Inorganic geochemistry of Albian sediments from the Lower Saxony Basin NW germany: palaeoenvironmental constraints and orbital cycles

Within the international ALBICORE project, Upper Albian sediments from a 24 5 rn core (Kirchrode I) drilled in Hannover were studied by inorganic geoch emical methods. More than 650 samples (minimum sample spacing 0.5 m) were a nalysed for major and minor element concentrations. This chemical database was used to reconstruct the depositional environment and to investigate the influence of high frequency Milankovitch cycles on the chemical composition of the sediments. The chemical composition of the sedimentary sequence is primarily controlled by variations in carbonate con tent, corresponding to Ca/Al ratios. Variations in both Ti/Al and Si/Al rat ios are interpreted as fluctuations in detrital composition resulting from increases in smectite and quartz content in the upper section of the core. High amounts of reactive iron, but low pyrite concentrations, the lack of e nrichment of redox-sensitive trace elements (V, Cr), and high Mn concentrat ions indicate oxic bottom water conditions and the absence of an oxygen min imum zone for the Upper Albian at the Kirchrode site. The nutrient-related trace elements (Ba, P, Zn) suggest a generally oligotrophic setting with sl ightly elevated productivity only in the uppermost Upper Albian. Only one l ayer was identified as containing volcanogenic material (at 87.20 m). This layer is characterised by elevated montmorillonite, Zr and Nb. Mass balance calculations for the entire core suggest that the high Mn values cannot be explained by riverine input alone. Therefore, a coastal oxygen minimum zon e south of the Kirchrode site is postulated as an additional Mn source. Cyclic variations in both carbonate content and Al-normalised concentration s of predominantly detrital elements, like Si and Ti, are seen in a 60 m. i nterval (40-100 m), where sedimentological, palaeontological and geochemica l data suggest stable depositional conditions. Although carbonate cyclicity , cyclicity in detrital input and nutrient-related element abundances do no t correlate with each other, three strong peaks in the results of spectral analysis of these chemical data for this interval appear to correspond to M ilankovitch frequencies in the eccentricity, obliquity and precession bands . (C) 2001 Elsevier Science B.V. All rights reserved.