Constraints on the duration of the early Toarcian T-OAE and evidence for carbon-reservoir change from the High Atlas (Morocco)

Abstract

Abstract The Toarcian oceanic anoxic event (T-OAE, ~183\u202fMa) 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 δ 13 C 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\u202fkyr. Here, we present an early Toarcian cyclostratigraphic record from the High Atlas in Morocco (Talghemt section), based on high-resolution δ 13 C and %CaCO 3 data, which capture the Pliensbachian-Toarcian (Pl-To) transition event and the T-OAE, and strongly correlate to previous δ 13 C key records. Orbital tuning based on the short and long, stable 405\u202fkyr (g2–g5) eccentricity cycles, provides a duration of ~400 to ~500\u202fkyr for the T-OAE. This duration is very close to that previously inferred from the Sancerre Core in the Paris Basin (300 to 500\u202fkyr), and similar to that recently revised from the Peniche section (Lusitanian Basin, Portugal) (~472\u202fkyr). In addition, the 405\u202fkyr%CaCO 3 timescale at Talghemt calibrates high-frequency δ 13 C variations at the Pl-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\u202fkyr calibrated O1 obliquity period (~30\u202fkyr) is shorter than the astronomically predicted one (~35\u202fkyr), 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 %CaCO 3 and δ 13 C 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 Pl-To event implying different causal mechanisms on the carbon cycle perturbation between the Pl-To and T-OAE events.