The Permian-Triassic boundary (251 million years ago) is characterized by one of the greatest mysteries in earth's history: a mass extinction of devastating proportions. This mass extinction is the most severe incision in the history of life on this planet, estimated 75 to 90% of the marine biota disappeared; the terrestrial synapsids lost their dominant role in the Permian ecosystems to the archosaurs (which will rule the planet until the end-Cretaceous extinction) and the forests of conifers were replaced by a landscape dominated by lycopods and ferns (and maybe also fungi, as explained in a fascinating post by "The Artful Amoeba" Blog).
Fig.1. The Bletterbach gorge in the Dolomites is one of only five major tracksites worldwide dated to the Late Permian (260-251 million years). The tracks of synapsids, archosaurs and other reptiles were found in sandstone-layers deposited in the alluvial plain and channels of a river system that regularly flooded the former semi-desertic landscape. Similar reddish sediments were found all over the globe, suggesting that the Permian was largely characterized by hostile environments.
The causes of this profound environmental changes are still unknown, however one hypothesis involved the large scale volcanic eruptions that deposited the rocks of the Siberian Traps, one of the largest igneous provinces recognized on earth.
Fig.2. Main igneous provinces recognized on earth (red areas) and corresponding hotspots (yellow dots, click to enlarge map). Hotspots are stationary regions of increased igneous activity; the spatial discrepancy of these spots and lava deposits is explained by the slow movement of the tectonic plates. The radiometric ages of some of the igneous provinces correlate with major mass extinction events during earth's history, as shown by the number of families recognized in the geological record (data after SEPKOSKI).
Large Igneous Provinces (or "LIPs") are deposits of igneous rocks with impressive dimensions, some cover thousands of square kilometers and are hundred of meters thick. Every LIP is connected to a hotspot - stationary regions where hot plumes of material are upwelling in the mantle of earth. The energy of the plume causes partial melting of the uppermost parts of the mantle and the crust and provides so the quantities of magma needed for the superficial eruptions. LIPs formation occurs apparently as sudden as mass extinctions and are huge events releasing gigantic amounts of lava, ashes and gases into the atmosphere in geological short periods (only some million years). These gases and aerosols can affect the climate of earth - this would explain the effects on terrestrial ecosystems - and modify the chemistry of the oceans - this would explain the disastrous effects on marine ecosystems.
The dates of mass extinctions in part coincide also with the radiometric ages achieved from the volcanic rocks of the LIPs.
However there is a problem with the supposed age of the Permian-Triassic extinction and the age of the Siberian Traps. Research published in 2008 by SAHNEY & BENTON suggested that the extinction at the end of the Permian was not a single and unique event, but a succession of minor events dated to approximately 272, 262 and 250 million years ago. During this time interval also the rate of diversification of the surviving species into new species was reduced.
The formation of the Siberian Traps is well dated to approximately 250 million years ago and apparently the volcanic eruptions happened too late to explain the earlier onset of the mass extinction. The new research by SOBOLEV et al. 2011 proposes now a possibly mechanism that could explain at least some of this temporal discrepancy.
Studying the petrological composition of samples of the Siberian basalt and grains of the mineral Olivine the researchers suggest that the source rock of the magma and lava was not only the melted continental crust of the Eurasian plate, but also mantle material from the plume contaminated with ancient oceanic crust. Continental crust and ocean crust differ significantly in chemical composition and display also different physical properties; one of the most important is the relative density. Continental crust is composed of rocks with a lower density when compared to oceanic crust, the heavy oceanic crusts therefore tends to sink in the mantle of earth, where it is melted and becomes recycled into a new plume.
The contaminated plume of the Jan Mayen hotspot that formed the Siberian Traps was more dense and heavier than clean mantle material, therefore it ascended slowly and remained blocked under the continental Eurasian crust. Trough fractures and magmatic conduits in the overlying crust gases like carbon dioxide, methane and other volatiles were however slowly outgassing over hundred and thousands of years from the plume and changing the composition of earth's atmosphere.
The plume finally melted the base of the crust, initiating the main phase of volcanic eruptions that produced the Siberian Traps and concluded the Permian-Triassic mass extinction.
COURTILLOT, V.E. (1990): A volcanic eruption? Scientific American Vol.263: 85-92
SAHNEY & BENTON (2008): Recovery from the most profound mass extinction of all time. Proc. R. Soc. B (2008) 275, 759-765
SOBOLEV, S.V.; SOBOLEV, A.V.; KUZMIN, D.V.; KRIVOLUTSKAYA, N.A.; PETRUNIN, A.G.; ARNDT, N.T.; RADKO, V.A. & VASILIEV, Y.R. (2011): Linking mantle plumes, large igneous provinces and environmental catastrophes. Nature Vol.477: 312-316