Pacing of the Toarcian Oceanic Anoxic Event (Early Jurassic) from astronomical correlation of marine sections

Abstract

The Early Toarcian Oceanic Anoxic Event (OAE) in the Early Jurassic Period is associated with a major negative carbon isotope excursion (CIE), mass extinction, marine transgression and global warming. The Toarcian OAE is thought to have been caused by flood basalt magmatism, and may have been a trigger for mass extinction. However, these proposed causes of the Toarcian OAE and associated biotic crisis are not adequately resolved by a precise chronology. The duration of the Toarcian OAE has been estimated to be anywhere from ~0.12 to ~0.9Myr, most recently 0.74 to 3.26Myr from U–Pb dating. The CIE associated with the Toarcian OAE has a similar pattern at numerous localities, and there is evidence that the marine carbon isotope variations recorded astronomical forcing signals. Here we estimate a duration of ~620kyr for the main negative CIE, ~860kyr for the polymorphum zone and >1.58Myr for the levisoni zone based on 405-kyr astronomical eccentricity tuning of the marine section at Peniche (Portugal). This 405-kyr tuned series provides a ~2.5Myr continuous high-resolution chronology through the Early Toarcian. There are 6, or possibly 7 short eccentricity cycles in the main CIE interval at Peniche. To confirm this astronomically based estimate, we analyzed three other sections at Yorkshire (UK), Dotternhausen (Germany), and Valdorbia (Italy) from marine carbon isotopic series. These four stratigraphic sections from Early Jurassic western Tethys record the Toarcian OAE with ~6 prominent carbon isotope cycles in the CIE that span a 600±100kyr duration. The Peniche 405-kyr tuned series indicates that the pre- and post-CIE intervals experienced strong precession–eccentricity-forced climate change, whereas the CIE interval is marked by dominant obliquity forcing. These dramatic and abrupt changes in astronomical response in the carbon isotopes point to fundamental shifting in the Early Toarcian paleoclimate system that was directly linked to the global carbon cycle.