A. Introduction
Before appearance of life on earth it was degassing from volcanisms, metamorphisms and weathering of silicate rocks that were regulating the carbon cycle. Volcanisms were supplying greenhouse gases like CO2 to atmosphere and weathering of Ca and Mg- Silicate rocks were drawing down CO2 from atmosphere to deposit them in form of carbonate rocks (Berner, 1983). The appearance of life on earth added biotic factor in global carbon cycle. One of the biggest events in geological history and also in evolution of Carbon cycle was the evolution of trees and seed plants which resulted in forestation and deep weathering of Silicate rocks (Beerbower et al. 1992). The forestation changed climate by working as a carbon sink and caused rapid drawdown of pCO2 that led to continental glaciations at the end of Devonian (Caputo 1985; Berner 1992, 1994). The rise of forests is correlated with widespread bottom water anoxia that was catastrophic for tropical benthos and coral reef communities (McGhee 1996). Sinking of organic matter and anoxia also resulted in deposition of Barite in Late Devonian Period (Paul, et al. 1994). The vegetative cover of land deeply affected the flow of water in terrestrial environments by strengthening of river banks, bars and changing the morphology of land. Meandering river system become more frequent as compared to prior vegetation cover of land when braided river system were dominating (Neil, et al. 2010). Prior to evolution of larger land plants in Silurian, virtually all river systems had a braided platform (Cotter 1978).
Thomas Algeo and Stephen Scheckler has recently (July, 2010) published a model to link all these events into a feedback system. Though this model covers beautifully most of the aspects of major impacts, tree evolution in a feedback system that changed both marine and terrestrial environments of Devonian Period however, this model is very brief with missing some important changes in feedback systems. There is no mention of forest fire though it was an important addition in Devonian ecosystem and perhaps climate. The rise of forests not only provided fuel but also oxygen level (more than 13%) for forest fire. A opposed to desertification, forestation was a main event that impacted the rate of evaporation and albedo. Eustatic sea level changes that many authors suggested a cause of mass extinctions in Late Devonian was totally ignored. The role of Arcadian orogeny is also totally ignored while Acadian orogeny may have played a vital role in accelerating silicate weathering of plants. All these events are important factors in marine-terrestrial teleconnections that was the main purpose of this flow chart model. Besides, Algeo-Scheckler model’s is showing that all subsequent events are related to Pedogenesis. Though Pedogenesis was an important outcome of arborescence and seed habit however we can’t overlook the roles of forestation and orogeny which were not only the main causes of Pedogenesis and silicate weathering but also had their independent role in marine-terrestrial teleconnections. For example, the landscape stabilization is shown as a result of intensified Pedogenesis while it is as a result of forestation and vegetative cover. Lastly it is needed to acknowledge the complications from parallel mechanisms for climate change and mass extinctions like orogeny, meteorite impacts and even orbital forcing of climate that is really difficult to link in distant geological time. Based on the above arguments I suggest for remodeling of this flow chart and I have redrawn it. Following is Algeo-Scheckler’s flow chart model and next to it, is my effort to amend this flow chart.
