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The Toarcian oceanic anoxic event (T-OAE, similar to 183 Ma) marks a geologically brief and severe global warming, associated with a profound perturbation in the global carbon cycle. The carbon cycle perturbation has been documented worldwide in marine and continental sedimentary records with a pronounced negative carbon isotope excursion (CIE) in the long-term delta C-13 profile. However, the cyclostratigraphically inferred duration of the CIE, which was mainly derived from the Paris (France) and Lusitanian (Portugal) basins, remains controversial, resulting in two notably different estimates of 300-500 and 900 kyr.

Here, we present an early Toarcian cyclostratigraphic record from the High Atlas in Morocco (Talghemt section), based on high-resolution delta C-13 and %CaCO3 data, which capture the Pliensbachian-Toarcian (Pl-To) transition event and the T-OAE, and strongly correlate to previous delta C-13 key records. Orbital tuning based on the short and long, stable 405 kyr (g2-g5) eccentricity cycles, provides a duration of similar to 400 to similar to 500 kyr for the T-OAE. This duration is very close to that previously inferred from the Sancerre Core in the Paris Basin (300 to 500 kyr), and similar to that recently revised from the Peniche section (Lusitanian Basin, Portugal) (similar to 472 kyr).

In addition, the 405 kyr%CaCO3 timescale at Talghemt calibrates high-frequency delta C-13 variations at the PI-To transition and the initiation part of the T-OAE to the obliquity cycle band, thus concuring with previous studies for obliquity forcing during these time intervals. The 405 kyr calibrated O1 obliquity period (similar to 30 kyr) is shorter than the astronomically predicted one (similar to 35 kyr), hence supporting the hypothesis of shortened obliquity periods during the Early Jurassic, and providing constraints on Earth's tidal dissipation factor during this geologic epoch.

Finally, a remarkable phase change between %CaCO3 and delta C-13 orbitally paced cycles is observed for the first time at the T-OAE, suggesting a change in the carbon reservoir in relation with volcanically released greenhouse gases and major carbonate crisis. However, this phase shift is not observed at the PI-To event implying different causal mechanisms on the carbon cycle perturbation between the PI-To and T-OAE events.