Paleoclimatology is the attempt to reconstruct ancient
climates and their causes.nLike all reconstructions, they are based
on specific assumptions that are worthy of reconsideration.
Glaciation: ice sheets wide-spread over separate continents or
localized on part of a supercontinent.
Wegener looked at evidence for glaciation around the
world, especially where the average person doesn't think of it as
existing, in the Southern hemisphere. He came to believe that
climatic events that could affect a large part of a super-continent
like Pangaea would nevertheless--from a global perspective--seem
relatively small and localized when compared to the same events
distributed over continents separated by oceans. Advancing and
retreating ice sheet s would be examples of such events.
To see what he meant, look at the image of the globe
below. It shows current relationships among continents. The darkened
areas (blobs) in the southern regions all around the globe show areas
that appear to have been covered with ice sheets an estimated 250
million years ago, during the Permian Period at the end of the
Paleozoic era. How could one or several ice sheets cover so much, so
separated ground? Wegener suggested that perhaps they didn't.
To follow Wegener's thinking, imagine pulling those
southern-most areas towards one central point at the center of the
very bottom. Pulling them together creates one Pangaea-like mass. It
has an Antarctic-like area as part of it, covering what are now those
darkened areas.
So, Wegener also drew the conclusion that in the late
Paleozoic Era (Permian Period), just before the presumed break-up of
the super-continent Pangaea, glaciation spread out from the Antarctic
area of the super-continent. It left traces (those darkened areas) on
those parts of Pangaea that later separated out to become Northern
Australia, southern Africa, India and a strip of South America. (Of
course, Wegener actually reasoned from glaciation evidence in those
areas back to the centralized mass that became called Pangaea) It is
commonly recognized that during the Paleozoic Era (570 to 225 million
years ago) North America was much nearer the equator.
It's possible that the existence of a large mass like
Pangaea allowed free circulation of warm equatorial waters to the
poles, making the development of a truly massive ice sheet
impossible.
But what about evidences of Pleistocene glaciation?
Specialists recognize roughly 20 cycles of glaciation during roughly
2 million years.
