Damage structures in leaf epidermis and cuticle as an indicator of elevated atmospheric sulphur dioxide in early Mesozoic floras

• Mesozoic fern and gymnosperm analogues were grown in controlled conditions.
• Plants were fumigated with zero or continuous elevated (0.2 ppm) sulphur dioxide.
• SO2 resulted in distinctive damage structures to leaf epidermis and cuticle.
• If observed in fossils, this could allow timing of high SO2 episodes in the past.
• Distinctive SO2 damage features could therefore be used as a palaeo-SO2 proxy.

Volcanic episodes are considered to be potentially important drivers of many mass extinction events in Earth history, when a sharp decrease in the diversity and abundance of macroscopic organisms occurred. Such events are hypothesised to be associated with volcanic sulphur dioxide (SO2) release. The effect of volcanism on atmospheric composition varies widely and is related to the magnitude and duration of the volcanic episode. Currently, there are limited methods for detecting the timing of SO2 release in the geological past. In field conditions, the influence of SO2 on plants can be difficult to separate from the effects of other volcanic emissions, which enter via stomata and can affect plant physiology, anatomy and morphology. In order to assess the direct effects of SO2 associated with palaeo-volcanic episodes, we conducted a six month growth chamber experiment growing plants under control (zero parts per million (ppm)) and continuous elevated (0.2 ppm) sulphur dioxide atmospheres. Leaf morphological responses of ten species representative of Mesozoic gymnosperm and fern fossil floras were examined using cryo-scanning electron microscopy. Here we show that expanded, fully mature leaves record unambiguous damage structures associated with injury from SO2 fumigation. In SO2 treated plants, leaves were smaller and did not persist; distinct raised areas of cuticle surrounding stomata appeared; surface waxes altered; blistering of the cuticle occurred; and the stomatal complex became distorted. These results have clear implications for application to the fossil record as many of the observed damage structures have the potential to be preserved in fossil plant cuticle and thus allow precise pinpointing of elevated SO2 episodes in the geological past.