Land Plant Evolution

The rise of large vascular land plants during the Devonian perturbed the long-term carbon cycle not only by accelerating the weathering of silicate rocks, but also by bringing about the removal of atmospheric CO2 and production of O2 by increased burial of organic matter in sediments. The increased burial was due primarily to the creation by the plants of a new microbially resistant form of organic matter, lignin. Lignin is unique to woody land plants, and it is decomposed only by a very limited biota, such as white rot fungi (Hedges et al., 1988; Hedges and Oades, 1997). Ligniferous plant debris was deposited in terrestrial coal swamps, but also in the marine environment after transport there by rivers, where it joined the burial of marine-derived organics. Evidence that large amounts of lignin were buried in the Carboniferous and Permian is indicated by the vast coal reserves of this time span. Coal is formed primarily during deep burial by the transformation of lignin to more carbon-rich aromatic substances (Thomas, 2002).

There is some suggestion that initially lignin burial was unusually large because of the relative absence of the appropriate decomposing fungi (Robinson, 1990, 1991), which evolved somewhat later in the Paleozoic. This could account for the fact that combined Permian and Carboniferous coal reserves outweigh those for all other periods (Bestougeff, 1980), even though these coals are the oldest and have been subjected to loss by erosion over the longest time. This is illustrated in table 3.2. A uniquely high rate of burial of organic matter during the Permo-Carboniferous is also shown by the relative abundance of coal basin sediments at that time relative to subsequent periods (figure 3.5; Ronov, 1976).

Table 3.2. Abundance of potentially recoverable coal as a function of geological age.

Period

Coal mass (1015 mol/my)

Cambrian

0

Ordovician

0

Silurian

0

Devonian

1

Carboniferous

31

Permian

53

Triassic

1

Jurassic

30

Cretaceous

15

Tertiary

19

Masses are normalized per unit time to correct for different lengths of the geological periods. Data from Bestiougeff (1980) and Raiswell and Berner (1983).

Masses are normalized per unit time to correct for different lengths of the geological periods. Data from Bestiougeff (1980) and Raiswell and Berner (1983).

fraction as coal basins

0 tmmmtu

-400

-100

Figure 3.5. Abundance of coal basin sediments as a function of time expressed as a fraction of total terrigenous sediments (sandstones and shales). (Data from Ronov, 1976.)

Another factor contributing to the high abundance of Permian and Carboniferous organic matter deposition is the paleogeography of that time (Stanley, 1999). There was an abundance of flat land on the supercontinent of Pangaea, especially as coastal lowlands. Because sea level was relatively low at the time, areas that would ordinarily be covered by epeiric seas had become low-lying land, and, combined with high rainfall and low relief, very shallow groundwater levels gave rise to abundant freshwater and brackish water swamps. Preservation of deposited organic matter on land is appreciable only in O2-free environments, and swamps provided a place for this to happen. Little organic matter in normally well-drained soils is preserved and buried into the geologic record because of its oxidation to CO2 by atmospheric oxygen. Therefore, most paleosols are very low in organic matter (Retallack, 1990).

Burial of organic matter in swamps is accompanied by limited pyrite (FeS2) formation, resulting in a high ratio of organic carbon to pyrite sulfur in freshwater sediments as compared to marine sediments. This is because there is so much less dissolved sulfate, the source of pyrite sulfur, in fresh water than in seawater (Berner and Raiswell, 1983). As a result, the high rate of burial of nonmarine organic matter during the Permo-Carboniferous is accompanied by a high C/S ratio (figure 3.2).

There is no clear evidence that the rise of angiosperms during the Cretaceous resulted in greater global rates of organic matter burial. Cretaceous and Cenozoic coal is not markedly more abundant compared to coal deposited earlier. A notable anomaly is the Triassic, when coal deposition (table 3.2; figure 3.5), and organic matter burial in general (Berner and Canfield, 1989), was extremely low. This may have been due to a lack of proper climatic and topographic conditions for the formation of coal swamps on Pangaea, as demonstrated by the paucity of Triassic coal basin sediments (figure 3.5). The rise of C-4 plants during the late Miocene (Cerling et al., 1997), as typified by warm-climate grasslands, is an additional factor to be considered in organic matter burial. The C-4 plants fractionate carbon isotopes less than most other (C-3) plants (813C averages around -15%o), and contribution of C-4 plant debris to total global organic matter burial could have played a role in the latest portion of the observed drop in the mean value of A13C (Hayes et al., 1999) during the Cenozoic (figure 3.4).

Organic Gardening

Organic Gardening

Gardening is also a great way to provide healthy food for you and your loved ones. When you buy produce from the store, it just isnt the same as presenting a salad to your family that came exclusively from your garden worked by your own two hands.

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