Past climate change - Climate Change: Biological and Human Aspects - page 86

end-Permian and the Eocene CIE) of a decrease in 13 C is that at the time the sediments
were formed there must have been comparatively far more 12 C in the atmospheric car-
bon pool, which was then fixed by living creatures into the carbonate in the strata being
laid down. As organic matter is the major 12 C-rich source then it is likely that there
was an injection of organically sourced carbon (be it fast or deep) into parts of the fast
carbon cycle from which the deep carbon cycle sediments were formed and which we
analyse today. This carbon conceivably could have come from forests (although this
is unlikely). (If it had all come from forests then woodlands planet-wide would have
needed to contribute the carbon, but we do not see such an extinction of tree species.)
Alternatively, the 12 C could have come from organic carbon that had originally been
buried in soils, or the combustion of carbon-rich sediments (such as coal) or methane
hydrates in the ocean. This last is discussed further in section 3.3.9 with respect to the
Eocene.
The Toarcian CIE (as we will also see with the Eocene CIE) also suggests that
the time was one of elevated atmospheric methane and carbon dioxide. (Note: even
if initially there was an increase in atmospheric methane, much would then have
become oxidised to carbon dioxide.) Of course, both methane and carbon dioxide are
greenhouse gases and there is evidence of a simultaneous rise in global temperatures.
The Toarcian CIE was also a time of a 400-800% increase in global levels of erosion
(suggesting a marked increase in global precipitation) and elevated levels of carbon
burial (which again one would expect in a warmer and wetter greenhouse world).
There was also a terrestrial and marine mass extinction, and strata from present-day
Yorkshire in the UK reveal a decline in marine invertebrate species of greater than
50%. Although the duration of the most significant climate change part of the event
lasted 120 000 years, overall the event lasted some 200 000 years and is coincident
with the geological formation of the Karoo-Ferrar large igneous province in South
Africa and Antarctica. Here, there is slowly mounting evidence suggesting that the
relationship between these two is causal. It could be that magma from the Karoo-
Ferrar province intruded into Gondwana coal seams, so igniting or driving off methane
(McElwain et al., 2005). It could be that the subsequent warming destabilised marine
methane hydrates, so releasing more 12 C into the atmosphere. Much depends on
the estimated size of the methane hydrate reservoir at the time and whether there
was enough to account for the Toarcian CIE. There is also a notion that matters
within the CIE event might have been further timed (or exacerbated) by Milankovitch
astronomical pacing (Kemp et al., 2005), but this last is fine detail (although it is
likely to be important when considering critical thresholds in biosphere stability; see
section 6.6.8).
In short, what is known is that the Toarcian extinction was caused by a perturbation
in the global carbon cycle that resulted in a period of greenhouse warming (above that
of the average greenhouse warming of the age) and a perturbation in ocean chemistry.
We will return to the question of CIEs in sections 3.3.9 and 6.6.4.
3.3.8 Cretaceous-Tertiaryextinction(65.5mya)
Evidence abounds that the Earth during the mid-Cretaceous was warmer than it
is today. For example, the breadfruit tree ( Artocarpus dicksoni ) has present-day
relatives that flourish in a tropical 15-38 ◦ C, and fossil remains of this plant from the