Past climate change - Climate Change: Biological and Human Aspects - page 82

which is greatly facilitated by the gas having a higher atmospheric partial pressure.
Metabolic models of these insects have been constructed. We know their size from
fossils and can reasonably assume a similar density to modern-day insects; hence we
can deduce their mass. We can therefore estimate the energy they would have required
for flight and so, in turn, their oxygen requirements. This gives us another way of
estimating the oxygen excess 300 mya: such estimates seem to fit those derived from
the carbon model.
3.3.5 End-Permianextinction(251mya)
The largest of all the mass extinctions of the past 600 million years was the end-
Permian or Permo-Triassic event, which occurred 251 mya. The marked turnover
of species - fauna and flora, marine and terrestrial - is used to mark the boundary
between the Palaeozoic and Mesozoic eras. At that time more than 60% of animal
species, both marine and terrestrial, disappeared. Some 90% of ocean plant and
animal species vanished, as did 70% of vertebrate families on land.
The tremendous volcanic activity of the Siberian Traps at the end of the Per-
mian undoubtedly played a major part in this extinction. The Siberian Traps are a
large igneous province that formed as a result of a mantle plume. They are centred
around the present-day Siberian city of Tura and also encompass Yakutsk, Noril'sk
and Irkutsk. This was a continental-flood basalt event. The Traps area, including
associated pyroclastics (rocks formed by materials thrown out by explosive volcanic
eruptions), is just fewer than 2 million km 2 and greater than that of present-day
Central and Western Europe combined. Estimates of the original volume of the Traps
range from 1 to 4 million km 3 (3 million km 3 is often cited). The Siberian Traps
eruptions lasted at full intensity for about 1 million years, which coincides with the
end-Permian extinction. The most accurate dating method available at the moment
is argon-argon radiometric dating which still has too many uncertainties to conclus-
ively prove the exact timing. The eruptions ejected volcanic ash and sulphur aerosols
into the atmosphere. This would have caused global cooling that probably lasted for
decades or even centuries after the main eruptions ceased. To put this into some kind
of present-day context, the largest eruption of the twentieth century, that of Mount
Pinatubo, was tiny compared to the Siberian Traps but even that caused a 0.5 ◦ Cdrop
in global temperatures in the following year. The largest eruption in historic memory
occurred in Iceland, 1783-4, and generated some 12 km 3 of lava on the island: the
Siberian Traps by comparison released 1-4 million km 3 . The poisonous gases given
out by the Icelandic event are recorded as killing most of the island's crops and foliage
and lowering global temperatures by about 1 ◦ C. It is therefore most likely that the
end-Permian event caused a short, sharp period of cooling that killed off much life. In
addition to ash and sulphur aerosols there would have been the release of considerable
quantities of greenhouse gases such as carbon dioxide, methane and water vapour.
Carbon dioxide stays in the atmosphere much longer than dust and sulphur aerosols,
especially if the Earth's photosynthetic pump is disabled, so its climate-changing
effects last longer. The warming from any major carbon dioxide release would have
become apparent only once the cooling dust and aerosols had been removed from the
atmosphere.