Two major problems that humankind have in our time is the global warming and role of men in changing the chemistry of atmosphere. In Devonian times it were trees (not human) that changed the chemistry of atmosphere, water bodies and land.
The relevance between these two phenomenons come from feedback mechanisms due to changing chemistry. In Devonian Period when evolution of trees changed the chemistry of atmosphere it caused cooling and glaciation. Cooling and glaciation is not much important to us but what is important is warming after glaciation because glaciation as a result of tree evolution buried a large amount of organic material under ice sheets. Following warming released these organic carbon.
Now we are living in interglacial period and for sure ice sheets in Arctic and Antartic areas have covered a large amount of organic carbon. So the rapid change of atmosphere by human is melting these ice sheets and as a result exposing these organic carbon (esp, methane gas trapped frozen lakes is quick to emit out). These emissions is speeding the rate of warming.
Certainly changed chemistry of atmosphere and oceans hadn't only caused cooling in Devonian Period but also extinction. The warming is not a single process but it initiate a series of feedback mechanisms that is affecting climate, biosphere, land and ocean and these are what we depend on.............
Following is the cartoonic presentation of the impact of forestation on Devonian marine-terrestrial feedback systems,
Forests increased O2 level in atmosphere and meanwhile removed CO2 from atmosphere which is a greenhouse gas. The removal of greenhouse gas resulted in glaciation at the end of Devonian Period.
Ecology:
Middle Devonian (Givetian) forests of Appalachian Basin were dominately on flood plains. Trees have adapted diverse environments from deltaic-coastal plain swamps and proximal fluvial environments to distal well-drained upland fllood plains (Mintz et al, 2010).
It is suggested that forests have evolved as response to increased level of O2 in atmosphere as well as a response to overgrazing. The lignification (woody part of tree) make trees able to grow taller as well as to penetrate their roots deeper. This made trees able to grow in higher and drier places especially after adapting seed bearing which made trees free from dependance on water for their reproduction. Certainly, the higher level of oxygen and evolution of grazers and their adaptation to land were facilitated with vegatations prior to tree forest during Silurian Period and earlier Devonian Period. The forests may have facilated fires which made able to non-woody plants to regrow and hence started what is known as succession (cycle of non-woody plants and then woody plants or gradual replacements of fast growing plants by slow growing trees ).
Though there is debate on what caused the late Devonian extinction, however one most likely possibility of extinction is global ocean anoxia. Evolution of Forests of resulted in sinking of a lot of large amount of CO2 as well as the symbiont relationships of trees with nitrogen fixing bacteria resulted in fixation of N2 into organic material. When the decomposed organic material reached dumped in large amounts in ocean, it caused algal bloom which drawn out dissolved oxygen from oceans. However, this couldn't be able to cause mass extinction unless the global cycle of oceans, stopped which also stop mixing of water which the main way of dissolving oxygen into water.
Two main factors that control sea level rise is plate tectonics and Climate change. As previously mentioned that evolutioin of forest declined the CO2 which is a greenhouse gas which resulted in glaciation. Glaciation cause drop in sea level which is known as regression. Regression facilate erosion and influx of terrigenous material. On the other hand volcanoes continue to add CO2 into atmosphere which result into global warming and sea level rise which results in flooding surfaces and mud size sediment deposition. The sediments that is bounded between two unconformities as a result of sea regression called sequence and the study of sequences are called sequence stratigraphy. Sequences are the main evidence of sea levels in earth history.
It is a common observation that the lands with less vegetative cover have higher rate of erosion. The dusts in deserts and heavy floody during rainy seasons are common observation in these areas. The evolution of plants and forests have not only changed the geomorphology but also flow of water and deposition of sediment. One of the main shifts that is noted as shifts of rivers from braided rivers to meandering rivers. Prior to vegetations mean befor middle Silurian Period, braided rivers were dominant but after the evolution of forest the meadering rivers have dominated.
The glaciation resulted because of forestation during Devonian might have buried organic matter which after glaciation, combined with volcanoes have pored large amounts of CO2 in atmosphere to which trees have responded and it is why have our largest cool deposits in next period of Carboniferous.
