Letter abstract
Nature Geoscience 1, 247 - 252 (2008)
Published online: 23 March 2008 | doi:10.1038/ngeo154
Subject Categories: Atmospheric science | Palaeoclimate and palaeoceanography
End-Permian ozone shield unaffected by oceanic hydrogen sulphide and methane releases
Michael B. Harfoot1, John A. Pyle1,2 & David J. Beerling3
Earth experienced repeated episodes of widespread surface and deep-ocean anoxia with a significant accumulation of sulphide-rich waters over the past two billion years1, 2. The resulting massive releases of hydrogen sulphide from the oceans, together with methane from the geosphere, have been suggested as a cause for mass extinctions through destruction of the ozone shield and a lethal accumulation of hydrogen sulphide at the surface1, 2, 3, 4. Here, we use a two-dimensional atmospheric chemistry-transport model5, 6, 7 with representative climate8 and atmospheric composition9 to simulate the effect of large hydrogen sulphide and methane releases at the time of the end-Permian mass extinction 
251 million years ago. In our simulations, the integrity of the stratospheric ozone shield is maintained for oceanic hydrogen sulphide releases up to 15,000 Tg S yr-1, a limit far exceeding the threshold for ozone collapse identified previously1 (2,000–4,000 Tg S yr-1). Scenarios of simultaneous hydrogen sulphide and methane injections also failed to significantly deplete the Earth's ozone shield, and generated non-lethal hydrogen sulphide concentrations (1–2 p.p.m.) at the surface. In our two-dimensional model simulations, the high photolysis environment in the tropics maintains the oxidizing capacity of the tropical troposphere, with high local hydroxyl radical concentrations, and greatly diminishes hydrogen sulphide entry into the stratosphere. We suggest that given current constraints on possible hydrogen sulphide and methane releases from anoxic oceans, and the geosphere, over the past 0.5 billion years, these gases seem unlikely to be the cause of coincident terrestrial biotic mass extinctions.
- Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge CB1 1EW, UK
- National Centre for Atmospheric Science, Global Composition, University of Cambridge, Cambridge CB2 1EW, UK
- Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
Correspondence to: David J. Beerling3 e-mail: d.j.beerling@sheffield.ac.uk
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