Middle and Late Albian geography, oceanography, and climate and the setting of the Kirchrode I and II borehole sites
Abstract The paper reviews the literature on the geography, oceanography, and climate belts during the Middle and Late Albian. Emphasis is placed on the northern hemisphere and the Lower Saxony basin. The Middle and Late Albian represents a time with a greenhouse climate. The N–S temperature gradient and the temperature gradient between the shallow and deep waters of the oceans were lower than at present, meaning higher temperatures especially at middle and high latitudes, a more sluggish ocean circulation than today, and the absence of polar ice caps. Warming from the Early to Late Albian is documented in the marine sediments by oxygen isotope studies and the poleward expansion of rudist-dominated carbonate platforms. In the terrigenous sediments of the northern hemisphere it is documented by northward migration of low-latitude, relatively arid flora. The opening North Atlantic was still narrow at the beginning of the Albian. It widened and deepened through the Albian. A shallow seaway between the South Atlantic and the western Tethys developed during the Late Albian. Wide shelf seas existed on the continental plates. With rising sea level the basins of the western epicontinental seas of Eurasia became increasingly connected and sedimentation gradually became more calcareous. Nevertheless, these basins received a large clastic input from the land, documenting the weathering conditions on the surrounding land masses. They received also a considerable biogenic particle flux from the surface waters, documenting productivity of the marine plankton. Because of these different origins of the sediment components, as well as the connections with neighbouring seas, the sediments in these shelf basins were expected to document changes in climate as well as palaeo-oceanographic changes. Earlier interpretations based on ammonite studies that indicate changing connections between the Arctic Ocean and the Tethys during the Albian can thus be tested. The Lower Saxony basin was a shelf sea in the southern part of the warm, humid climate belt at mid-latitudes in the northern hemisphere in which thick deposits of hemipelagic sediments accumulated. Mapping previous to our study indicated that a complete, undisturbed Albian basinal sequence ca. 400 m thick is present below a thin Quaternary cover in a sub-basin between the NNE–SSW-trending Benthe and Lehrte salt structures in the central part of the Lower Saxony basin. The boreholes at the Kirchrode I and II sites (52°22.35′N, 9°49.87′E, and 52°22.16′N, 9°49.12′E, respectively, in Hannover) were drilled into this sub-basin. The high sedimentation rates known for the Albian of the Lower Saxony basin are ideal for studying climatic changes with a cyclicity of a few ten thousand to a few hundred thousand years (Milankovitch cyclicity).