Oxygen isotope composition of Upper Cretaceous chalk at Lägerdorf (NW Germany): its original environmental signal and palaeotemperature interpretation

Abstract

The oxygen isotopic composition of bulk chalk samples, the planktonic foraminifers Globotruncana and Rugoglobigerina, and the benthic bivalve Inoceramus are reported from the standard section of the Upper Cretaceous white chalk of Lägerdorf-Kronsmoor, NW Germany (Middle Coniacian to Lower Maastrichtian). The section shows the increasing impact of burial diagenesis with depth expressed by a significant negative trend in the oxygen isotopic values and increasing amounts of secondary, precipitated microspar. However, the biogenic components studied show different diagenetic characteristics. The planktonic foraminifers are completely recrystallized, whereas the prisms of Inoceramus are well preserved and only slightly overgrown by negligible amounts of secondary calcite. The original oxygen isotopic composition of the chalk is estimated on the basis of numerical elimination of the diagenetic trend and by use of a normalized carbonate content. It appears that the corrected isotopic signal of the bulk sediment, which primarily consisted of calcareous nannoplankton, significantly correlates with the isotopic composition of the Inoceramus prisms. Thus, the benthic bivalve Inoceramus and the nannoplankton probably lived in a water mass of the same isotopic composition. The Inoceramus oxygen isotope values suggest a mean temperature of approximately 16°C for the NW European Basin during the late Campanian and early Maastrichtian.