Biomarker evidence from Demerara Rise for surface and deep water redox conditions in the Mid Cretaceous western Equatorial Atlantic

Author(s): Beckmann, B.; Hofmann, P.; Schouten, S.; Sinninghe Damsté, J. S.; Wagner, T.
Author Affiliation(s): Primary:
University of Cologne, Department of Geology and Mineralogy, Cologne, Federal Republic of Germany
Royal Netherlands Institute for Sea Research, Netherlands
University of Newcastle, United Kingdom
Volume Title: AGU 2006 fall meeting
Source: Eos, Transactions, American Geophysical Union, 87( Fall Meeting Suppl.); American Geophysical Union 2006 fall meeting, San Francisco, CA, Dec. 11-15, 2006. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0096-3941 CODEN: EOSTAJ
Note: In English
Summary: Oceanic Anoxic Events (OAEs) provide deep insights into rapid climate change and atmosphere-land ocean interactions during an extremely warm mode of the Earth system. We present results from ODP Leg 207 at Demerara Rise deposited in the western tropical Atlantic during transition from the Turonian OAE 2 to the Santonian OAE 3. Molecular markers in organic matter-rich black shale identify the composition of primary producers and provide detailed information on the oxygenation state of surface and deep waters. This information is relevant to infer the dynamics and controls of sedimentation leading to black shale in the tropical Atlantic. Bulk organic geochemical data suggest the dominance of lipid-rich marine organic matter throughout the study section. Biomarkers from the aliphatic fraction instead reveal variable contributions of e.g., archaea, diatoms, and dinoflagellates supporting changes in the community of primary producers that thrived in the oxic part of the photic zone in response to changing environmental conditions similar to modern high productive areas along continental margins. Also comparable to modern high productive areas the sea floor remained generally oxygen-depleted throughout the Turonian to Santonian as supported by elevated lycopane contents along with an enrichment of redox-sensitive elements and documented by persistent high TOC concentrations (1 to 14%). Isorenieratane derivates indicative of photic zone euxinia (PZE) were only detected in low abundances in the lowest part of the study section. This observation contrasts biomarker records from the eastern low latitude Atlantic where PZE was a temporal feature determining black shale formation. The new biomarker data from Leg 207 support progressive weakening of upwelling intensity along with oxygenation of surface and possibly mid waters from the upper Coniacian on. Different from black shale sites in many semi-sheltered sub-basins along the Equatorial Atlantic, Demerara Rise was fully exposed to open marine currents throughout the mid-Cretaceous. Increasing ocean circulation along with the widening of the Equatorial Atlantic probably had a significant effect on shallow ocean oxygenation off tropical S-America. Notably deep ocean oxygenation was decoupled from these processes posing the general question what maintained anoxia at the sea floor over millions of years in the aftermath of OAE 2 at Demerara Rise.
Year of Publication: 2006
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Anaerobic environment; Atlantic Ocean; Biomarkers; Black shale; Clastic rocks; Cretaceous; Deep-water environment; Demerara Rise; Equatorial Atlantic; Leg 207; Lithofacies; Marine environment; Mesozoic; Middle Cretaceous; North Atlantic; Northwest Atlantic; Ocean Drilling Program; Organic compounds; Sedimentary rocks; South America; Surinam; Total organic carbon; West Atlantic
Coordinates: N090200 N092800 W0541100 W0544400
Record ID: 2009021094
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