After Algeo et al, 2010
Above: Algeo-Scheckler’s flow chart model of marine-terrestrial teleconnections
Below: Redrawn flow chart model of Devonian marine-terrestrial-teleconnections
B. Discussion:
I. Physiological innovations and forestation:
During Early to middle Silurian bryophytic plants have started invading terrestrial environment. Although it was a big step in evolution of land plants however the reproductive and physiological limitations of the bryophytes kept them close to water bodies. Bryophytes are non-vascular, rootless, without stems and leafless thalloid plants that reproduce by spores (Beerbower et al. 1992). Lack of wood, kept them small and limited to wet environment so their invasion of terrestrial environments didn’t result in forestation and their role were limited in silicate weathering as product of their weathering were protosols. These physiological limitations of bryophytes limited their impacts on global carbon cycle, global climate and their role in marine-terrestrial teleconnections. It was Late Silurian and Early Devonian that vascular plants evolved and diversified (Gray, 1985). Though these land plants were small, shallowly rooted and limited to moist land areas but in Middle to Late Devonian Large trees with deep root systems like Archeopteride appeared that had colonized uplands. Algeo-Scheckler (Algeo et al, 2010) has presented artistically the evolution of vascular trees from early to late Devonian. The main innovation that led to differentiation of different plant systems (roots, branches, leaf) was appearance of wood. Raven (Raven, 2005) has pictured this phenomenon beautifully, “Scientists believe that once a lycophyte tree was stabilized by its shallow, forking, rootlike axes, it pushed rapidly skyward……”. By focusing more on impact of forestation of marine-terrestrial teleconnections, my focus will be on , a. environmental diversity of plants and on their size,
Early Devonian (Siegenian/Pragnian-Emsian) upland floodplain
After Algeo et al, 2010
Middle Devonian (Eifelian-Givetian) upland floodplain
After Algeo et al, 2010
Environmental diversity of land plants:
The range of environments that land plants have covered is key to understand the impact of their evolutionary innovations. Algeo-Scheckler’s reconstructions of environments of small, shallow rooted vascular plants in early Devonian and middle to large sized trees in Middle to Late Devonian show their environments as upland floodplains. Their reconstructions are supported by their own works on exposed beds of Franklinian Geosyncline that have exposures at High Arctic island of Canada and Famennian beds in Appalachian USA (Scheckler 1986a; Streel & Scheckler 1990). The flood plains that these trees have covered had diverse environments ranging from fluvial-deltaic to shore lines.
Scheckler (Scheckler et al, 1990) have compared the flora from meandering and braided river system and those of lowland and found that upland vegetation was similar to those of lowland but less diverse.
New York is famous for it’s in place tree stumps especially those of Gilboa village. In place tree stumps in New York regions are mostly found in Fluvial-deltaic, chiefly comprises of two formations of Plattekill and ManorKill formations which are successions of mudstone to sandstone that deposited in diverse range of environments. Bridge-Willis (Bridge et al, 1994) listed environments of Catskill formation from (i) Storm-wave-dominated muddy marine shelf with sandy shoals; (ii) sandy, tide influenced channels with wave- and tide-influenced mouth bars; (iii) sandy and muddy tidal flats, including channels, mouth bars, and washovers ; and (iv) muddy brackish bays, lakes and flood plains.
Late Devonian (Famennian) upland plain
After Algeo et al, 2010
2. Tree sizes (stem and roots)
Along with diversity of environment the size of trees and their rooting systems is another measure of impacts of evolutionary innovations. The large trunk sizes play as large reservoirs of carbon sinks and extensive rooting systems play main key role in weathering of Silicate rocks.
We see a rapid increase in trunk sizes, rooting systems and differentiation of different plant organs from simplest vascular plant of Early Devonian plants like Rhyniophytes to giant trees with extensive rooting like of Archeopterids of Late Devonian. Following I provide some representative plant assemblage of Devonian Period and also representative sizes just to show, how small plants turned into huge trees. This increase of size was a response to terrestrial environment.
i. Early Devonian assemblage:
Early Devonian plant assemblage includes Rhyniophytes, Trimerophytes and Zosterophylls. Rhyniophytes is thought to be oldest vascular plants and it is the simplest of all known vascular plants (Taylor, 1993). Rhyniophytes were small plants and they could reach the height of 30 cm (Renalia) though most of them were much shorter. Zosterophylls that are believed to be the ancestor of Lycopods could attain a height up to 50 cm (Gosslingia breconensis). Trimerophytes which were more complex than Rhyniophytes and Zosterophylls and their size could exceeds than a meter in height. Hence the plant assemblage of early Devonian was leafless with dichotomous braches ranging from few cm to a meter tall.
ii. Middle Devonian assemblage:
The Middle Devonian plant assemblage included Cladoxylalean ferns, Aneurophytes progymnosperms and Drepanophycales lycopods. Lycopods have originated from zosterophylls, had small leafs known as microphylls and most species were herbaceous. The diameter of their stem could reach 6.5 cm and leaves up to 4 cm long and they could reach to height of 50 cm tall. Cladoxylales are fern-like group of small trees; some of them (Pseudosporochnus) could reach 3 meters of height with a trunk that bore large roots and atleast three order of branches. Aneurophytes are the most primitive group (order) within progymnosperms, had three dimensional branching and their trunks from Gilboa, NY (Eospermatopteris), and is believed to 9 to 12 m tall.
