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

Boulila, Slah, Galbrun, Bruno, Sadki, Driss, Gardin, Silvia, Bartolini, Annachiara
Global and planetary change 2019 v.175 pp. 113-128
Toarcian age, basins, carbon, carbon cycle, carbonates, global warming, greenhouse gases, phase transition, stable isotopes, France, Morocco, Portugal
The Toarcian oceanic anoxic event (T-OAE, ~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 δ13C 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 δ13C and %CaCO3 data, which capture the Pliensbachian-Toarcian (Pl-To) transition event and the T-OAE, and strongly correlate to previous δ13C key records. Orbital tuning based on the short and long, stable 405 kyr (g2–g5) eccentricity cycles, provides a duration of ~400 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) (~472 kyr).In addition, the 405 kyr%CaCO3 timescale at Talghemt calibrates high-frequency δ13C 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 kyr calibrated O1 obliquity period (~30 kyr) is shorter than the astronomically predicted one (~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 δ13C 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.