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New high resolution geochemistry of Lower Jurassic marine sections in western North America : a global positive carbon isotope excursion in the Sinemurian?

Porter, S.J. and Smith, P.L. and Carruthers, A.H. and Hou, P. and Gröcke, D.R. and Selby, D. (2014) 'New high resolution geochemistry of Lower Jurassic marine sections in western North America : a global positive carbon isotope excursion in the Sinemurian?', Earth and planetary science letters., 397 . pp. 19-31.


Recognising variations in the carbon isotope compositions of marine organic-rich sedimentary rocks can provide insight into changes in ocean chemistry throughout geological time. Further, identification of global excursions in the carbon isotope record has proved to be valuable as a chronostratigraphic correlation tool. This investigation presents new high-resolution organic carbon isotope data (View the MathML source) for marine sediments from 2 regions in North America (Last Creek, British Columbia, Canada and Five Card Draw, Nevada, USA). The carbon isotope profiles demonstrate that there were significant differences between the carbon reservoirs at Five Card Draw and Last Creek, notably in the upper part of the Leslei Zone. The View the MathML source values show a gradual positive CIE (∼2‰) at Last Creek in the upper part of the Leslei Zone. This corresponds to a coeval positive CIE of similar duration in Dorset, UK (upper Turneri Zone; Jenkyns and Weedon, 2013), suggesting that this may be a global marine carbon isotope signature, and likely reflects a widespread increase in primary productivity during the Early Sinemurian. In addition, a brief negative CIE is observed in the uppermost Lower Sinemurian at Last Creek. This negative excursion is not recorded in the Dorset section, suggesting localised upwelling of 12C-rich bottom-waters at Last Creek. Further, the signals identified at Last Creek are not present in coeval sections at Five Card Draw, thus highlighting a significant difference between these localities. Osmium (Os) isotope data (initial 187Os/188Os values) provide a quantitative determination of the contrasting depositional environments of Five Card Draw and Last Creek (at least partially restricted with high levels of continental inundation and open-ocean, respectively). This demonstrates that basinal restriction may act as a major factor that controls isotopic stratigraphic signatures, thus preventing the identification of global or widespread regional excursions.

Item Type:Article
Keywords:Stable carbon isotopes, Carbon isotope excursions (CIEs), Osmium isotopes, Lower Jurassic, Sinemurian, North America.
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Publisher statement:NOTICE: this is the author’s version of a work that was accepted for publication in Earth and Planetary Science Letters. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Earth and Planetary Science Letters, 397, 1 July 2014, 10.1016/j.epsl.2014.04.023.
Date accepted:10 April 2014
Date deposited:14 January 2015
Date of first online publication:05 May 2014
Date first made open access:No date available

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