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TitleHigh-resolution carbon isotope records of the Toarcian Oceanic Anoxic Event (early Jurassic) from North America and implications for the global drivers of the Toarcian carbon cycle
AuthorThem, R T; Gill, B C; Caruthers, A H; Grocke, D R; Tulsky, E T; Martindale, R C; Poulton, T P; Smith, P L
SourceEarth and Planetary Science Letters vol. 459, 2017 p. 118-126,
Alt SeriesEarth Sciences Sector, Contribution Series 20150492
Mediapaper; on-line; digital
File formatpdf
AreaYorkshire; Sancerre; Brody-Lubienia; United Kingdom; France; Poland
Subjectsstratigraphy; Toarcian; climate; oceanic anoxic event; methanogenesis; carbon cycle; Jurassic
Illustrationslocation maps; stratigraphic columns; graphs
ProgramShale Reservoir Characterization, Geoscience for New Energy Supply (GNES)
AbstractThe Mesozoic Era experienced several instances of abrupt environmental change that are associated with instabilities in the climate, reorganizations of the global carbon cycle, and elevated extinction rates. Often during these perturbations, oxygen-deficient conditions developed in the oceans resulting in the widespread deposition of organic-rich sediments - these events are referred to as Oceanic Anoxic Events or OAEs. Such events have been linked to massive injections of greenhouse gases into the ocean-atmosphere system by transient episodes of voluminous volcanism and the destabilization of methane clathrates within marine environments. Nevertheless, uncertainty surrounds the specific environmental drivers and feedbacks that occurred during the OAEs that caused perturbations in the carbon cycle; this is particularly true of the Early Jurassic Toarcian OAE (~183.1 Ma). Here, we present biostratigraphically constrained carbon isotope data from western North America (Alberta and British Columbia, Canada) to better assess the global extent of the carbon cycle perturbations. We identify the large negative carbon isotope excursion associated with the OAE along with high-frequency oscillations and steps within the onset of this excursion. We propose that these high-frequency carbon isotope excursions reflect changes to the global carbon cycle and also that they are related to the production and release of greenhouse gases from terrestrial environments on astronomical timescales. Furthermore, increased terrestrial methanogenesis should be considered an important climatic feedback during Ocean Anoxic Events and other similar events in Earth history after the proliferation of land plants.
Summary(Plain Language Summary, not published)
One of the Earth's major episodes of global oceanic anoxia, which also coincides in time with a major volcanic event, is recorded in the Early Toarcian stage (about 180 million years ago) in western Alberta. This report provides documentation of this event in the form of a major shift in the geochemistry of the shale (carbon isotopes). It also speculates that high-frequency variation in these isotope variations is due to the release of methane from Greenhouse-type wetlands, and suggests that this process can be applied also to at least one other major global anoxia-volcanic event.