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Australia: The Land Where Time Began |
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The Devonian
World – Biogeography and Climate At the beginning of the
Devonian more than
400 Ma the world was very different from that of the present, in which
the atmospheric oxygen levels differed from the present, that according
to some models were about half the present levels, and the carbon
dioxide levels were about 10 times higher than now. Berner et
al. have estimated (Berner,
1993, 1999) atmospheric O2 and CO2 levels based on
factors such as solar radiation that has been increasing over the last
570 My, and the effect that has had on rates of weathering of carbonates
and silicates, and how the increasing vegetation cover has changed the
weathering rate as well as the uptake of CO2 from the
atmosphere. Evidence obtained from carbon isotope studies of fossil
soils has supported these models, the effects meaning that the total
density of the air was less than the present density.
Throughout the Devonian the
atmospheric CO2 levels have generally been estimated to have
been high than at the present, then in the Early and Middle Devonian
dropping from about 0.35% to about 0.3% by the close of the Famennian.
The increased rates of organic carbon burial and enhanced
weathering of silicates
have been suggested to have been the causes of this drop, and these
processes are also suggested to have had an effect on the cooling of the
climate (Algeo et al., 2001).
By the Middle
Carboniferous (late Mississippian) the present levels of atmospheric
CO2 had been reached. According to Clack1 it is an
example of the influence on the climate exerted by the increased
vegetative cover of the Earth, extracting CO2 from the
atmosphere and putting O2 into the atmosphere. It is suggested by those estimates that atmospheric
O2 levels were somewhat lower than the present level of 21%,
then by the middle of the Frasnian dropping slightly from about 19% to
about 17%, after which they rose again steadily, eventually reaching
about 33% by the end of the period (Berner, 1999; Berner et
al.,. 2003), though a new
model of estimating O2 has been produced that combines and
refines previous models, giving a new estimate for the Devonian. These
are based on a GEOCARBSULF model (Berner, 2006). Factors such as rates
of weathering of calcium and magnesium silicates and pyrite, decay of
organic matter, degree of exposure of coastal lands, and the variability
of the weathering of volcanic rocks are used by this computer-based
model to obtain its estimate. It is suggested by the new figures that
during the Early Devonian atmospheric O2 levels first rose
from about 22% to nearer to 25%, after which they dropped precipitously
through most of the remainder of the Devonian until the close of the
Frasnian, when they reached a low of 13%. The levels then rose slowly to
about 16% by the end of the Famennian, and by the Middle Viséan reaching
present levels. The rest of life on Earth was profoundly influenced by
the results of these changes in way that have only recently been
appreciated (e.g., Dudley, 1998; Graham et
al., 1997; Ward et
al., 2006; Clack, 2007).
Clack1 stresses that these are only models, so are
provisional. Other conclusions can be reached as techniques change and
other data can be used, future research possibly suggesting new views of
the Palaeozoic climate and atmospheric regimes. According to Clack1 in the Devonian the
continents of the present were separate chunks that were scattered
around the oceans. As the spinning of the Earth has been gradually
slowing since it formed the day length was shorter in the Devonian than
at present, though the Sun is thought to have been less bright at that
time. In the Early Devonian the land was essentially bare, with
terrestrial biota undergoing explosive evolution throughout the late
Palaeozoic, and forests that had essentially modern structure flourished
by the Middle Carboniferous, the interactions between plants and animals
were comparable to those of the present, though there were no
terrestrial vertebrate herbivores before the latest Carboniferous. Clack1
mentions the chapter by DiMichele & Hooks (1992) in the volume
Terrestrial Ecosystems Through
Time edited by Behrensmeyer et
al., is a comprehensive
summary of terrestrial ecosystems through the Palaeozoic. Though details
have changed since it was published in 1992 the overall sweep of the
conclusions is still a good picture of the times.
