Rise of calcispheres during the Carnian Pluvial Episode (Late Triassic)

Abstract

It has been argued that the beginning of significant pelagic calcification could have been linked to the Carnian Pluvial Episode (CPE), a climate change in the Late Triassic (~234–232 Ma) that was marked by C-cycle disruption and global warming. Nevertheless, abundant calcareous nannofossils have been described so far only in post-CPE rocks, and therefore no conclusive hypotheses can be drawn on possible causal links with it. Here we show that in deep-water successions of the Western Tethys, Orthopithonella calcispheres interpreted as calcareous dinocysts became an important component of carbonate sedimentation from the onset of the CPE, and could constitute up to 8% of hemipelagic limestones. Before the CPE, in similar depositional environments, calcispheres are rare or absent, and never constitute a significant part of the sediment. This change in the deep-water carbonate sedimentation, is mirrored in the shallow water environments by the rise of the reefs built by Scleractinia corals. These important innovations in Earth's carbonate systems may indicate a deep modification in the ocean biogeochemistry during the CPE. • Calcispheres were studied in successions across the Carnian Pluvial Episode (CPE). • In Western Tethys Calcispheres are abundant from the onset of the CPE. • They belong to the genus Orthopithonella. • They were a significant component of CPE hemipelagic–pelagic limestones.