Title: Abnormal Fern Spores Reveal Extreme Environmental Stress during the End-Triassic Mass Extinction Event
An international team of scientists from the Netherlands, China, Denmark, Britain, and the Czech Republic has discovered new insights into the causes of the end-Triassic mass extinction event, one of the most catastrophic events in Earth’s history that resulted in the demise of up to 80% of all species. This study reveals unprecedented evidence of extreme environmental stress, as indicated by a profusion of abnormal fern spores in sediment samples from Northern Germany, Denmark, and Sweden.
The research, published in the journal Science, was led by Remco Bos of Utrecht University, who explains: “Seeing the sheer amount and different types of malformed fern spores in sediment samples from a coastal lagoon, dating back 201 million years ago, is truly astonishing. It means there must have been very many ferns being stressed. It is also not something we see regularly during other periods that also contain many fern fossils, making it a true signal connected to the end-Triassic mass-extinction event.”
The study of pollen and spore abundances revealed various malformations, such as abnormalities in wall structure and evidence for botched meiotic divisions, leading to unseparated, dwarfed, and fused fern spores. This phenomenon suggests mass-scale ecological disruption of ferns due to dramatic environmental changes likely caused by heat stress, enhanced monsoonal rainfall, and increased forest fire activity.
Interestingly, the ferns that produced these abnormal spores survived the mass-extinction event, demonstrating their high resilience, possibly due to their different mercury tolerance compared to other plants. This finding supports earlier work by the research team, who discovered a similar pattern of fern spread across coastal lowlands in Northwestern Europe in response to widespread deforestation.
The research also highlights four additional episodes of high mercury (Hg) concentrations and high numbers of malformed spores in the 1.3 to 2 million years following the extinction interval. These periods correspond closely to the long eccentricity cycle, a major variation in the shape of Earth’s orbit that moves Earth closer or further away from the Sun every 405 thousand years. During these cycles, as the Earth’s atmosphere was already supercharged with carbon dioxide from the volcanism, the modulation of the climate system repeatedly triggered forest dieback, allowing for the renewed spread of pioneer ferns.
A crucial data set generated at Tianjin University (China) reveals isotopic variations that allow for linking the initial pulse in Hg enrichment at the Triassic-Jurassic boundary to the emission of mercury from flood basalt volcanism. However, the four other pulses in mercury had a different isotopic composition, indicating they were mainly driven by Hg input from soil erosion and photochemical reduction.
The findings offer a complex and drawn-out sequence of events, starting with massive volcanism driving climate change and releasing toxic pollutants, followed by episodic pulses of disturbance that lasted for at least 1.3 million years. Furthermore, the study concludes that coastal ecosystems likely suffered the most by receiving large amounts of mobilized mercury from vast catchment areas, leading to continued ecosystem perturbation.
The research provides a significant step towards understanding the complex and long-lasting events that take place during mass-extinction events, with crucial implications for understanding the ongoing effects of human-induced climate change and pollution. As the study shows, greenhouse warming and pollution led to continued ecosystem perturbation, and the Earth took time to clean up the mess and restore stable forested biomes.