Wetlands Through Time,
another volume mentioned by Clack1, collected papers edited
by Greb & DiMichele (2006) has details of sites that have been
intensively studied, provides a picture that is updated, mainly from the
late Palaeozoic. Plants Invade
the Land, another volume, which is edited by Gensel & Edwards
(2001), includes studies of flora and fauna from the Silurian and
Devonian. The period from the Late Devonian to the close of the Permian,
which coincides with the close of the Palaeozoic, was a time of enormous
but gradual changes that occurred in the world. Over this period of time
the continents moved into new configurations, and these changed
configurations affected the climate. As different continents slid over
the poles the result was greater ice thicknesses than when there is only
sea ice at the poles, as is seen today by comparing the Antarctic with
the Arctic, the latter being covered by only sea ice, and this in turn
affects the entire global climate (Erwin, 1995, 1996; Erwin et
al., 2002). The circulation
of water around the poles was also profoundly affected by the changed
positions of the continents, and this in turn affects the distribution
of warm and cold water and therefore the climate. The continents had combined into 2 major landmasses
by the Early Devonian, Laurussia to the north and Gondwana to the south,
which were separated by the
Iapetus Ocean.
Laurussia became part of
Pangaea and when Pangaea split up it associated with Laurasia. The
total length of the seacoast of the combined continents was reduced as
they coalesced which limited the shore and nearshore habitats for biota
to inhabit. Another change for the life of the time was that more
communities are brought into more contact with each other, the effect of
this being to increase competition and decrease the diversity as species
with similar requirements that competed for dominance, and hence
survival. Communities that are isolated from each other tend to evolve
away from each other, becoming more localised specialists and unique
species, and when these communities are brought together, each species
has the opportunity to colonise greater areas, becoming more
cosmopolitan. During the late
Palaeozoic the
effect on marine species was therefore a gradual decrease of the
diversity of species, with a corresponding increase in the distribution
area of those remaining (Briggs & Crowther, 2001). Laurussia and Gondwana gradually moved towards each
other throughout the Devonian, though it was in the Permian that they
ultimately collided to form the supercontinent Pangaea, a process which
eventually squeezed the Iapetus Ocean out of existence. A number of
profound consequences for life resulted by the closing of this ocean. Clack1 suggests many organisms may have
been forced to adapt to a terrestrial life or die out by the loss of
marginal marine and nearshore habitats. What is certain is that with the
reduction of aquatic habitats more terrestrial habitats became
available. Throughout the Devonian this process of continental collision
with the accompanying coastline habitat loss appears to have been
continuing, with North America and northwestern Europe being brought
together in the Middle Devonian to form Laurasia, aka the Old Red
Sandstone Continent. There are vast areas of terrestrial sediments that
are associated with this continent, which include the fossils of the
first known vascular plants and insects. This is also where the first
tetrapod from the Devonian was found, in Greenland, though fossil finds
since then have shown that by this time they were also present in
eastern North America, that was also part of Laurasia in the Devonian,
and they are now known to have also been present in China and Australia.
Australia was far away on the eastern end of the southern continental
landmass and China was on an isolated landmass to the northeast. The large continental landmass that comprised the
present-day continents of Antarctica, South America, and South Africa
were in polar regions, resulting in episodic glaciations that were
gradually increasing in scale throughout the late Palaeozoic. The
development of large ice caps, in their turn, locked up vast amounts of
water which lowered sea levels generally, which was another cause of the
loss of marine habitats throughout the interval. The glaciation
gradually came to an end towards the close of the
Permian, and
Pangaea had formed by the
merging of the northern continents with
Gondwana in the
south. The vast area that resulted experienced continental climates with
extremes of temperature, and the global climate was affected by this
heating, as land gets hotter than water, and desert conditions came into
existence over large areas of the land surface. There were also periodic fluctuations in the
position of the Earth in relation to the Sun, with combinations of
circumstances in its orbit and declination, added to cyclical changes in
the Sun which combined to form cycles, Milankovitch cycles, that were
slowly repeating cycles in the temperature and insolation of the Earth.
The Milankovitch cycles still operate at the present. It has been found
that they are the cause of cyclical changes, such as periodic
glaciation, and the rocks record their periodicity. They can be seen in
the alternating red and green bands in the so-called red-bed deposits in
East Greenland that date to the Devonian (Olsen, 1994; Astin et
al., 2010), and they are also
partly responsible for cyclical deposition that is found in coal
deposits. These cycles that are relatively short term, having timescales
of 10s or 100s of thousands of years, add to the effects of changes in
oceanic currents and the positions of the continents that took place
over millions of years in the Palaeozoic. Links
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |