Onset of significant pelagic carbonate accumulation after the Carnian Pluvial Event (CPE) in the western Tethys

Abundant calcispheres occur in Upper Carnian and Norian hemipelagic limestone successions of the Southern Apennines and Sicily. They exhibit a variety of morphologies that were investigated with optical and scanning electron microscopy (SEM). The most common morphology is that of a full solid sphere of radiaxial calcite crystals, 20–22 μm in diameter on average, with or without a minor hollow in the center. Smaller forms may be clusters of sub-micron crystals, only rarely disposed as to form a spherical test with a diameter of 10 μm or less. Larger forms are similar to small forms (clusters or spheres of sub-micron crystals) with an epitaxial calcite overgrowth. The taxonomic attribution of these calcispheres is uncertain, mostly because of their poor preservation, but a comparison is possible with some Mesozoic calcispheres attributed to calcareous dinocysts. The amount of epitaxial overgrowth is variable, but in most cases much larger than the original sphere. This prevents a significant evaluation of the contribution of calcispheres to carbonate pelagic sedimentation by point counting in thin-section. However, it can be shown that calcispheres become abundant only after a major climatic perturbation dated at the end of the Early Carnian, known as the Carnian Pluvial Event (CPE). This event involved a strong and prolonged enhancement of the hydrological cycle, with consequent supply of excess hydrogen carbonate to the oceans and increased seawater alkalinity. Although calcispheres of this type are known at least from the Middle Triassic, it is only shortly after the CPE that they become abundant, and the first common occurrence of calcareous nannoplankton in the western Tethys is thus Late Carnian in age.