Mid-Cretaceous cyclic oscillations in oxygenation: The story told by benthic foraminifera and XRD data (Ultrahelvetics, Austria)
|Location||International Geological Congress,oslo 2008|
|Author||Wendler, Ines۱; Wendler, Jens۱; Neuhuber, Stephanie۲; Wagreich, Michael۲|
|Holding Date||21 September 2008|
Lower and Middle Turonian limestones and marls of the Buchberg section in the Eastern Alps were deposited on the distal European continental margin of the Tethys. The carbonates represent upper- to middle bathyal sediments as is indicated by the benthic foraminifera association. The section displays a general development in sediment colours from light grey in the Lower Turonian part to red in the Middle Turonian part. The colour transition occurs stepwise with repeated grey-red changes which are related to a cyclic sedimentary pattern. These sediment packages consist of four marlstone-limestone couplets with upward increasing bedding thickness, red carbonates at the base, and a grey limestone bed in the top. Based on carbon isotope stratigraphy, the packages are interpreted to reflect the 400 ka eccentricity cycles, with the four marlstone-limestone couplets representing the 100 ka cycles. The total organic carbon content is very low throughout the section, but benthic foraminifera associations, especially high abundance of Tappanina laciniosa, indicate repeated phases of enhanced organic matter flux and less aerated bottom waters at the end of the cycles. There is indication for the temporary presence of a local oxygen minimum zone during enhanced production at the end of each 400 ka cycle, possibly linked to sea-level fluctuations. Organic carbon fluxes seem to have decreased immediately above the cycle boundaries. The reddish layers overlying the 400 ka cycle boundaries are enriched in iron oxides/hydroxides and pyrite and may represent paleo-redox fronts that were related to periods of well oxygenated bottom waters, reduced surface water production and sedimentation rates, and degradation of organic matter in the underlying sediments. The recycling of organic carbon could be responsible for the distinct carbon isotope minima at the base of each cycle. Accumulation rates based on our orbital model suggest a switch from relatively uniform sedimentation rates in the Early Turonian to cyclic fluctuations of sedimentation rates in the Middle Turonian. The occurrence of three 400 ka cycles between the main carbon isotope excursions points to a 1.2 my periodicity, which may reflect the Earth’s long obliquity cycle.