Past climate change - Climate Change: Biological and Human Aspects - page 75

could be processed in greater quantities, and more quickly. Greater carbon processing
could have led to a dip in atmospheric carbon dioxide. This would explain the global
cooling thought to have taken place between 850 and 635 mya. We know this cooling
took place because it was so marked that large ice caps formed, leaving geological
traces that can be seen today. The large ice caps reflected sunlight. This further cooled
the Earth, so enlarging the ice caps, just as happened in Snowball Earth I, and this led
to a second Snowball Earth, Snowball Earth II in the aptly named Cryogenian period
(meaning ice-generating). It should be pointed out that this was not one single event
but a clutch of at least three main events taking place within the Cryogenian period.
An interesting possibility as to one aspect to the extra biological carbon processing
taking place prior to Snowball Earth II is that life colonised the land around 850 mya.
US researchers Paul Knauth and Martin Kennedy in 2009 used 13 C/ 12 C isotope ratios
from many old sediment samples originally deposited by freshwater run-off from
continents. The results suggested that the continental run-off contained carbon that
was biological in origin, which in turn implies terrestrial life. In short, they suggest
that the continents were covered by a thin green carpet in many places before Snowball
Earth II. However, the problem with this theory, compelling as it is (it fits in with the
new theories of evolution and the Earth system and its climate), is that the evidence is
somewhat circumstantial. Algae covering the land, unlike larger vascular plants with
good structure such as leaves, do not leave much in the way of fossils. Nonetheless,
something must have altered the 13 C/ 12 C isotope ratios and terrestrial life is arguably
the best candidate. However, there is some fossil evidence for very early terrestrial
life prior to Snowball Earth II from a small team of British and US researchers -
Paul Strother, Leila Battison, Martin D. Brasier and Charles Wellman - in 2011. They
reported the discovery of large populations of diverse microfossils with cell walls that
contained organic compounds. These assemblages contained multicellular structures,
some approaching 1 mm in diameter (which is large). The fossils offer direct evidence
of eukaryotes living in freshwater aquatic and sub-aerially exposed habitats during
the Proterozoic era. The apparent dominance of eukaryotes in non-marine settings
by 1 bya indicates that eukaryotic evolution on land may have commenced far earlier
than previously thought.
As with Snowball Earth I, there is considerable debate as to whether Snowball
Earth II saw heavy glaciation across the equator. One idea (Pollard and Kasting,
2005) is that the Snowball Earth came about slowly and at the equator ice may have
formed at rates as low as 7 mm a year and reached the equator, at least in part, by
glaciers flowing from higher latitudes. In this case, the researchers speculate, the sea
glacier ice formed would have been far more transparent than we find in many of
today's glaciers. Such transparent ice may have allowed photosynthesis to take place
in the water below. Whatever the case, there is little doubt that Snowball Earth I and
II were extremely large events for the Earth's biosphere.
As with Snowball Earth I, it is thought that volcanic carbon dioxide came to the
rescue of Snowball Earth II by building up over several million years to atmospheric
concentrations a few hundred times today's levels. As with the first Snowball event,
this led to considerable global warming that ended the massive glaciation. However,
there is one final detail worth mentioning, because we will see similar events later.
There is evidence to suggest that the end of Snowball Earth II was accelerated