Assessing pelagic palaeoenvironments using foraminiferal assemblages — A case study from the late Campanian Radotruncana calcarata Zone (Upper Cretaceous, Austrian Alps)
Abstract Two upper Campanian sections in the Austrian Alps representing the north western Tethyan biogeographic realm from either sides of the Penninic Ocean (Alpine Tethys) have been examined aiming at a high-resolution assessment of foraminiferal assemblages: the Postalm section from the Northern Calcareous Alps (southern active margin) and the Oberhehenfeld section from the Ultrahelvetics (northern passive margin). This study focuses on plankton biostratigraphy and foraminiferal palaeoecology of the Radotruncana calcarata Total Range Zone. The Postalm section displays cyclic red deposits with marls and marly limestones, while we find uniform grey marls at Oberhehenfeld. The Oberhehenfeld section from the Ultrahelvetics can be correlated stratigraphically to the Postalm section using foraminifera, calcareous nannoplankton and stable isotope stratigraphy, and provides a point of comparison from the northern margin of the Penninic Ocean. The two sections show minimal difference in faunal composition and few distinct local stratigraphic signals. Palaeoenvironmental trends from the late Campanian can be recognised relating the two sections from the Austrian Alps. The depositional water depth can be reconstructed as some 500–800 m. Plankton assemblages show a remarkable stability despite the sudden appearance and disappearance of R. calcarata, hinting at the late Campanian as a time interval of general foraminiferal stasis without significant evolutionary events. We speculate that the origin and extinction of R. calcarata are related to the prolonged evolution of ocean stratification during the Campanian from the mid-Cretaceous sluggish hothouse during a time of general slow greenhouse climate decline.