Middle and Late Albian geography, oceanography, and climate and the setting of the Kirchrode I and II borehole sites

The paper reviews the literature on the geography, oceanography, and climat e belts during the Middle and Late Albian. Emphasis is placed on the northe rn hemisphere and the Lower Saxony basin. The Middle and Late Albian repres ents a time with a greenhouse climate. The N-S temperature gradient and the temperature gradient between the shallow and deep waters of the oceans wer e lower than at present, meaning higher temperatures especially at middle a nd high latitudes, a more sluggish ocean circulation than today, and the ab sence of polar ice caps. Warming from the Early to Late Albian is documente d in the marine sediments by oxygen isotope studies and the poleward expans ion of rudist-dominated carbonate platforms. In the terrigenous sediments o f the northern hemisphere it is documented by northward migration of low-la titude, relatively and 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 S outh Atlantic and the western Tethys developed during the Late Albian. Wide shelf seas existed on the continental plates. With rising sea level the ba sins of the western epicontinental seas of Eurasia became increasingly conn ected and sedimentation gradually became more calcareous. Nevertheless, the se basins received a large elastic input from the land, documenting the wea thering conditions on the surrounding land masses. They received also a con siderable biogenic particle flux from the surface waters, documenting produ ctivity of the marine plankton. Because of these different origins of the s ediment components, as well as the connections with neighbouring seas, the sediments in these shelf basins were expected to document changes in climat e as well as palaeo-oceanographic changes. Earlier interpretations based on ammonite studies that indicate changing connections between the Arctic Oce an 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, hu mid climate belt at mid-latitudes in the northern hemisphere in which thick deposits of hemipelagic sediments accumulated. Mapping previous to our stu dy 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 t he Lower Saxony basin. The boreholes at the Kirchrode I and II sites (52 de grees 22.35'N, 9 degrees 49.87'E, and 52 degrees 22.16'N, 9 degrees 49.12'E , respectively, in Hannover) were drilled into this sub-basin. The high sed imentation rates known for the Albian of the Lower Saxony basin are ideal f or studying climatic changes with a cyclicity of a few ten thousand to a fe w hundred thousand years (Milankovitch cyclicity). (C) 2001 Elsevier Scienc e B.V. All rights reserved.