Organic geochemistry and depositional history of Upper Albian sediments from the Kirchrode I borehole, northern Germany

In the framework of a multidisciplinary research program, an organic geoche mical study was carried out on a drill core which comprises a 245 m thick s equence of light-colored, Upper Albian marlstones that were deposited in th e central part of the Lower Saxony basin (northern Germany). For part of the Upper Albian sequence, high-resolution measurements of carb onate contents reveal cycles which can be related to earth orbital forcing. Based on these data, sediment accumulation rates were calculated to be in the order of 15 g/m(2) yr(1). These high accumulation rates contrast with v ery low organic carbon contents and an extremely poor preservation of the a utochthonous organic matter. Most of the sedimentary organic matter is of t errigenous origin and mainly derived from the erosion of older sedimentary rocks. Organic petrography reveals only a very small fraction of marine org anic particles. Carbon/sulphur ratios, pristane/phytane ratios as well as t he predominance of resedimented organic particles over autochthonous organi c particles suggest that aerobic degradation processes rather than anaerobi c processes (sulphate reduction) were responsible for the degradation of th e organic matter. Furthermore, the scarcity of terrigenous organic particle s (vitrinite) indicates that there was little vegetation on nearby land are as. To explain these analytical results, a depositional model was developed whi ch could explain the scarcity of organic matter in the Upper Albian sedimen ts. This model is based on downwelling of oxygen-rich, saline waters of Tet hyan origin, which reduces the nutrient content of surface waters and thus primary bioproductivity while degradation of primary organic matter in the water column is enhanced at the same time. These conditions contrast to tho se which existed in Barremian and early Aptian times in this basin, when li mited water exchange with adjacent oceans caused oxygen deficiency and the deposition of numerous organic carbon-rich black shales. The thick, organic matter-poor Upper Albian sequence of northern Germany also contrasts with comparatively thin, time-equivalent, deep-sea black shales from Italy. This discrepancy indicates that local and regional oceanographic factors (at le ast in this case) have a greater influence on organic matter deposition tha n global events. (C) 2001 Elsevier Science B.V. All rights reserved.