Integrated stratigraphy of Lower Cretaceous sediments (Ryazanian–Hauterivian) from North-East Greenland
Abstract The reconstruction of past climates and oceanography requires a solid stratigraphic framework ideally applicable on a global scale. The earliest Cretaceous, however, was a time of strong faunal provincialism, making supra-regional correlation of biostratigraphical zonations difficult. The step-by-step correlations between neighbouring provinces/subprovinces that are commonly utilized bear the risk of loosing accuracy in every step. Here we present 87Sr/86Sr- and stable isotope-data (δ13C, δ18O) from belemnite rostra of the Rodryggen section in North-East Greenland. The integrated stratigraphy based on Sr-isotope ratios, ammonite and calcareous nannofossil biostratigraphy offers the opportunity for a direct comparison of the different stratigraphic zonations. These are complemented by δ13Cbel-data recording the positive carbon isotope excursion of the Valanginian Weissert Event, which is a reliable stratigraphic event. The geochemical data furthermore allow a reliable correlation of Tethyan and Boreal strata. The stratigraphic range of the Rodryggen section resulting from Sr-isotope stratigraphy (Ryazanian–Barremian) is in agreement with the biostratigraphic findings. Mismatches regarding stage/substage boundaries demand a reconsideration of the nannofossil biostratigraphy of the Boreal Lower Cretaceous. Our findings suggest stratigraphic ranges for two nannofossil index species (Sollasites arcuatus, Micrantholithus speetonensis) different from published ranges. The new observations imply changes in the Boreal Ryazanian–Valanginian nannofossil zonation scheme. Specifically the base of calcareous nannofossil zone BC3, originally defined as uppermost Ryazanian, is shifted to the lower Valanginian. Based on these new stratigraphic interpretations a decrease in the abundance of nannoconids observed in the Rodryggen section can now be identified as the Valanginian nannoconid crises. This nannoconid decline has been observed in Tethyan sections along with the Weissert Event. A positive trend in the δ18Obel-data agrees with a late Valanginian cooling that has been postulated based on independent proxies from the Boreal Realm and the Tethys.