Do spores herald the Toarcian Oceanic Anoxic Event?

Abstract

The environmental perturbations of the Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) were associated with the widespread deposition of black shales, such as the Posidonia Shale Formation in southwest Germany. We recently published a detailed palynological analysis of the deposits of the Posidonia Shale Formation at Dormettingen, investigating pollen, spores, and marine plankton. In addition to the comprehensive palynological analysis, we point out the occurrence of possible teratological features (i.e. unseparated spore tetrads and dark-coloured sporomorphs) before the T-OAE. These features have been tentatively linked to environmental changes, such as increased UV-B irradiation or heavy metal pollution associated with the large-scale volcanism of the Karoo Ferrar province (Galasso et al., 2022). Our interpretation has drawn criticism (Riding et al., 2022). Here we present a more substantiated discussion of why spore tetrads and darkened spores might not be sedimentological artefacts but indicators of aggravating environmental conditions prior to the T-OAE. The combined occurrence of several teratomorphic features (i.e. spore tetrads, dark sporomorph walls, and even polyploidy) during the earliest Toarcian (Tenuicostatum Zone) is interpreted as a signal of environmental stress in land plants, although for each individual morphological variation a much easier explanation exists.