iii. Late Devonian assemblage:
The Late Devonian plant assemblage included Zygopterid ferns, Archeopterids progymnosperms, Sphenophyll vines and Tree Lycopods. The elaborately frond bearing Zygopterid ferns were almost tree size as Austroclepsis from Lower Carboniferous of Australia had a trunk of 30 cm in diameter, consisting of numerous leaf bearing stems and intertwined roots. Archeopterids were large trees with extensive root system. Some specimen (Callixylon) had a woody of stem of up to 150 cm and a height exceeding 10 meters. Sphenophyll vines were small trees less than a meter and formed the understory of forests in Devonian and Carboniferous forests.
II. Forest Fire and marine-terrestrial teleconnections:
The most important point that is missing in Algeo-Schekler’s model is forest fire. Forest fire is an important event for Geology, Ecology, Climate and Paleobotany. Forest Fires could be traced by fossilized charcoal known as Fusian. Fusian are dominantly characoalified secondary wood but it may include other plant parts like leaves and seed (Scott et al, 1991) and hence they are important for Paleobotanists as they provide valuable information about the Plants that were constituted forest. Fusian is very important for paleoclimatologists because, forest fire is only possible when atmospheric oxygen level exceeds 13% and it charcoal will not form if atmospheric oxygen content exceeds that of 35% as it burn out the whole woods (Scott et al, 1991). The Fusian along with spores and megafossil may also tell the succession of trees.
Forest fire is important in marine-terrestrial teleconnections of Devonian period to explain the episodic erosional surfaces of black of Devonian. Schieber (Schieber et al al, 2004) identified four regional erosional surfaces in Chattanooga Shale. Algeo-Scheckler’s model linked the black shale to intensified Pedogenesis during Devonian Period. Though intensified Pedogenesis must have played a key role in deposition of black shale however it can’t explain the erosional surfaces of black shale because Pedogenesis was an increasingly intensifying event with evolution of trees. Erosional surfaces could be better explained by transgressive-regressive cycle, orogeny and perhaps also forest fires which are episodic and occur more frequent in hor and climate or during freuquent volcanisms. Pedogenesis, Arcadian orogeny, eustatic sea-level changes and forest fire combined provide a plausible explanation for episodic black shale deposits. Black shale basin of Late Devonian used to measure the rate of Arcadian orogeny (Ettensohn et al 1987). Black shale was deposited in foreland basin and the rate of Arcadian orogeny was exceeding 7 cm/year, so we would expect high terrigenous influx which is adding complicate linear explanation of events.
III. Forestation cover, Fluvial systems, Pedogenesis and Silicate weathering:
Though forestation was not the only big event in Devonian Period but it was the single only event that caused a series of big events of Devonian Period that were related to it like, landscape stabilization, decrease of desertification, intensified Pedogenesis, forest fire and increased O2/CO2. Appearance of root system and their rapid lateral and downward growth and anchorage not only helped in enlargement of plants to tree size but also intensification of the Pedogenesis.
Change in Fluvial Systems:
One of the most obvious impacts of forestation was the landscape stabilization. Big differences in alluvial and fluvial sedimentation and fluvial system have been reported. Davies-Gibling (Davis-Gibling et al, 2010) have compiled the case studies and made a database of subject from Cambrian to Devonian. They constructed a flow chart of feedback system showing inter-relationship of fluvial systems prior evolution of root system of terrestrial plants. It make easy to understand major changes in alluvial system and sediment characteristics after the evolution of root systems. Pre-Devonian fluvial succession characterized by lack of fine grained sediments, were dominated by bedload-dominated transport, unstable banks and flashy discharges showing braided river system characters (Schumm, 1968, Eriksson et al., 2006). Cotter reviewed (Cotter, 1978) 39 published studies on fluvial system from Precambrian to Devonian and found only two reports of pre-Silurian meandering system. Though some authors like Bridge (Bridge, 2006, p.156) has criticized this approach by Cotter however, Vandenberghe (2001, 2003) has provided more evidence on impact of vegetation on fluvial system from preglacial river systems. Whether a system adopts a meandering system or braided systems depends on patchiness of vegetation.
