EFFECTS OF VOLCANISM AND ASTRONOMICAL CYCLES ON THE TOARCIAN OCEANIC ANOXIC EVENTS (T-OAES; EARLY JURASSIC)

One of the most profound environmental changes in the Mesozoic took place during Pliensbachian–Toarcian (Early Jurassic), including oceanic anoxia (Toarcian Oceanic Anoxic Event; T-OAE). The T-OAE is thought to have been caused by increased atmospheric CO2 triggered by Karoo–Ferrar volcanism. This idea, however, remains debated, primarily due to uncertainties in their age constraints of the relevant sedimentary sequences. To examine their temporal relationships, herein, we provide the carbon isotope records and astronomical time scale of the Lower Jurassic deep-sea bedded chert sequences from the pelagic Panthalassa superocean, which are exposed in the Inuyama area, central Japan. Two well-laminated black bedded chert inervals (T-OAE1 and T-OAE2) are coincide with negative shift of organic carbon isotope and enrichment of redox-sensitive metals. A 405-kyr tuned astrochronology, anchored to the end-Triassic extinction as 201.4 ± 0.2 Ma (Ikeda and Tada, 2013, 2014), allows us to constrain the ages of T-OAE1 and T-OAE2. The ages of these T-OAEs overlap U–Pb ages of Karoo-Ferrar volcanic rocks. These temporal relationships support the potential impact of Karoo–Ferrar volcanism on T-OAEs on a global scale. On the other hand, the onset of T-OAEs occur at the maxima of ~40 kyr, ~100 kyr, and 405 kyr cycles of chert thickness, and the termination of T-OAEs and the recovery to oxic conditions in pelagic ocean coincide with the minima of ~40 kyr, ~100 kyr, and 405 kyr cycles of chert thickness.These temporal relationships imply that orbital-scale productivity cycles were important for paceing the onset and termination of T-OAEs through the carbon cycle dynamics, which have been already amplified by Karoo–Ferrar volcanism.