© American Geophysical Union
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ISI Journal Citation Reports © Ranking: 2014: 1/50 (Paleontology); 4/61 (Oceanography); 14/175 (Geosciences Multidisciplinary)
Online ISSN: 1944-9186
Was ocean acidification responsible for history's greatest extinction?
Two hundred and fifty million years ago, the world suffered the greatest recorded extinction of all time. More than 90% of marine animals and a majority of terrestrial species disappeared, yet the cause of the Permian-Triassic boundary (PTB) die-off remains unknown. Various theories abound, with most focusing on rampant Siberian volcanism and its potential consequences: global warming, carbon dioxide poisoning, ocean acidification, or the severe drawdown of oceanic dissolved oxygen levels, also known as anoxia. To narrow down the range of possible causes, Montenegro et al. (2011) ran climate simulations for the PTB using the University of Victoria Earth System Climate Model, a carbon cycle-climate coupled general circulation model. The model's highlights include dynamic representations of terrestrial vegetation, ocean carbon fluxes, and net primary production. They ran nine simulations, using three different concentrations of atmospheric carbon dioxide, three modes of ocean floor topography, and two competing theories for the geography of the time.The authors found that varying the ocean floor topography by adding deep ocean ridges increased the strength of the Meridional Overturning Circulation (MOC)-a convective cycle that mixes ocean waters. Also, the presence of the MOC was not abated by elevated atmospheric carbon dioxide as was found in previous research, suggesting that the ocean would have been well mixed and well oxygenated, restricting the chances of widespread deep ocean anoxia.Further, the authors found that if atmospheric carbon dioxide concentrations were 3000 parts per million by volume or higher, fitting within estimates for PTB, the ocean pH would have been 7.34 or lower. At those levels, the authors say the ocean's acidity would have had significant negative impacts on mollusks, corals, and other species that rely on oceanic calcium carbonate, suggesting ocean acidification may have been the main culprit in the PTB extinction.