Siberian Traps - End Permian Mass Extinction

The end-Permian mass extinction is the largest known. Some 95% of marine species were wiped out. There were major losses among the bryozoa, corals, crinoids, brachiopods, and foraminifera. In addition, terrestrial vertebrates, insects, and plants also suffered large loss of diversity. Details of the extinction, and the history of research, can be found in informative books by Erwin (2005) and Benton (2003). A summary of the mass extinction, and its possible causes, was written by White (2002) and recently by Saunders and Reichow (2009).

The duration of the main extinction event is unknown, but was probably no longer than 200,000 years, and possibly a lot shorter (see, e.g. Bowring et al., 1998 and Mundil et al., 2004). Imprecision in the dating techniques currently precludes further resolution.

The timing of the extinction is, with the resolution of current dating methods, identical with the main eruptions of the Siberian Traps. See here for more information.

The reduction in animal and plant species was not the only feature of the late Permian and early Triassic. There was a pronounced shallow-ocean anoxic event, that appears to have been global in scale, affecting the continental shelves. There was an increase in the amount of fungal (or algal) remains at the P-Tr boundary, and indirect evidence shows significant global warming and climate change. Perhaps most importantly, there was a significant shift in the ratio of isotopically 'heavy' ¹³carbon to 'light' ¹²C in the marine realm, as shown on the adjacent figure.

Figure Caption. Profile through the global stratotype section at Meishan, China. The Permo-Triassic boundary is defined on the first appearance of conodont Hindeodus parvus, and occurs in Bed 27 . Dates are based on Ar-Ar measurmeent sof feldpars in ash bed 25 (Renne et al., 1995) and Bed 28 (Reichow et al., 2009). Carbon isotope data from Cao et al. (2002) and Riccardi et al. (2007); note the excursion to isotopically 'light' carbon close to the extinction horizon, although the main carbon isotope excursion (CIE) post-dates the extinction horizon (see compressed section, right). Modified from Saunders and Reichow (2009). Bed thicknesses based on field observations by ADS.

This 'excursion' of 4 parts per thousand does not seem much, but because the world's oceans contain vast amounts of carbon (currently about 50,000 billion tonnes), to change the isotopic ratio by even this small amount requires a massive change in the carbon cycle. One way of doing this is to increase the amount of isotopically light carbon in the atmosphere and oceans. Volcanic carbon released from the magma has a ¹³C/¹²C ratio slightly lower than that of seawater, so it has been argued that it cannot create the observed CIE. However, this does not preclude volcanic CO₂ having played an important role in global heating and the mass extinction. The total volume of CO₂ released by volcanic degassing is immense and, although the annual addition to the atmosphere may be small compared with current, manmade additions, because CO₂ has a long average lifetime in the atmosphere, it will steadily accumulate over the eruption of the province as a whole (Saunders and Reichow, 2009). Another source, perhaps organic, isotopically very 'light' carbon stored in methane hydrates on the seafloor, or carbon stored in sedimentary basins, may be required to explain rapid CIEs. It is the trigger for the release of this carbon that is currently of great interest. Humankind does it by drilling. Warming of oceanic bottom waters, or injection of basaltic sills (Svensen et al., 2004; 2009; McElwain et al., 2005) may be Nature's mechanisms.