A blast from the gas in the past

From the AGU today, I find they are moving the cause of ancient planetary disaster from comets impacts and volcanoes or other big events to CO2 causing acidification of the oceans, literally they have a blast from the gas, to make CO2 the villain here. Of course, it’s just another modeling exercise in uncertainty.

From Wikipedia, a clear cut case of “we don’t know“:

There are several proposed mechanisms for the extinctions; the earlier phase was likely due to gradual environmental change, while the latter phase has been argued to be due to a catastrophic event. Suggested mechanisms for the latter include large or multiple bolide impact events, increased volcanism, and sudden release of methane clathrate from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.

From the AGU Highlights:

1. 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 percent 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. 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. The researchers 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 find that varying the ocean floor topography by adding deep ocean ridges increases 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 researchers find that if atmospheric carbon dioxide concentrations were 3000 parts per million by volume or higher, fitting within estimates for the Permian-Triassic boundary, 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 Permian-Triassic boundary extinction.

Source: Paleoceanography, doi:10.1029/2010PA002058, 2011 http://dx.doi.org/10.1029/2010PA002058

Title: Climate simulations of the Permian-Triassic boundary: Ocean acidification and the extinction event

Authors: A. Montenegro: Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada, and Environmental Sciences Research Centre, St. Francis Xavier University, Antigonish, Nova Scotia, Canada;

P. Spence and K. J. Meissner: Climate Change Research Centre, University of New South Wales, Sydney, New South Wales, Australia;

M. J. Melchin: Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada;

M. Eby and S. T. Johnston: School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada.