Duration of the Early Toarcian carbon isotope excursion deduced from spectral analysis: Consequence for its possible causes

作者：Suan Guillaume; Pittet Bernard^*; Bour Ivan; Mattioli Emanuela; Duarte Luis V; Mailliot Samuel

来源：Earth and Planetary Science Letters, 2008, 267(3-4): 666-679.

DOI：10.1016/j.epsl.2007.12.017

• cyclostratigraphy
• carbon cycle
• Jurassic
• Toarcian oceanic anoxic event

The marked 3-8 parts per thousand negative carbon isotope excursion associated with the Early Toarcian oceanic anoxic event (OAE; similar to 183 myr ago) in the Early Jurassic period is thought to represent one of the most important perturbations of the C-cycle in the last 200 myr. However, the origin of this excursion remains strongly debated, primarily due to uncertainties in the estimation of its duration, which ranges from similar to 200 kyr to 1 myr. Here we present a new orbital calibration of the Early Toarcian carbon isotope excursion, based on spectral analyses of two independent datasets generated from the sedimentary record of two hemipelagic sections from Portugal (Peniche) and SW Germany (Dotternhausen), in order to better constrain the timescale and hence the origin of this excursion. These analyses reveal that orbital cycles exert a strong influence on both the calcium carbonate content in Portugal and on the greyscale of black shales in Germany, which allow us to propose a duration of >= 1.9 myr for the Early Toarcian and of similar to 900 kyr for the entire carbon isotope excursion. The shift towards lower carbon isotope values lasted similar to 150 kyr, and carbon isotope values remained low for similar to 450 kyr; the subsequent increase of carbon isotope values lasted similar to 300 kyr. This calibration suggests that the sustained input of isotopically light carbon at the origin of the excursion occurred over similar to 600 kyr and thus dismisses causal mechanisms implying relatively small source reservoirs such as the massive dissociation of methane hydrates. In the light of our new cyclostratigraphic timescale, the massive input of isotopically light carbon associated with the emplacement of the Karoo-Ferrar basaltic province appears as the most likely cause of the Toarcian global carbon isotope excursion. We also show that the C-isotope perturbation coincided with a transition from precession-eceentricity-dominated cycles to obliquity-eccentricity-dominated cycles, suggesting that the OAE was marked by a fundamental change in the response of the climate system, which allowed the obliquity signal, normally better recorded at high latitudes, to be a dominant forcing factor of short-term sedimentary cycles at tropical latitudes.

• 出版日期2008-3-30