The cause of terrestrial biodiversity turnover, during the Triassic_Jurassic boundary, using spore darkness as a climate proxy

Abstract

The End Triassic mass-extinction was caused by large scale volcanism in the Central American Magmatic Province (CAMP). The events following the volcanism which led to the mass extinction on land remain unclear. While doing research into the end-Triassic mass extinction in the terrestrial realm, a band of darker spores under the Triassic Jurassic boundary was found. It is hypothesized that this so called “Dark zone” is caused by stress in the terrestrial ecosystem. To research what caused the palynomorphs to become darker this project focused on obtaining a record of the darkness of smooth trilete spores across the end-Triassic extinction event in the Scandelah-1 core. To get an indication of the difference in chemical composition between dark and light spores, spores were also examined with Fourier transform infrared (FTIR) spectrometry. There was no significant difference between light and dark spores visible in the FTIR spectra, leading to those results being discarded for the purpose of this research. The record of the palynomorph darkness index (PDI) was more useful. The PDI record seems to be closely following the spore/pollen ratio and δ13C record. The PDI also showed some similar trends to the Charcoal abundance record. There also appears to be some cyclicity in the records. These results lead to the conclusion that high temperatures caused by forest fires causes spores to become darker. Charcoal is a proxy for forest fires and the spore pollen ratio is also in agreement with the trends that the charcoal data shows. The mechanism to explain the darkening caused by forest fires is that with high temperatures the sporopollenin in the spore wall changes chemically to get a darker color. The cyclicity could be an indication that these fires are forced by Milankovitch cycles.