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Abstract

A section from the Ultrahelvetic units of Upper Austria comprises the transition from light-gray, upper-bathyal to middle-bathyal carbonates of Early Turonian age towards red-colored carbonates of the Middle Turonian. 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 decreased oxygen concentrations at the seafloor. The overlying reddish layers are enriched in iron oxides and hydroxides and pyrite and may represent paleo-redox fronts that were related to periods of well oxygenated bottom waters, reduced sedimentation rates, and degradation of organic matter in the underlying sediments. Cyclic sedimentary packages consist of four marlstone-limestone couplets with upward-increasing bedding thickness, red carbonates at the base, and a gray limestone bed at the top. Based on carbon isotope stratigraphy, these packages reflect the 400 kyr eccentricity cycle, with the four marlstone-limestone couplets representing the 100 kyr cycle. Three 400-kyr-cycle boundaries are preceded by horizons with high T.laciniosa abundance and are followed by a paleo-redox front and red-bed deposition. Thus, there is indication of the temporary presence of a local oxygen-minimum zone during enhanced production at the end of each 400 kyr cycle, possibly linked to sea-level fluctuations. Organic-carbon fluxes seem to have decreased immediately above the cycle boundaries. 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 kyr cycles between the main carbon-isotope excursions points to a 1.2 Myr periodicity, which may reflect the Earth’s long obliquity cycle. A long-term increase in bottom-water aeration from the Early to the Middle Turonian is evident from the increasing dominance of red sediments.

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