Geochemical Cycles in the Albian of NW-Germany

V. Rachold
Alfred Wegener Institute for Polar and Marine Research,
Research Department Potsdam, Telegrafenberg A 43, 14473 Potsdam, FRG
H.-J. Brumsack
Institute for Chemistry and Biology of the Sea
University of Oldenburg, P.O. Box 2503, 26111 Oldenburg, FRG
Under the framework of the international ALBICORE project the Boreal Cretaceous Cycles Project (BCCP) concentrates on orbitally induced sedimentary cycles in the Albian of NW-Germany. Within this project a 250 m long core (Kirchrode 1/91) was analysed in 0.5 m intervals for major and minor element concentrations to demonstrate the influence of high frequency Milankovitch cycles on the chemical composition of the sediment.
Cyclic variations in carbonate content and Al-normalised concentrations of predominantly detrital elements, like Si and Ti, occur in a distinct depth interval of 60 m length where geochemical and sedimentological data indicate stable conditions during sedimentation. In a selected section that was studied at higher resolution (0.1 m intervals) the chemical data almost exactly display one cycle of 12 m length. In agreement with this spectral analysis of the chemical data sets clearly manifest three strong frequencies of 12 m, 5.8 m, and 3 m length corresponding to eccentricity, obliquity, and precession, respectively.
Although elements combined with the nutrient cycles, i.e. Ba and P, and elements affected by changes in redox conditions do not exhibit cyclic variations, carbonate fluctuations are related to productivity. Variation in Si/Al- and Ti/Al-ratios on the other hand are mainly linked to climatically induced changes of weathering processes. Since productivity almost immediately responds on climatic changes, whereas the "weathering system" reacts more tardy, there is no direct correlation between carbonate and detrital cycles and variations in detrital material composition are dominated by lower frequency cycles.

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