Complications:
1. Warming-Cooling due to plate tectonics:
Volcanisms are one of the main sources of CO2 in atmosphere. Subduction of oceanic plate under continental plate create Magmatic arcs (Oceanic arcs) as a result of partial melting of oceanic crust (due to friction, water and increasing heat). This partially molten magma intrude the continents and explode as volcanoes. During Devonian Period the micro-continent (Avalonia) drifted towards Northern American continents and caused series of volcanisms. In Late Devonian Period the Avalonia micro-continent colloided with North American continents and resulted in Arcadian Orogeny. Uplift of lands due to volcanism expose fresh rocks for weathering and weathering consume atmospheric CO2 which result in cooling. As Arcadian orogeny is occuring during Late Devonian (360 Ma) so there is two possibilities,
i) Increased rock weathering intensified the on going cooling because of draw down of CO2 by evolution of trees.
ii) The Late Devonian cooling (glaciation) is due to rock weathering like the previous glaciations. Orogeny not only expose new rocks for weathering but also intensifies raining in the area (One good example is intensification of Himalyan orogeny in Late Miocene (10 Million Years ago as evidenced from thick deposits of Siwaliks stretched from Pakistan to India and Nepal ) and intensification of Monsoon system. Both uplift and increase in raining increase erosion.
CaSiO3 + 2 CO2 + 3H2O → Ca+2 + H4SiO4 + 2 HCO-3 OR (3H2O + 2CO2 → 2 HCO-3 +4H+)
Ca+2 + H4SiO4 + 2 HCO-3 → CaCO3 +SiO2 + 3H2O + CO2
If you notice, it is the Ca ion (and also Mg ion) from weathering of silicate rocks that bind with atmospheric CO2 that draw down atmospheric CO2 but it is not only Silicate rocks that weathers but Carbonate rocks also weathers which increase the CO2 levels. It is reported that Silicate weathering is 3-10% higher in Himalayan-Tibetan region that of continental areas. It is a significant amount. Furthermore riverine analysis for silicate weathering flux from Tibet and Himalayan region is equal to 16 to 33% of global silicate weathering from only 4.5% land area. This amount of weathering continental crust may cool down temperature from 1 to 6 degrees ( Bickle et al, 2002)
Unlike Himalayan-Tibetan orogeny which is the result of collision of Indian subcontinent and Eurasian Continent, the Arcadian collision was a result of collision of micro-continent with North American continent. This micro-continent was a very small (approximately the size of Japan) as compared to subcontinent India so Arcadian orogeny was much smaller as to Himalayan and we can't expect the same rate of weathering and CO2 draw down by this orogeny but we can't ignore its role.
The complication is in determining the share of orogeny and trees in atmospheric CO2 draw down..........
2. Orbital forcing of climate:
Due to gravitional forces in Solar system, the earth axis and orbit changes in cyclic manner and it directly affects the amount of insulation that different geographical regions of earth get. It is very important whether the northern hemisphere get more insulation or Southern Hemisphere. As northern hemisphere has more land mass than southern hemisphere so less insulation in Northern Hemisphere may initiate glaciation and more insulation may cause global warming. The orbital forcing of climate is basically of three forms,
i. Axial Precession (Or wobbling of earth)
ii. Precession of equinoxes (Or the shifting of earth's orbital revolution around the sun)
iii. Accentricity (The shape of orbit around sun)
The orbital forcing of climate is usually known as Milankovitch theory of climate in honor of Serbian Scientist, Milutun Milancovitch.
Though Milancovitch cycles have evidential support from ice cores of Greenland, however it is difficult to infer them from Devonian deposits................?
References:
- Bickle, M. J., 2002, "Impact of the Himalayan Orogeny on Global Climate", American Geophysical Union, Fall Meeting 2002, abstract #GC61A-04
Time Table of Devonian Period
- Berner, R. A., Lasaga, A. C. & Garrels R. M. (1983) Am. J. Sci. 283, 641-683.
Devonian Period
Devonian Period (416 -359 Ma = 57 Ma)
Early Devonian Epoch (416 – 398 Ma )
Lochkovian Age (416-411 Ma = 5 Ma)
Pragian Age (411 -407 Ma = 4 Ma)
Emsian Age (407 – 398 Ma = 9 Ma)
Middle Devonian Epoch
( 398 – 385 Ma )
Eifelian Age (398 -392 Ma = 6 Ma)
Givetian Age (407 – 398 Ma = 7 Ma )
Late Devonian Epoch (385 – 359 Ma )
Frasnian Age (385 – 375 Ma = 10 Ma)
Famennian Age (375 – 359 Ma = 16 Ma)
External Links to Devonian:
- Black Shale,
http://www.shale-mudstone-research-schieber.indiana.edu/devonian-black-shales.htm
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