The onset of anoxic conditions in the early Barremian of the Boreal Realm evidenced by benthic foraminifera

Abstract

The Frielingen clay pit, situated approximately 20km northwest of Hannover (northern Germany) exposes sediments of Early Cretaceous age (Hauterivian, Barremian). Rhythmic alternations of dark mudstones and pale marls of late Hauterivian age (Simbirskites discofalcatus ammonite Zone) dominate the lithology. These bedding rhythms are overlain by laminated dark shales and lighter mudstones, assigned to the lowermost Barremian (Praeoxyteuthis pugio belemnite Zone). For this study 27 samples covering the Hauterivian/Barremian boundary interval were investigated with respect to their CaCO3 and total organic contents (TOC), stable isotope composition (δ13Corg, δ13CCaCO3, δ18OCaCO3) and their micropalaeontology (foraminifera). The samples were analysed in order to better understand the onset of the Barremian anoxic conditions in the Boreal Realm. The data obtained show that shifts of the environmental parameters are reflected by the mudstone-marl alternation. The dark mudstones reflect cooler surface water temperatures and a lower oxygen content, while the pale marls were deposited under warm surface water conditions. These bedding rhythms are thought to be the result of climatic variations in the Milankovitch Band (10Ka–1Ma). This study further documents that the Hauterivian/Barremian transition is characterised by a deterioration of the living conditions for benthic organisms. This is indicated by the decline of several foraminiferal genera, by decreasing generic richness and by a coeval increase in the abundances of specific taxa. The TOC content and the isotopic composition of the organic carbon support these findings. A morphogroup analysis of the benthic foraminifera indicates changes of the habitats from mainly infaunal in the late Hauterivian to more epifaunal in the early Barremian. Due to this dramatic decline of infaunal forms in the laminated, TOC-rich mudstones of the lowermost Barremian, suboxic to anoxic conditions are postulated for these sediments. The bottom water must, however, have contained some oxygen at least, as is indicated by a few epifaunal genera that became very abundant (e.g. Ammodiscus). A major regression around the Hauterivian/Barremian boundary caused a closing of the seaway to the Tethys, resulting in a significantly reduced water exchange and the establishment of anoxic conditions.