Mid-Cretaceous paleoenvironmental changes in the western Tethys

Abstract

We present a continuous record of surface water temperature and fertility variations through the latest Barremian–Cenomanian interval (ca. 27 Myr) based on calcareous nannofossil abundances from the western Tethys. The nannofossil temperature index, calibrated with TEX86 sea surface temperatures, suggests that warmest (34–36 °C) conditions were reached during oceanic anoxic event (OAE) 1a onset, the Aptian–Albian boundary interval hyperthermals (113, Kilian level and Urbino level OAE 1b) and during a ca. 4 Myr long phase in the middle Albian. Coolest temperatures (29 °C) correspond instead to the late Aptian. Generally warm conditions characterized the Albian followed by a progressive cooling trend that started in the latest Albian (at the Marne a Fucoidi–Scaglia Bianca Formation transition). Temperate conditions occurred in the Cenomanian with frequent short-term variations highlighted by abundance peaks of the cold-water nannofossil species E. floralis and R. parvidentatum. Mid-Cretaceous surface water fertility was rather fluctuating and mostly independent from climatic conditions as well as from black shales intervals. Intense warming and fertility spikes were systematically associated only with black shales of OAE 1a and of the Aptian–Albian boundary hyperthermals. The Albian–Cenomanian rhythmic black shales are, in fact, associated with varying long-term climatic and fertility conditions. The similarity of western Tethys climatic and fertility fluctuations during OAE 1a, OAE 1b, the middle Albian and OAE 1d with nannofossil-based records from other basins indicated that these paleoenvironmental conditions were affecting the oceans at supra-regional to global scale.