Flow chart of feedback loops of alluvial system prior to vegetation.
After Davies and Gibling et al, 2010
Root systems and vegetation cover of the trees and plants of forest, stabilized river banks and helped to change the dominant braided river system to increasing meandering river system.
2. Silicate Weathering and Pedogenesis:
Algeo-Scheckler (Algeo et al, 2010) have illustrated it beautifully, showing that soil penetration was shallow in Middle Devonian by less 20 cm which increased by Late Devonian to more 80 cm. Vascular plants affect silicate weathering by multiple processes, (i) acidification of soil by organic acids, (ii) increasing residence of water (iii) Increasing the depth of weathering by deep penetration of root systems.
After Algeo-Scheckler et al, 2010
The evidence for intensification of chemical weathering of silicate rocks comes from difference of sedimentation prior vegetation and after the vegetation cover. Prior to vegetation and in absence of K-chelation, K-feldspar have been more stable in terrigenous sediments (Ranganathan, 1983) and hence arkosic and to subarkosic sandstone were more common in fluvial sandstone (Hiscott et al., 1984).
IV. Eutrophication, Glaciation, Sea level variation and Mass extinction:
Due to opposing reports and disagreements among researchers about causes of Late Devonian eustatic sea-level changes and mass extinctions it is not easy to establish an acceptable mechanism for Anoxia and sea level changes and extinctions. However, the coincidence of certain events makes us able to have a general explanation. There were two extinction events in Late Devonian, the first one between Frasnian-Famennian (F-F) and second one between Devonian-Carboniferous (D-C) boundary (Sepkoski, 1996). F-F mass extinction is one of the big fives of mass extinctions (MaGhee,1996). One important aspect of this mass extinction is that most of its victims were shallow, warm-water taxa, reef taxa and pelagic taxa (Hallam and Wignall, 1997) however deep and cold water taxa survived mass extinction (McGhee, 1996).
The episodic mass extinctions as well as survival of cold, deep-water taxa make it difficult to explain the mass extinction as a result of eutrophication from increased nutrient flow as a result of Pedogenesis alone. However, eutrophication becomes a likely factor when it coincides with other processes to accelerate the process of mass killings. Some authors like Newell (Newell, 1967) and Johnson (Johnson, 1974) suggested a rapid regression was the cause of F-F mass extinction, however extinction process was started during high sea level and continued to regression during F-F mass extinction event. There are reports of two regressive-transgressive cycles during mass extinction event. Based on these two cycles Buggisch (Buggisch, 1991) provided an attractive mechanism to link eustatic sea-level changes with marine-terrestrial events to F-F mass extinction.
After Hallam-Wignall et al., 1999
The event begins with transgression which starts two simultaneous processes. Firstly, transgression floods the shelf area and makes a deep and anoxic environment which results in mass killings of benthos. Secondly, transgression bury large amount of organic carbon which initiate global cooling that triggers glaciations. Glaciations cause the sea level drops and expose the organic carbon for oxidation. The oxidation of organic carbon increase CO2 level in atmosphere which in turn results in warming and melting of ice and hence another transgression. Though this mechanism seems explains well the rapid icehouse and greenhouse cycles during F-F mass extinction however, evidences from Morocco and Poland do not show evidences for transgression and regression cycles as you can see in figure of eustatic sea level curves of the period and it is only North America and South China that support the suggested mechanism.
V. Conclusion:
Despite of somewhat contradicting evidences and explanations, the evidences for two stages mass extinctions, two cycles of eustatic sea level changes at least in large part of globe if not all the whole globe, black shale deposits, sulfide and sulfate {Barite deposits (Jewell et al., 1994)} deposits, increased Pedogenesis and change in characteristics fluvial sediments and fluvial systems show unusual conditions in Late Devonian that could be link to rise of forests. Having said that it is noteworthy that all these events were not linear and solely as a result of rise forestation but forestation played a major role to accelerate these events. Hence it is more logical to not overlook other processes during Devonian when constructing a model for marine-terrestrial teleconnections.
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