First high-resolution δ13C-records across black shales of the Early Aptian Oceanic Anoxic Event 1a within the mid-latitudes of northwest Europe (Germany, Lower Saxony Basin)a
Heldt, M.; Mutterlose, J.; Berner, U.; Erbacher, J.
Newsletters on Stratigraphy Volume 45 Number 2 (2012), p. 151 - 169
published: Apr 1, 2012
The late Early Aptian Fischschiefer, a laminated black shale horizon deposited under anoxic conditions in a shallow epicontinental sea in northwest Europe (Lower Saxony Basin), has been investigated in detail over the last decades. Biostratigraphic studies indicate, that the Fischschiefer horizon corresponds to the Oceanic Anoxic Event 1a (OAE 1a). So far high resolution carbon isotope records across the Fischschiefer horizon are still missing. These may help to improve the stratigraphy of the OAE 1a in northwest Europe. In order to fill this gap, we have analyzed two sediment cores of Aptian age (Hoheneggelsen KB 9 and 40) from the Brunswick area, north Germany, for their lithostratigraphy, geochemistry (CaCO3, TOC), biostratigraphy (coccoliths, nannoliths) and high resolution chemostratigraphy (δ13Ccarb and δ13Corg). The data allow to establish a detailed stratigraphy for the cores, suggesting an Early to early Late Aptian age. The isotope curves are divided into characteristic segments (C2-C7), which were originally described from marine sediments of the Tethys Ocean. Some of these segments are difficult to recognise in the investigated cores, which might be due to diagenesis. Strong δ13Ccarb depletion in some intervals of the core Hoheneggelsen KB 9 indicate an influence of early diagenesis at times (sulfate reduction and methanogenesis, accompanied by authigenic carbonate formation). The combination of bio- and chemostratigraphy, however, allows to correlate the Fisch schiefer horizon with the OAE 1a. The high-resolution stratigraphy sheds new light on previous publications of the Fischschiefer horizon. Independent evidence suggests, that anoxic and sulfidic conditions prevailed during the early phase of the OAE 1a in sediment and bottom waters of the Lower Saxony Basin. This was accompanied by high sulfate reduction and authigenic carbonate formation rates. The environmental conditions were probably triggered by a prominent global warming trend near the onset of the OAE 1a and associated processes (e.g.establishment of a freshwater cap, increased primary productivity/organic matter supply to the sediments and stable water stratification). Sulfate reduction and diagenetic carbonate formation rates then gradually decreased over the course of the OAE 1a.