Effects of Karoo–Ferrar volcanism and astronomical cycles on the Toarcian Oceanic Anoxic Events (Early Jurassic)

• Astrochronology of the Lower Jurassic deep-sea sequence was established.
• Timing of T-OAEs were synchronous with Karoo–Ferrar volcanism.
• Onset/recovery of T-OAEs was at the maxima/minima of orbital cycles, respectively.

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 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. A 405-kyr tuned astrochronology, anchored to the end-Triassic extinction as 201.4 ± 0.2 Ma (Ikeda and Tada, 2013), allows us to constrain the ages of two black bedded cherts (T-OAE1 and T-OAE2). The ages of these T-OAEs overlap U–Pb ages of Karoo volcanic rocks. T-OAE in the European region is also synchronous with the Karoo–Ferrar volcanism, based on radiolarian and ammonite biostratigraphic correlation. 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 variation. 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. Moreover, the termination of final black chert and grey chert deposition coincide with the minima of ~ 1800 kyr cycles of chert thickness. These temporal relationships imply that orbital-scale productivity cycles were important in controlling the onset and termination of T-OAEs through the carbon cycle dynamics, which have been already amplified by Karoo–Ferrar volcanism.

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