Palaeoceanographic and climatic changes during the Albian, summary of the results from the Kirchrode boreholes
Abstract Analysis of the Albian sediments from the Kirchrode site by different disciplines yielded information about depth, productivity and oxygenation of the water in the deeper parts of the Lower Saxony basin, one of the mid-latitude shelf sea basins in the northern hemisphere. In addition information was obtained about phases during which inflow of water from the neighboring oceans (Tethys, North Atlantic) and circulation within the basin was intensified, about sea level changes, as well as about climate changes occurring during the Albian in this mid-latitude region. This information was derived from observations of the effects the changes in the palaeoenvironment had on the fossil and mineral assemblages, the sediment facies, and the geochemical composition of the sediment. The increase in terrigenous input during the Upper Albian, the relative increase in detrital montmorillonite and silt-sized minerals, e.g. quartz and zircon, reflect tectonic movements (e.g. uplift of the Rhenish–Bohemian massif and subsidence of the Lower Saxony basin, locally intensified by halokinetic movements) and long-term regional changes in climate. A change to a more humid climate occurred approximately at the stratigraphic first occurrence of the calcareous nannofossil genus Eiffellithus in this region and at the start of a new transgressional period, and it led to increased runoff from the land. Possibly, a second change, this time to a more arid climate, occurred in the latest Albian. During times of high sedimentation rates of terrigenous components the marine productivity in the shelf sea is also increased as a consequence of the increased input of dissolved and particulate matter. During the latest Albian, upwelling may have been the cause of a further increase in productivity. A global sea level change overlying the effects of these regional and local processes was recognised e.g. in changes in sedimentation rates in the monotonous deep-shelf sediments (decreasing during transgression and highstand times, increasing during regression and lowstand times). Global sea level changes are best documented in changes in the abundances of the main plankton and benthos groups, and changes in the relative abundance of benthos groups with different feeding strategies: suspension-feeders and detritus-feeders. Transgressional periods start with an abundance maximum in species that immigrated from the Tethys. Highstand conditions at lower nutrient levels are documented in the sediment record by a maximum in planktonic foraminifera and calcareous nannoplankton. When nutrient levels under highstand conditions were higher, like at the end of the Albian, calcareous nannofossils and siliceous plankton dominate. Milankovitch cyclicity was identified using spectral analysis of the regularly and closely spaced data sets of sedimentological, palaeontological, and geochemical parameters. The higher abundance of terrigenous plant material in the dark parts of the sedimentary cyles in the Upper Albian suggest a change in humidity was the main climatic change and changes in benthos foraminifera assemblages indicate cyclic productivity changes in at least part of the sequence. Among planktonic foraminifera, species that immigrated from the Tethys have a stronger precessional signal in their abundance fluctuations, while the ‘Boreal’ species show a stronger obliquity signal. For the deep part of the shelf basin, the strong precessional signal in the cyclicity of benthic foraminifera abundances suggests that conditions were influenced for most of the Upper Albian by processes triggered in the low latitudes. Based on the identified Milankovitch cyclicity, sedimentation rates were found to increase through the Upper Albian from 1.5–6.5 to 7–13 cm/ka. The duration of the Callihoplites auritus Ammonite Subzone was determined to be 2.1 Ma and the minimum duration of the Late Albian 4 Ma (possibly 4.5–4.8 Ma).