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Plant Stomatal Response to Volcanogenic SO2
Tanner, L.H., Smith, D.L. and Allan, A. 2007. Stomatal response of swordfern to volcanogenic CO2 and SO2 from Kilauea volcano. Geophysical Research Letters 34: 10.1029/2007GL030320.

The authors note that "the release of CO2 from the outgassing of a large volume of basaltic lava has been cited as triggering an interval of intense greenhouse warming that resulted in a mass extinction at the Triassic-Jurassic boundary (200 million years ago)," citing the studies of Yapp and Poths (1996), McElwain et al. (1999) and McHone (2003), and they write that "similar transient increases in paleo-pCO2 have been identified at other important paleontologic boundaries using fossil stomatal data, including the Permian-Triassic (250 million years ago) and the Cretaceous-Tertiary (65 million years ago)," citing the work of Retallack (2001) and Beerling et al. (2002). However, they report that in addition to releasing CO2 into the air, "large flood basalt eruptions may include the loading of the atmosphere with SO2," which could also produce mass extinctions. Hence, they decided to collect a set of real-world measurements designed to determine if elevated concentrations of atmospheric SO2 can lower plant stomatal index in the same way that elevated concentrations of CO2 do, which if found to be the case would imply that calculations of pCO2 based on the stomatal frequency of fossil leaves may actually be due in large part to SO2, implying lower concentrations of atmospheric CO2 during past mass extinction events, which would imply less intense greenhouse warming and possibly reveal SO2 to be the primary culprit behind the mass die-offs.

What was done
Tanner et al. measured the stomatal index (SI) of the common swordfern (Nephrolepis exaltata) in various plumes of actively outgassing vents of Kilauea, a basaltic shield volcano on the southeastern flank of Hawaii, as well as in other locations chosen in such a way as to provide situations where CO2 was high and SO2 low, where CO2 was low and SO2 high, and where both CO2 and SO2 were either high or low together.

What was learned
The researchers' protocol revealed that "SO2 alone causes a lowering of SI similar to that well documented for CO2."

What it means
Tanner et al. conclude that "long-term eruptions may have produced a similar phenotypic response in plant species across large areas of the globe that is indistinguishable from that produced by elevated CO2." As a result, they further conclude that it is "necessary to examine more closely the impact of volcanogenic SO2 on the global environment and the potential role of SO2 outgassing as a mechanism of extinction."

Beerling, D.J., Lomax, B.H., Royer, D.L., Upchurch, G.R. and Kump, L.R. 2002. An atmospheric pCO2 reconstruction across the Cretaceous-Tertiary boundary from leaf megafossils. Proceedings of the National Academy of Sciences USA 99: 7836-7840.

McElwain, J.C., Beerling, D.J. and Woodward, F.I. 1999. Fossil plants and global warming at the Triassic-Jurassic boundary. Science 285: 1386-1390.

McHone, J.G. 2003. Volatile emissions from central Atlantic magmatic province basalts: Mass assumptions and environmental consequences. In: Hames, W.E. et al. (Eds.) The Central Atlantic Magmatic Province: Perspectives from the Rifted Fragments of Pangea. Geophysical Monograph Series 136: 241-254.

Retallack, G.J. 2001. A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles. Nature 411: 287-290.

Yapp, C.J. and Poths, H. 1996. Carbon isotopes in continental weathering environments and variations in ancient atmospheric CO2 pressure. Earth and Planetary Science Letters 137: 71-82.

Reviewed 7 November 2007