HR: 0830h
AN: PP41B-0840    [PDF]
TI: The Global Significance of a Deep-sea Isotopic Event During the Toarcian Oceanic Anoxic Event Recorded in Japan
AU: * Grocke, D R
EM: grocke@mcmaster.ca
AF: McMaster University, School of Geography & Geology, Hamilton, ON L8S 4K1 Canada
AU: Hori, R S
EM: shori@sci.ehime-u.ac.jp
AF: Ehime University, Department of Earth Sciences, Matsuyama, 790-8577 Japan
AU: Arthur, M A
EM: arthur@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AB: The record of the Toarcian oceanic anoxic event (OAE) is considered global in nature due to the occurrence of organic carbon-rich sediments worldwide, although its expression in the carbon isotopic record (marine carbonate and organic matter and terrestrial plants) has only been recorded in European localities. We have investigated the carbon-isotope record of marine organic matter in deep-sea cherts of the Inuyama area in Japan. This stratigraphic chert sequence records the Toarcian OAE through a transition from red to black to red-green cherts and radiolarian biostratigraphy indicates a Pliensbachian-Toarcian age range. A major negative carbon-isotope excursion ($\sim$6$\permil$) is recorded just after the Pliensbachain/Toarcian boundary based on radiolarian biostratigraphy, where $\delta^{13}$C$_{org}$ fluctuate and peak at a value of $\sim$-32$\permil$ that is concurrent with the highest TOC values ($\sim$3.5 wt %). $\delta^{13}$C$_{org}$ values gradually return to pre-excursion values but continue to rise to more positive values of $\sim$-21$\permil$ just prior to the Toarcian Radiolarian Event. After this point carbon-isotope values return to pre-Toarcian values of $\sim$-25$\permil$. This dataset confirms that the Toarcian OAE did affect the entire global carbon reservoir and was not isotopically expressed only in Europe, shallow seas or continental margin settings. The nature and cause of the excursion has been proposed to be caused by the upwelling of oxidized organic matter, a massive dissociation of continental margin CH$_{4}$-gas hydrates and/or increased volcanism all leading to a surface-water productivity event. Based on sedimentation rates the onset of the negative $\delta^{13}$C$_{org}$ excursion occurs in less than $\sim$100kyr, which is certainly within the realm of a CH$_{4}$-gas hydrate event, and the total duration of the negative excursion is $\sim$500kyr. The duration of the negative and positive $\delta^{13}$C$_{org}$ excursion inclusive is $\sim$2Myr. Although a definitive answer as to the driving mechanism for the negative and positive excursion and high rates of carbon burial in the deep-sea is still a matter of debate, a model of the carbon-isotope curve suggests that both CH$_{4}$-gas hydrate and volcanism are involved in driving the global Toarcian carbon burial event. Due to the apparent synchroneity of biostratigraphy between shallow and deep-water sequences, upwelling of $^{12}$C-rich organic matter could not explain the negative carbon-isotope event on a global scale.
DE: 4267 Paleoceanography
DE: 4802 Anoxic environments
DE: 4806 Carbon cycling
DE: 4825 Geochemistry
DE: 4870 Stable isotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting