teaching - Earth Science Teachers' Association

TEACHING EARTH SCIENCES ● Volume 31 ● Number 4, 2006
results can then be tested against the other kinds of
palaeoclimatic indicators.
Carboniferous Icehouse Earth
From a palaeoclimatic perspective, the latter part of the
Carboniferous period, 315-290 million years ago, may
be one of the most interesting phases of Earth history.
Like today, Carboniferous climate was cold with large
polar ice caps. Geologists refer to this kind of climate as
an Icehouse world. When Alfred Wegener proposed his
theory of continental drift in the early twentieth century
he used evidence for these ice caps to support his
argument. He noticed that Carboniferous tillites
occurred widely across Antarctica, South America,
Africa, India and Australia, and proposed that they had
once comprised a single landmass (Gondwanaland)
positioned over the South Pole. The remaining continents
of Europe and North America comprised
Euramerica, which lay over the equator.
Every geologist knows that British Carboniferous
rocks are rich in coal (Carboniferous literally means
coal-bearing), the compacted remains of primitive
tropical rainforests (Figure 1). One interesting feature
of these successions is that they contain distinct cycles
of coal interbedded with marine bands. These
‘cyclothems’ record repeated sea-level fluctuations of
about 70 metres, and average cycles are estimated to
have been about 100,000 years in duration. They were
probably formed as the Gondwanan ice cap waxed and
waned in size through successive ice ages. The cause of
cyclic ice build up and retreat was probably linked to
wobbles in the Earth orbit (Milankovitch cycles),
which changed the amount of solar energy that reached
the surface. The Quaternary ice ages were driven by the
same phenomenon.
What was the effect of these ice ages on the Carboniferous
coal forests? Lying on the equator, they were
far away from the nearest ice sheets, nevertheless there
is evidence that climate exerted a major influence. During
ice ages, the Earth’s atmosphere becomes drier as
cold air cannot carry so much moisture, and this is
notably the case in the tropics. Tracking fossil plants
through Carboniferous cyclothems shows that coal
forests flourished during the warm, wet interglacial
phases, but during subsequent ice ages were replaced by
drought-adapted vegetation. One big debate raging at
the moment is what happened to the Amazon rainforest
during the height of the last ice age, 18,000 years ago.
Some say it contracted, while others argue that it
remained intact. Carboniferous studies contribute to
this debate showing that the earliest rainforests to
evolve did indeed respond to ice age fluctuations.
Cretaceous Greenhouse Earth
Another interesting period for palaeoclimatologists is
the Cretaceous, some 144-65 million years ago. At this
time, our planet appears to have experienced the other
climatic extreme, a Greenhouse phase. Computer
models predict that global mean annual temperature
may have been 10°C greater compared to today. So what
was Greenhouse Earth like? For one thing, the polar ice
caps appear to have almost entirely melted, as there are
no known tillites and only limited evidence of icerafted
debris near the Cretaceous poles. Consequently,
sea level was raised by as much as 200 metres. The
familiar Chalk seas of Europe, and similar marine
deposits in North America, are some of the most tangible
evidence for such a global sea-level rise.
Today, the formation of polar sea ice is the main driver
of ocean circulation. As seawater freezes, salt is
expelled from the ice lattice, and cold, dense brines form
that sink to the ocean floors creating currents. In a more
or less ice-free Earth, as envisaged for the Cretaceous,
one might expect that ocean circulation would be much
reduced. This has, in fact, proved to be the case, as Cretaceous
successions contain common black shales.
These organic-rich deposits accumulated in stagnant
seas where there was too little oxygen to break down
organic matter. These relics of Cretaceous oceanic stagnation
are of enormous economic importance, as black
shales are source rocks for many oil fields.
If an ice-free Earth with stagnant oceans sounds
pretty hard to believe, one final aspect of the Cretaceous
world was even more bizarre: the existence of temperate
rainforests over both poles, similar to those of present
day Chile (Figure 2). Some of the best fossil forests
have been discovered on Antarctica, which, like today,
lay over the South Pole in Cretaceous times. Growing
at more than 75° of latitude, an intriguing feature of
these ecosystems is the fact that they would have had to
endure months of continuous darkness during the winter.
However, studies of tree-rings in fossil woods show
that the trees actually thrived in these environments.
Annual rings are typically more than 2 mm in width,
and Antarctic forests were as productive as any English
woodland.
Figure 2
Did Antarctica
once look like
this? Monkeypuzzle
forests in
Chile are the
nearest living
relatives to
Cretaceous polar
forests.
11 www.esta-uk.org