Simulated charcoalification of Lycopodium spores: The usefulness of spore colour and chemistry for understanding the fossil record

Abstract

The fossil pollen and spore (sporomorph) record includes occurrences of darkened grains typically attributed to thermal maturation from geological processes. However, zones of sporomorph darkening and colour variability within samples sometimes coincide with mass extinction events. Although bimodal sporomorph coloration is relatively common, its abundance often increases markedly during such intervals. These observations have prompted alternative explanatory hypotheses suggesting either environmental stresses on parent plants or possibly reworking of sporomorphs. Here, we propose another explanation: variation in sporomorph colour and darkness may result from combustion in wildfires during large-scale ecological disturbances prior to fossilisation. To test this hypothesis, we investigate how pyrolysis might impact Lycopodium spore colour and darkness. Untreated, intact spores were combusted at temperature increments from 150 to 800 °C. We quantified spore colour by measuring red, green and blue (RGB) intensities and by converting them to Palynomorph Darkness Index (PDI) values. As well as measuring various physical attributes, we used Fourier-transform infrared (FTIR) spectroscopy to determine spore chemistry. As pyrolysis temperature increased, spores darkened, lost mass, and shrank. FTIR analysis revealed three distinct chemical states between non-pyrolysed spores and those heated to 375 °C. Physical changes correlated more strongly with temperature, forming different groupings than those of the chemical data, both partially explaining colour change due to pyrolysis. With these data, we establish a baseline for comparison in a future artificial thermal maturation study, which will help determine whether pre-diagenetic combustion could influence, and be preserved in, the physical and chemical properties of fossil sporomorphs.