LEAF PHYSIOGNOMIC VARIATIONS AS A PROXY FOR ELEVATED ATMOSPHERIC SO2 ACROSS THE TRIASSIC–JURASSIC BOUNDARY

The Triassic–Jurassic boundary (~200 million years ago) is marked by a doubling of atmospheric CO₂ concentrations, rising temperatures and ecosystem instability. This appears to have been driven by a major perturbation in the global carbon cycle likely due to massive volcanism in the Central Atlantic Magmatic Provence. This volcanism also likely delivered huge amounts of sulphur dioxide (SO₂) to the atmosphere; however, the role that this may have played in leading to ecosystem instability at the time has received less attention. To date relatively little direct evidence has been presented from the fossil record able to suggest SO₂ as a cause of plant extinctions at this time. In order to address this, we performed a physiognomic leaf analysis of groups of fossil leaves, including Ginkgoales, bennettites and conifers, from nine well-preserved plant beds that span the Tr–J boundary at Astartekløft, East Greenland. The physiognomic responses of these key taxa were then compared to the leaf size and shape variations observed in nearest living equivalent taxa exposed to simulated palaeoatmospheric conditions in the controlled environment chambers of the Programme for Experimental Atmospheres and Climates (PÉAC) facility. The modern taxa all showed a statistically significant increase in leaf roundness when exposed to elevated SO₂, regardless of the proportion of other gases (O₂ and CO₂) in each experiment. A similar increase in leaf roundness was also observed in the TJ fossil taxa immediately prior to a sudden decrease in relative abundance (up to local absence) at Astartekløft. These findings suggest, firstly, that the sulphur dioxide can be traced in the fossil record using physiognomic changes in plant leaves, primarily via the increase in roundness with elevated atmospheric SO₂, which supports the pre-existing suggestion that SO₂ had a significant role in the Tr–J extinction and secondly, that the role of SO₂ in plant biodiversity declines across major geological boundaries coinciding with global scale volcanism may currently have been underestimated.