Geochemical and palaeontological studies of the δ 13C anomaly in boreal and North Tethyan cenomanian-turonian sediments in Germany and adjacent areas

Abstract

In two sections of the Upper Cenomanian and Lower Turonian of Northwest Germany (boreal realm, North Sea Basin) and southeast Germany (North Tethys, Regensburg area) stable isotopes ( 13C, 18O), acid-soluble manganese, carbonate contents and foraminifera were investigated. In addition, other sections across Norttwest- and Middle Europe are reviewed with respect to the detailed stratigraphy of a positive δ 13C anomaly which was found earlier to be of global importance in the archaeocretacea foraminiferal zone (Cenomanian-Turonian transition). The δ 13C excursion was found to be isochronous, with the lower limit marked by the extinction of the foraminiferal genus Rotalipora. The upper limit is found immediately above the entry of the foraminiferal Praeglobotruncana helvetica Bolli. In the macrofossil zonation, the base is marked by the occurrence of the belemnite Actinocamax plenus (Blainville) in the upper part the Metoicoceras geslinianum ammonite zone. The upper limit falls in the Watinoceras coloradoense ammonite zone just above the entry of the inoceramid genus Mytiloides. In some cases the “δ 13C excursion” refers to a maximum of a more or less well expressed oscillation of the δ 13C values with minima in the lowermost Cenomanian and the Upper Turonian. In other cases δ 13C is constant with a marked “spike” at the position of the anomaly. The locally different structures of the δ 13C trends point to incomplete mixing of water masses between basins in Northwest Europe. A manganese anomaly associated with the δ 13C excursion in the Anglo-Paris Basin was found at a higher position in northwest Germany and is apparently absent from the German North Tethys. In both cases high Mn-contents appear to be associated with a markedly increased terrigenous input expressed by an increase in grainsize of the acid-insoluble residue. The stratigraphical value of the Mn anomaly is therefore limited. The palaeoecological conditions are discussed in terms of foraminiferal diversity and the occurrences of macrofossils. A shift to higher δ 18O values coincident with the δ 13C excursion is possibly a response to cooling of ocean water for which evidence is provided by a low diversity globular planktonic assemblage. The correlation of δ 18O and foraminiferal diversity is suggested to provide a tool for bathymetric interpretations of sections, but needs further investigations.