Upper Hettangian vegetational disturbance: recovery after a mass-extinction or long-term phytotoxicity?

Abstract

The Early Jurassic Hettangian stage is known as the recovery phase after the end-Triassic massextinction (ETE), which was one of the largest mass extinctions of the Phanerozoic era. This event occurred synchronously with volcanic emplacements from the Central Atlantic Magmatic Province, which has been implicated as the main driver of the ETE. Ecosystem stability uctuated in the Upper Hettangian, with indications in vegetational disturbance similar to the ETE. This study aims to understand the severity of Upper Hettangian terrestrial ecosystem disturbance around the North German Basin, and to ?nd indications for a volcanic source as a potential cause for this disturbance. A palynological study using the classi?cation from Lindstr om et al., 2019, revealed evidence for a heavy disturbed and toxic terrestrial ecosystem, as high abundances of mutagenic spores are found in two intervals within the Upper Hettangian, synchronous with recurrent fern spikes. In addition, high molecular weight polycyclic aromatic hydrocarbons (PAHs) were measured as a potential source of mutagenesis due to their phytotoxic properties, often associated with (volcanic) biomass burning. However, relatively low PAH concentrations are found in the intervals with increased mutagenesis, indicating a discrepancy between plant mutagenesis and PAH pollution. Although late CAMP pulses could be responsible for the Upper Hettangian aberrations, no direct link to a volcanic source can be established. An alternative explanation for the observed Upper Hettangian disturbances is astronomically driven climate forcing on the vulnerable, recovering European ecosystems. Potentially increased aridity caused soil destabilisation and erosion, while long term ozone depletion was harmful for life during periods with increased insolation. An astronomical tuning to the 405 kyr eccentricity cycle suggests a regular pacing of disturbance which ends at the base of the Sinemurian.