Radiolarian response to environmental changes at the Sinemurian–Pliensbachian transition in the Northern Calcareous Alps, Austria

Abstract

Major environmental, climate and sealevel changes occurred in the Western Tethyan Realm during the late Sinemurian to early Pliensbachian time interval. Here, we examine how these changes affected the taxonomic composition of radiolarian fauna. Radiolarian assemblages were collected on Mount Rettenstein (Northern Calcareous Alps) from a siliceous limestone and marl succession, deposited in a well‐oxygenated basin a few hundred metres in depth on the continental shelf at the western edge of the Neotethys Ocean. Radiolarian research was complemented with elemental and isotope geochemistry on bulk carbonate samples. The siliceous microfaunas below and above the stage boundary consist of more than 80% sponge spicules and less than 20% radiolarians, with a strong predominance of the Order Spumellaria. The Nassellaria to Spumellaria abundance ratio ranges from 1:5 to 1:3. At the Sinemurian–Pliensbachian transition, a significant drop in diversity occurred, accompanied by a substantial change in relative abundances of radiolarian taxa. The most severely affected groups were surface‐dwelling radiolarians (Angulobrachiidae, Hagiastridae, Pantanelliidae; mostly Gorgansium, Poulpidae and Ultranaporidae), which almost or completely disappeared. In contrast, Archaeocenosphaera, Praeconocaryomma, Zhamoidellum and Lantus became abundant and were apparently the most resistant to environmental stress. The changes in radiolarian assemblages were local and probably induced by the end‐Sinemurian sealevel drop that transformed the area into a semi‐enclosed basin with restricted ocean circulation. The exchange of water masses and thus radiolarian faunas with the open sea was reduced and their productivity may have been lowered by the lower inflow of fertile waters from the ocean.