One thing to ruin them all
Authors:   , Bos Remco 
Published: January 2025

This thesis presents new data on ecological variability and disturbance in the terrestrial realm across the end-Triassic mass-extinction (ETME) within the well-studied region of NW Europe. A key innovation of this thesis lies in the high-resolution palynofloral reconstructions with an emphasis on malformed pollen and spores that provides an extra dimension of mutagenic disturbance. This approach is predicated on the assumption that volcanic pollution, in particular Hg-toxicity, contributed to plant mutagenesis and large-scale deforestation.

From a palynofloral perspective, this thesis confirms the two-phased structure of the ETME which consistently varies in concert with negative excursions in δ13Corg records. Evidence for a stepwise decline in vegetation, particular upper canopy conifers, marks the onset of the ETME in NW Europe and corresponds to the Marshi CIE. The second extinction pulse marks the transition to the Early Jurassic with several Triassic taxa noting their last appearance and corresponds clearly with the Spelae CIE. Increased abundances of charcoal and reworked palynomorphs provides evidence of increased wildfires and elevated soil erosion during the interlude phase between extinction events. This has been interpreted to reflect major changes in the hydrological cycle that were likely forced increased atmospheric CO2.

Intervals with increased palynofloral malformations (mutagenesis) seem to follow a similar two-phased progression that coincides with negative carbon isotope excursions suggesting that vegetation was strongly influenced by pulses in CAMP volcanism. It can be clearly demonstrated that the malformations in fern spores are closely tied with sedimentary mercury (Hg) enrichments suggesting that heavy metal pollution from volcanic activity could have caused mass-poisoning of the pioneering fern vegetation after forests experienced a decline and extinction. Evidence supports that Hg-enrichment was mainly volcanically-sourced through a positive shift in stable Hg-isotopes (199Hg).

The consequences of volcanic Hg-enrichment at the Triassic-Jurassic boundary seem to have persisted in the Early Jurassic interval (Hettangian) with several repeated sedimentary Hg-anomalies. Palynological reconstructions indicate that terrestrial vegetation did not immediately recover from major environmental upheaval across the ETME. Indications of repeated vegetation collapse and reworking during the Hettangian suggests that Hg-enrichments are derived from terrestrial (i.e. soil and bedrock) reservoirs. The idea is supported by Hg-stable isotope records showing repeated positive shifts in MIF (199Hg) and MDF (202Hg) values indicating that remobilization of terrestrial Hg was periodically enhanced during the Hettangian in open coastal/wetland areas, due to the loss of canopy cover.

By examining modern sites of anthropogenic Hg-pollution and assessing the local ferns for Hg-contamination, a causal link can be established between Hg and mutagenesis in plants. Hg-concentrations in root systems of sampled Dryopteris-ferns reflect topsoil contamination, inferring that fern root systems mainly absorb Hg through soil-interaction. In contrast, Hg-concentration in fern foliage is not directly related to topsoil contamination but instead is mainly driven by atmospheric (gaseous) uptake through stomata. Moreover, high bio-accumulation of Hg in the spore-producing sori relative to the other foliage components potentially promotes the mutagenic effects in spores.