The Devonian World – Biogeography and Climate

Australia: The Land Where Time Began

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 CO₂levels 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.

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