Deep impacts vs mega-eruptions – the extinction debate goes on

Many extinction events are observed in the palaeontological record, but two of the biggest (and best understood) happened at the end of the Permian (the P-Tr extinction at 250 m.y.) and at the end of the Cretaceous (the K-T extinction at 65 m.y.).  There is compelling evidence that the K-T extinction was primarily caused by a large impact (either a comet or a very large meteorite), although a major basaltic eruption, the Deccan Traps in India, occurred at the same time.  There are many who believe that the Deccan eruption at least contributed to the K-T extinction, and some who believe that the eruption was the main factor. 

The P-Tr extinction was the most severe of all documented extinction events, resulting in the demise of approximately 50% of families and 90% of species.  While there is some evidence of a major impact at this time, much of that evidence is ambiguous, and it is not nearly as convincing as that for an impact at the K-T boundary.  Again, however, there was a very large basaltic eruption at the P-Tr boundary – the Siberian Traps in central Russia.  Recent work by geologists from Britain and Russia have led them to suggest that the Siberian Traps are at least 2 times (and possibly more than 3 times) as extensive as previously thought, and also that their timing appears to perfectly overlap the P-Tr boundary (Reichow et al., 2002).  The apparent huge extent of the Siberia Traps would make them the largest of the continental basalt-flood deposits on earth, and the coincidence with the P-Tr boundary is consistent with them being the primary cause of the extinction. 

Large volcanic eruptions can have three important implications for climate.  Firstly a great deal of fine volcanic dust and ash is released and this can have an immediate, but relatively short-lived, cooling effect.  Secondly huge volumes of gases are released – the most important being CO₂ and SO₂.  

SO₂ is converted to solid and liquid sulphate aerosol (tiny drops and particles) in the atmosphere.  These aerosols block incoming sunlight and promote cooling.  Some recent large eruptions (such as Pinatubo in 1991) have led to significant atmospheric cooling (ca. 0.5° C) for a couple of years or more.  It follows that sustained large eruptions, over a few hundred thousand years, could lead to longer-term and much more pronounced cooling.  SO₂ emissions also lead to acidification of the atmosphere and to surface and ground waters if sufficiently sustained. 

CO₂, on the other hand, leads to greenhouse warming, and since the atmospheric residence time of CO₂ is much longer than that of sulphate aerosols (decades versus years) it would follow that significant global warming could result from a prolonged flood basalt eruption. 

In fact, the data on warming and/or cooling following major flood basalt episodes are ambiguous.  Some studies suggest that intense global warming has predominated, while others suggest that significant cooling was the main long-term effect.  We have never witnessed a major flood basalt eruption (and we never could since they go on for thousands of years).  The1783 basalt eruption at Laki Iceland was the largest eruption in historical times.  It lasted for 8 months and produced 12 km³ of lava. Northern hemisphere temperatures dropped by about 1° C for a few years, and there was significant acidification.  In comparison, the eruption of the Siberian Traps lasted for several hundred thousand years, and is estimated to have produced between 2 and 3 million km³ of lava (Renne, 2002).

This new evidence of the massive size of the Siberian Traps provides support for the theory that the extinction at the end of the Permian was caused by intense and sustained basaltic volcanism. 

References

Reichow, M., Saunders, A., White, R., Pringle, M., Al’Mukhamedov, A., Medvedev, A. and Kirda, N., 2002, ⁴⁰Ar/³⁹Ar dates from the West Siberia Basin: Siberian flood basalt province doubles, Science, V. 296, p. 1846-1849. (June 2002) 

Renne, P., 2002, Flood basalts – bigger and badder, Science, V. 296, p. 1812-1813. (June 2002)

Steven Earle, 2002. Return to Earth Science News