ATMOSPHERIC COMPOSITION AS A TAPHONOMIC FILTER FOR FOSSIL PLANTS IN THE GEOLOGICAL RECORD – AN EXAMPLE FROM THE TRIASSIC–JURASSIC BOUNDARY OF EAST GREENLAND

The Triassic–Jurassic boundary (~200 million years ago) was a period of intense climate change marked by a doubling of background atmospheric CO₂ concentrations, rising temperatures and ecosystem instability likely due to volcanic activity from Central Atlantic Magmatic Provence volcanism. The impact of these environmental stresses upon the plant community have been well-documented for the fossil flora of East Greenland; however, the potential impact of atmospheric composition change on the process of fossilisation and the resulting impact on interpreting biodiversity change across the event have not been considered in detail. Plants directly incorporate carbon from the atmosphere into their structures, and atmospheric composition directly affects the functioning and development of plants. One often-reported effect is an increase in leaf mass per area (LMA) with increasing levels of atmospheric CO₂, which could imply an increase in the preservation potential of leaves due to thicker, stronger leaves at times of high atmospheric CO₂. To test this, a group of evolutionarily diverse plants was grown in simulated palaeoatmospheric treatments with differing atmospheric CO₂ and O₂ levels in the controlled environment chambers of the Programme for Experimental Atmospheres and Climates (PÉAC) facility. All nine species, including gymnosperms, ferns and angiosperms, increased LMA and leaf thickness significantly when exposed to high CO₂ and even more when exposed to high CO₂ and low O₂. This suggests that at times of high atmospheric CO₂ and low atmospheric O₂, leaves may have a greater preservation potential under the same geological and biological conditions than at times of lower atmospheric carbon dioxide. This has implications for the interpretation of biodiversity recorded in the fossil record. To test this, we compared fossil preservation quality across the Tr–J boundary of Astartekløft and identified beds where current biodiversity estimates may represent over- or under-estimates based on atmospheric composition at the time. Overall, the findings of this study suggest that atmospheric composition should be considered as having a potentially significant influence on the preservation of leaf cuticle in the fossil record and interpretations of biodiversity change should take this into account.