| Title: | The Cenomanian-Turonian Oceanic Anoxic Event: crystallizing the concept |
| Author: | Hugh C Jenkyns |
| Date Submitted: | 04/25/2002 |
| Address: | Parks Road
OXFORD
UK
OX1 4LY
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| Phone: | 44-1865-272023 |
| Email: | hughj@earth.ox.ac.uk |
| Co-Authors: | Tsikos, Harry, Department of Earth Sciences, Oxford University, harryt@earth.ox.ac.uk, C/T Net Group |
| Affiliation: | Department of Earth Sciences, Oxford University |
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| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725490 |
| Author Project webpage: | http://www.icbm.de/ctnet/ |
| Keywords: | Oceanic Anoxic Event, Cenomanian-Turonian, OAE |
| Abstract: | The discovery of black carbon-rich shales in deep-sea drilling sites from the Atlantic, Indian and Pacific Oceans (from DSDP Leg 1 onwards) led in the mid-nineteen-seventies to the concept of Oceanic Anoxic Events. Such events, whatever their exact nature and cause, were hypothesized to foster deposition of coeval carbon-rich sediments across environments ranging from deep oceans to shelf seas. The original concept was primarily stratigraphic in nature, being based on the implicit assumption that the world ocean underwent a fundamental chemical and/or biological change during such events: enhanced productivity of organic-walled microfossils and bacteria and/or enhanced preservation of organic matter were both suggested as likely causes. Although early stratigraphic compilations attributed black shales of this approximate age to either the late Cenomanian and/or the early Turonian, increased stratigraphic resolution has demonstrated a remarkable synchroneity of these characteristic deposits around and across the stage boundary. The advent of carbon-isotope stratigraphy in the nineteen eighties, with the recognition of a characteristic positive excursion related to excess global carbon burial, offered an independent means of high-resolution correlation on a global basis, independent of biostratigraphy. Recent carbon-isotope studies from England, Italy, Morocco and elsewhere now demonstrate that the stratigraphic position of Cenomanian-Turonian black shales, or of their most carbon-rich portions, is not fixed in exactly the same position with respect to carbon-isotope curve: in some localities the onset of the positive excursion coincides with the beginning of black-shale deposition, in others it does not. Hence, at these high levels of resolution, the "stratigraphic" concept of the Oceanic Anoxic Event breaks down. The event is thus more usefully defined with respect to the carbon-isotope excursion alone. Broadly coincident increases in bulk nitrogen-isotope ratios and total organic carbon over the interval of the carbon-isotope excursion suggest that in most, if not all localities enhanced productivity was a more important mechanism than was enhanced preservation for the generation of black shales during the Cenomanian-Turonian Oceanic Anoxic Event.
This work is supported by the European Community's Improving Human Potential Program under contract HPRN-CT-1999-00055, C/T-NET. |
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