succession is reconstructed as sediments on the lower slope of a mid-Panthalassic seamount. The major extinction event of Permian radiolarians (MEE) occurs at the top of the Changhsingian chert. Our analysis shows that the TOC content is markedly higher in the Induan black claystone than the Changhsingian chert. TOC values rapidly increase across the PTB. The onset of the increase in TOC values corresponds to MEE. The rapid and profound increase in TOC values implies the sudden onset of oceanic anoxia as well as the rapid increase in primary production at the PTB. δ13Corg values are generally higher in the Changhsingian chert than the Induan claystone and chert. The stepwise drop of δ13Corg values characterizes their excursion of the study section. The largest-scale drop occurs prior to MEE. The minimum δ13Corg value is recorded at the base of the Induan claystone. The flux of δ13C-depleted gasses related to the Siberian Traps could have significantly contributed to the marked drop of δ13Corg values. Due to the weakened ocean circulation under the predominant condition of global warming, oceanic anoxia suddenly took place at the end of the Permian. The anoxic condition in the surface layer was the most likely kill-mechanism for Permian radiolarians. The anoxic condition persisted into the Induan, but was intermittently weakened. The episodic relaxation of the anoxic condition in the Induan resulted in intermittent and transient blooming of radiolarians and deposition of radiolarian chert with lowered TOC values. These biotic and depositional records of the study section imply the mid-Panthalassic Ocean was under the more hospitable conditions favored by the earlier recovery of radiolarians, relative to the Tethys Ocean and Pangean marginal seas.