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

Abstract

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 d 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 kyr. Here, we present an early Toarcian cyclostratigraphic record from the High Atlas in Morocco (Talghemt section), based on high-resolution d 13 C and %CaCO 3 data, which capture the Pliensbachian-Toarcian (Pl-To) transition event and the T-OAE, and strongly correlate to previous d 13 C 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 %CaCO 3 timescale at Talghemt calibrates high-frequency d 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 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 %CaCO 3 and d 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.

A major sea-level drop briefly precedes the Toarcian oceanic anoxic event: implication for Early Jurassic climate and carbon cycle

“…We suggest that in Peniche, hiatuses are most likely associated with the condensed intervals documented at the Pliensbacian–Toarcian boundary and within the onset of the T-OAE carbon isotope excursion 43,44 . In summary our best estimate for the duration of the latest Polymorphum sea-level fall is at least 100 kyr, and possibly longer given the likelihood of the presence of hiatuses or extreme condensation in the Peniche section 43,44,72 .…”

“…The duration of the Polymorphum Zone has been estimated to about 1 Myr 70–72 . It is possible to infer the duration for the forced regression based on the correlation of discontinuities between the Central High Atlas and the Lusitanian Basins (Portugal).…”

Phanerozoic Large Igneous Province, Petroleum System, and Source Rock Links

“…The Toarcian OAE is marked by a negative CIE bracketed by 182 and 183 Ma based on expanded sections from North America (Them et al, 2017), and the Atlas to western Europe (Ait-Itto et al, 2018) with a cyclostratigraphically tuned duration of 500-600 kyr (Boulila et al, 2019). Rhenium-osmium geochronology of the Toarcian Posidonia Shale, southwest Germany, yield a 183 ± 2 Ma isochron age for the shales with low 187 Os/ 188 Os i values (0.38) consistent with weathering of the Karoo-Ferrar LIP (van Acken et al, 2019).…”

Ocean acidification and photic‐zone anoxia at the Toarcian Oceanic Anoxic Event: Insights from the Adriatic Carbonate Platform

“…5). Using the 400 to 500 kyr estimation for the duration of the T-OAE (Boulila et al, 2019), the carbonate accumulation rate on the outer northwest Adriatic Platform (Kovk), not accounting for compaction or pressure dissolution, was approximately 0.6 cm kyr −1 (500 kyr assumption). By comparison, normal accumulation rates of pure carbonate platforms of the Jurassic are approximately 8.5 cm kyr −1 (40.5 m over 500 kyr) (Bosscher & Schlager, 1993), implying that erosion or winnowing could not have been solely responsible for the thin T-OAE interval.…”

“…The end of the broad positive excursion, defined at the level of peak positive carbon-isotope values following the negative excursion, signals the end of the T-OAE and coincides with the exaratum-falciferum Subzone boundary of the mid serpentinum Zone in northern European sections (Jenkyns et al, 2002;Xu et al, 2018), approximately equivalent to the mid levisoni Zone (Mediterranean). Cyclostratigraphic estimates of the duration of the carbonisotope excursion demarcating the T-OAE range from 300 to 900 kyr (Suan et al, 2008a;Boulila et al, 2014;Huang & Hesselbo, 2014;Boulila & Hinnov, 2017;Boulila et al, 2019).…”