Astronomical calibration of the Toarcian Stage: Implications for sequence stratigraphy and duration of the early Toarcian OAE

The Toarcian Oceanic Anoxic Event (T-OAE) of the early Jurassic period involves one of the largest perturbations of the carbon cycle in the past 250 Ma, recorded by a pronounced negative carbon-isotope excursion (CIE). Numerous studies have focused on potential causes of the T-OAE and CIE, but are hampered by an uncertain timescale. Here we present high-resolution (similar to 2 kyr) magnetic susceptibility (MS) measurements from the marine mans of the Sancerre-Couy drill-core, southern Paris Basin, spanning the entire Toarcian Stage. The MS variations document a rich series of sub-Milankovitch to Milankovitch frequencies (precession, obliquity and eccentricity) with the periodic g(2)-g(5) (405 kyr) and quasi-periodic g(4)-g(3) (similar to 2.4 Myr Cenozoic mean periodicity) eccentricity terms being the most prominent. The MS-related g(4)-g(3) variation reflects third-order eustatic sequences, and constrains the sequence stratigraphic framework of the Toarcian Stage. In addition, MS variations reveal a modulation of g(2)-g(5) by g(4)-g(3) eccentricity related cycles, suggesting that sea-level change was the main control on the deposition of the Toarcian Sancerre marls, in tune with the astro-climatic frequencies. The stable 405 kyr cyclicity constrains a minimum duration of the Toarcian Stage to similar to 8.3 Myr, and the well documented CIE, associated with the T-OAE, to similar to 300 to 500 kyr. The 405 kyr MS timescale calibrates the periodicity of the prominent high-frequency delta C-13 cycles that occur in the decreasing part of the CIE to 30 to 34 kyr, consistent with the Toarcian obliquity period predicted for an Earth experiencing sustained tidal dissipation. (C) 2013 Elsevier B.V. All rights reserved.