Stable isotope (O180, O13C) records across the Cretaceous/Tertiary boundary at Geulhemmerberg, southern Netherlands

Abstract

The stable isotopic (δ18O, δ13C) records for bulk samples and well-preserved, monospecific, benthic foraminiferal samples show no or only small variations across the Cretaceous/Tertiary (K/T) boundary at Geulhemmerberg, southern Netherlands. The site represents an inner shelf environment, where calcarenites and clay layers formed. Comparisons with previously established isotopic records from Danish K/T boundary sections, where significant isotopic changes (2-3 °C cooling and 1.5‰ negative δ13C shift) occur in the basalmost P0 Zone, indicate a small hiatus at the base of the Danian at Geulhemmerberg. This is consistent with other data, such as absence of a strong Ir anomaly and shocked quartz. The oxygen-isotopic values of planktonic, mid-depth-dwelling Heterohelix globulosa foraminifera, recovered from two early Danian Geulhemmerberg clay layers, are similar to, or more positive than the benthic values from the same layers. The origin of these apparently anomalous water-column ð18O gradients is enigmatic. The inverted gradient may reflect sporadic development of an unusual water-mass stratification, such as the occurrence of an upper water mass with a slightly lower (1-2‰) salinity and with a few degrees lower temperature than bottom water. Alternatively, it may reflect different provenance areas of the planktonic and benthic foraminifera during turbulent conditions, while storm and back-wash deposits formed. Overall, the whole-rock and benthic oxygen-isotopic records across the Geulhemmerberg section indicate fully marine (> 33‰) conditions throughout.