Early Toarcian (Early Jurassic) oceanic anoxic event recorded in the shelf deposits in the northwestern Panthalassa: Evidence from the Nishinakayama Formation in the Toyora area, west Japan
Abstract The early Toarcian oceanic anoxic event (T-OAE) was marked by the worldwide burial of organic-rich black shales and a coincident pronounced negative carbon-isotope excursion of carbonates, organic matter, and fossil wood. This characteristic carbon-isotope perturbation and other geochemical records have been recognized in many sections in the European domain and in selected sections in the central and eastern Panthalassa, but not in the western Panthalassa. We present herein, for the first time, data of carbon isotope ratios of organic carbon, total organic carbon (TOC) contents, and sedimentary fabrics from the uppermost Pliensbachian to the lower Toarcian sequence in the Sakuraguchi-dani Valley, Toyora area, west Japan, that was deposited in the shelf basin of the northwestern Panthalassa. The carbon-isotope profile displays a similar trend to those in contemporaneous strata in the Tethys and in the eastern and central Panthalassic regions, which is characterized by a distinct negative excursion around the Tenuicostatum/Falciferum Zone boundary in the early Toarcian. This evidence suggests that the widely recognized negative carbon-isotope excursion represents a nature of the global carbon-cycle perturbation. Analyses of total organic carbon contents and sedimentary fabrics in the study section reveal that accumulation of organic carbon occurred and anoxic bottom-water was prevalent during the early Toarcian. The most organic-rich strata and bottom-water anoxia described in the study section, however, exist in the interval showing the carbon-isotope positive recovery after the abrupt negative shift, and this fact suggests that the accumulation of organic matters and development of marine anoxia were globally diachronous, even though the negative CIE was interpreted as a synchronous event. Our study also demonstrates that the early Toarcian anoxia in the northwestern Panthalassic basin was not a single and continuous anoxic event but was interrupted by episodic short-term oxygenation events.