Synchronization of the astronomical time scales in the Early Toarcian: A link between anoxia, carbon-cycle perturbation, mass extinction and volcanism

Abstract

The Late Pliensbachian-Early Toarcian is a pivotal time in the Mesozoic era, marked by pronounced carbon-isotope excursions, biotic crises and major climatic and oceanographic changes. Here we present new high-resolution carbon-isotope and magnetic-susceptibility measurements from an expanded hemipelagic Late Pliensbachian-Early Toarcian section from the Middle Atlas Basin (Morocco). Our new astronomical calibration allows the construction of an orbital time scale based on the 100-kyr eccentricity cycle. The Early Toarcian Polymorphum Zone contains 10 to 10.5 repetitions of the 100-kyr eccentricity both in the carbon-isotope and the magnetic-susceptibility data, leading to an average duration of 1.00 ± 0.08 myr. We also show that the Late Pliensbachian-Early Toarcian global carbon-cycle perturbation has an average duration of 0.24 ± 0.02 myr. These durations are comparable to previous astrochronological time scales provided for this time interval in the most complete 2 sections of the Tethyan area, and longer than what has been provided in condensed sections. Anchoring this framework on published radiometric ages and astrochronological time scales, we estimate that the carbon-cycle perturbation of the Late Pliensbachian-Early Toarcian corresponds with the early phase of the Karoo and Chonke Aike large igneous provinces. Likewise, our new age constraints confirm that the Toarcian oceanic anoxic event is synchronous to the main phase of the Ferrar volcanic activity. Thus, these successive and short phases of the volcanic activity may have been at the origin of the successive phases of the mass extinctions observed in marine biotas in the Pliensbachian and Toarcian times.

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

“…7). Thus, this duration is very close to the 1 ±0.08 Myr duration inferred from the relatively more expanded Issouka section (Ait-Itto et al, 2018). Accordingly, significant condensation or hiatus at Talghemt section is unlikely.…”

“…7). Previous cyclostratigraphic estimates from Foum Tillicht section in the central High Atlas Basin, and Issouka section in the Middle Atlas Basin provided durations for the Pl-To CIE of respectively 0.18-0.27 Myr (Martinez et al, 2017), and 0.24 ±0.02 Myr (Ait-Itto et al, 2018).…”

“…Accordingly, significant condensation or hiatus at Talghemt section is unlikely. The same study of Ait-Itto et al (2018) claimed that the longer 900 kyr duration (e.g., Suan et al, 2008b) of the T-OAE is more appropriate than the shorter 300-500 kyr duration (e.g., , while their studied Issouka section does not record the T-OAE time interval. They further went to estimate ages for the base and end of T-OAE to draw conclusions on implications of these ages for LIP phases.…”

“…For instance, the Polymorphum ammonite zone interval is almost 30 m thick at Talghemt section. The same equivalent interval is almost 65 m thick at Amellago section (Bodin et al, 2010(Bodin et al, , 2016, and almost 40 m thick at Issouka section (Ait-Itto et al, 2018). The Issouka section was used for cyclostratigraphy to assess the duration of Polymorphum Zone at 1 ±0.08 Myr (Ait-Itto et al, 2018).…”

Orbital pacing and secular evolution of the Early Jurassic carbon cycle

“…Therefore, these δ 13 C shifts have largely been interpreted as standalone events, linked to a release of 12 C from as-yet-undefined sources, reduced organic productivity (leaving more 12 C in the ocean-atmosphere system) and/or 13 C-depleted carbon sequestration and orbitally forced environmental change affecting the carbon cycle on the scale of Milankovitch cyclicity (17,20,24,25). Evidence for the latter is so far limited to the Hettangian to early Sinemurian and the early Toarcian, where high-resolution isotope records provide the basis for cyclostratigraphic analysis (17)(18)(19)(29)(30)(31).…”